2,889 research outputs found
Complete bond-operator theory of the two-chain spin ladder
The discovery of the almost ideal, two-chain spin-ladder material
(C_5H_12N)_2CuBr_4 has once again focused attention on this most fundamental
problem in low-dimensional quantum magnetism. Within the bond-operator
framework, three qualitative advances are introduced which extend the theory to
all finite temperatures and magnetic fields in the gapped regime. This
systematic description permits quantitative and parameter-free experimental
comparisons, which are presented for the specific heat, and predictions for
thermal renormalization of the triplet magnon excitations.Comment: 12 pages, 10 figure
Circulating-current states and ring-exchange interactions in cuprates
We consider the consequences for circulating-current states of a cyclic,
four-spin, ``ring-exchange'' interaction of the type shown recently to be
significant in cuprate systems. The real-space Hartree-Fock approach is used to
establish the existence of charge-current and spin-current phases in a
generalized Hubbard model for the CuO_2 planes in cuprates. We compare the
results of the Hartree-Fock approximation with the correlated states
renormalized by Gutzwiller projection factors which allows us to gauge the
qualitative effects of projection to no double site occupancy. We find that
charge flux states may be competitive in cuprates, whereas spin flux states are
suppressed in the strongly correlated regime. We then include the ring-exchange
interaction and demonstrate its effect on current-carrying states both at and
away from half-filling.Comment: 14 pages, 11 figure
Nonlinear sigma Model Treatment of Quantum Antiferromagnets in a Magnetic Field
We present a theoretical analysis of the properties of low-dimensional
quantum antiferromagnets in applied magnetic fields. In a nonlinear sigma model
description, we use a spin stiffness analysis, a 1/N expansion, and a
renormalization group approach to describe the broken-symmetry regimes of
finite magnetization, and, in cases of most interest, a low-field regime where
symmetry is restored by quantum fluctuations. We compute the magnetization,
critical fields, spin correlation functions, and decay exponents accessible by
nuclear magnetic resonance experiments. The model is relevant to many systems
exhibiting Haldane physics, and provides good agreement with data for the
two-chain spin ladder compound CuHpCl.Comment: 14 pages, 6 figures, full paper to accompany cond-mat/980415
Multi-triplet bound states and finite-temperature dynamics in highly frustrated quantum spin ladders
Low-dimensional quantum magnets at finite temperatures present a complex
interplay of quantum and thermal fluctuation effects in a restricted phase
space. While some information about dynamical response functions is available
from theoretical studies of the one-triplet dispersion in unfrustrated chains
and ladders, little is known about the finite-temperature dynamics of
frustrated systems. Experimentally, inelastic neutron scattering studies of the
highly frustrated two-dimensional material SrCu(BO) show an almost
complete destruction of the one-triplet excitation band at a temperature only
1/3 of its gap energy, accompanied by strong scattering intensities for
apparent multi-triplet excitations. We investigate these questions in the
frustrated spin ladder and present numerical results from exact diagonalization
for the dynamical structure factor as a function of temperature. We find
anomalously rapid transfer of spectral weight out of the one-triplet band and
into both broad and sharp spectral features at a wide range of energies,
including below the zero-temperature gap of this excitation. These features are
multi-triplet bound states, which develop particularly strongly near the
quantum phase transition, fall to particularly low energies there, and persist
to all the way to infinite temperature. Our results offer valuable insight into
the physics of finite-temperature spectral functions in SrCu(BO)
and many other highly frustrated spin systems.Comment: 22 pages, 19 figures; published version: many small modification
Static impurities in the kagome lattice: dimer freezing and mutual repulsion
We consider the effects of doping the S = 1/2 kagome lattice with static
impurities. We demonstrate that impurities lower the number of low-lying
singlet states, induce dimer-dimer correlations of considerable spatial extent,
and do not generate free spin degrees of freedom. Most importantly, they
experience a highly unconventional mutual repulsion as a direct consequence of
the strong spin frustration. These properties are illustrated by exact
diagonalization, and reproduced to semi-quantitative accuracy within a dimer
resonating-valence-bond description which affords access to longer length
scales. We calculate the local magnetization induced by doped impurities, and
consider its implications for nuclear magnetic resonance measurements on known
kagome systems.Comment: 9 pages, 12 figure
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