Quantum Kagome antiferromagnet ZnCu3(OH)6Cl2

The frustration of antiferromagnetic interactions on the loosely connected kagome lattice associated to the enhancement of quantum fluctuations for S=1/2 spins was acknowledged long ago as a keypoint to stabilize novel ground states of magnetic matter. Only very recently, the model compound Herbersmithite, ZnCu3(OH)6Cl2, a structurally perfect kagome antiferromagnet, could be synthesized and enables a close comparison to theories. We review and classify various experimental results obtained over the past years and underline some of the pending issues.Comment: 23 pages, 16 figures, invited paper in J. Phys. Soc. Jpn, special topics issue on "Novel States of Matter Induced by Frustration", to be published in Jan. 201

Influence of electrical stimulation on regeneration of the radial nerve in dogs

The effects of biphasic electric fields on nerve regeneration that follows injury to the left radial nerve were studied in dogs by electromyography (EMG). Left and right radial nerves were crushed with a serrated haemostat. Stimulating electrodes were positioned proximally and distally to the site of the injury. The left nerves received rectangular, biphasic and current pulses (30 µA, 0.5 Hz) through the injury for two months. The right radial nerves were treated as controls and regenerated without electrical stimulation. EMG activities were recorded intramuscularly from the left and right musculus extensor digitalis communis (MEDC). Results obtained at the end of the two-month stimulation period showed a significant difference in EMG activity between the left (stimulated) and the right (non-stimulated) MEDC, suggesting that electrical treatment enhanced nerve regeneration

Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite

We report $^{35}$Cl NMR, ESR, $\mu$SR and specific heat measurements on the $S=1/2$ frustrated kagom\'e magnet kapellasite, $\alpha-$Cu$_3$Zn(OH)$_6$Cl$_2$, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction $J_1$ and give an energy scale for the competing interactions $|J| \sim 10$ K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term $D$, with $|D/J|\sim 3$%. These findings validate a posteriori the use of the $J_1 - J_2 - J_d$ Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagom\'e lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by $^{35}$Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and $\mu$SR in the low-$T$, correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes.Comment: 15 pages, 11 figures. To appear in Phys. Rev.

Experimental signatures of quantum and topological states in frustrated magnetism

Frustration in magnetic materials arising from competing exchange interactions can prevent the system from adopting long-range magnetic order and can instead lead to a diverse range of novel quantum and topological states with exotic quasiparticle excitations. Here, we review prominent examples of such emergent phenomena, including magnetically-disordered and extensively degenerate spin ices, which feature emergent magnetic monopole excitations, highly-entangled quantum spin liquids with fractional spinon excitations, topological order and emergent gauge fields, as well as complex particle-like topological spin textures known as skyrmions. We provide an overview of recent advances in the search for magnetically-disordered candidate materials on the three-dimensional pyrochlore lattice and two-dimensional triangular, kagome and honeycomb lattices, the latter with bond-dependent Kitaev interactions, and on lattices supporting topological magnetism. We highlight experimental signatures of these often elusive phenomena and single out the most suitable experimental techniques that can be used to detect them. Our review also aims at providing a comprehensive guide for designing and investigating novel frustrated magnetic materials, with the potential of addressing some important open questions in contemporary condensed matter physics

X-Band ESR Determination of Dzyaloshinsky-Moriya Interaction in 2D SrCu2_2(BO3_3)2_2 System

X-band ESR measurements on a single crystal of SrCu$_2$(BO$_3$)$_2$ system in a temperature range between 10 K and 580 K are presented. The temperature and angular dependence of unusually broad ESR spectra can be explained by the inclusion of antisymmetric Dzyaloshinsky-Moriya (DM) interaction, which yields by far the largest contribution to the linewidth. However, the well-accepted picture of only out-of-plane interdimer DM vectors is not sufficient for explanation of the observed angular dependence. In order to account for the experimental linewidth anisotropy we had to include sizable in-plane components of interdimer as well as intradimer DM interaction in addition to the out-of-plane interdimer one. The nearest-neighbor DM vectors lie perpendicular to crystal anisotropy c-axis due to crystal symmetry. We also emphasize that above the structural phase transition occurring at 395 K dynamical mechanism should be present allowing for instantaneous DM interactions. Moreover, the linewidth at an arbitrary temperature can be divided into two contributions; namely, the first part arising from spin dynamics governed by the spin Hamiltonian of the system and the second part due to significant spin-phonon coupling. The nature of the latter mechanism is attributed to phonon-modulation of the antisymmetric interaction, which is responsible for the observed linear increase of the linewidth at high temperatures.Comment: 17 pages, 4 figures, submitted to PR

Spin dynamics and disorder effects in the S = 1/2 kagome Heisenberg spin liquid phase of kapellasite

International audienceWe report 35 Cl NMR, ESR, µSR and specific heat measurements on the S = 1/2 frustrated kagomé magnet kapellasite, α−Cu3Zn(OH)6Cl2, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction J1 and give an energy scale for the competing interactions |J| ∼ 10 K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term D, with |D/J| ∼ 3 %. These findings validate a posteriori the use of the J1 − J2 − J d Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagomé lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by 35 Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and µSR in the low-T , correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low energy modes

The role of an interface in stabilizing reaction intermediates for hydrogen evolution in aprotic electrolytes

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electro-reduction reactions of HF, water and methanesulfonic acid (MSA) on the single crystal (111) facets of Au, Pt, Ir and Cu in organic aprotic electrolytes, 1 M LiPF(6) in EC/EMC 3:7W (LP57), the aprotic electrolyte commonly used in Li-ion batteries, 1 M LiClO(4) in EC/EMC 3:7W and 0.2 M TBAPF(6) in 3 : 7 EC/EMC. In our previous work, we have established that LiF formation, accompanied by H(2) evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li(+) adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having adsorbed Li(+). Li(+) is needed to facilitate the dissociation of HF which is the source of protons. Further experiments on other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F(−) or HO(−), a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy

Spin dynamics and disorder effects in the S = 1/2 kagome Heisenberg spin liquid phase of kapellasite

International audienceWe report 35 Cl NMR, ESR, µSR and specific heat measurements on the S = 1/2 frustrated kagomé magnet kapellasite, α−Cu3Zn(OH)6Cl2, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction J1 and give an energy scale for the competing interactions |J| ∼ 10 K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term D, with |D/J| ∼ 3 %. These findings validate a posteriori the use of the J1 − J2 − J d Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagomé lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by 35 Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and µSR in the low-T , correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low energy modes

Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR

We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x = 0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for all nuclei also scale with one another, despite the fact that the form factors for each site sample different regions of q-space. This result suggests a lack of any q-space structure in the dynamical spin susceptibility that might be expected in the presence of antiferromagnetic correlations. Rather, our results are more compatible with simple quasi-particle scattering. Furthermore, we find that the increase in the electric field gradient at the As cannot be accounted for by LDA calculations, suggesting that structural changes, in particular the position of the As in the unit cell, dominate the NQR response.Comment: 17 pages, 6 figure

Origin of magnetic moments in carbon nanofoam

A range of carbon nanofoam samples was prepared by using a high-repetition-rate laser ablation technique under various Ar pressures. Their magnetic properties were systematically investigated by dc magnetization measurements and continuous wave (cw) as well as pulsed EPR techniques. In all samples we found very large zero-field cooled-field-cooled thermal hysteresis in the susceptibility measurements extending up to room temperature. Zero-field cooled (ZFC) susceptibility measurements also display very complex behavior with a susceptibility maximum that strongly varies in temperature from sample to sample. Low-temperature magnetization curves indicate a saturation magnetization MS ≈0.35 emu g at 2 K and can be well fitted with a classical Langevin function. MS is more than an order of magnitude larger than any possible iron impurity, proving that the observed magnetic phenomena are an intrinsic effect of the carbon nanofoam. Magnetization measurements are consistent with a spin-glass type ground state. The cusps in the ZFC susceptibility curves imply spin freezing temperatures that range from 50 K to the extremely high value of >300 K. Further EPR measurements revealed three different centers that coexist in all samples, distinguished on the basis of g -factor and relaxation time. Their possible origin and the role in the magnetic phenomena are discussed