2,108 research outputs found
Nature of the Quantum Phase Transition in Quantum Compass Model
In this work, we show that the quantum compass model on an square lattice can
be mapped to a fermionic model with local density interaction. We introduce a
mean-field approximation where the most important fluctuations, those
perpendicular to the ordering direction, are taken into account exactly. It is
found that the quantum phase transition point at marks a first order
phase transition. We also show that the mean field result is robust against the
remaining fluctuation corrections up to the second order.Comment: 7 pages, 10 fig
Phase transition and critical properties of spin-orbital interacting systems
Phase transition and critical properties of Ising-like spin-orbital
interacting systems in 2-dimensional triangular lattice are investigated. We
first show that the ground state of the system is a composite spin-orbital
ferro-ordered phase. Though Landau effective field theory predicts the
second-order phase transition of the composite spin-orbital order, however, the
critical exponents obtained by the renormalization group approach demonstrate
that the spin-orbital order-disorder transition is far from the second-order,
rather, it is more close to the first-order, implying that the widely observed
first-order transition in many transition-metal oxides may be intrinsic. The
unusual critical behavior near the transition point is attributed to the
fractionalization of the composite order parameter.Comment: Accepted to Phys. Lett.
Spin-Orbital Entanglement and Phase Diagram of Spin-orbital Chain with Symmetry
Spin-orbital entanglement in quantum spin-orbital systems is quantified by a
reduced von Neumann entropy, and is calculated for the ground state of a
coupled spin-orbital chain with symmetry. By analyzing the
discontinuity and local extreme of the reduced entropy as functions of the
model parameters, we deduce a rich phase diagram to describe the quantum phase
transitions in the model. Our approach provides an efficient and powerful
method to identify phase boundaries in a system with complex correlation
between multiply degrees of freedom.Comment: 4 pages, 3 figure
Finite temperature spin-dynamics and phase transitions in spin-orbital models
We study finite temperature properties of a generic spin-orbital model
relevant to transition metal compounds, having coupled quantum Heisenberg-spin
and Ising-orbital degrees of freedom. The model system undergoes a phase
transition, consistent with that of a 2D Ising model, to an orbitally ordered
state at a temperature set by short-range magnetic order. At low temperatures
the orbital degrees of freedom freeze-out and the model maps on to a quantum
Heisenberg model. The onset of orbital excitations causes a rapid scrambling of
the spin spectral weight away from coherent spin-waves, which leads to a sharp
increase in uniform magnetic susceptibility just below the phase transition,
reminiscent of the observed behavior in the Fe-pnictide materials.Comment: 4 page
N\'eel and Spin-Peierls ground states of two-dimensional SU(N) quantum antiferromagnets
The two-dimensional SU(N) quantum antiferromagnet, a generalization of the
quantum Heisenberg model, is investigated by quantum Monte Carlo simulations.
The ground state for is found to be of the N\'eel type with broken
SU(N) symmetry, whereas it is of the Spin-Peierls type for with broken
lattice translational invariance. No intermediate spin-liquid phase was
observed in contrast to previous numerical simulations on smaller lattices
[Santoro et al., Phys. Rev. Lett. {\bf 83} 3065 (1999)].Comment: 4 pages, 4 figure
Doping dependence of the exchange energies in bilayer manganites: Role of orbital degrees of freedom
Recently, an intriguing doping dependence of the exchange energies in the
bilayer manganites has been observed in the neutron
scattering experiments. The intra-layer exchange only weakly changed with
doping while the inter-layer one drastically decreased. Here we propose a
theory which accounts for these experimental findings. We argue, that the
observed striking doping dependence of the exchange energies can be attributed
to the evaluation of the orbital level splitting with doping. The latter is
handled by the interplay between Jahn-Teller effect (supporting an axial
orbital) and the orbital anisotropy of the electronic band in the bilayer
structure (promoting an in-plane orbital), which is monitored by the Coulomb
repulsion. The presented theory, while being a mean-field type, describes well
the experimental data and also gives the estimates of the several interesting
energy scales involved in the problem.Comment: Added references, corrected typos. To appear in Phys. Rev.
First integrals of Ginzburg-Landau equations and stability criteria for vortex-free state in unconventional superconductors
The first integrals of the Ginzburg-Landau equations for a vortex-free state
of superconductors with different mixed symmetries of the order parameter are
found. The general boundary conditions for the order parameter at the ideal
interface between the superconductor and vacuum are derived. Based on these
integrals and boundary conditions, we analyze the stability criteria for
vortex-free state in unconventional superconductors. The threshold field above
which the Abrikosov vortices can enter the superconductor is found to be higher
or equal to the thermodynamic critical field for all states under study.Comment: 8 pages, pdf file, no figure
Numerical study of the one-dimensional quantum compass model
The ground state magnetic phase diagram of the one-dimensional quantum
compass model (QCM) is studied using the numerical Lanczos method. A detailed
numerical analysis of the low energy excitation spectrum is presented. The
energy gap and the spin-spin correlation functions are calculated for finite
chains. Two kind of the magnetic long-range orders, the Neel and a type of the
stripe-antiferromagnet, in the ground state phase diagram are identified. Based
on the numerical analysis, the first and second order quantum phase transitions
in the ground state phase diagram are identified.Comment: 6 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1105.211
Thermal/Electronic Transport Properties and Two-Phase Mixtures in La_{5/8-x}Pr_{x}Ca_{3/8}MnO_{3}
We measured thermal conductivity, k, thermoelectric power, S, and dc electric
conductivity, sigma, of La_{5/8-x}Pr_{x}Ca_{3/8}MnO_{3}, showing an intricate
interplay between metallic ferromagnetism (FM) and charge ordering (CO)
instability. The change of k, S and sigma with temperature (T) and x agrees
well with the effective medium theories for binary metal-insulator mixtures.
This agreement clearly demonstrates that with the variation of T as well as x,
the relative volumes of FM and CO phases drastically change and percolative
metal-insulator transition occurs in the mixture of FM and CO domains.Comment: 8 pages, 4 eps figures included, to appear in Phys. Rev. Let
Manganites at Quarter Filling: Role of Jahn-Teller Interactions
We have analyzed different correlation functions in a realistic spin-orbital
model for half-doped manganites. Using a finite-temperature diagonalization
technique the CE phase was found in the charge-ordered phase in the case of
small antiferromagnetic interactions between electrons. It is shown
that a key ingredient responsible for stabilization of the CE-type spin and
orbital-ordered state is the cooperative Jahn-Teller (JT) interaction between
next-nearest Mn neighbors mediated by the breathing mode distortion of
Mn octahedra and displacements of Mn ions. The topological phase
factor in the Mn-Mn hopping leading to gap formation in one-dimensional models
for the CE phase as well as the nearest neighbor JT coupling are not able to
produce the zigzag chains typical for the CE phase in our model.Comment: 16 pages with 16 figures, contains a more detailed parameter estimate
based on the structural data by Radaelli et al. (accepted for publication in
Phys. Rev. B
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