Evolution of Two-Gap Behavior of the Superconductor FeSe1-x

R. Khasanov et al, Physical Review Letters 104, 087004 (2010)

The superfluid density, ρs, of the iron chalcogenide superconductor, FeSe1-x, was studied as a function of pressure by means of muon-spin rotation. The analysis of ρs(T) within the two-gap scheme reveals that the effect on both, the transition temperature Tc and ρs(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.104.087004

Pressure Induced Static Magnetic Order in Superconducting FeSe1-x

M. Bendele et al, Physical Review Letters 104, 087003 (2010)

We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure ?0.8  GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above Tc and bulk superconductivity coexists and competes on short length scales with the magnetic order below Tc. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.104.087003

Strong H···F Hydrogen Bonds as Synthons in Polymeric Quantum Magnets: Structural, Magnetic, and Theoretical Characterization of [Cu(HF2)(pyrazine)2]SbF6, [Cu2F(HF)(HF2)(pyrazine)4](SbF6)2, and [CuAg(H3F4)(pyrazine)5](SbF6)2

J.L. Manson et al, Journal of the American Chemical Society 131, 6733 (2009)

Three Cu2+-containing coordination polymers were synthesized and characterized by experimental (X-ray diffraction, magnetic susceptibility, pulsed-field magnetization, heat capacity, and muon-spin relaxation) and electronic structure studies (quantum Monte Carlo simulations and density functional theory calculations). Read more...

Facility: SμS
Read full article:

http://pubs.acs.org/doi/full/10.1021/ja808761d

Characterization of the Antiferromagnetism in Ag(pyz)2(S2O8) (pyz = Pyrazine) with a Two-Dimensional Square Lattice of Ag2+ Ions

J.L. Manson et al, Journal of the American Chemical Society 131, 4590 (2009)

X-ray powder diffraction and magnetic susceptibility measurements show that Ag(pyz)2(S2O8) consists of 2D square nets of Ag2+ ions resulting from the corner-sharing of axially elongated AgN4O2 octahedra and exhibits characteristic 2D antiferromagnetism. Nevertheless, μ+SR measurements indicate that Ag(pyz)2(S2O8) undergoes 3D magnetic ordering below 7.8(3) K

Facility: SμS
Read full article:

http://pubs.acs.org/doi/full/10.1021/ja9005223

Electric-Field-Enhanced Neutralization of Deep Centers in GaAs

D. G. Eshchenko et al, Physical Review Letters 103, 216601 (2009)

The charge dynamics of hydrogenlike centers in semi-insulating GaAs have been studied by muon spin resonance in the presence of electric field and RF excitation. Electric-field-enhanced neutralization of deep electron and hole traps by track-induced hot carriers results in an increase of the excess electron's or hole's lifetimes. Similar processes may take place in semiconductor devices working at high voltages and/or under irradiation. As a consequence of the deep traps neutralization, the muonium (µ⁺+e^-) center can capture a hole.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.216601

Li Diffusion in LixCoO2 Probed by Muon-Spin Spectroscopy

J. Sugiyama, Physical Review Letters 103, 147601 (2009)

The diffusion coefficient of Li+ ions (DLi) in the battery material LixCoO2 has been investigated by muon-spin relaxation (µ+SR). Based on experiments in zero and weak longitudinal fields at temperatures up to 400 K, we determined the fluctuation rate (nu) of the fields on the muons due to their interaction with the nuclear moments. Combined with susceptibility data and electrostatic potential calculations, clear Li+ ion diffusion was detected above ~150 K. The DLi estimated from 'nu' was in very good agreement with predictions from first-principles calculations, and we present the µ+SR technique as an optimal probe to detect DLi for materials containing magnetic ions.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.147601

Spin Amplitude Modulation Driven Magnetoelectric Coupling in the New Multiferroic FeTe2O5Br

http://link.aps.org/doi/10.1103/PhysRevLett.103.147202

Superfluid Density and Energy Gap Function of Superconducting PrPt4Ge12

http://link.aps.org/doi/10.1103/PhysRevLett.103.147002

Pressure-Induced Insulator-to-Metal Transition in TbBaCo2O5.48

D. Chernyshov et al, Physical Review Letters 103, 125501 (2009)

TbBaCo₂O_5.48 has been studied by high-pressure synchrotron x-ray diffraction together with resistivity measurements as a function of temperature and pressure. It was found that under pressure a structural phase transition takes place corroborating with a sluggish insulator-to-metal transition. An onset of the metallic state was deduced from a gradual drop of resistivity at the range 3–10 GPa culminating into the change in sign of dR/dT, from negative to positive, at P ≥10 GPa; at the same pressure range there is a change of lattice strain components calculated from the unit cell dimensions. The changes in structural and transport properties are very similar to those found on heating at ambient pressure implying a common mechanism.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.125501

Orbital Order at Mn and O Sites and Absence of Zener Polaron Formation in Manganites

M. Garcia-Fernandez et al, Physical Review Letters 103, 097205 (2009)

We report the doping dependence of the ground state of A-site ordered manganites below and above half doping. Energy and polarization dependence of the orbital reflection, taken by resonant soft-x-ray powder diffraction, at both Mn L_2,3 and O K edges, provides direct evidence for orbital order at Mn³⁺ and oxygen sites and absence of Zener polaron formation. For x ≥ 0.2 anomalous melting of the orbital order is observed, which is coupled neither to magnetic ordering nor to a structural transition, indicating a two-dimensional character of the interactions.

Facility: SLS, SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.097205

Test of Lorentz Invariance with Spin Precession of Ultracold Neutrons

I. Altarev et al, Physical Review Letters 103, 081602 (2009)

A clock comparison experiment, analyzing the ratio of spin precession frequencies of stored ultracold neutrons and 199Hg atoms, is reported. No daily variation of this ratio could be found, from which is set an upper limit on the Lorentz invariance violating cosmic anisotropy field b_{perpendicular}<2×10^-20 eV (95% C.L.). This is the first limit for the free neutron. This result is also interpreted as a direct limit on the gravitational dipole moment of the neutron |g_n|<0.3 eV/c2 m from a spin-dependent interaction with the Sun. Analyzing the gravitational interaction with the Earth, based on previous data, yields a more stringent limit |g_n|<3×10^-4 eV/c²m.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.081602

Direct Observation of Charge Order and an Orbital Glass State in Multiferroic LuFe2O4

A. Mulders et al, Physical Review Letters 103, 077602 (2009)

Geometrical frustration of the Fe ions in LuFe₂O₄ leads to intricate charge and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the two types of Fe ions equals 4.0(1) eV corresponding to full charge separation into Fe²⁺ and Fe. The polarization and azimuthal angle dependence of the superlattice reflections demonstrate the absence of differences in anisotropic scattering revealing random orientations of the Fe2+ orbitals characteristic of an orbital glass state.

Facility: SLS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.077602

Superconductivity and Field-Induced Magnetism in SrFe1.75Co0.25As2

R. Khasanov et al, Physical Review Letters 103, 067010 (2009)

Using muon-spin rotation, we studied the in-plane (lambda_ab) and the out of plane (lambda_c) magnetic field penetration depth in SrFe_1.75Co_0.25As₂ (T_c approx 13.3 K). The penetration depth anisotropy gamma_lambda=lambda_c/lambda_ab increases from gamma_lambda approx 2.1 at T_c to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.067010

Collective Magnetic Excitations in the Spin Ladder Sr14Cu24O41 Measured Using High-Resolution Resonant Inelastic X-Ray Scattering

J. Schlappa et al, Physical Review Letters 103, 047401 (2009)

We investigate magnetic excitations in the spin-ladder compound Sr₁₄Cu₂₄O₄₁ using high-resolution Cu L₃ edge resonant inelastic x-ray scattering (RIXS). Our findings demonstrate that RIXS couples to two-triplon collective excitations. In contrast to inelastic neutron scattering, the RIXS cross section changes only moderately over the entire Brillouin zone, revealing high sensitivity also at small momentum transfers, allowing determination of the two-triplon energy gap as 100±30 meV. Our results are backed by calculations within an effective Hubbard model for a finite-size cluster, and confirm that optical selection rules are obeyed for excitations from this spherically symmetric quantum spin-liquid ground state.

Facility: SLS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.047401

Impurity-Induced Magnetic Order in Low-Dimensional Spin-Gapped Materials

J. Bobroff et al, Physical Review Letters 103, 047201 (2009)

We have studied the effect of nonmagnetic Zn impurities in the coupled spin ladder Bi(Cu_1-xZn_x)₂PO₆ using ³¹P NMR, muon spin resonance (µSR), and quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane, or spin-Peierls chains. This allows us to propose a simple common framework to explain the generic low-temperature impurity-induced freezings observed in low-dimensional spin-gapped materials.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.047201

Magnetic-Field-Enhanced Incommensurate Magnetic Order in the Underdoped High-Temperature Superconductor YBa2Cu3O6.45

http://link.aps.org/doi/10.1103/PhysRevLett.103.017001

Next-To-Leading Order QCD Corrections to pp -> tt'bb'+X at the LHC

A. Bredenstein et al, Physical Review Letters 103, 012002 (2009)

We report on the calculation of the full next-to-leading-order QCD corrections to the production of tt'bb' final states at the LHC, which deliver a serious background contribution to the production of a Higgs boson (decaying into a bb' pair) in association with a tt' pair. While the corrections significantly reduce the unphysical scale dependence of the leading-order cross section, our results predict an enhancement of the tt'bb' production cross section by a K factor of about 1.8.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.103.012002

On the Microstructure of Nanoporous Gold: An X-ray Diffraction Study

S. van Petegem et al, Nanoletters 9, 1158 (2009)

The evolution of the grain structure, internal strain, and the lattice misorientations of nanoporous gold during dealloying of bulk (3D) Ag−Au alloy samples was studied for the first time by various in situ and ex situ X-ray diffraction techniques including powder and Laue diffraction. The experiments reveal that the dealloying process preserves the original crystallographic structure but leads to a small spread in orientations within individual grains. Initially, most grains develop in-plane tensile stresses, which are partly released during further dealloying. Simultaneously, the feature size of the developing nanoporous structure increases with increasing dealloying time. Finally, microdiffraction experiments on dealloyed micron-sized nanoporous pillars reveal significant surface damage introduced by focused ion beam milling.

Read full article:

http://pubs.acs.org/doi/full/10.1021/nl803799q

Quantum Effects in a Weakly Frustrated S=1/2 Two-Dimensional Heisenberg Antiferromagnet in an Applied Magnetic Field

N. Tsyrulin et al, Physical Review Letters 102, 197201 (2009)

We have studied the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)2(ClO4)2 (where pz denotes pyrazine), using neutron inelastic scattering and series expansion calculations. We show that the presence of antiferromagnetic next-nearest-neighbor interactions enhances quantum fluctuations associated with resonating valence bonds. Intermediate magnetic fields lead to a selective tuning of resonating valence bonds and a spectacular inversion of the zone-boundary dispersion, providing novel insight into 2D antiferromagnetism in the quantum limit.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.197201

Two-Gap Superconductivity in Ba1-xKxFe2As2: A Complementary Study of the Magnetic Penetration Depth by Muon-Spin Rotation and Angle-Resolved Photoemission

R. Khasanov et al, Physical Review Letters 102, 187005 (2009)

We investigate the magnetic penetration depth lambda in superconducting Ba1-xKxFe2As2 (Tc approx 32 K) with muon-spin rotation (µSR) and angle-resolved photoemission (ARPES). Using µSR, we find the penetration-depth anisotropy gamma_lambda = lambda_c / lambda_ab and the second-critical-field anisotropy gamma_Hc2 to show an opposite T evolution below Tc. This dichotomy resembles the situation in the two-gap superconductor MgB2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth lambda_ab around 7 K. The complementarity of µSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T evolution of lambda_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.187005

The preparation and structure of salty ice VII under pressure

S. Klotz et al, Nature Materials 8, 405 (2009)

It is widely accepted that ice, no matter what phase, is unable to incorporate large amounts of salt into its structure. This conclusion is based on the observation that on freezing of salt water, ice expels the salt almost entirely as brine. Here, we show that this behaviour is not an intrinsic physico-chemical property of ice phases. We demonstrate by neutron diffraction that substantial amounts of dissolved LiCl can be built homogeneously into the ice VII structure if it is produced by recrystallization of its glassy (amorphous) state under pressure. Such ‘alloyed’ ice VII has significantly different structural properties compared with pure ice VII, such as an 8% larger unit cell volume, 5 times larger displacement factors, an absence of a transition to an ordered ice VIII structure and plasticity. Our study suggests that there could be a whole new class of ‘salty’ high-pressure ice forms.

Facility: SINQ
Read full article:

http://dx.doi.org/10.1038/NMAT2422

Anomalous Magnetic Excitations of Cooperative Tetrahedral Spin Clusters

K. Prsa et al, Physical Review Letters 102, 177202 (2009)

An inelastic neutron scattering study of Cu2Te2O5X2 (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above TN, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2O5Cl2 and the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach.

Facility: SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.177202

Magnetic-Field-Induced Soft-Mode Quantum Phase Transition in the High-Temperature Superconductor La1.855Sr0.145CuO4: An Inelastic Neutron-Scattering Study

J. Chang et al, Physical Review Letters 102, 177006 (2009)

Inelastic neutron-scattering experiments on the high-temperature superconductor La1.855Sr0.145CuO4 reveal a magnetic excitation gap Delta that decreases continuously upon application of a magnetic field perpendicular to the CuO2 planes. The gap vanishes at the critical field required to induce long-range incommensurate antiferromagnetic order, providing compelling evidence for a field-induced soft-mode driven quantum phase transition.

Facility: SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.177006

Measurement of the Transverse Polarization of Electrons Emitted in Free-Neutron Decay

A. Kozela et al, Physical Review Letters 102, 172301 (2009)

Both components of the transverse polarization of electrons (sigma_T1, sigma_T2) emitted in the beta-decay of polarized, free neutrons have been measured. The T-odd, P-odd correlation coefficient quantifying sigma_T2, perpendicular to the neutron polarization and electron momentum, was found to be R=0.008±0.015±0.005. This value is consistent with time reversal invariance and significantly improves limits on the relative strength of imaginary scalar couplings in the weak interaction. The value obtained for the correlation coefficient associated with sigma_T1, N=0.056±0.011±0.005, agrees with the Standard Model expectation, providing an important sensitivity test of the experimental setup.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.172301

Three- to Two-Dimensional Transition of the Electronic Structure in CaFe2As2: A Parent Compound for an Iron Arsenic High-Temperature Superconductor

C. Liu et al, Physical Review Letters 102, 167004 (2009)

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2—parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (Ts) Fermi surfaces around Gamma and X points are cylindrical and quasi 2D. Below Ts, the Gamma pocket forms a 3D ellipsoid, while the X pocket remains quasi 2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.

Facility: SLS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.167004

Muon Spin Rotation and Relaxation in the Superconducting Ferromagnet UCoGe

A. de Visser et al, Physical Review Letters 102, 167003 (2009)

We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature TC=3 K. The µ+ precession frequency persists below the bulk superconducting transition temperature Tsc=0.5 K, where it measures a local magnetic field Bloc=0.015 T. The amplitude of the µSR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.167003

Quantitative Radiography of Magnetic Fields Using Neutron Spin Phase Imaging

F.M. Piegsa et al, Physical Review Letters 102, 145501 (2009)

We report on a novel neutron radiography technique that uses the Ramsey principle, a method similar to neutron spin echo. For the first time quantitative imaging measurements of magnetic objects and fields could be performed. The strength of the spin-dependent magnetic interaction is detected by a change in the Larmor precession frequency of the neutron spins. Hence, one obtains in addition to the normal attenuation radiography image a so-called neutron spin phase image, which provides a two-dimensional projection of the magnetic field integrated over the neutron flight path.

Facility: SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.145501

Electronic Phase Separation in the Slightly Underdoped Iron Pnictide Superconductor Ba1-xKxFe2As2

J.T. Park et al, Physical Review Letters 102, 117006 (2009)

Here we present a combined study of the slightly underdoped novel pnictide superconductor Ba1-xKxFe2As2 by means of x-ray powder diffraction, neutron scattering, muon-spin rotation (µSR), and magnetic force microscopy (MFM). Static antiferromagnetic order sets in below Tm70 K as inferred from the neutron scattering and zero-field-µSR data. Transverse-field µSR below Tc shows a coexistence of magnetically ordered and nonmagnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting- or normal-state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.

Facility: SINQ, SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.117006

Direct Observation of Magnon Fractionalization in the Quantum Spin Ladder

B. Thielemann et al, Physical Review Letters 102, 107204 (2009)

We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua.

Facility: SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.107204

Fermi Surface and Order Parameter Driven Vortex Lattice Structure Transitions in Twin-Free YBa2Cu3O7

J.S. White et al, Physical Review Letters 102, 097001 (2009)

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

Facility: SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.097001

Quantized Hyperfine Field at an Implanted µ+ Site in PrPb3: Interplay between Localized f Electrons and an Interstitial Charged Particle

T.U. Ito et al, Physical Review Letters 102, 096403 (2009)

The local effect of an interstitial hydrogenlike particle on localized f electrons was studied in PrPb3 by means of µ+ spin rotation and relaxation. Spontaneous µ+ spin precession with harmonic frequencies was observed for the first time in f electron compounds. We demonstrate that the signal is derived from a coupling between the µ+ spin and the hyperfine-enhanced nuclear spin of nearest neighbor (NN) 141Pr with Ising-like anisotropy. The signal also suggests a marked suppression of spin dynamics of the NN 141Pr in comparison with that of the bulk 141Pr. These facts strongly indicate modification of the f electronic state due to the interstitial charged particle.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.096403

Tuning the Interlayer Spacing of High-Tc Bi-Based Superconductors by Intercalation: Measuring the Penetration Depth and the Two-Dimensional Superfluid Density

P.J. Baker et al, Physical Review Letters 102, 087002 (2009)

Substantial control of the interlayer spacing in Bi-based high temperature superconductors has been achieved through the intercalation of guest molecules between the superconducting layers. Measurements using implanted muons reveal that the penetration depth increases with increasing layer separation while Tc does not vary appreciably, demonstrating that the bulk superfluid density is not the determining factor controlling Tc. Our results strongly suggest that for Bi-based high temperature superconductors the superfluid density appearing in the Uemura scaling relation rho_s prop Tc should be interpreted as the two-dimensional density within the superconducting layers, which we find to be constant for each class of system investigated.

Facility: SμS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.102.087002

Coexistence of static magnetism and superconductivity in SmFeAsO1-xFx as revealed by muon spin rotation

A. Drew et al, Nature Materials 8, 310 (2009)

The recent observation of superconductivity with critical temperatures (Tc) up to 55 K in the pnictide RFeAsO1-xFx, where R is a lanthanide, marks the first discovery of a non-copper-oxide-based layered high-Tc superconductor. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprate superconductors, as both families exhibit superconductivity following charge doping of a magnetic parent material. In this context, it is important to follow the evolution of the microscopic magnetic properties of the pnictides with doping and hence to determine whether magnetic correlations coexist with superconductivity. Here, we present a muon spin rotation study on SmFeAsO1-xFx, with x=0–0.30 that shows that, as in the cuprates, static magnetism persists well into the superconducting regime. This analogy is quite surprising as the parent compounds of the two families have rather different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott–Hubbard insulator in the cuprates. Our findings therefore suggest that the proximity to magnetic order and associated soft magnetic fluctuations, rather than strong electronic correlations in the vicinity of a Mott–Hubbard transition, may be the key ingredients of high-Tc superconductors.

Facility: SμS
Read full article:

http://dx.doi.org/10.1038/NMAT2396

Competition between the pseudogap and superconductivity in the high-Tc copper oxides

T. Kondo et al, Nature 457, 296 (2009)

In a classical Bardeen–Cooper–Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature (T c), giving rise to a gap in the electronic energy spectrum. In the high-T c copper oxide superconductors, however, a pseudogap extends above T c. The relationship between the pseudogap and superconductivity is one of the central issues in this field. Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive. The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle-resolved photoemission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non-monotonicity reveals the presence of a competing state. We demonstrate a direct correlation, for different values of momenta and doping, between the loss in the low-energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing.

Read full article:

http://www.nature.com/nature/journal/v457/n7227/full/nature07644.html

The electronic phase diagram of the LaO1-xFxFeAs superconductor

H. Luetkens et al, Nature Materials 8, 305 (2009)

The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates1, heavy fermions2 and organic superconductors3. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors4, 5, 6. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state7, 8 and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO1-xFxFeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

Facility: SμS
Read full article:

http://dx.doi.org/10.1038/NMAT2397

Giant superconductivity-induced modulation of the ferromagnetic magnetization in a cuprate–manganite superlattice

J. Hoppler et al, Nature Materials 8, 315 (2009)

Artificial multilayers offer unique opportunities for combining materials with antagonistic orders such as superconductivity and ferromagnetism and thus to realize novel quantum states. In particular, oxide multilayers enable the utilization of the high superconducting transition temperature of the cuprates and the versatile magnetic properties of the colossal-magnetoresistance manganites. However, apart from exploratory work, the in-depth investigation of their unusual properties has only just begun. Here we present neutron reflectometry measurements of a [Y0.6Pr0.4Ba2Cu3O7 (10 nm)/La2/3Ca1/3MnO3 (10 nm)]10 superlattice, which reveal a surprisingly large superconductivity-induced modulation of the vertical ferromagnetic magnetization profile. Most surprisingly, this modulation seems to involve the density rather than the orientation of the magnetization and is highly susceptible to the strain, which is transmitted from the SrTiO3 substrate. We outline a possible explanation of this unusual superconductivity-induced phenomenon in terms of a phase separation between ferromagnetic and non-ferromagnetic nanodomains in the La2/3Ca1/3MnO3 layers.

Facility: SINQ
Read full article:

http://dx.doi.org/10.1038/NMAT2383

Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation

A.J. Drew et al, Nature Materials 8, 109 (2009)

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.

Facility: SµS
Read full article:

http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat2333.html

Spin-State Polarons in Lightly-Hole-Doped LaCoO3

A. Podlesnyak et al, Physical Review Letters 101, 247603 (2008)

Inelastic neutron scattering (INS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly-hole-doped La1-xSrxCoO3, x~0.002. Both INS and ESR low temperature spectra show intense excitations with large effective g factors ~10–18. NMR data indicate the creation of extended magnetic clusters. From the Q dependence of the INS magnetic intensity, we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions. The present INS, ESR, and NMR data give evidence for two regimes in the lightly-hole-doped samples: (i) T<35 K dominated by spin polarons; (ii) T>35 K dominated by thermally activated magnetic Co3+ ions.

Facility: SINQ
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.101.247603

Thermodynamics of the Spin Luttinger Liquid in a Model Ladder Material

C. Rüegg et al, Physical Review Letters 101, 247202 (2008)

The phase diagram in temperature and magnetic field of the metal-organic, two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the specific heat and the magnetocaloric effect. We demonstrate the presence of an extended spin Luttinger-liquid phase between two field-induced quantum critical points and over a broad range of temperature. Based on an ideal spin-ladder Hamiltonian, comprehensive numerical modeling of the ladder specific heat yields excellent quantitative agreement with the experimental data across the entire phase diagram.

Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.101.247202

Evidence for a Competition between the Superconducting State and the Pseudogap State of (BiPb)2(SrLa)2CuO(6+delta) from Muon Spin Rotation Experiments

R. Khasanov et al, Physical Review Letters 101, 227002 (2008)

The in-plane magnetic penetration depth lambda_(ab) in optimally doped (BiPb)2(SrLa)2CuO6+ (OP Bi2201) was studied by means of muon-spin rotation. The measurements of lambda_(ab)^(-2) (T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett. 98, 267004 (2007)], which suggest that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. The remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate.

Facility: SµS
Read full article:

http://link.aps.org/doi/10.1103/PhysRevLett.101.227002

Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation

A.J. Drew et al, Nature Materials doi:10.1038/nmat2333 (2008)

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.

Facility: SµS
Read full article:

http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat2333.html

Microscopic Understanding of Negative Magnetization in Cu, Mn, and Fe Based Prussian Blue Analogues

A. Kumar et al, Physical Review Letters 101, 207206 (2008)

A crossover of the field-cooled magnetization from positive to negative has been observed below the magnetic ordering temperature (17.9 K) in a multimetal Prussian Blue analogue (PBA), Cu0.73Mn0.77[Fe(CN)6]·zH2O. The reverse Monte Carlo (RMC) modeling (using the program RMCPOW) has been used to derive the various scattering contributions (e.g., nuclear diffuse, nuclear Bragg, magnetic diffuse, and magnetic Bragg) from the observed neutron diffraction patterns. The RMC analysis combined with the Rietveld refinement technique show an antiferromagnetic ordering of Mn moments with respect to the Cu as well as the Fe moments. Our study gives the first neutron magnetic structure evidence towards the microscopic understanding of the negative magnetization in the PBAs. This information can be effectively utilized to design suitable PBAs for making multifunctional devices.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v101/e207206

Avoided crossing of rattler modes in thermoelectric materials

M. Christensen et al, Nature Materials 7, 811 (2008)

Engineering of materials with specific physical properties has recently focused on the effect of nano-sized 'guest domains' in a 'host matrix' that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the 'phonon glass–electron crystal' concept and may be realized in host–guest systems. The guest entities are believed to have independent oscillations, so-called rattler modes, which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass–electron crystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low kappa_L is provided.

Facility: SINQ
Read full article:

http://www.nature.com/nmat/journal/v7/n10/full/nmat2273.html

Coupled Superconducting and Magnetic Order in CeCoIn5

M. Kenzelmann et al, Science 321, 1652 (2008)

Strong magnetic fluctuations can provide a coupling mechanism for electrons that leads to unconventional superconductivity. Magnetic order and superconductivity have been found to coexist in a number of magnetically mediated superconductors, but these order parameters generally compete. We report that close to the upper critical field, CeCoIn5 adopts a multicomponent ground state that simultaneously carries cooperating magnetic and superconducting orders. Suppressing superconductivity in a first-order transition at the upper critical field leads to the simultaneous collapse of the magnetic order, showing that superconductivity is necessary for the magnetic order. A symmetry analysis of the coupling between the magnetic order and the superconducting gap function suggests a form of superconductivity that is associated with a nonvanishing momentum.

Facility: SINQ
Read full article:

http://www.sciencemag.org/cgi/content/full/321/5896/1652?rss=1

Nature of the Magnetic Order in Ca3Co2O6

S. Agrestini et al, Physical Review Letters 101, 097207 (2008)

We present a detailed powder and single-crystal neutron diffraction study of the spin chain compound Ca3Co2O6. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order coexists with a shorter-range order with a correlation length scale of ~180 Å in the ab plane. Remarkably, on cooling, the volume of material exhibiting short-range order increases at the expense of the long-range order.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v101/e097207

Field and Temperature Dependence of the Superfluid Density in LaFeAsO1-xFx Superconductors: A Muon Spin Relaxation Study

H. Luetkens et al, Physical Review Letters 101, 097009 (2008)

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda_ab(0)=254(2) nm for x=0.1 and lambda_ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v101/e097009

Coherent d-Wave Superconducting Gap in Underdoped La2-xSrxCuO4 by Angle-Resolved Photoemission Spectroscopy

M. Shi et al, Physical Review Letters 101, 047002 (2008)

We present angle-resolved photoemission spectroscopy data on moderately underdoped La1.855Sr0.145CuO4 at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1.895Sr0.105CuO4 also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.

Facility: SLS
Read full article:

http://link.aps.org/abstract/PRL/v101/e047002

Quantum Hall Effect of Massless Dirac Fermions in a Vanishing Magnetic Field

K. Nomura et al, Physical Review Letters 100, 246806 (2008)

The effect of strong long-range disorder on the quantization of the Hall conductivity sigma_(xy) in graphene is studied numerically. It is shown that increasing Landau-level mixing progressively destroys all plateaus in sigma_(xy) except the plateaus at sigma_(xy)= F e^2/2h (per valley and per spin). The critical state at the Dirac point is robust to strong disorder and belongs to the universality class of the conventional plateau transitions in the integer quantum Hall effect. We propose that the breaking of time-reversal symmetry by ripples in graphene can realize this quantum critical point in a vanishing magnetic field.

Read full article:

http://link.aps.org/abstract/PRL/v100/e246806

Dislocation Cross-Slip in Nanocrystalline fcc Metals

E. Bitzek et al, Physical Review Letters 100, 235501 (2008)

Constant strain rate molecular dynamics simulations of nanocrystalline Al demonstrate that a significant amount of dislocations that have nucleated at the grain boundaries, exhibit cross-slip via the Fleischer mechanism as they propagate through the grain. The grain boundary structure is found to strongly influence when and where cross-slip occurs, allowing the dislocation to avoid local stress concentrations that otherwise can act as strong pinning sites for dislocation propagation.

Read full article:

http://link.aps.org/abstract/PRL/v100/e235501

Effect of the alpha-gamma Phase Transition on the Stability of Dislocation Loops in bcc Iron

S.L. Dudarev et al, Physical Review Letters 100, 135503 (2008)

Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the alpha-gamma phase transition temperature Tc=912 °C that is associated with the dramatic reduction of the shear stiffness constant c'=(c11-c12)/2 near Tc. This reduction of c' has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a<100> and a/2<111> prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a<100>, as opposed to the a/2<111>, Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures.

Read full article:

http://link.aps.org/abstract/PRL/v100/e135503

Irrational Versus Rational Charge and Statistics in Two-Dimensional Quantum Systems

C. Chamon et al, Physical Review Letters 100, 110405 (2008)

We show that quasiparticle excitations with irrational charge and irrational exchange statistics exist in tight-binding systems described, in the continuum approximation, by the Dirac equation in (2+1)-dimensional space and time. These excitations can be deconfined at zero temperature, but when they are, the charge rerationalizes to the value 1/2 and the exchange statistics to that of “quartons” (half-semions).

Read full article:

http://link.aps.org/abstract/PRL/v100/e110405

Magnetic and Orbital Ordering in the Spinel MnV2O4

V.O. Garlea et al, Physical Review Letters 100, 066404 (2008)

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2 O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

Read full article:

http://link.aps.org/abstract/PRL/v100/e066404

Manipulating the Magnetic Structure with Electric Fields in Multiferroic ErMn2O5

Y. Bodenthin et al, Physical Review Letters 100, 027201 (2008)

Based on measurements of soft x-ray magnetic diffraction under in situ applied electric field, we report on significant manipulation and exciting of commensurate magnetic order in multiferroic ErMn2O5. The induced magnetic scattering intensity arises at the commensurate magnetic Bragg position whereas the initial magnetic signal almost persists. We demonstrate the possibility to imprint a magnetic response function in ErMn2O5 by applying an electric field.

Facility: SLS
Read full article:

http://link.aps.org/abstract/PRL/v100/e027201

Unexpected High Stiffness of Ag and Au Nanoparticles

Q.F. Gu et al, Physical Review Letters 100, 045502 (2008)

We studied the compressibility of silver (10 nm) and gold (30 nm) nanoparticles, n-Ag and n-Au, suspended in a methanol-ethanol mixture by x-ray diffraction (XRD) with synchrotron radiation at pressures up to 30 GPa. Unexpectedly for that size, the nanoparticles show a significantly higher stiffness than the corresponding bulk materials. The bulk modulus of n-Au, K0=290(8) GPa, shows an increase of ca. 60% and is in the order of W or Ir. The structural characterization of both kinds of nanoparticles by XRD and high-resolution electron microscopy identified polysynthetic domain twinning and lamellar defects as the main origin for the strong decrease in compressibility.

Facility: SLS
Read full article:

http://link.aps.org/abstract/PRL/v100/e045502

Commensurate Spin Density Wave in LaFeAsO: A Local Probe Study

H. Klauss et al, Physical Review Letters 101, 077005 (2008)

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe Mössbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)µB at the iron site below TN=138 K, well separated from a structural phase transition at TS=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below TN are reproduced.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v101/e077005

Oxygen Isotope Effects on the Superconducting Transition and Magnetic States Within the Phase Diagram of Y1-xPrxBa2Cu3O7-delta

R. Khasanov et al, Physical Review Letters 101, 077001 (2008)

The various phases observed in all cuprate superconductors [superconducting (SC), spin-glass (SG), and antiferromagnetic (AFM)] were investigated with respect to oxygen-isotope (16O/18O) effects, using here as a prototype system of cuprates Y1-xPrxBa2Cu3O7-delta. All phases exhibit an isotope effect which is strongest where the respective phase terminates. In addition, the isotope effects on the magnetic phases (SG and AFM) are sign reversed as compared to the one on the superconducting phase. In the coexistence regime of the SG and SC phase a two-component behavior is observed where the isotope induced decrease of the superfluid density leads to a corresponding enhancement in the SG related density.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v101/e077001

Spatially Resolved Inhomogeneous Ferromagnetism in (Ga,Mn)As Diluted Magnetic Semiconductors: A Microscopic Study by Muon Spin Relaxation

V.G. Storchak et al, Physical Review Letters 101, 027202 (2008)

Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-µSR) as well as by transport and magnetization measurement techniques. LE-µSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v101/e027202

Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho

H. Luetkens et al, Physical Review Letters 101, 017601 (2008)

We report muon-spin relaxation measurements on the magnetic structures of RBaCo2O5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co3+ of CoO6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v101/e017601

Depth-Dependent Spin Dynamics of Canonical Spin-Glass Films: A Low-Energy Muon-Spin-Rotation Study

E. Morenzoni et al, Physical Review Letters 100, 147205 (2008)

We have performed depth dependent muon-spin-rotation and -relaxation studies of the dynamics of single layer films of AuFe and CuMn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (5–70 nm). A significant reduction in the muon-spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5–10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to, e.g., 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v100/e147205

Easy-Axis Kagome Antiferromagnet: Local-Probe Study of Nd3Ga5SiO14

A. Zorko et al, Physical Review Letters 100, 116601 (2008)

We report a local-probe investigation of the magnetically anisotropic kagome compound Nd3Ga5SiO14. Our zero-field muon spin relaxation (µSR) results provide direct evidence of a fluctuating collective paramagnetic state down to 60 mK, supported by a wipeout of the Ga nuclear magnetic resonance (NMR) signal below 25 K. At 60 mK a dynamics crossover to a much more static state is observed by µSR in magnetic fields above 0.5 T. Accordingly, the NMR signal is recovered at low T above a threshold field, revealing a rapid temperature and field variation of the magnetic fluctuations.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v100/e147201

Intrinsic Mobility Limit for Anisotropic Electron Transport in Alq3

A.J. Drew et al, Physical Review Letters 100, 116601 (2008)

Muon spin relaxation has been used to probe the charge carrier motion in the molecular conductor Alq3 (tris[8-hydroxy-quinoline] aluminum). At 290 K, the magnetic field dependence of the muon spin relaxation corresponds to that expected for highly anisotropic intermolecular electron hopping. Intermolecular mobility in the fast hopping direction has been found to be 0.23±0.03 cm2 V-1 s-1 in the absence of an electric- field gradient, increasing to 0.32±0.06 cm2 V-1 s-1 in an electric field gradient of 1 MV m-1. These intrinsic mobility values provide an estimate of the upper limit for mobility achievable in bulk material.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v100/e116601

Two-Step Magnetic Ordering in Quasi-One-Dimensional Helimagnets: Possible Experimental Validation of Villain's Conjecture about a Chiral Spin Liquid Phase

F. Cinti et al, Physical Review Letters 100, 057203 (2008)

Low-temperature specific heat, magnetic susceptibility, and zero-field muon spin resonance (µSR) measurements have been performed in the quasi-one-dimensional molecular helimagnetic compound Gd(hfac)3NITEt. The specific heat presents two anomalies at T0=2.19±0.02 K and TN=1.88±0.02 K, which both disappear upon the application of a weak magnetic field. Conversely, magnetic susceptibility and µSR data show the divergence of two-spin correlation functions only at TN=1.88±0.02 K. These results suggest an experimental validation of Villain's conjecture of a two-step magnetic ordering in quasi-one-dimensional XY helimagnets; i.e., the paramagnetic phase and the helical spin solid phase are separated by a chiral spin liquid phase, where translational invariance is broken without violation of rotational invariance.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v100/e057203

Direct Link between Low-Temperature Magnetism and High-Temperature Sodium Order in NaxCoO2

T.F. Schulze et al, Physical Review Letters 100, 026407 (2008)

We prove the direct link between low-temperature (T) magnetism and high-T Na+ ordering in NaxCoO2 using the example of a so far unreported magnetic transition at 8 K which involves a weak ferromagnetic moment. The 8 K feature is characterized in detail and its dependence on a diffusive Na+ rearrangement around 200 K is demonstrated. Applying muons as local probes this process is shown to result in a reversible phase separation into distinct magnetic phases that can be controlled by specific cooling protocols. Thus the impact of ordered Na+ Coulomb potential on the CoO2 physics is evidenced opening new ways to experimentally revisit the NaxCoO2 phase diagram.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v100/e026407

Anomalous Temperature Evolution of the Internal Magnetic Field Distribution in the Charge-Ordered Triangular Antiferromagnet AgNiO2

T. Lancaster et al, Physical Review Letters 100, 017206 (2008)

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet AgNiO2 are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of TN=19.9(2) K we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v100/e017206

Electronic Liquid Crystal State in the High-Temperature Superconductor YBa2Cu3O6.45

V. Hinkov et al, Science 319, 597 (2008)

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at millikelvin temperatures. We report the spontaneous onset of a one- dimensional, incommensurate modulation of the spin system in the high- transition- temperature superconductor YBa2Cu3O6.45 upon cooling below similar to 150 kelvin, whereas static magnetic order is absent above 2 kelvin. The evolution of this modulation with temperature and doping parallels that of the in- plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route toward electronic liquid crystals and that nematic order can coexist with high- temperature superconductivity in underdoped cuprates.

Facility: SµS
Read full article:

http://www.sciencemag.org/cgi/content/full/319/5863/597

Neutron Decoherence Imaging for Visualizing Bulk Magnetic Domain Structures

C. Grünzweig et al, Physical Review Letters 101, 025504 (2008)

Here we introduce a novel neutron imaging method, which is based on the effect that the spatial coherence of the neutron wave front can be changed through small-angle scattering of neutrons at magnetic domain walls in the specimen. We show that the technique can be used to visualize internal bulk magnetic domain structures that are difficult to access by other techniques. The method is transferable to a wide variety of specimens, extendable to three dimensions, and well suited for investigating materials under the influence of external parameters, as, e.g., external magnetic field, temperature, or pressure.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v101/e025504

Quantum Magnets under Pressure: Controlling Elementary Excitations in TlCuCl3

C. Rüegg et al, Physical Review Letters 100, 205701 (2008)

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v100/e205701

Uncovering Flux Line Correlations in Superconductors by Reverse Monte Carlo Refinement of Neutron Scattering Data

M. Laver et al, Physical Review Letters 100, 107001 (2008)

We describe the use of reverse Monte Carlo refinement to extract structural information from angle-resolved data of a Bragg peak. Starting with small-angle neutron scattering data, the positional order of an ensemble of flux lines in superconducting Nb is revealed. We discuss the uncovered correlation functions in the light of topical theories, in particular, the “Bragg glass” paradigm.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v100/e107001

Size Effect in the Spin Glass Magnetization of Thin AuFe Films as Studied by Polarized Neutron Reflectometry

M. Saoudi et al, Physical Review Letters 100, 057204 (2008)

We used polarized neutron reflectometry to determine the temperature dependence of the magnetization of thin AuFe films with 3% Fe concentration. We performed the measurements in a large magnetic field of 6 T in a temperature range from 295 to 2 K. For the films in the thickness range from 500 to 20 nm we observed a Brillouin-type behavior from 295 K down to 50 K and a constant magnetization of about 0.9µB per Fe atom below 30 K. However, for the 10 nm thick film we observed a Brillouin-type behavior down to 20 K and a constant magnetization of about 1.3µB per Fe atom below 20 K. These experiments are the first to show a finite-size effect in the magnetization of single spin-glass films in large magnetic fields. Furthermore, the ability to measure the deviation from the paramagnetic behavior enables us to prove the existence of the spin-glass state where other methods relying on a cusp-type behavior fail.

Read full article:

http://link.aps.org/abstract/PRL/v100/e057204

Superconducting Vortices in CeCoIn₅ - Towards the Pauli-Limiting Field

A. Bianchi et al, Science 319, 177 (2008)

Many superconducting materials allow the penetration of magnetic fields in a mixed state where the superfluid is threaded by a regular lattice of Abrikosov vortices each carrying one quantum of magnetic flux. While the phenomenological Ginzburg-Landau theory based on the concept of characteristic length scales has generally provided a good description of the Abrikosov vortex lat- tice state, the present neutron scattering measurements of the vortex lattice form factor in the heavy-fermion superconductor CeCoIn 5 , increases with increasing field - opposite to the expectations from the Abrikosov-Ginzburg- Landau paradigm. It is proposed that the anomalous field-dependence of the form factor arises from Pauli paramagnetic effects around the vortex cores and the proximity of the superconducting state to a quantum critical point.

Facility: SINQ
Read full article:

http://www.sciencemag.org/cgi/content/full/319/5860/177

Quantum Magnetism in the Paratacamite Family: Towards an Ideal Kagomé Lattice

P. Mendels et al, Physical Review Letters 98, 077204 (2007)

We report muon spin rotation measurements on the S=1/2 (Cu2+) paratacamite ZnxCu4-x(OH)6Cl2 family. Despite a Weiss temperature of ~ -300 K, the x=1 compound is found to have no transition to a magnetic frozen state down to 50 mK as theoretically expected for the kagomé Heisenberg antiferromagnet. We find that the limit between a dynamical and a partly frozen ground state occurs around x=0.5. For x=1, we discuss the relevance to a singlet picture.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v98/e077204

Formation of Hydrogen Impurity States in Silicon and Insulators at Low Implantation Energies

T. Prokscha et al, Physical Review Letters 98, 227401 (2007)

The formation of hydrogenlike muonium (Mu) has been studied as a function of implantation energy in intrinsic Si, thin films of condensed van der Waals gases (N 2, Ne, Ar, Xe), fused and crystalline quartz, and sapphire. By varying the initial energy of positive muons between 1 and 30 keV the number of electron-hole pairs generated in the ionization track of the muon can be tuned between a few and several thousand. The results show the strong suppression of the formation of those Mu states that depend on the availability of excess electrons. This indicates that the role of H-impurity states in determining electric properties of semiconductors and insulators depends on the way in which atomic H is introduced into the material.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v98/e227401

Muon Spin Rotation Studies of Spin Dynamics at Avoided Level Crossings in LiY0.998Ho0.002F4

M. Graf et al, Physical Review Letters 99, 267203 (2007)

We have studied the Ho3+ spin dynamics for LiY0.998Ho0.002F4 via the positive muon (µ+) transverse field depolarization rate TF as a function of temperature and magnetic field. We find sharp minima in TF(H) at fields for which the Ho3+ ion system has field-induced (avoided) level crossings. The reduction scales with calculated level repulsions, suggesting that µ+ depolarization by slow fluctuations of nonresonant Ho3+ spin states is partly suppressed when resonant tunneling opens new fluctuation channels at frequencies much greater than the muon precession frequency.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v99/e267203

Magnetic Phase Diagram of Layered Cobalt Dioxide LixCoO2

K. Mukai et al, Physical Review Letters 99, 087601 (2007)

The magnetism of Li xCoO 2 (LCO), which has a similar structure to Na xCoO 2 (NCO), has been investigated by muon-spin spectroscopy and susceptibility measurements using samples with x=0.1–1 prepared by an electrochemical reaction. In the x range below 0.75, LCO was found to be Pauli paramagnetic down to 1.8 K, suggesting an intermediate- or weak-coupling regime, although disordered local moments, with volume fractions below ~20%, appear at low T for LCO with x = 0.5. The phase diagram and interactions of LCO are thus strikingly different from NCO, while the differences cannot be explained simply by structural differences between the two systems.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v99/e087601

Multiple Gap Symmetries for the Order Parameter of Cuprate Superconductors from Penetration Depth Measurements

R. Khasanov et al, Physical Review Letters 99, 237601 (2007)

The temperature dependence of the London penetration depth 'lambda' was measured for an untwinned single crystal of YBa2Cu3O7- along the three principal crystallographic directions (a, b, and c). Both in-plane components ( lambda_a^-2 and lambda_b^-2) show an inflection point in their temperature dependence which is absent in the component along the c direction (lambda_c^-2). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of (s+d)-wave symmetry whereas it is mainly s wave along the c direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v99/e237601

Magnetism in Geometrically Frustrated YMnO3 under Hydrostatic Pressure Studied with Muon Spin Relaxation

T. Lancaster et al, Physical Review Letters 98, 197203 (2007)

The ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below TN70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no purely relaxing signals resolvable (which would require different coexisting spin distributions). The transition temperature TN increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v98/e197203

Experimental Estimates of Dephasing Time in Molecular Magnets

A. Keren et al, Physical Review Letters 98, 257204 (2007)

Muon spin relaxation measurements in isotropic molecular magnets (MM) with a spin value S ranging from 7/2 to 27/2 are used to determine the magnitude and origin of dephasing time of molecular magnets. It is found that the dephasing time ~10 nsec with no S or ligand dependence. This indicates a nuclear origin for the stochastic field. Since the dephasing time is a property of the environment, we argue that it is a number common to similar types of MM. Therefore, depahsing time is shorter than the Zener and tunneling times of anisotropic MM such as Fe8 or Mn4 for standard laboratory sweep rates. Our findings call for a stochastic Landau-Zener theory in this particular case.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v98/e257204

Experimental Evidence for Two Gaps in the High-Temperature La1.83Sr0.17CuO4 Superconductor

R. Khasanov et al, Physical Review Letters 98, 057007 (2007)

The in-plane magnetic field penetration depth (lambda_ab) in single-crystal La1.83Sr0.17CuO4 was investigated by muon-spin rotation (µSR). The temperature dependence of lambda_ab^-2 has an inflection point around 10–15 K, suggesting the presence of two superconducting gaps: a large gap (Delta_1) with d-wave and a small gap (Delta_2) with s-wave symmetry. The zero-temperature values of the gaps at µ0H=0.02 T were found to be Delta_1(0)=8.2(1) meV and Delta_2(0)=1.57(8) meV.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v98/e057007

Chiral-Like Critical Behavior in the Antiferromagnet Cobalt Glycerolate

F. Pratt et al, Physical Review Letters 99, 017202 (2007)

Critical exponents closely matching those of the N=2 chiral universality class have been obtained for the layered magnetic system cobalt glycerolate using muon spin relaxation. This class was originally introduced to represent geometrically frustrated triangular stacked-layer XY magnets with chiral noncollinear spin structures. Since the present magnetic system is a canted XY system without geometrical frustration or chiral degeneracy, the results indicate that the order parameter for canting in this system plays a similar role to the chiral order parameter in the geometrically frustrated systems, strongly suggesting that both types of noncollinear system share the same universality class.

Facility: SµS
Read full article:

http://link.aps.org/abstract/PRL/v99/e017202

Alternating Vorticity Bands in a Solution of Wormlike Micelles

V. Herle et al, Physical Review Letters 99, 158302 (2007)

Vorticity bands formed in a wormlike micellar solution has been structurally characterized by Rheo – small angle neutron scattering and video imaging experiments. Below a critical shear stress in Newtonian and shear-thinning regime, only a minor flow alignment of the micelles is observed. Above , in the shear-thickening regime, alternating transparent and turbid bands are formed. Triggered small-angle neutron scattering shows different anisotropic patterns in both bands indicating strongly aligned structures. By high-speed video imaging, it is shown that such an alignment of micelles does not correspond to a phase of lower viscosity.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v99/e158302

Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6

C. Rüegg et al, Physical Review Letters 98, 017202 (2007)

The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S=1/2 moments on a quasi-2D square lattice. We have investigated its spin dynamics by inelastic neutron scattering experiments on single crystals with an energy resolution considerably higher than in an earlier study. We observe multiple magnon modes, indicating clearly the presence of magnetically inequivalent dimer sites. The more complex spin Hamiltonian revealed in our study leads to a distinct form of magnon Bose-Einstein condensate phase with a spatially modulated condensate amplitude.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v98/e017202

Polymer Chain Dynamics in a Random Environment: Heterogeneous Mobilities

K. Niedzwiedz et al, Physical Review Letters 98, 168301 (2007)

We present a neutron scattering investigation on a miscible blend of two polymers with greatly different glass-transition temperatures Tg. Under such conditions, the nearly frozen high-Tg component imposes a random environment on the mobile chain. The results demand the consideration of a distribution of heterogeneous mobilities in the material and demonstrate that the larger scale dynamics of the fast component is not determined by the average local environment alone. This distribution of mobilities can be mapped quantitatively on the spectrum of local relaxation rates measured at high momentum transfers.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v98/e168301

New Insight into Cataract Formation: Enhanced Stability through Mutual Attraction

A. Stradner et al, Physical Review Letters 99, 198103 (2007)

Small-angle neutron scattering experiments and molecular dynamics simulations combined with an application of concepts from soft matter physics to complex protein mixtures provide new insight into the stability of eye lens protein mixtures. Exploring this colloid-protein analogy we demonstrate that weak attractions between unlike proteins help to maintain lens transparency in an extremely sensitive and nonmonotonic manner. These results not only represent an important step towards a better understanding of protein condensation diseases such as cataract formation, but provide general guidelines for tuning the stability of colloid mixtures, a topic relevant for soft matter physics and industrial applications.

Facility: SINQ
Read full article:

http://link.aps.org/abstract/PRL/v99/e198103

Cold Neutron Energy Dependent Production of Ultracold Neutrons in Solid Deuterium

F. Atchison et al, Physical Review Letters 99, 262502 (2007)

A measurement of the production of ultracold neutrons from velocity-selected cold neutrons on gaseous and solid deuterium targets is reported. The expected energy dependence for two-particle collisions with well defined neutron and Maxwell-Boltzmann distributed molecular velocities is found for the gas target. The solid target data agree in shape with the phonon density-of-states curve and provide strong evidence for the phonon model including multiphonon excitations.

Read full article:

http://link.aps.org/abstract/PRL/v99/e262502

Spinodal Decomposition in a Model Colloid-Polymer Mixture in Microgravity

A.E. Bailey et al, Physical Review Letters 99, 205701 (2007)

We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.

Read full article:

http://link.aps.org/abstract/PRL/v99/e205701

Noncentral Forces in Crystals of Charged Colloids

D. Reinke et al, Physical Review Letters 98, 038301 (2007)

The elastic properties of fcc crystals consisting of charge stabilized colloidal particles are determined from real space imaging experiments using confocal microscopy. The normal modes and the force constants of the crystal are obtained from the fluctuations of the particles around their lattice sites using the equipartition theorem. We show that the Cauchy relation is not fulfilled and that only noncentral many-body forces can account for the elastic properties.

Read full article:

http://link.aps.org/abstract/PRL/v98/e038301

Magnetic-Field-Induced Spin Excitations and Renormalized Spin Gap of the Underdoped La1.895Sr0.105CuO4 Superconductor

J. Chang et al, Physical Review Letters 98, 077004 (2007)

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La1.895Sr0.105CuO4 (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum transfer QIC=(0.5,0.5±,0),(0.5±,0.5,0) exhibits an anomaly at the superconducting Tc which broadens and shifts to lower temperature upon the application of a magnetic field along the c axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.

Read full article:

http://link.aps.org/abstract/PRL/v98/e077004

Nature of the Magnetic Order in the Charge-Ordered Cuprate La1.48 Nd0.4 Sr0.12 CuO4

N. Christensen et al, Physical Review Letters 98, 197003 (2007)

Using polarized neutron scattering we establish that the magnetic order in La1.48Nd0.4Sr0.12CuO4 is either (i) one dimensionally modulated and collinear, consistent with the stripe model or (ii) two dimensionally modulated with a novel noncollinear structure. The measurements rule out a number of alternative models characterized by 2D electronic order or 1D helical spin order. The low-energy spin excitations are found to be primarily transversely polarized relative to the stripe ordered state, consistent with conventional spin waves.

Read full article:

http://link.aps.org/abstract/PRL/v98/e197003

Surface of Strontium Titanate

R. Herger et al, Physical Review Letters 98, 76102 (2007)

We report the first complete determination, using surface x-ray diffraction, of the surface structure of TiO2-terminated SrTiO3(001), both at room temperature in vacuum, and also hot, under typical conditions used for thin film growth. The cold structure consists of a mixture of a (1×1) relaxation and (2×1) and (2×2) reconstructions. The latter disappear over several minutes upon heating. The structures are best modeled by a TiO2-rich surface similar to that proposed by Erdman et al. [Nature (London) 419, 55 (2002).]. Both reconstructions have been shown by density functional theory to be energetically favorable. The calculated (1×1) surface energy is higher, indicating that it may be a disordered mixture of the reconstructions. Atomic displacements are significant down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO3.

Read full article:

http://link.aps.org/abstract/PRL/v98/e076102

Experimental Evidence for a Crossover between Two Distinct Mechanisms of Amorphization in Ice Ih under Pressure

T. Strässle et al, Physical Review Letters 99, 175501 (2007)

We report neutron scattering data which reveal the central role of phonon softening leading to a negative melting line, solid-state amorphization, and negative thermal expansion of ice. We find that pressure-induced amorphization is due to mechanical melting at low temperatures, while at higher temperatures amorphization is governed by thermal melting (violations of Born's and Lindemann's criteria, respectively). This confirms earlier conjectures of a crossover between two distinct amorphization mechanisms and provides a natural explanation for the strong annealing observed in high-density amorphous ice.

Read full article:

http://link.aps.org/abstract/PRL/v99/e175501

Two energy scales in the spin excitations of the high-temperature superconductor La2-x Srx CuO4

B. Vignolle et al, Nature Physics 3, 163 (2007)

The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons. A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission and optical conductivity. Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La2-xSrxCuO4 are more structured than previously thought. The excitations have a two-component structure with a low-frequency component strongest around 18 meV and a broader component peaking near 40–70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission in the range 50–90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles.

Read full article:

http://www.nature.com/nphys/journal/v3/n3/full/nphys546.html

Structural Basis for the Conducting Interface between LaAlO3 and SrTiO3

P.R. Willmott et al, Physical Review Letters 99, 155502 (2007)

The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La1-xSrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti4+/Ti3+ across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.

Read full article:

http://link.aps.org/abstract/PRL/v99/e155502

Superfluid-Helium Converter for Accumulation and Extraction of Ultracold Neutrons

O. Zimmer et al, Physical Review Letters 99, 104801 (2007)

We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation.

Read full article:

http://link.aps.org/abstract/PRL/v99/e104801

Electron Fractionalization in Two-Dimensional Graphenelike Structures

C.Y. Hou et al, Physical Review Letters 98, 186809 (2007)

Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. In this Letter, we show that fractionally charged topological excitations exist on graphenelike structures, where quasiparticles are described by two flavors of Dirac fermions and time-reversal symmetry is respected. The topological zero modes are mathematically similar to fractional vortices in p-wave superconductors. They correspond to a twist in the phase in the mass of the Dirac fermions, akin to cosmic strings in particle physics.

Read full article:

http://link.aps.org/abstract/PRL/v98/e186809

Dual Character of the Electronic Structure of YBa2Cu4O8: The Conduction Bands of CuO2 Planes and CuO Chains

T. Kondo et al, Physical Review Letters 98, 157002 (2007)

We use microprobe angle-resolved photoemission spectroscopy (µARPES) to separately investigate the electronic properties of CuO2 planes and CuO chains in the high temperature superconductor, YBa2Cu4O8. For the CuO2 planes, a two-dimensional (2D) electronic structure is observed and, in contrast to Bi2Sr2CaCu2O8+, the bilayer splitting is almost isotropic and 50% larger, which strongly suggests that bilayer splitting has no direct effect on the superconducting properties. In addition, the scattering rate for the bonding band is about 1.5 times stronger than the antibonding band and is independent of momentum. For the CuO chains, the electronic structure is quasi-one-dimensional and consists of a conduction and insulating band. Finally, we find that the conduction electrons are well confined within the planes and chains with a nontrivial hybridization.

Read full article:

http://link.aps.org/abstract/PRL/v98/e157002

Time-Resolved Laue Diffraction of Deforming Micropillars

R. Maaß et al, Physical Review Letters 99, 145505 (2007)

We demonstrate real-time resolved white beam Laue diffraction during compression of micron-sized focused ion beam milled single crystals Au pillars, revealing the dynamical correlation between microstructure and plasticity. The evolution of the Laue patterns of the Au pillars demonstrates the occurrence of crystal rotation and strengthening is explained by plasticity starting on a slip system that is geometrically not predicted but selected because of the character of the preexisting strain gradient.

Read full article:

http://link.aps.org/abstract/PRL/v99/e145505

Z_2 Topological Term, the Global Anomaly, and the Two-Dimensional Symplectic Symmetry Class of Anderson Localization

S. Ryu et al, Physical Review Letters 99, 116601 (2007)

We discuss, for a two-dimensional Dirac Hamiltonian with a random scalar potential, the presence of a Z_2 topological term in the nonlinear sigma model encoding the physics of Anderson localization in the symplectic symmetry class. The Z_2 topological term realizes the sign of the Pfaffian of a family of Dirac operators. We compute the corresponding global anomaly, i.e., the change in the sign of the Pfaffian by studying a spectral flow numerically. This Z_2 topological effect can be relevant to graphene when the impurity potential is long ranged and, also, to the two-dimensional boundaries of a three-dimensional lattice model of Z_2 topological insulators in the symplectic symmetry class.

Read full article:

http://link.aps.org/abstract/PRL/v99/e116601

Million-atom molecular dynamics simulations of magnetic iron

P. Derlet et al, Progress in Materials Science 52, 299 (2007)

The problem of large-scale molecular dynamics simulations of iron has recently attracted attention in connection with the need to understand the microscopic picture of radiation damage in ferritic steels. In this paper we review the development of a new interatomic potential for magnetic iron, and describe the first large-scale atomistic simulations performed using the new method. We investigate the structure and thermally activated mobility of self-interstitial atom clusters and show that the spatial distribution of magnetic moments around a cluster is well correlated with the distribution of hydrostatic pressure, highlighting the significant part played by magneto-elasticity in the treatment of radiation damage. We show that self-interstitial atom clusters exhibit a transition from relatively immobile configurations containing 1 1 0-like groups of atoms to 1 1 1-like configurations occurring at a critical cluster size Nc 5 atoms. We discuss implications of this finding for the treatment of cascade damage effects, and the possibility of observing new low-temperature resistivity recovery stages in neutron-irradiated α-iron.

Read full article:

doi:10.1016/j.pmatsci.2006.10.011

Using spin polarised positive muons for studying guest molecule partitioning in soft matter structures

A. Martyniak et al, Physical Chemistry Chemical Physics 8, 4723 (2006)

Fully polarised positive muons substituted for protons in organic free radicals can be used as spin labels which reveal information about the structure, dynamics and environment of these radicals. In applications via the technique of avoided-level-crossing muon spin resonance (ALC-µSR), the positive muon has been used to study the partitioning of phenyl alcohols in lamellar phase colloidal dispersions of a cationic dichain surfactant. It was demonstrated the ALC capability in the study of partitioning of cosurfactant molecules in surfactant bilayers in order to elucidate the main factors which contribute to cosurfactant ordering at interfaces.

Facility: SµS
Read full article:

http://www.rsc.org/Publishing/Journals/CP/article.asp?doi=b610414b

Phase separation in superoxygenated La2-x Srx Cu O4+y

H.E. Mohottala et al, Nature Materials 5, 377 (2006)

The complex interplay between superconducting and magnetic phases remains poorly understood. This paper reports on the phase separation of doped holes into separate magnetic and superconducting regions in superoxygenated La 2-xSr xCuO 4+y, with various Sr contents. Irrespective of Sr-doping, excess oxygen raises the superconducting onset to 40 K with a coexisting magnetic spin-density wave that also orders near 40 K in each of our samples. The magnetic region is closely related to the anomalous, 1/8-hole-doped magnetic versions of La 2CuO 4, whereas the superconducting region is optimally doped. The two phases are probably the only truly stable ground states in this region of the phase diagram. This simple two-component system is a candidate for electronic phase separation in cuprate superconductors, and a key to understanding seemingly conflicting experimental observations.

Facility: SµS
Read full article:

http://www.nature.com/nmat/journal/v5/n5/full/nmat1633.html

Nitrogen Diffusion in Amorphous Silicon Nitride Isotope Multilayers Probed by Neutron Reflectometry

H. Schmidt et al, Physical Review Letters 96, 055901 (2006)

Spin-State Transition in LaCoO3: Direct Neutron Spectroscopic Evidence of Excited Magnetic States

A. Podlesnyak et al, Physical Review Letters 97, 247208 (2006)

Bose-Einstein Condensation of S=1 Nickel Spin Degrees of Freedom in NiCl2-4SC(NH2)2

V.S. Zapf et al, Physical Review Letters 96, 077204 (2006)