113 research outputs found
Effect of Hund coupling in the one-dimensional SU(4) Hubbard model
The one-dimensional SU(4) Hubbard model perturbed by Hund coupling is
studied, away from half-filling, by means of renormalization group and
bosonization methods. A spectral gap is always present in the spin-orbital
sector irrespective of the magnitude of the Coulomb repulsion. We further
distinguish between two qualitatively different regimes. At small Hund
coupling, we find that the symmetry of the system is dynamically enlarged to
SU(4) at low energy with the result of {\it coherent} spin-orbital excitations.
When the charge sector is not gapped, a superconducting instability is shown to
exist. At large Hund coupling, the symmetry is no longer enlarged to SU(4) and
the excitations in the spin sector become {\it incoherent}. Furthermore, the
superconductivity can be suppressed in favor of the conventional charge density
wave state.Comment: 10 pages, 1 figur
Quantum Phase Transitions in the One-Dimensional S=1 Spin-Orbital Model: Implications for Cubic Vanadates
We investigate ground-state properties and quantum phase transitions in the
one-dimensional S=1 spin-orbital model relevant to cubic vanadates. Using the
density matrix renormalization group, we compute the ground-state energy, the
magnetization and the correlation functions for different values of the Hund's
coupling and the external magnetic field. It is found that the
magnetization jumps at a certain critical field, which is a hallmark of the
field-induced first-order phase transition. The phase transition driven by
is also of first order. We also consider how the lattice-induced
ferro-type interaction between orbitals modifies the phase diagram, and discuss
the results in a context of the first-order phase transition observed in
YVO at 77K.Comment: 7 pages, 7 figur
Magnetic impurities in the one-dimensional spin-orbital model
Using one-dimensional spin-orbital model as a typical example of quantum spin
systems with richer symmetries, we study the effect of an isolated impurity on
its low energy dynamics in the gapless phase through bosonization and
renormalization group methods. In the case of internal impurities, depending on
the symmetry, the boundary fixed points can be either an open chain with a
residual spin or (and) orbital triplet left behind, or a periodic chain.
However, these two fixed points are indistinguishable in the sense that in both
cases, the lead-correction-to-scaling boundary operators (LCBO) only show
Fermi-liquid like corrections to thermodynamical quantities. (Except the
possible Curie-like contributions from the residual moments in the latter
cases.) In the case of external (Kondo) impurities, the boundary fixed points,
depending on the sign of orbital couplings, can be either an open chain with an
isolated orbital doublet due to Kondo screening or it will flow to an
intermediate fixed point with the same LCBO as that of the two-channel Kondo
problem. Comparison with the Kondo effect in one-dimensional (1D) Heisenberg
spin chain and multi-band Hubbard models is also made.Comment: 7 pages, No figur
Spin-orbital gapped phase with least symmetry breaking in the one-dimensional symmetrically coupled spin-orbital model
To describe the spin-orbital energy gap formation in the one-dimensional
symmetrically coupled spin-orbital model, we propose a simple mean field theory
based on an SU(4) constraint fermion representation of spins and orbitals. A
spin-orbital gapped phase is formed due to a marginally relevant spin-orbital
valence bond pairing interaction. The energy gap of the spin and orbital
excitations grows extremely slowly from the SU(4) symmetric point up to a
maximum value and then decreases rapidly. By calculating the spin, orbital, and
spin-orbital tensor static susceptibilities at zero temperature, we find a
crossover from coherent to incoherent magnetic excitations as the spin-orbital
coupling decreasing from large to small values.Comment: 10 pages, Revtex file, 5 figure
Effect of symmetry breaking perturbations in the one-dimensional SU(4) spin-orbital model
We study the effect of symmetry breaking perturbations in the one-dimensional
SU(4) spin-orbital model. We allow the exchange in spin () and orbital
() channel to be different and thus reduce the symmetry to SU(2)
SU(2). A magnetic field along the direction is also applied. Using
the formalism developped by Azaria et al we extend their analysis of the
isotropic , h=0 case and obtain the low-energy effective theory near
the SU(4) point in the asymmetric case. An accurate analysis of the
renormalization group flow is presented with a particular emphasis on the
effect of the anisotropy. In zero magnetic field, we retrieve the same
qualitative low-energy physics than in the isotropic case. In particular, the
massless behavior found on the line extends in a large
anisotropic region. We discover though that the anisotropy plays its trick in
allowing non trivial scaling behaviors of the physical quantities. When a
magnetic field is present the effect of the anisotropy is striking. In addition
to the usual commensurate-incommensurate phase transition that occurs in the
spin sector of the theory, we find that the field may induce a second
transition of the KT type in the remaining degrees of freedom to which it does
not couple directly. In this sector, we find that the effective theory is that
of an SO(4) Gross-Neveu model with an h-dependent coupling that may change its
sign as h varies.Comment: 14 pages, 5 Figs, added referenc
Elementary excitations in one-dimensional spin-orbital models: neutral and charged solitons and their bound states
We study, both numerically and variationally, the interplay between different
types of elementary excitations in the model of a spin chain with anisotropic
spin-orbit coupling, in the vicinity of the "dimer line" with an exactly known
dimerized ground state. Our variational treatment is found to be in a
qualitative agreement with the exact diagonalization results. Soliton pairs are
shown to be the lowest excitations only in a very narrow region of the phase
diagram near the dimer line, and the phase transitions are always governed by
magnon-type excitations which can be viewed as soliton-antisoliton bound
states. It is shown that when the anisotropy exceeds certain critical value, a
new phase boundary appears. In the doped model on the dimer line, the exact
elementary charge excitation is shown to be a hole bound to a soliton. Bound
states of those "charged solitons" are studied; exact solutions for N-hole
bound states are presented.Comment: 11 pages revtex, 6 figure
Crossover Phenomena in the One-Dimensional SU(4) Spin-Orbit Model under Magnetic Fields
We study the one-dimensional SU(4) exchange model under magnetic fields,
which is the simplest effective Hamiltonian in order to investigate the quantum
fluctuations concerned with the orbital degrees of freedom in coupled
spin-orbit systems. The Bethe ansatz approaches and numerical calculations
using the density matrix renormalization group method are employed. The main
concern of the paper is how the system changes from the SU(4) to the SU(2)
symmetric limit as the magnetic field is increased. For this model the
conformal field theory predicts an usual behavior: there is a jump of the
critical exponents just before the SU(2) limit. For a finite-size system,
however, the orbital-orbital correlation functions approach continuously to the
SU(2) limit after interesting crossover phenomena. The crossover takes place in
the magnetization range of 1/3 1/2 for the system with 72 sites studied
in this paper.Comment: 8 pages, 6 Postscript figures, REVTeX, submitted to Phys. Rev.
Histogram Monte Carlo study of multicritical behavior in the hexagonal easy-axis Heisenberg antiferromagnet
The results of a detailed histogram Monte-Carlo study of critical-fluctuation
effects on the magnetic-field temperature phase diagram associated with the
hexagonal Heisenberg antiferromagnet with weak axial anisotropy are reported.
The multiphase point where three lines of continuous transitions merge at the
spin-flop boundary exhibits a structure consistent with scaling theory but
without the usual umbilicus as found in the case of a bicritical point.Comment: 7 pages (RevTex 3.0), 1 figure available upon request, CRPS-93-1
Numerical Study of the One-Dimensional Spin-Orbit Coupled System with SU(2)SU(2) Symmetry
We numerically study the SU(2)SU(2) symmetric spin-orbit coupled
model as a lower symmetric generalization of the SU(4) exchange model. On the
symmetric line with respect to the spin and orbit, our result shows the
essentially singular gap formation in consistent with the analytic approach,
which is different from the previous numerical calculation. Furthermore, we
find new critical phases around the SU(4) point, surrounding the previously
known gapless symmetric line. In these novel phases either spin or orbital
excitations around momentum form massless continua which split from the
excitations belonging to the irreducible representation at the SU(4)
point. On the critical symmetric line, the additional coupled spin-orbit
excitations around originating from [] become critical, too.Comment: 6 pages with 5 figures, submitted to J. Phys. Soc. Jp
- …