102 research outputs found
Elementary excitations of the symmetric spin-orbital model: The XY limit
The elementary excitations of the 1D, symmetric, spin-orbital model are
investigated by studying two anisotropic versions of the model, the pure XY and
the dimerized XXZ case, with analytical and numerical methods. While they
preserve the symmetry between spin and orbital degrees of freedom, these models
allow for a simple and transparent picture of the low--lying excitations: In
the pure XY case, a phase separation takes place between two phases with
free--fermion like, gapless excitations, while in the dimerized case, the
low-energy effective Hamiltonian reduces to the 1D Ising model with gapped
excitations. In both cases, all the elementary excitations involve simultaneous
flips of the spin and orbital degrees of freedom, a clear indication of the
breakdown of the traditional mean-field theory.Comment: Revtex, two figure
Entropy Driven Dimerization in a One-Dimensional Spin-Orbital Model
We study a new version of the one-dimensional spin-orbital model with spins
S=1 relevant to cubic vanadates. At small Hund's coupling J_H we discover
dimerization in a pure electronic system solely due to a dynamical spin-orbital
coupling. Above a critical value J_H, a uniform ferromagnetic state is
stabilized at zero temperature. More surprisingly, we observe a temperature
driven dimerization of the ferrochain, which occurs due to a large entropy
released by dimer states. This dynamical dimerization seems to be the mechanism
driving the peculiar intermediate phase of YVO_3.Comment: 5 pages, 4 figure
Quantum Monte Carlo Loop Algorithm for the t-J Model
We propose a generalization of the Quantum Monte Carlo loop algorithm to the
t-J model by a mapping to three coupled six-vertex models. The autocorrelation
times are reduced by orders of magnitude compared to the conventional local
algorithms. The method is completely ergodic and can be formulated directly in
continuous time. We introduce improved estimators for simulations with a local
sign problem. Some first results of finite temperature simulations are
presented for a t-J chain, a frustrated Heisenberg chain, and t-J ladder
models.Comment: 22 pages, including 12 figures. RevTex v3.0, uses psf.te
Order from disorder: Quantum spin gap in magnon spectra of LaTiO_3
A theory of the anisotropic superexchange and low energy spin excitations in
a Mott insulator with t_{2g} orbital degeneracy is presented. We observe that
the spin-orbit coupling induces frustrating Ising-like anisotropy terms in the
spin Hamiltonian, which invalidate noninteracting spin wave theory. The
frustration of classical states is resolved by an order from disorder
mechanism, which selects a particular direction of the staggered moment and
generates a quantum spin gap. The theory explains well the observed magnon gaps
in LaTiO_3. As a test case, a specific prediction is made on the splitting of
magnon branches at certain momentum directions.Comment: 5 pages, 2 figures, final versio
Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2 Chains with Bond Disorder
One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as . We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.Comment: 7 pages, 5 eps-figures included, typeset using JPSJ.sty, accepted for
publication in J. Phys. Soc. Jpn. 68, Vol. 3. (1999
Numerical renormalization-group study of spin correlations in one-dimensional random spin chains
We calculate the ground-state two-spin correlation functions of spin-1/2
quantum Heisenberg chains with random exchange couplings using the real-space
renormalization group scheme. We extend the conventional scheme to take account
of the contribution of local higher multiplet excitations in each decimation
step. This extended scheme can provide highly accurate numerical data for large
systems. The random average of staggered spin correlations of the chains with
random antiferromagnetic (AF) couplings shows algebraic decay like ,
which verifies the Fisher's analytic results. For chains with random
ferromagnetic (FM) and AF couplings, the random average of generalized
staggered correlations is found to decay more slowly than a power-law, in the
form close to . The difference between the distribution functions of
the spin correlations of the random AF chains and of the random FM-AF chains is
also discussed.Comment: 14 pages including 8 figures, REVTeX, submitted to Physical Review
Orbital liquid in three dimensional Mott insulator:
We present a theory of spin and orbital states in Mott insulator .
The spin-orbital superexchange interaction between ions in cubic
crystal suffers from a pathological degeneracy of orbital states at classical
level. Quantum effects remove this degeneracy and result in the formation of
the coherent ground state, in which the orbital moment of level is
fully quenched. We find a finite gap for orbital excitations. Such a disordered
state of local degrees of freedom on unfrustrated, simple cubic lattice is
highly unusual. Orbital liquid state naturally explains observed anomalies of
.Comment: 5 pages, 3 figure
Lightly Doped t-J Three-Leg Ladders - an Analog for the Underdoped Cuprates
The three-leg ladder has one odd-parity and two even-parity channels. At low
doping these behave quite differently. Numerical calculations for a t-J model
show that the initial phase upon hole doping has two components - a conducting
Luttinger liquid in the odd-parity channel, coexisting with an insulating (i.e.
undoped) spin liquid phase in the even-parity channels. This phase has a
partially truncated Fermi surface and violates the Luttinger theorem. This
coexistence of conducting fermionic and insulating paired bosonic degrees of
freedom is similar to the recent proposal of Geshkenbein, Ioffe, and Larkin for
the underdoped spin-gap normal phase of the cuprates. A mean field
approximation is derived which has many similarities to the numerical results.
One difference however is an induced hole pairing in the odd-parity channel at
arbitrary small dopings, similar to that proposed by Geshkenbein, Ioffe, and
Larkin for the two-dimensional case. At higher dopings, we propose that a
quantum phase transition will occur as holes enter the even-parity channels,
resulting in a Luther-Emery liquid with hole pairing with essentially d-wave
character. In the mean field approximation a crossover occurs which we
interpret as a reflection of this quantum phase transition deduced from the
numerical results.Comment: RevTex, 36 pages with 16 figure
Quantum magnetism in the stripe phase: bond- versus site order
It is argued that the spin dynamics in the charge-ordered stripe phase might
be revealing with regards to the nature of the anomalous spin dynamics in
cuprate superconductors. Specifically, if the stripes are bond ordered much of
the spin fluctuation will originate in the spin sector itself, while site
ordered stripes require the charge sector as the driving force for the strong
quantum spin fluctuations.Comment: 4 pages, 3 figures, LaTe
Three-leg Antiferromagnetic Heisenberg Ladder with Frustrated Boundary Condition; Ground State Properties
The antiferromagnetic Heisenberg spin systems on the three-leg ladder are
investigated. Periodic boundary condition is imposed in the rung direction. The
system has an excitation gap for all antiferromagnetic inter-chain coupling
(). The estimated gap for the strong coupling limit
() is 0.28. Although the interaction is
homogeneous and only nearest-neighbor, the ground states of the system are
dimerized and break the translational symmetry in the thermodynamic limit.
Introducing the next-nearest neighbor coupling (), we can see that the
system is solved exactly. The ground state wave function is completely
dimer-ordered. Using density matrix renomalization group algorithm, we show
numerically that the original model () has the same nature with the
exactly solvable model. The ground state properties of the ladder with a higher
odd number of legs are also discussed.Comment: 15 pages, LaTeX, to be published in J.Phys.Soc.Jpn. Vol. 66 No. 1
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