The first-order charge density wave (CDW) phase transition of Er 2 Ir 3 Si 5 is characterized by a crystal structure analysis, and electrical resistivity, magnetic susceptibility and specific heat measurements. The incommensurate CDW is accompanied by a strong lattice distortion, from which it is shown that the CDW resides on zigzag chains of iridium atoms. The CDW transition affects the magnitude of the local magnetic moments on Er 3+ , implying strong coupling between CDW and magnetism. This could account for the observation that magnetic order is suppressed down to at least 0.1 K in the high-quality single crystal presently studied. Any disorder in the crystallinity, as in ceramic material, broadens and suppresses the CDW transition, while magnetic order appears at 2.1 K.

We report on electron spin resonance (ESR) studies of the spin relaxation in Cs2CuCl4. The main source of the ESR linewidth at temperatures T ≤ 150 K is attributed to the uniform DzyaloshinskiiMoriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. Both the angular and temperature dependence of the ESR linewidth have been analyzed using a selfconsistent quantum-mechanical approach. In addition analytical expressions based on a quasiclassical picture for spin fluctuations are derived, which show good agreement with the quantumapproach for temperatures T ≥ 2J/kB ≈ 15 K. A small modulation of the ESR linewidth observed in the ac-plane is attributed to the anisotropic Zeeman interaction, which reflects the two magnetically nonequivalent Cu positions.

We present a high-temperature series expansion code for spin-1/2 Heisenberg models on arbitrary lattices. As an example we demonstrate how to use the application for an anisotropic triangular lattice with two independent couplings J 1 and J 2 and calculate the high-temperature series of the magnetic susceptibility and the static structure factor up to 12 th and 10 th order, respectively. We show how to extract effective coupling constants for the triangular Heisenberg model from experimental data on Cs 2 CuBr 4 . Nature of problem: Calculation of thermodynamic properties (magnetic susceptibility and static structure factor) for quantum magnets on arbitrary lattices. A particularly hard problem pose quantum magnets on so frustrated lattice geometries, as they can not be solved efficently by Quantum Monte Carlo methods. Solution method: High-temperature series expansions employing a linked-cluster expansion allow to obtain a high-order series in the inverse temperature for thermodynamic quantities in the thermodynamic limit. The resulting high-temperature series are exact up to the expansion order. We implement the calculation of high-temperature series for the zero-field magnetic susceptibility and static magnetic structure factor for the spin-1/2 Heisenberg model on arbitrary infinite lattices in arbitrary dimension. Program code and examples: http://www.comp-phys.org/lcse/

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We present a structural analysis of the substituted system (Ba1−xSrx)CuSi2O6, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with lowtemperature neutron and synchrotron powder diffraction, room-temperature and cryogenic highresolution NMR, as well as magnetic-and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x = 0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x = 0.1 and magnetic measurements prove that the Cu-dimers are preserved, yet with a slightly reduced intradimer coupling Jintra. Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einsteincondensation (BEC) of triplons, within the tetragonal crystal structure of I 41/acd. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

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The spin excitations in the spin-liquid phase of the anisotropic triangular lattice quantum antiferromagnet Cs2CuCl4 have been shown to propagate dominantly along the crystallographic b axis. To test this dimensional reduction scenario, we have performed ultrasound experiments in the spin-liquid phase of Cs2CuCl4 probing the elastic constant c22 and the sound attenuation along the b axis as a function of an external magnetic field along the a axis. We show that our data can be quantitatively explained within the framework of a nearest-neighbor spin-1/2 Heisenberg chain, where fermions are introduced via the Jordan-Wigner transformation and the spin-phonon interaction arises from the usual exchange-striction mechanism.PACS numbers: 43.35.+d, 75.10.Kt, 75.10.Pq, 72.55.+s Spin-liquid behavior can occur either in a spin-liquid ground state or in a spin-liquid phase at finite temperatures. One of the characteristic properties of spin liquids are strong short-range spin correlations in the absence of long-range magnetic order. Such a behavior has been observed in the magnetic insulator Cs 2 CuCl 4 , for example in inelastic neutron scattering experiments, 1 at temperatures between 0.6 K and 2.6 K in magnetic fields below the saturation field B c = 8.5 T. Experimentally, the boundary between the spin-liquid phase and the conventional paramagnetic phase has been characterized by broad peaks in the specific heat 2 and in the magnetic susceptibility. 3 Cs 2 CuCl 4 can be modeled by a spin-1/2 Heisenberg antiferromagnet on a spatially anisotropic triangular lattice with nearest-neighbor exchange couplings J = 4.34 K along the crystallographic b axis and J = 1.49 K ≈ J/3 along the diagonal links within the bc plane (see Fig. 1). 4 The interplane interaction J = 0.2 K and the Dzyaloshinskii-Moriya interaction D = 0.23 K can be neglected in the temperature range of the spinliquid phase. Given the fact that the diagonal coupling J is nonnegligible, one would naively expect an anisotropic two-dimensional spin liquid state. However, several independent calculations 5-15 found that the spin excitations in the anisotropic triangular lattice antiferromagnet are quasi-one-dimensional and propagate dominantly along the direction corresponding to the largest exchange coupling, which is the crystallographic b axis in Cs 2 CuCl 4 . In this work we shall give further evidence for this dimensional reduction scenario 16 by showing that ultrasound experiments probing the sound propagation along the b axis can be quantitatively explained using a onedimensional Heisenberg chain which is coupled to lattice vibrations via the usual exchange-striction mechanism. 17The spin-phonon interaction and the ultrasonic attenuation in two-dimensional spin liquids have recently been discussed by Zhou and Lee, 18 and by Serbyn and Lee. 19 In one-dimensional Heisenberg 20 and XY chains 21 the c 22 c b J J B x y FIG. 1. Part of the anisotropic triangular lattice formed by the spins in Cs2CuCl4. The largest exchange coupling J connects nearest-neighbor spi...

We present the temperature dependence of the electrical resistivity, the magnetic susceptibility, and the specific heat of a high-quality single crystal of La 3 Co 4 Sn 13 . As opposed to earlier reports on this system, these bulk properties exhibit clear anomalies at the phase transition at T * = 151(1) K, while the present data confirm the second-order character of this transition. X-ray diffraction with synchrotron radiation is used to solve the fourfold superstructure in space group I2 1 3 as it exists below T * in the charge-density-wave (CDW) state of La 3 Co 4 Sn 13 . Unlike conventional CDW systems, we have observed hysteresis between the zero-field-cooled and field-cooled magnetization below the CDW transition. This discrepancy can be attributed to a possible magnetic instability arising out of correlations of Co in the lattice, developing at the CDW transition. The crystal structure shows that any modifications of the electronic state of Co might be due to modified binding characteristics of the Sn atoms comprising the trigonal prismatic coordination of Co, while the coordination of Co itself is hardly changed at the phase transition. The superconducting transition is observed at T sc = 2.85(2) K. The superconducting energy gap is estimated as 5 K on the basis of the specific heat measured down to 0.1 K. These results suggest that La 3 Co 4 Sn 13 is a conventional weak-electron-phonon-coupling superconductor.