379 research outputs found
Lower bound for the segregation energy in the Falicov-Kimball model
In this work, a lower bound for the ground state energy of the
Falicov-Kimball model for intermediate densities is derived. The explicit
derivation is important in the proof of the conjecture of segregation of the
two kinds of fermions in the Falicov-Kimball model, for sufficiently large
interactions. This bound is given by a bulk term, plus a term proportional to
the boundary of the region devoid of classical particles. A detailed proof is
presented for density n=1/2, where the coefficient 10^(-13) is obtained for the
boundary term, in two dimensions. With suitable modifications the method can
also be used to obtain a coefficient for all densities.Comment: 8 pages, 2 figure
The 3-d Random Field Ising Model at zero temperature
We study numerically the zero temperature Random Field Ising Model on cubic
lattices of various linear sizes in three dimensions. For each random field
configuration we vary the ferromagnetic coupling strength . We find that in
the infinite volume limit the magnetization is discontinuous in . The energy
and its first derivative are continuous. The approch to the thermodynamic
limit is slow, behaving like with for the gaussian
distribution of the random field. We also study the bimodal distribution , and we find similar results for the magnetization but with a
different value of the exponent . This raises the question of the
validity of universality for the random field problem.Comment: 8 pages, 3 PostScript Figure
Comment on ``Spin Polarization and Magnetic Circular Dichroism in Photoemission from the 2p Core Level of Ferromagnetic Ni''
Although the Ni_4 cluster includes more information regarding the Ni band
structure with respect to the Anderson impurity model, it also favors very
peculiar ground states which are incompatible with a coherent picture of all
dichroism experiments.Comment: 1 page, RevTeX, 1 epsf figur
Charge-density-wave order parameter of the Falicov-Kimball model in infinite dimensions
In the large-U limit, the Falicov-Kimball model maps onto an effective Ising
model, with an order parameter described by a BCS-like mean-field theory in
infinite dimensions. In the small-U limit, van Dongen and Vollhardt showed that
the order parameter assumes a strange non-BCS-like shape with a sharp reduction
near T approx T_c/2. Here we numerically investigate the crossover between
these two regimes and qualitatively determine the order parameter for a variety
of different values of U. We find the overall behavior of the order parameter
as a function of temperature to be quite anomalous.Comment: (5 pages, 3 figures, typeset with ReVTeX4
On the Origin of the -4.4 eV Band in CdTe(100)"
We calculate the bulk- (infinite system), (100)-bulk-projected- and
(100)-Surface-projected Green's functions using the Surface Green's Function
Matching method (SGFM) and an empirical tight-binding hamiltonian with
tight-binding parameters (TBP) that describe well the bulk band structure of
CdTe. In particular, we analyze the band (B--4) arising at --4.4 eV from the
top of the valence band at according to the results of Niles and
H\"ochst and at -4.6 eV according to Gawlik {\it et al.} both obtained by
Angle-resolved photoelectron spectroscopy (ARPES). We give the first
theoretical description of this band.Comment: 17 pages, Rev-TEX, CIEA-Phys. 02/9
The use of interpretive phenomenological analysis in couple and family therapy research
This article proposes a research methodology that is newer to the field of couple and family therapy research called Interpretive Phenomenological Analysis (IPA). Researchers exploring couple and family therapy research continue to establish the efficacy of couple and family interventions in a context that favors a positivist view of phenomena. This research continues to be critical for establishing the role of couple and family therapy in the field of mental health as well as further clarifying which interventions are best for specific clinical issues and when. IPA offers researchers the opportunity to explore how couples and families make meaning of their experiences from an intersubjective perspective. Meaning making is central to understanding couples and families as well as part of the many clinical approaches to working with couples and families. Despite the importance of meaning, few research methodologies allow for this central concept in couple and family therapy to be the focus of exploration. The following article outlines one such methodology and the possible use of IPA in couple and family therapy research
Refugees' views of the effectiveness of support provided by their host countries.
BACKGROUND: The war in former Yugoslavia, which commenced in 1990, caused the biggest refugee crisis in Europe since World War II. There are numerous research investigations into the trauma and associated problems. However, there is no available publication concerning refugees' own perception of the provided support in host countries. AIMS: To investigate how refugees evaluated support received (helpful or detrimental) and what kinds of support they wish to receive in the future. METHOD: The study participants were 854 refugees from former Yugoslavia settled in the United Kingdom, Germany, and Italy. Alongside demographic data, they were assessed using International Neuropsychiatric Interview (MINI), Life Stressor Checklist-Revised (LSC-R), Manchester Short Assessment of Quality of Life (MANSA), Matrix for Recording Health Care, Social Interventions (MACSI), and an open questions interview. RESULTS: Data revealed that 99.3% of refugees received some kind of support. The most frequent support (98.7%) was primary health care and the least frequent (34.7%) was support in employment and further training. The most helpful (27.5%) was primary health care, and the most detrimental (11.6%) was legal support. The most desired types of support were help in employment (31.8%) and further education/training (20.5%). The educational level of refugees affected their perceptions of support as detrimental or desired. CONCLUSIONS: There are different levels of received and desired support among host countries. There are also differences in the perception of received and desired support with regard to the refugees' educational levels
Exact ground states for the four-electron problem in a two-dimensional finite Hubbard square system
We present exact explicit analytical results describing the exact ground
state of four electrons in a two dimensional square Hubbard cluster containing
16 sites taken with periodic boundary conditions. The presented procedure,
which works for arbitrary even particle number and lattice sites, is based on
explicitly given symmetry adapted base vectors constructed in r-space. The
Hamiltonian acting on these states generates a closed system of 85 linear
equations providing by its minimum eigenvalue the exact ground state of the
system. The presented results, described with the aim to generate further
creative developments, not only show how the ground state can be exactly
obtained and what kind of contributions enter in its construction, but
emphasize further characteristics of the spectrum. On this line i) possible
explications are found regarding why weak coupling expansions often provide a
good approximation for the Hubbard model at intermediate couplings, or ii)
explicitly given low lying energy states of the kinetic energy, avoiding double
occupancy, suggest new roots for pairing mechanism attracting decrease in the
kinetic energy, as emphasized by kinetic energy driven superconductivity
theories.Comment: 37 pages, 18 figure
Electronic Theory for the Nonlinear Magneto-Optical Response of Transition-Metals at Surfaces and Interfaces: Dependence of the Kerr-Rotation on Polarization and on the Magnetic Easy Axis
We extend our previous study of the polarization dependence of the nonlinear
optical response to the case of magnetic surfaces and buried magnetic
interfaces. We calculate for the longitudinal and polar configuration the
nonlinear magneto-optical Kerr rotation angle. In particular, we show which
tensor elements of the susceptibilities are involved in the enhancement of the
Kerr rotation in nonlinear optics for different configurations and we
demonstrate by a detailed analysis how the direction of the magnetization and
thus the easy axis at surfaces and buried interfaces can be determined from the
polarization dependence of the nonlinear magneto-optical response, since the
nonlinear Kerr rotation is sensitive to the electromagnetic field components
instead of merely the intensities. We also prove from the microscopic treatment
of spin-orbit coupling that there is an intrinsic phase difference of
90 between tensor elements which are even or odd under magnetization
reversal in contrast to linear magneto-optics. Finally, we compare our results
with several experiments on Co/Cu films and on Co/Au and Fe/Cr multilayers. We
conclude that the nonlinear magneto-optical Kerr-effect determines uniquely the
magnetic structure and in particular the magnetic easy axis in films and at
multilayer interfaces.Comment: 23 pages Revtex, preprintstyle, 2 uuencoded figure
Charge-ordered ferromagnetic phase in manganites
A mechanism for charge-ordered ferromagnetic phase in manganites is proposed.
The mechanism is based on the double exchange in the presence of diagonal
disorder. It is modeled by a combination of the Ising double-exchange and the
Falicov-Kimball model. Within the dynamical mean-field theory the charge and
spin correlation function are explicitely calculated. It is shown that the
system exhibits two successive phase transitions. The first one is the
ferromagnetic phase transition, and the second one is a charge ordering. As a
result a charge-ordered ferromagnetic phase is stabilized at low temperature.Comment: To appear in Phys. Rev.
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