26 research outputs found
Some exact results for the zero-bandwidth extended Hubbard model with intersite charge and magnetic interactions
The extended Hubbard model in the zero-bandwidth limit is studied. The
effective Hamiltonian consists of (i) on-site interaction and intersite
(ii) density-density interaction and (iii) Ising-like magnetic exchange
interaction (between the nearest-neighbors). We present rigorous (and
analytical) results obtained within the transfer-matrix method for 1D-chain in
two particular cases: (a) and ; (b) and
(, ). We obtain the exact formulas for the partition
functions which enables to calculate thermodynamic properties such as entropy,
specific heat (), and double occupancy per site. In both cases the system
exhibits an interesting temperature dependence of involving a
characteristic two-peak structure. There are no phase transitions at finite
temperatures and the only transitions occur in the ground state.Comment: 4 pages, 2 figures; pdf-ReVTeX; updated references; presented at The
European Conference PHYSICS OF MAGNETISM 2014 (PM'14), June 23-27, 2014,
Poznan, Poland; submitted to Acta Physica Polonica
Monte Carlo study of phase separation in magnetic insulators
In this work we focus on the study of phase separation in the zero-bandwidth
extended Hubbard with nearest-neighbors intersite Ising-like magnetic
interactions and on-site Coulomb interactions . The system has been
analyzed by means of Monte Carlo simulations (in the grand canonical ensemble)
on two dimensional square lattice (with sites) and the
results for as a function of chemical potential and electron
concentration have been obtained. Depending on the values of interaction
parameters the system exhibits homogeneous (anti-)ferromagnetic (AF) or
non-ordered (NO) phase as well as phase separation PS:AF/NO state. Transitions
between homogeneous phases (i.e. AF-NO transitions) can be of first or second
order and the tricritical point is also present on the phase diagrams. The
electron compressibility is an indicator of the phase separation and that
quantity is of particular interest of this paper.Comment: 4 pages, 3 figures; pdf-ReVTeX; updated references; presented at The
European Conference PHYSICS OF MAGNETISM 2014 (PM'14), June 23-27, 2014,
Poznan, Poland; submitted to Acta Physica Polonica
Some properties of two dimensional extended repulsive Hubbard model with intersite magnetic interactions - a Monte Carlo study
In this paper the two dimensional extended Hubbard model with intersite
magnetic Ising-like interaction in the atomic limit is analyzed by means of the
classical Monte Carlo method in the grand canonical ensemble. Such an effective
simple model could describe behavior of insulating (anti)ferromagnets. In the
model considered the Coulomb interaction () is on-site and the magnetic
interactions in -direction (, antiferromagnetic) are restricted to
nearest-neighbors. Simulations of the model have been performed on a square
lattice consisting of sites () in order to obtain the
full phase diagram for . Results obtained for on-site repulsion
() show that, apart from homogeneous non-ordered (NO) and ordered magnetic
(antiferromagnetic, AF) phases, there is also a region of phase separation (PS:
AF/NO) occurrence. We present a phase diagram as well as some thermodynamic
properties of the model for the case of (and arbitrary chemical
potential and arbitrary electron concentration). The AF-NO transition can be
second-order as well as first-order and the tricritical point occurs on the
diagram.Comment: 5 pages, 4 figures, pdf-ReVTeX, presented at 16th National School of
Superconductivity: Unconventional superconductivity and strongly correlated
systems, Zakopane, Poland, October 7-12, 2013, submitted to Acta Physica
Polonica
Interplay between charge and magnetic orderings in the zero-bandwidth limit of the extended Hubbard model for strong on-site repulsion
A simple effective model of charge ordered and (or) magnetically ordered
insulators is studied. The tight binding Hamiltonian analyzed consists of (i)
the effective on-site interaction U, (ii) the intersite density-density
interaction W and (iii) intersite magnetic exchange interaction Jz (or Jxy)
between nearest-neighbors. The intersite interaction are treated within the
mean-field approximation. One shows that the systems considered can exhibit
very interesting multicritical behaviors, including among others bicritical,
tricritical, tetracritical and critical end points. The analysis of the model
has been performed for an arbitrary electron concentration as well as an
arbitrary chemical potential in the limit of strong on-site repulsion. The
phase diagrams obtained in such a case are shown to consist of at least 9
different states, including four homogenous phases: nonordered (NO),
ferromagnetic (F), charge ordered (CO), ferrimagnetic (intermediate, I) and
five types of phase separation: NO-NO, F-NO, F-F, CO-F, CO-I.Comment: 3 pages, 5 figures, pdf-ReVTeX, presented at The European Conference
Physics of Magnetism 2011, June 27 - July 1, 2011, Poznan, Polan
The effects of the next-nearest-neighbour density-density interaction in the atomic limit of the extended Hubbard model
We have studied the extended Hubbard model in the atomic limit. The
Hamiltonian analyzed consists of the effective on-site interaction U and the
intersite density-density interactions Wij (both: nearest-neighbour and
next-nearest-neighbour). The model can be considered as a simple effective
model of charge ordered insulators. The phase diagrams and thermodynamic
properties of this system have been determined within the variational approach,
which treats the on-site interaction term exactly and the intersite
interactions within the mean-field approximation. Our investigation of the
general case taking into account for the first time the effects of
longer-ranged density-density interaction (repulsive and attractive) as well as
possible phase separations shows that, depending on the values of the
interaction parameters and the electron concentration, the system can exhibit
not only several homogeneous charge ordered (CO) phases, but also various phase
separated states (CO-CO and CO-nonordered). One finds that the model considered
exhibits very interesting multicritical behaviours and features, including
among others bicritical, tricritical, critical-end and isolated critical
points.Comment: 12 pages, 7 figures; final version, pdf-ReVTeX; corrected typos in
reference; submitted to Journal of Physics: Condensed Matte
Effects of diagonal disorder on Charge Density Wave and Superconductivity in local pair systems
We analyse the influence of diagonal disorder (random site energy) on Charge
Density Wave (CDW) and Superconductivity (SS) in local pair systems which are
described by the model of hard core charged bosons on a lattice. This problem
was previously studied within the mean field approximation for the case of half
filled band (n = 1). Here we extend that investigation to the case of arbitrary
particle concentration (0 < n < 2) and examine the phase diagrams of the model
and the behaviour of superfluid density as a function of n and the increasing
disorder. Depending on the strength of random on-site energies, the intersite
density-density repulsion and the concentration the model can exhibit several
various phases, including homogeneous phases: CDW, SS and Bose-glass (NO) as
well as the phase separated states: CDW-SS, CDW-NO and particle droplets. The
obtained results for SS phase are in qualitative agreement with the available
Monte Carlo calculations for two dimensional lattice. Also, in a definite range
of parameters the system exhibits the phenomena which we call a disorder
induced superconductivity and a disorder induced charge ordering.Comment: 21 pages, 8 figure