15 research outputs found
Comparison of perturbative expansions using different phonon bases for two-site Holstein model
The two-site single-polaron problem is studied within the perturbative
expansions using different standard phonon basis obtained through the Lang
Firsov (LF), modified LF (MLF) and modified LF transformation with squeezed
phonon states (MLFS). The role of these convergent expansions using the above
prescriptions in lowering the energy and in determining the correlation
functions are compared for different values of coupling strength. The
single-electron energy, oscillator wave functions and correlation functions are
calculated for the same system. The applicability of different phonon basis in
different regimes of the coupling strength as well as in different regimes of
hopping are also discussed.Comment: 24 pages (RevTEX), 12 postscript figures, final version accepted in
PRB(2000) Jornal Ref: Phys. Rev. B, 61, 4592-4602 (2000
Polarons and bipolarons in strongly interacting electron-phonon systems
The Holstein Hubbard and Holstein t--J models are studied for a wide range of
phonon frequencies, electron--electron and electron--phonon interaction
strengths on finite lattices with up to ten sites by means of direct Lanczos
diagonalization. Previously the necessary truncation of the phononic Hilbert
space caused serious limitations to either very small systems (four or even two
sites) or to weak electron--phonon coupling, in particular in the adiabatic
regime. Using parallel computers we were able to investigate the transition
from `large' to `small' polarons in detail. By resolving the low--lying
eigenstates of the Hamiltonian and by calculating the spectral function we can
identify a polaron band in the strong--coupling case, whose dispersion deviates
from the free--particle dispersion at low and intermediate phonon frequencies.
For two electrons (holes) we establish the existence of bipolaronic states and
discuss the formation of a bipolaron band. For the 2D Holstein t--J model we
demonstrate that the formation of hole--polarons is favoured by strong Coulomb
correlations. Analyzing the hole--hole correlation functions we find that hole
binding is enhanced as a dynamical effect of the electron--phonon interaction.Comment: 23 pages (Revtex) with 13 figures (ps, uuencoded
Spin and Charge Structure Factor of the 2-d Hubbard Model
The spin and charge structure factors are calculated for the Hubbard model on
the square lattice near half-filling using a spin-rotation invariant six-slave
boson representation. The charge structure factor shows a broad maximum at the
zone corner and is found to decrease monotonically with increasing interaction
strength and electron density and increasing temperature. The spin structure
factor develops with increasing interaction two incommensurate peaks at the
zone boundary and along the zone diagonal. Comparison with results of Quantum
Monte Carlo and variational calculations is carried out and the agreement is
found to be good. The limitations of an RPA-type approach are pointed out.Comment: 18 pages, revtex, 13 postscript figures, submitted to Phys. Rev.
Optical absorption and single-particle excitations in the 2D Holstein t-J model
To discuss the interplay of electronic and lattice degrees of freedom in
systems with strong Coulomb correlations we have performed an extensive
numerical study of the two-dimensional Holstein t-J model. The model describes
the interaction of holes, doped in a quantum antiferromagnet, with a
dispersionsless optical phonon mode. We apply finite-lattice Lanczos
diagonalization, combined with a well-controlled phonon Hilbert space
truncation, to the Hamiltonian. The focus is on the dynamical properties. In
particular we have evaluated the single-particle spectral function and the
optical conductivity for characteristic hole-phonon couplings, spin exchange
interactions and phonon frequencies. The results are used to analyze the
formation of hole polarons in great detail. Links with experiments on layered
perovskites are made. Supplementary we compare the Chebyshev recursion and
maximum entropy algorithms, used for calculating spectral functions, with
standard Lanczos methods.Comment: 32 pages, 12 figures, submitted to Phys. Rev.
A Study of the Antiferromagnetic Phase in the Hubbard Model by means of the Composite Operator Method
We have investigated the antiferromagnetic phase of the 2D, the 3D and the
extended Hubbard models on a bipartite cubic lattice by means of the Composite
Operator Method within a two-pole approximation. This approach yields a fully
self-consistent treatment of the antiferromagnetic state that respects the
symmetry properties of both the model and the algebra. The complete phase
diagram, as regards the antiferromagnetic and the paramagnetic phases, has been
drawn. We firstly reported, within a pole approximation, three kinds of
transitions at half-filling: Mott-Hubbard, Mott-Heisenberg and Heisenberg. We
have also found a metal-insulator transition, driven by doping, within the
antiferromagnetic phase. This latter is restricted to a very small region near
half filling and has, in contrast to what has been found by similar approaches,
a finite critical Coulomb interaction as lower bound at half filling. Finally,
it is worth noting that our antiferromagnetic gap has two independent
components: one due to the antiferromagnetic correlations and another coming
from the Mott-Hubbard mechanism.Comment: 20 pages, 37 figures, RevTeX, submitted to Phys. Rev.
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
How long should older people take antidepressants to prevent relapse? Por quanto tempo os idosos devem tomar antidepressivos para evitar recaĂdas?
Patients with depressive disorder have a high risk of relapse after recovery from a depressive episode. Can the relapse of depressive disorder be prevented or delayed for older adults? This paper reviews the evidence from randomised clinical trials and open label trials of the effectiveness of maintenance antidepressant therapy for older adults with depressive disorder. It also examines the evidence for the effectiveness of psychosocial and psychotherapeutic interventions. The paper concludes with recommendations for clinical practice and future research.<br>Pacientes com transtorno depressivo apresentam alto risco de recorrĂŞncia e recaĂda. É possĂvel prevenir a recaĂda ou a recorrĂŞncia do episĂłdio depressivo ou retardá-lo em fases tardias da vida? Este artigo revisa ensaios clĂnicos aleatorizados e nĂŁo-aleatorizados com o objetivo de estabelecer se o tratamento antidepressivo de manutenção reduz o risco de recaĂda e recorrĂŞncia de depressĂŁo em idosos. O artigo tambĂ©m examina a evidĂŞncia atualmente disponĂvel sobre a eficácia das intervenções psicossociais e psicoterapĂŞuticas. O artigo conclui com recomendações para a prática clĂnica e pesquisas futuras