The layered honeycomb magnet α-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled j eff = 1/2 Ru 3+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on un-twinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the j eff = 1/2 picture once spin orbit coupling and electron correlations are included. Of the three nearest neighbour Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the b-axis non-equivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering [Banerjee et al.], in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction show a single magnetic transition upon cooling below TN ≈13 K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly-anisotropic exchange interactions.

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It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.

This article reviews experimental and theoretical work on Bose-Einstein condensation in quantum magnets. These magnets are natural realizations of gases of interacting bosons whose relevant parameters such as dimensionality, lattice geometry, amount of disorder, nature of the interactions, and particle concentration can vary widely between different compounds. The particle concentration can be easily tuned by applying an external magnetic field which plays the role of a chemical potential. This rich spectrum of realizations offers a unique possibility for studying the different physical behaviors that emerge in interacting Bose gases from the interplay between their relevant parameters. The plethora of other bosonic phases that can emerge in quantum magnets, of which the Bose-Einstein condensate is the most basic ground state, is reviewed. The compounds discussed in this review have been intensively studied in the last two decades and have led to important contributions in the area of quantum magnetism. In spite of their apparent simplicity, these systems often exhibit surprising behaviors. The possibility of using controlled theoretical approaches has triggered the discovery of unusual effects induced by frustration, dimensionality, or disorder.

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation (BEC) and superfluidity, which have been tested experimentally in a variety of different systems. When bosons are interacting, disorder can destroy condensation leading to a so-called Bose glass. This phase has been very elusive to experiments due to the absence of any broken symmetry and of a finite energy gap in the spectrum.Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (Br-doped dichloro-tetrakis-thiourea-Nickel, DTN).The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand-canonical ensemble; Br-doping introduces disorder in the hoppings and interaction strengths, leading to localization of the bosons into a Bose glass down to zero field, where it acquires the nature of an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the BEC (corresponding to a magnetically ordered phase) is marked by a novel, universal exponent governing the scaling on the critical temperature with the applied field, in excellent agreement arXiv:1109.4403v2 [cond-mat.str-el] 21 Sep 2011 2 with theoretical predictions. Our study represents the first, quantitative account of the universal features of disordered bosons in the grand-canonical ensemble.PACS numbers: 03.75. Lm, 71.23.Ft, 68.65.Cd, 72.15.Rn Introduction. Disorder can have a very strong impact on quantum fluids. Due to their wave-like nature, quantum particles are subject to destructive interference when scattering against disordered potentials. This leads to their quantum localization (or Anderson localization), which prevents e.g.electrons from conducting electrical currents in strongly disordered metals [1], and non-interacting bosons from condensing into a zero-momentum state [2]. Yet interacting bosons represent a matter wave with arbitrarily strong non-linearity, whose localization properties in a random environment cannot be deduced from the standard theory of Anderson localization. For strongly interacting bosons it is known that Anderson localization manifests itself in the Bose glass: in this phase the collective modes of the system -and not the individual particles -are Anderson-localized over arbitrarily large regions, leading to a gapless energy spectrum, and a finite compressibility of the fluid [3, 4]. Moreover nonlinear bosonic matter waves can undergo a localization-delocalization quantum phase transition in any spatial dimension when the interaction strength is varied [3, 4]; the transition brings the system from a non-interacting Anderson insulator to an interacting superfluid condensate, or from a superfluid to a Bose glass. Such a transition is relevant for a large variety of physical systems, including superfluid helium in porous media [6], Cooper pairs in disordered superconductors [7], and cold atoms in random optical potenti...

NiCl2-4SC(NH2)2 (DTN) is a quantum S = 1 chain system with strong easy-pane anisotropy and a new candidate for the Bose-Einstein condensation of the spin degrees of freedom. ESR studies of magnetic excitations in DTN in fields up to 25 T are presented. Based on analysis of the single-magnon excitation mode in the high-field spin-polarized phase and previous experimental results [ Phys. Rev. Lett. 96, 077204 (2006)], a revised set of spin-Hamiltonian parameters is obtained. Our results yield D = 8.9 K, Jc = 2.2 K, and J a,b = 0.18 K for the anisotropy, intrachain, and interchain exchange interactions, respectively. These values are used to calculate the antiferromagnetic phase boundary, magnetization and the frequency-field dependence of two-magnon bound-state excitations predicted by theory and observed in DTN for the first time. Excellent quantitative agreement with experimental data is obtained. PACS numbers: 75.40.Gb, 75.10.Jm Antiferromagnetic (AFM) quantum spin-1 chains have been the subject of intensive theoretical and experimental studies, fostered especially by the Haldane conjecture [1]. Due to quantum fluctuations, an isotropic spin-1 chain has a spin-singlet ground state separated from the first excited state by a gap ∆ ∼ 0.41J [2], where J is the exchange interaction. As shown by Golinelli et al. [3], the presence of a strong easy-plane anisotropy D can significantly modify the excitation spectrum, so that the gap size is not determined by the strength of the AFM quantum fluctuations exclusively, but depends on the dimensionless parameter ρ = D/J. The Haldane phase is predicted to survive up to ρ c = 0.93 [4], where the system undergoes a quantum phase transition. For ρ > ρ c the gap reopens, but its origin is dominated by the anisotropy D, and the system is in the so-called large-D regime. While the underlying physics of Haldane chains is fairly well understood, relatively little is known about the magnetic properties (and particularly the elementary excitation spectrum) of nonHaldane S = 1 AFM chains in the large-D phase. Intense theoretical work and numerous predictions [3,4,5,6,7,8,9,10] make the experimental investigation of large-D spin-1 chains a topical problem in low-dimensional magnetism.Recently, weakly-coupled spin-1 chains have attracted renewed interest due to their possible relevance to the fieldinduced Bose-Einstein condensation (BEC) of magnons. When the field H, applied perpendicular to the easy plane, exceeds a critical value H c1 (defined at T = 0), the gap closes and the system undergoes a transition into an XY -like AFM phase with a finite magnetization and AFM magnon excitations. If the spin Hamiltonian has axial symmetry with respect to the applied field, the AFM ordering can be described as BEC of magnons by mapping the spin-1 system into a gas of semi-hard-core bosons [11]. The applied field plays the role of a chemical potential, changing the boson population. In accordance with mean-field BEC theory [12,13,14], the phasediagram boundary for a three-dimensional system sh...

We report the magnetic field dependence of the low temperature specific heat of single crystals of the first Pr-based heavy fermion superconductor PrOs4Sb12. The low temperature specific heat and the magnetic phase diagram inferred from specific heat, resistivity and magnetisation provide compelling evidence of a doublet ground state and hence superconductivity mediated by quadrupolar fluctuations. This establishes PrOs4Sb12 as a very strong contender of superconductive pairing that is neither electron-phonon nor magnetically mediated.PACS numbers: 74.70. Tx, 7127,+a, 75.30.Mb Superconductivity mediated by a pairing potential other than a conventional electron-phonon interaction has been the subject of a very large number of theoretical and experimental investigations over the decades. In recent years, finally, intermetallic compounds have been discovered which, together with the high-T c cuprates, represent prime candidates for magnetically mediated pairing. Surprisingly, however, magnetically mediated pairing thus far has been considered the only serious alternative to electron-phonon mediated pairing, while excitonic and polaronic mechanisms have also been proposed.We have recently reported the discovery of the first Pr-based heavy fermion superconductor PrOs 4 Sb 12 [1], for which the nonmagnetic ground state appeared best described as a crystalline electric field (CEF) doublet. This in turn suggested that the heavy electron liquid is of quadrupolar origin and that consequently quadrupolar fluctuations mediate the superconductive pairing. PrOs 4 Sb 12 hence is a candidate for being the first material in which neither electron-phonon nor magnetic interactions mediate the pairing. However, a magnetic origin could not be completely ruled out [1] and recent low temperature specific heat measurements are reported to be consistent with a singlet CEF ground state hence questioning the hypothesis of quadrupolar pairing [2].In order to settle the question of the ground state and thus the possibility of the first example of quadrupolar mediated superconductive pairing demands definitive establishement of (i) the CEF level scheme, (ii) the unconventional nature of the superconductivity, and (iii) coupling of the CEF excitations to the conduction electrons. Here we report specific heat measurements of high quality single crystals of PrOs 4 Sb 12 at low T and high magnetic field. We show that the zero field data of the single crystals and the magnetic phase diagram as established from the specific heat, resistivity and magnetisation, as well as the observation of a novel high field ground state, provide compelling evidence of a nonmagnetic doublet CEF ground state intimately linked to the conduction electrons. This unambiguously establishes quadrupolar fluctuations as the most likely pairing mechanism. Moreover, we observe two superconducting transitions giving evidence of two distinct superconducting phases.Previous experiments on pressed pellets of tiny single crystals of PrOs 4 Sb 12 [1] revealed superconductivity ...

We present the first example of magnetic ordering-induced multiferroic behavior in a metal-organic framework magnet. This compound is [CH3NH3][Co(HCOO)3] with a perovskite-like structure. The A-site [CH3NH3](+) cation strongly distorts the framework, allowing anisotropic magnetic and electric behavior and coupling between them to occur. This material is a spin canted antiferromagnet below 15.9 K with a weak ferromagnetic component attributable to Dzyaloshinskii-Moriya (DM) interactions and experiences a discontinuous hysteretic magnetic-field-induced switching along [010] and a more continuous hysteresis along [101]. Coupling between the magnetic and electric order is resolved when the field is applied along this [101]: a spin rearrangement occurs at a critical magnetic field in the ac plane that induces a change in the electric polarization along [101] and [10-1]. The electric polarization exhibits an unusual memory effect, as it remembers the direction of the previous two magnetic-field pulses applied. The data are consistent with an inverse-DM mechanism for multiferroic behavior.