51 research outputs found
Observation of Field-Induced Transverse N\'{e}el Ordering in the Spin Gap System TlCuCl
Neutron elastic scattering experiments have been performed on the spin gap
system TlCuCl in magnetic fields parallel to the -axis. The magnetic
Bragg peaks which indicate the field-induced N\'{e}el ordering were observed
for magnetic field higher than the gap field T at with odd in the 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(CuMg)Cl
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(CuMg)Cl with . The magnetic Bragg reflections
indicative of the ordering were observed at with integer
and odd below K. It was found that the spin structure
of the impurity-induced antiferromagnetic ordered phase on average in
Tl(CuMg)Cl with is the same as that of the
field-induced magnetic ordered phase for in the parent
compound TlCuCl. The triplet magnetic excitation was clearly observed in
the - 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 with integer
and odd , as observed in TlCuCl. It was also found that the spin gap
increases steeply below upon decreasing temperature. This strongly
indicates that the impurity-induced antiferromagnetic ordering coexists with
the spin gap state in Tl(CuMg)Cl with .Comment: 24 pages, 7 figures, 11 eps files, revtex style, will appear in Phys.
Rev.
The susceptibility and excitation spectrum of (VO)PO 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 and . 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)PO, gives the parameters meV and meV. With these values we predict a singlet-triplet energy gap of
meV, and give a numerical estimate of the ladder triplet
dispersion relation . In contrast, a fit to the dimer chain model
leads to meV and meV, which predicts a gap of 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 CsFeBr
The ground state of the stacked triangular antiferromagnet CsFeBr is a
spin singlet due to the large single ion anisotropy . 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 -axis. Unexpectedly, two
phase transitions were observed in the magnetic field 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
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