Observation of Field-Induced Transverse N\'{e}el Ordering in the Spin Gap System TlCuCl3_3

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic Bragg peaks which indicate the field-induced N\'{e}el ordering were observed for magnetic field higher than the gap field $H_{\rm g}\approx 5.5$ T at $Q=(h, 0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced N\'{e}el ordering as a Bose-Einstein condensation of magnons.Comment: 4 pages, 5 eps figures, jpsj styl

The Magnetization of Cu_2(C_5H_{12}N_2)_2Cl_4 : A Heisenberg Spin Ladder System

We study the magnetization of a Heisenberg spin ladder using exact diagonalization techniques, finding three distinct magnetic phases. We consider the results in relation to the experimental behaviour of the new copper compound Cu_2(C_5H_{12}N_2)_2Cl_4 and deduce that the compound is well described by such a model with a ratio of `chain' to `rung' bond strengths (J/J^\prime) of the order of 0.2, consistent with results from the magnetic susceptibility. The effects of temperature, spin impurities and additional diagonal bonds are presented and we give evidence that these diagonal bonds are indeed of a ferromagnetic nature.Comment: Latex file (4 pages), related figures (encapsulated postscript) appende

Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl(Cu1−x_{1-x}Mgx_x)Cl3_3

Neutron elastic and inelastic scattering measurements have been performed in order to investigate the spin structure and the magnetic excitations in the impurity-induced antiferromagnetic ordered phase of the doped spin gap system Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$ with $x=0.03$. The magnetic Bragg reflections indicative of the ordering were observed at ${\pmb Q}=(h, 0, l)$ with integer $h$ and odd $l$ below $T_{\rm N}=3.45$ K. It was found that the spin structure of the impurity-induced antiferromagnetic ordered phase on average in Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$ with $x=0.03$ is the same as that of the field-induced magnetic ordered phase for ${\pmb H} \parallel b$ in the parent compound TlCuCl$_3$. The triplet magnetic excitation was clearly observed in the $a^*$-$c^*$ plane and the dispersion relations of the triplet excitation were determined along four different directions. The lowest triplet excitation corresponding to the spin gap was observed at ${\pmb Q}=(h, 0, l)$ with integer $h$ and odd $l$, as observed in TlCuCl$_3$. It was also found that the spin gap increases steeply below $T_{\rm N}$ upon decreasing temperature. This strongly indicates that the impurity-induced antiferromagnetic ordering coexists with the spin gap state in Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$ with $x=0.03$.Comment: 24 pages, 7 figures, 11 eps files, revtex style, will appear in Phys. Rev.

The susceptibility and excitation spectrum of (VO)2_2P2_2O7_7 in ladder and dimer chain models

We present numerical results for the magnetic susceptibility of a Heisenberg antiferromagnetic spin ladder, as a function of temperature and the spin-spin interaction strengths $J_\perp$ and $J_{||}$. These are contrasted with new bulk limit results for the dimer chain. A fit to the experimental susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate, (VO)$_2$P$_2$O$_7$, gives the parameters $J_\perp = 7.82$ meV and $J_{||} = 7.76$ meV. With these values we predict a singlet-triplet energy gap of $E_{gap} = 3.9$ meV, and give a numerical estimate of the ladder triplet dispersion relation $\omega(k)$. In contrast, a fit to the dimer chain model leads to $J_1=11.11$ meV and $J_2=8.02$ meV, which predicts a gap of $E_{gap} = 4.9$ meV.Comment: 16 pages, 6 figures available upon request, RevTex 3.0, preprint ORNL-CCIP-94-04 / RAL-94-02

Non-local space-time supersymmetry on the lattice

We show that several well-known one-dimensional quantum systems possess a hidden nonlocal supersymmetry. The simplest example is the open XXZ spin chain with \Delta=-1/2. We use the supersymmetry to place lower bounds on the ground state energy with various boundary conditions. For an odd number of sites in the periodic chain, and with a particular boundary magnetic field in the open chain, we can derive the ground state energy exactly. The supersymmetry thus explains why it is possible to solve the Bethe equations for the ground state in these cases. We also show that a similar space-time supersymmetry holds for the t-J model at its integrable ferromagnetic point, where the space-time supersymmetry and the Hamiltonian it yields coexist with a global u(1|2) graded Lie algebra symmetry. Possible generalizations to other algebras are discussed.Comment: 12 page

Field-Induced Two-Step Phase Transitions in the Singlet Ground State Triangular Antiferromagnet CsFeBr3_3

The ground state of the stacked triangular antiferromagnet CsFeBr$_3$ is a spin singlet due to the large single ion anisotropy $D(S^z)^2$. The field-induced magnetic ordering in this compound was investigated by the magnetic susceptibility, the magnetization process and specific heat measurements for an external field parallel to the $c$-axis. Unexpectedly, two phase transitions were observed in the magnetic field $H$ higher than 3 T. The phase diagram for temperature versus magnetic field was obtained. The mechanism leading to the successive phase transitions is discussed.Comment: 8 pages, 9 figures, 10 eps files, jpsj styl

Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3

Magnetization and neutron elastic scattering measurements under a hydrostatic pressure were performed on KCuCl3, which is a three-dimensionally coupled spin dimer system with a gapped ground state. It was found that an intradimer interaction decreases with increasing pressure, while the sum of interdimer interactions increases. This leads to the shrinkage of spin gap. A quantum phase transition from a gapped state to an antiferromagnetic state occurs at Pc ? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal lattice points equivalent to those for the lowest magnetic excitation at zero pressure. This confirms that the spin gap decreases and closes under applied pressure.Comment: 7 pages, 10 figures, submitted to J. Phys. Soc. Jp

Lattice fermion models with supersymmetry

We investigate a family of lattice models with manifest N=2 supersymmetry. The models describe fermions on a 1D lattice, subject to the constraint that no more than k consecutive lattice sites may be occupied. We discuss the special properties arising from the supersymmetry, and present Bethe ansatz solutions of the simplest models. We display the connections of the k=1 model with the spin-1/2 antiferromagnetic XXZ chain at \Delta=-1/2, and the k=2 model with both the su(2|1)-symmetric tJ model in the ferromagnetic regime and the integrable spin-1 XXZ chain at \Delta=-1/\sqrt{2}. We argue that these models include critical points described by the superconformal minimal models.Comment: 28 pages. v2: added new result on mapping to XXZ chai

Frustration-Induced Two Dimensional Quantum Disordered Phase in Piperazinium Hexachlorodicuprate

Piperazinium Hexachlorodicuprate (PHCC) is shown to be a frustrated quasi-two-dimensional quantum Heisenberg antiferromagnet with a gapped spectrum. Zero-field inelastic neutron scattering and susceptibility and specific heat measurements as a function of applied magnetic field are presented. At T = 1.5 K, the magnetic excitation spectrum is dominated by a single propagating mode with a gap, Delta = 1 meV, and bandwidth of approximately 1.8 meV in the (h0l) plane. The mode has no dispersion along the b* direction indicating that neighboring a-c planes of the triclinic structure are magnetically decoupled. The heat capacity shows a reduction of the gap as a function of applied magnetic field in agreement with a singlet-triplet excitation spectrum. A field-induced ordered phase is observed in heat capacity and magnetic susceptibility measurements for magnetic fields greater than H_c1 approximately equal to 7.5 Tesla. Analysis of the neutron scattering data reveals the important exchange interactions and indicates that some of these are highly frustrated.Comment: 13 pages with 14 figures, 7 pages of text, 6 pages of figures. Submitted to Phys. Rev. B 4/7/2001. email comments to [email protected] or [email protected]

Critical Properties of Condensation of Field-Induced Triplet Quasiparticles

A review on the field-induced magnetic ordering is given, together with some results of a quantum Monte Carlo simulation focused on the critical behevior near the quantum critical point.Comment: Proceedings of SPQS, Sendai, 200