Frustrated three-leg spin tubes: from spin 1/2 with chirality to spin 3/2

Motivated by the recent discovery of the spin tube [(CuCl$_2$tachH)$_3$Cl]Cl$_2$, we investigate the properties of a frustrated three-leg spin tube with antiferromagnetic intra-ring and inter-ring couplings. We pay special attention to the evolution of the properties from weak to strong inter-ring coupling and show on the basis of extensive density matrix renormalization group and exact diagonalization calculations that the system undergoes a first-order phase transition between a dimerized gapped phase at weak coupling that can be described by the usual spin-chirality model and a gapless critical phase at strong coupling that can be described by an effective spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in the gapped phase and slightly beyond. The implications for [(CuCl$_2$tachH)$_3$Cl]Cl$_2$ are discussed, with the conclusion that this system behaves essentially as a spin-3/2 chain.Comment: 8 pages, 9 figures, revised versio

Quantum Dimer Model on the triangular lattice: Semiclassical and variational approaches to vison dispersion and condensation

After reviewing the concept of vison excitations in Z_2 dimer liquids, we study the liquid-crystal transition of the Quantum Dimer Model on the triangular lattice by means of a semiclassical spin-wave approximation to the dispersion of visons in the context of a "soft-dimer" version of the model. This approach captures some important qualitative features of the transition: continuous nature of the transition, linear dispersion at the critical point, and \sqrt{12}x\sqrt{12} symmetry-breaking pattern. In a second part, we present a variational calculation of the vison dispersion relation at the RK point which reproduces the qualitative shape of the dispersion relation and the order of magnitude of the gap. This approach provides a simple but reliable approximation of the vison wave functions at the RK point.Comment: 12 pages, 10 figures. v2: minor changes, to appear in Phys. Rev.

Field-Induced Gap in a Quantum Spin-1/2 Chain in a Strong Magnetic Field

Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating $g$-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron spin resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field $H_{sat}=48.5$ T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG investigation of the spin-1/2 Heisenberg chain with a staggered transverse field

Dzyaloshinskii-Moriya anisotropy and non-magnetic impurities in the s=1/2s = 1/2 kagome system ZnCu_3(OH)_6Cl_2

Motivated by recent nuclear magnetic resonance experiments on ZnCu$_3$(OH)$_6$Cl$_2$, we present an exact-diagonalization study of the combined effects of non-magnetic impurities and Dzyaloshinskii-Moriya (DM) interactions in the $s = 1/2$ kagome antiferromagnet. The local response to an applied field and correlation-matrix data reveal that the dimer freezing which occurs around each impurity for $D = 0$ persists at least up to $D/J\simeq 0.06$, where $J$ and $D$ denote respectively the exchange and DM interaction energies. The phase transition to the ($Q = 0$) semiclassical, 120$^\circ$ state favored at large $D$ takes place at $D/J\simeq 0.1$. However, the dimers next to the impurity sites remain strong up to values $D \sim J$, far above this critical point, and thus do not participate fully in the ordered state. We discuss the implications of our results for experiments on ZnCu$_3$(OH)$_6$Cl$_2$.Comment: 11 pages, submitted to PR

Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum and thermodynamic properties are compatible with long range Neel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated Electron

Evidence of columnar order in the fully frustrated transverse field Ising model on the square lattice

Using extensive classical and quantum Monte Carlo simulations, we investigate the ground-state phase diagram of the fully frustrated transverse field Ising model on the square lattice. We show that pure columnar order develops in the low-field phase above a surprisingly large length scale, below which an effective U(1) symmetry is present. The same conclusion applies to the Quantum Dimer Model with purely kinetic energy, to which the model reduces in the zero-field limit, as well as to the stacked classical version of the model. By contrast, the 2D classical version of the model is shown to develop plaquette order. Semiclassical arguments show that the transition from plaquette to columnar order is a consequence of quantum fluctuations.Comment: 5 pages (including Supplemental Material), 5 figure

Spatially Resolved Magnetization in the Bose-Einstein Condensed State of BaCuSi2O6: Evidence for Imperfect Frustration

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC) phase in BaCuSi2O6, we performed detailed 63Cu and 29Si NMR above the critical magnetic field, Hc1= 23.4 T. The two different alternating layers present in the system have very different local magnetizations close to Hc1; one is very weak, and its size and field dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could only be determined by "on-site" 63Cu NMR, and the data are fully reproduced by a model of interacting hard-core bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed