245 research outputs found
Dynamical Structure Factors for Dimerized Spin Systems
We discuss the transition strength between the disordered ground state and
the basic low-lying triplet excitation for interacting dimer materials by
presenting theoretical calculations and series expansions as well as inelastic
neutron scattering results for the material KCuCl_3. We describe in detail the
features resulting from the presence of two differently oriented dimers per
unit cell and show how energies and spectral weights of the resulting two modes
are related to each other. We present results from the perturbation expansion
in the interdimer interaction strength and thus demonstrate that the wave
vector dependence of the simple dimer approximation is modified in higher
orders. Explicit results are given in 10th order for dimers coupled in 1D, and
in 2nd order for dimers coupled in 3D with application to KCuCl_3 and TlCuCl_3.Comment: 17 pages, 6 figures, part 2 is based on cond-mat/021133
Random Bond Effect in the Quantum Spin System (TlK)CuCl
The effect of exchange bond randomness on the ground state and the
field-induced magnetic ordering was investigated through magnetization
measurements in the spin-1/2 mixed quantum spin system
(TlK)CuCl for . Both parent compounds TlCuCl and
KCuCl are coupled spin dimer systems, which have the singlet ground state
with excitation gaps K and 31 K, respectively. Due to
bond randomness, the singlet ground state turns into the magnetic state with
finite susceptibility, nevertheless, the excitation gap remains. Field-induced
magnetic ordering, which can be described by the Bose condensation of excited
triplets, magnons, was observed as in the parent systems. The phase transition
temperature is suppressed by the bond randomness. This behavior may be
attributed to the localization effect.Comment: 19 pages, 7 figures, 12 eps files, revtex, will appear in PR
Dispersive magnetic excitations in the S=1 antiferromagnet BaMnO
We present powder inelastic neutron scattering measurements of the S=1
dimerized antiferromagnet BaMnO. The K magnetic spectrum
exhibits a spin-gap of meV and a dispersive spectrum with
a bandwidth of approximately 1.5 meV. Comparison to coupled dimer models
describe the dispersion and scattering intensity accurately and determine the
exchange constants in BaMnO. The wave vector dependent scattering
intensity confirms the proposed S=1 dimer bond. Temperature dependent
measurements of the magnetic excitations indicate the presence of both
singlet-triplet and thermally activated triplet-quintet excitations.Comment: 8 pages, 8 figures, Submitted to Physical Review B, Resubmited
versio
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
Spin-resonance modes of the spin-gap magnet TlCuCl_3
Three kinds of magnetic resonance signals were detected in crystals of the
spin-gap magnet TlCuCl_3.
First, we have observed the microwave absorption due to the excitation of the
transitions between the singlet ground state and the excited triplet states.
This mode has the linear frequency-field dependence corresponding to the
previously known value of the zero-field spin-gap of 156 GHz and to the closing
of spin-gap at the magnetic field H_c of about 50 kOe.
Second, the thermally activated resonance absorption due to the transitions
between the spin sublevels of the triplet excitations was found. These
sublevels are split by the crystal field and external magnetic field.
Finally, we have observed antiferromagnetic resonance absorption in the
field-induced antiferromagnetic phase above the critical field H_c. This
resonance frequency is strongly anisotropic with respect to the direction of
the magnetic field.Comment: v.2: typo correction (one of the field directions was misprinted in
the v.1
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.
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]
Quantum Critical Point of the XY Model and Condensation of Field-Induced Quasiparticles in Dimer Compounds
The quantum critical point of the three-dimensional XY model in a
symmetry-preserving field is investigated. The results of Monte Carlo
simulations with the directed-loop algorithm show that the quantum critical
behavior is characterized by the mean-field values of critical exponents. The
system-size dependence of various quantities is compared to a simple
field-theoretical argument that supports the mean-field scaling
Pressure-Induced Magnetic Quantum Phase Transitions from Gapped Ground State in TlCuCl3
Magnetization maesurements under hydrostatic pressure were performed on an
S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic
field H parallel to the [2,0,1] direction. With increasing applied pressure P,
the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3
undergoes antiferromagnetic ordering. A spin-flop transition was observed at
Hsf=0.7T. The spin-flop field is approximately independent of pressure,
although the sublattice magnetization increases with pressure. The gap and Neel
temperature are presented as function is attributed to to the relative
enhancement of the interdimer exchange interactions compared with the
intradimer exchange interaction.Comment: 4pages,3figures To be published in J. Phys. Soc. Jpn. Vol.73 No.1
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