419 research outputs found
Variational state based on the Bethe ansatz solution and a correlated singlet liquid state in the one-dimensional t-J model
The one-dimensional t-J model is investigated by the variational Monte Carlo
method. A variational wave function based on the Bethe ansatz solution is newly
proposed, where the spin-charge separation is realized, and a long-range
correlation factor of Jastrow-type is included. In most regions of the phase
diagram, this wave function provides an excellent description of the
ground-state properties characterized as a Tomonaga-Luttinger liquid; Both of
the amplitude and exponent of correlation functions are correctly reproduced.
For the spin-gap phase, another trial state of correlated singlet pairs with a
Jastrow factor is introduced. This wave function shows generalized Luther-Emery
liquid behavior, exhibiting enhanced superconducting correlations and
exponential decay of the spin correlation function. Using these two variational
wave functions, the whole phase diagram is determined. In addition, relations
between the correlation exponent and variational parameters in the trial
functions are derived.Comment: REVTeX 3.0, 27 pages. 7 figures available upon request
([email protected]). To be published in Phys. Rev. B 5
Formation of clusters in the ground state of the model on a two leg ladder
We investigate the ground state properties of the model on a two leg
ladder with anisotropic couplings () along rungs and
() along legs. We have implemented a cluster approach based
on 4-site plaqettes. In the strong asymmetric cases and
the ground state energy is well described by plaquette
clusters with charges . The interaction between the clusters favours the
condensation of plaquettes with maximal charge -- a signal for phase
separation. The dominance of Q=2 plaquettes explains the emergence of tightly
bound hole pairs. We have presented the numerical results of exact
diagonalization to support our cluster approach.Comment: 11 pages, 9 figures, RevTex
Phase separation at all interaction strengths in the t-J model
We investigate the phase diagram of the two-dimensional t-J model using a
recently developed Green's Function Monte Carlo method for lattice fermions. We
use the technique to calculate exact ground-state energies of the model on
large lattices. In contrast to many previous studies, we find the model phase
separates for all values of J/t. In particular, it is unstable at the hole
dopings and interaction strengths at which the model was thought to describe
the cuprate superconductors.Comment: Revtex, 4 pages, 3 figures. Some minor changes were made to the text
and figures, and some references were adde
Phase Separation of the Two-Dimensional t-J model
The boundary of phase separation of the two-dimensional t-J model is
investigated by the power-Lanczos method and Maxwell construction. The method
is similar to a variational approach and it determines the lower bound of the
phase separation boundary with in the limit . In
the physical interesting regime of high T_c superconductors where
there is no phase separation.Comment: LaTex 5 pages, 4 figure
Spin-triplet superconductivity in repulsive Hubbard models with disconnected Fermi surfaces: a case study on triangular and honeycomb lattices
We propose that spin-fluctuation-mediated spin-triplet superconductivity may
be realized in repulsive Hubbard models with disconnected Fermi surfaces. The
idea is confirmed for Hubbard models on triangular (dilute band filling) and
honeycomb (near half-filling) lattices using fluctuation exchange
approximation, where triplet pairing order parameter with f-wave symmetry is
obtained. Possible relevance to real superconductors is suggested.Comment: 5 pages, 6 figures, RevTeX, uses epsf.sty and multicol.st
Green's Function Monte Carlo for Lattice Fermions: Application to the t-J Model
We develop a general numerical method to study the zero temperature
properties of strongly correlated electron models on large lattices. The
technique, which resembles Green's Function Monte Carlo, projects the ground
state component from a trial wave function with no approximations. We use this
method to determine the phase diagram of the two-dimensional t-J model, using
the Maxwell construction to investigate electronic phase separation. The shell
effects of fermions on finite-sized periodic lattices are minimized by keeping
the number of electrons fixed at a closed-shell configuration and varying the
size of the lattice. Results obtained for various electron numbers
corresponding to different closed-shells indicate that the finite-size effects
in our calculation are small. For any value of interaction strength, we find
that there is always a value of the electron density above which the system can
lower its energy by forming a two-component phase separated state. Our results
are compared with other calculations on the t-J model. We find that the most
accurate results are consistent with phase separation at all interaction
strengths.Comment: 22 pages, 22 figure
The Phase Diagram of Correlated Electrons in a Lattice of Berry Molecules
A model for correlated electrons in a lattice with local additional spin--1
degrees of freedom inducing constrained hopping, is studied both in the low
density limit and at quarter filling. We show that in both 1D and 2D two
particles form a bound state even in presence of a repulsive U<U_c. A picture
of a dilute Bose gas, leading to off-diagonal long range order (LRO) in 2D
(quasi-LRO in 1D), is supported by quantitative calculations in 1D which allow
for a determination of the phase diagram.Comment: 7 pages + 2 ps figures, published versio
Application of Gradient-Corrected Density Functional Theory to the Structures and Thermochemistries of \mbox{ScF}_3, \mbox{TiF}_4, \mbox{VF}_5, and \mbox{CrF}_6
Density functional theory(DFT) and Hartree-Fock(HF) calculations are reported
for the family of transition metal fluorides \mbox{ScF}_3, \mbox{TiF}_4,
\mbox{VF}_5, and \mbox{CrF}_6. Both HF and the local-density-aproximation
(LDA) yield excellent agreement with experimental bond lengths, while the B-LYP
gradient-corrected density functional gives bond lengths \AA\ too
long. An investigation of various combinations of exchange and correlation
functionals shows that, for this series, the origin of this behavior lies in
the Becke exchange functional. Much improved bond distances are found using the
hybrid HF/DFT functional advocated by Becke. This approximation also leads to
much improved thermochemistries. The LDA overestimates average bond energies in
this series by kcal/mol, whereas the B-LYP functional overbinds by only
kcal/mol, and the hybrid HF/DFT method overbinds by only
kcal/mol. The hybrid method predicts the octahedral isomer of \mbox{CrF}_6 to
be more stable than the trigonal prismatic form by kcal/mol. Comparison of
theoretical vibrational frequencies with experiment supports the assignment of
an octahedral geometry.Comment: 18 pages, latex, 1 figure, Submitted to Journal of Chemical Physic
Finite-Temperature Properties across the Charge Ordering Transition -- Combined Bosonization, Renormalization Group, and Numerical Methods
We theoretically describe the charge ordering (CO) metal-insulator transition
based on a quasi-one-dimensional extended Hubbard model, and investigate the
finite temperature () properties across the transition temperature, . In order to calculate dependence of physical quantities such as the
spin susceptibility and the electrical resistivity, both above and below
, a theoretical scheme is developed which combines analytical
methods with numerical calculations. We take advantage of the renormalization
group equations derived from the effective bosonized Hamiltonian, where Lanczos
exact diagonalization data are chosen as initial parameters, while the CO order
parameter at finite- is determined by quantum Monte Carlo simulations. The
results show that the spin susceptibility does not show a steep singularity at
, and it slightly increases compared to the case without CO because
of the suppression of the spin velocity. In contrast, the resistivity exhibits
a sudden increase at , below which a characteristic dependence
is observed. We also compare our results with experiments on molecular
conductors as well as transition metal oxides showing CO.Comment: 9 pages, 8 figure
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