35 research outputs found
Perturbation expansion for 2-D Hubbard model
We develop an efficient method to calculate the third-order corrections to
the self-energy of the hole-doped two-dimensional Hubbard model in space-time
representation. Using the Dyson equation we evaluate the renormalized spectral
function in various parts of the Brillouin zone and find significant
modifications with respect to the second-order theory even for rather small
values of the coupling constant U. The spectral function becomes unphysical for
, where W is the half-width of the conduction band. Close to the
Fermi surface and for U<W, the single-particle spectral weight is reduced in a
finite energy interval around the Fermi energy. The increase of U opens a gap
between the occupied and unoccupied parts of the spectral function.Comment: 17 pages, 11 Postscript figures, Phys. Rev. B, accepte
Superconducting properties of the attractive Hubbard model
A self-consistent set of equations for the one-electron self-energy in the
ladder approximation is derived for the attractive Hubbard model in the
superconducting state. The equations provide an extension of a T-matrix
formalism recently used to study the effect of electron correlations on
normal-state properties. An approximation to the set of equations is solved
numerically in the intermediate coupling regime, and the one-particle spectral
functions are found to have four peaks. This feature is traced back to a peak
in the self-energy, which is related to the formation of real-space bound
states. For comparison we extend the moment approach to the superconducting
state and discuss the crossover from the weak (BCS) to the intermediate
coupling regime from the perspective of single-particle spectral densities.Comment: RevTeX format, 8 figures. Accepted for publication in Z.Phys.
Resistively-shunted superconducting quantum point contacts
We have studied the Josephson dynamics of resistively-shunted ballistic
superconducting quantum point contacts at finite temperatures and arbitrary
number of conducting modes. Compared to the classical Josephson dynamics of
tunnel junctions, dynamics of quantum point contacts exhibits several new
features associated with temporal fluctuations of the Josephson potential
caused by fluctuations in the occupation of the current-carrying Andreev
levels.Comment: 5 pages, RevTex, 3 postscript figures include
Calidad de vida y sus factores determinantes en universitarios españoles de Ciencias de la Salud
La calidad de vida en la población universitaria
adquiere una especial importancia ya que permite
obtener información sobre las condiciones de vida
de los universitarios y, sobre todo, de cómo éstos las perciben.
Objetivo: Evaluar la calidad de vida de los universitarios
que cursan estudios en ciencias de la salud y su
relación con diferentes factores tales como: hábitos de
vida, parámetros antropométricos y la influencia de las
distintas variables sobre su percepción.
Material y Método: Estudio transversal de una muestra
de 1.753 estudiantes de ciencias de la salud de nueve
universidades españolas con diseño muestral aleatorio y
estatrificado según curso y facultad al que se le aplicó
un cuestionaro ad hoc que recogía todas las variables a
estudio.
Resultados: La calidad de vida percibida por los participantes
fue Me = 75. Los factores explorados de la calidad
de vida se co-relacionaron significativamente con la
percepción global de calidad de vida de los estudiantes
(p<0,001). Se establecieron 3 dimensiones y el impacto de
cada una de ellas sobre la percepción de calidad de vida
global fue p<0,001. Los varones percibieron mejor calidad
de vida que las mujeres y también los estudiantes con
menor Índice de Masa Corporal (IMC).
Conclusión: Los universitarios son una población clave
para realizar actividades de promoción y prevención
de la salud por lo que resulta necesario crear mejores infraestucturas
y recursos educativos para mejorar la CV y
fomentar hábitos y estilos de vida saludable con especial
atención en la alimentación y la realización de una adecuada
actividad física.Abstract
Introduction: The quality of life of university students
acquires special importance because it provides information
about their life conditions and especially how they
perceive it.
Objetive: Evaluate the quality of life of students who
are enrolled in health science studies and its relation with
the following diverse factors: life and dietetic habits, anthropometric
parameters and the influence of distinct
variables on their perception.
Methods: Transversal study of a sample of 1753 health
science degree students of nine Spanish universities with
a randomized design and stratified by course and faculty
for which we applied an ad hoc questionnaire that considered
all study variables.
Results: The quality of life (QoL) perceived by the
participants had a Median of 75. The factors that were
explored about the quality of life correlated significantly
with their global perception of it (p<0.001). Three dimensions
were established and the impact of each one of them
on their global perception of QoL was p<0.001. Men perceived
better QoL then women and the students with lower
Body Mass Index (BMI).
Conclusions: University students are a key population
for realizing health promotion and prevention activities
therefore it is necessary to develop and provide better
infrastructures and educative resources in order to enhance
their QoL and to promote healthier habits and life
styles with special attention on dietetics habits and the
performance of an adequate physical activity
Modified Perturbation Theory Applied to Kondo-Type Transport through a Quantum Dot under a Magnetic Field
Linear conductance through a quantum dot is calculated under a finite
magnetic field using the modified perturbation theory. The method is based on
the second-order perturbation theory with respect to the Coulomb repulsion, but
the self-energy is modified to reproduce the correct atomic limit and to
fulfill the Friedel sum rule exactly. Although this method is applicable only
to zero temperature in a strict sense, it is approximately extended to finite
temperatures. It is found that the conductance near electron-hole symmetry is
suppressed by the application of the magnetic field at low temperatures.
Positive magnetoconductance is observed in the case of large electron-hole
asymmetry.Comment: 4pages, 5 figure
Subharmonic Shapiro steps and assisted tunneling in superconducting point contacts
We analyze the current in a superconducting point contact of arbitrary
transmission in the presence of a microwave radiation. The interplay between
the ac Josephson current and the microwave signal gives rise to Shapiro steps
at voltages V = (m/n) \hbar \omega_r/2e, where n,m are integer numbers and
\omega_r is the radiation frequency. The subharmonic steps (n different from 1)
are a consequence of the ocurrence of multiple Andreev reflections (MAR) and
provide an unambiguous signature of the peculiar ac Josephson effect at high
transmission. Moreover, the dc current exhibits a rich subgap structure due to
photon-assisted MARs.Comment: Revtex, 4 pages, 4 figure
Magnetic phase diagram of the Hubbard model
The competition between commensurate and incommensurate spin-density-wave
phases in the infinite-dimensional single-band Hubbard model is examined with
quantum Monte Carlo simulation and strong and weak coupling approximations.
Quantum fluctuations modify the weak-coupling phase diagram by factors of order
unity and produce remarkable agreement with the quantum Monte Carlo data, but
strong-coupling theories (that map onto effective Falicov-Kimball models)
display pathological behavior. The single-band model can be used to describe
much of the experimental data in Cr and its dilute alloys with V and Mn.Comment: 12 pages plus 3 uuencoded postscript figures, ReVTe
The low-energy scale of the periodic Anderson model
Wilson's Numerical Renormalization Group method is used to study the
paramagnetic ground state of the periodic Anderson model within the dynamical
mean-field approach. For the particle-hole symmetric model, which is a Kondo
insulator, we find that the lattice Kondo scale T_0 is strongly enhanced over
the impurity scale T_K; T_0/T_K ~ exp(1/3I), where I is the Schrieffer-Wolff
exchange coupling. In the metallic regime, where the conduction band filling is
reduced from one, we find characteristic signatures of Nozi\`eres exhaustion
scenario, including a strongly reduced lattice Kondo scale, a significant
suppression of the states available to screen the f-electron moment, and a
Kondo resonance with a strongly enhanced height. However, in contrast to the
quantitative predictions of Nozi\`eres, we find that the T_0 ~ T_K with a
coefficient which depends strongly on conduction band filling.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
Non-Equilibrium Quasiclassical Theory for Josephson Structures
We present a non-equilibrium quasiclassical formalism suitable for studying
linear response ac properties of Josephson junctions. The non-equilibrium
self-consistency equations are satisfied, to very good accuracy, already in
zeroth iteration. We use the formalism to study ac Josephson effect in a
ballistic superconducting point contact. The real and imaginary parts of the ac
linear conductance are calculated both analytically (at low frequencies) and
numerically (at arbitrary frequency). They show strong temperature, frequency,
and phase dependence. Many anomalous properties appear near phi = pi. We
ascribe them to the presence of zero energy bound states.Comment: 11 pages, 9 figures, Final version to appear in PR
Iterated perturbation theory for the attractive Holstein and Hubbard models
A strictly truncated (weak-coupling) perturbation theory is applied to the
attractive Holstein and Hubbard models in infinite dimensions. These results
are qualified by comparison with essentially exact Monte Carlo results. The
second order iterated perturbation theory is shown to be quite accurate in
calculating transition temperatures for retarded interactions, but is not as
accurate for the self energy or the irreducible vertex functions themselves.
Iterated perturbation theory is carried out thru fourth order for the Hubbard
model. The self energy is quite accurately reproduced by the theory, but the
vertex functions are not. Anomalous behavior occurs near half filling because
the iterated perturbation theory is not a conserving approximation. (REPLACED
WITH UUENCODED FIGURES AT THE END. THE TEXT IS UNCHANGED)Comment: 27 pages, RevTex (figures appended at end