1,213 research outputs found
Interaction of strongly correlated electrons and acoustical phonons
We investigate the interaction of correlated electrons with acoustical
phonons using the extended Hubbard-Holstein model in which both, the
electron-phonon interaction and the on-site Coulomb repulsion are considered to
be strong. The Lang-Firsov canonical transformation allows to obtain mobile
polarons for which a new diagram technique and generalized Wick's theorem is
used. This allows to handle the Coulomb repulsion between the electrons emerged
into a sea of phonon fields (\textit{phonon clouds}). The physics of emission
and absorption of the collective phonon-field mode by the polarons is discussed
in detail. Moreover, we have investigated the different behavior of optical and
acoustical phonon clouds when propagating through the lattice. In the
strong-coupling limit of the electron-phonon interaction, and in the normal as
well as in the superconducting phase, chronological thermodynamical averages of
products of acoustical phonon-cloud operators can be expressed by one-cloud
operator averages. While the normal one-cloud propagator has the form of a
Lorentzian, the anomalous one is of Gaussian form and considerably smaller.
Therefore, the anomalous electron Green's functions can be considered to be
more important than corresponding polarons functions, i.e., pairing of
electrons without phonon-clouds is easier to achieve than pairing of polarons
with such clouds.Comment: : 28 pages, 9 figures, revtex4. Invited paper for a special issue of
Low Temperature Physics dedicated to the 20th anniversary of HTS
Is the term "type-1.5 superconductivity" warranted by Ginzburg-Landau theory?
It is shown that within the Ginzburg-Landau (GL) approximation the order
parameters Delta1(r, T) and Delta2(r, T) in two-band superconductors vary on
the same length scale, the difference in the zero-T coherence lengths xi0_i
~vF_i/Delta_i(0), i = 1, 2 notwithstanding. This amounts to a single physical
GL parameter kappa and the classic GL dichotomy: kappa < 1/sqrt(2) for type-I
and kappa > 1/sqrt(2) for type-II.Comment: 5 pages, revised and extended version; previous title "Two-band
superconductors near Tc" change
Constraints on cosmic-ray propagation models from a global Bayesian analysis
Research in many areas of modern physics such as, e.g., indirect searches for
dark matter and particle acceleration in SNR shocks, rely heavily on studies of
cosmic rays (CRs) and associated diffuse emissions (radio, microwave, X-rays,
gamma rays). While very detailed numerical models of CR propagation exist, a
quantitative statistical analysis of such models has been so far hampered by
the large computational effort that those models require. Although statistical
analyses have been carried out before using semi-analytical models (where the
computation is much faster), the evaluation of the results obtained from such
models is difficult, as they necessarily suffer from many simplifying
assumptions, The main objective of this paper is to present a working method
for a full Bayesian parameter estimation for a numerical CR propagation model.
For this study, we use the GALPROP code, the most advanced of its kind, that
uses astrophysical information, nuclear and particle data as input to
self-consistently predict CRs, gamma rays, synchrotron and other observables.
We demonstrate that a full Bayesian analysis is possible using nested sampling
and Markov Chain Monte Carlo methods (implemented in the SuperBayeS code)
despite the heavy computational demands of a numerical propagation code. The
best-fit values of parameters found in this analysis are in agreement with
previous, significantly simpler, studies also based on GALPROP.Comment: 19 figures, 3 tables, emulateapj.sty. A typo is fixed. To be
published in the Astrophysical Journal v.728 (February 10, 2011 issue).
Supplementary material can be found at
http://www.g-vo.org/pub/GALPROP/GalpropBayesPaper
Diagrammatic theory for Periodic Anderson Model: Stationary property of the thermodynamic potential
Diagrammatic theory for Periodic Anderson Model has been developed, supposing
the Coulomb repulsion of localized electrons as a main parameter of the
theory. electrons are strongly correlated and conduction electrons
are uncorrelated. Correlation function for and mass operator for
electrons are determined. The Dyson equation for and Dyson-type equation
for electrons are formulated for their propagators. The skeleton diagrams
are defined for correlation function and thermodynamic functional. The
stationary property of renormalized thermodynamic potential about the variation
of the mass operator is established. The result is appropriate as for normal
and as for superconducting state of the system.Comment: 12 pages, 10 figure
Propagation of cosmic rays: nuclear physics in cosmic-ray studies
The nuclei fraction in cosmic rays (CR) far exceeds the fraction of other CR
species, such as antiprotons, electrons, and positrons. Thus the majority of
information obtained from CR studies is based on interpretation of isotopic
abundances using CR propagation models where the nuclear data and isotopic
production cross sections in p- and alpha-induced reactions are the key
elements. This paper presents an introduction to the astrophysics of CR and
diffuse gamma rays and discusses some of the puzzles that have emerged recently
due to more precise data and improved propagation models. Merging with
cosmology and particle physics, astrophysics of CR has become a very dynamic
field with a large potential of breakthrough and discoveries in the near
future. Exploiting the data collected by the CR experiments to the fullest
requires accurate nuclear cross sections.Comment: 6 pages, 13 figures, aip style files. Invited review talk at the Int.
Conf. on Nuclear Data-2004 (Santa Fe, Sep. 26 - Oct. 1, 2004). To appear in
AIP Conf. Pro
Photo-induced spin filtering in a double quantum dot
We investigate the spin-resolved electron dynamics in a double quantum dot
driven by ultrafast asymmetric electromagnetic pulses. Using a analytical model
we show that applying an appropriate pulse sequence allows to control
coherently the spin degree of freedom on the femtosecond time scale. It can be
achieved that the spin-up state is localized in a selected quantum dot while
the spin-down state remains in the other dot. We show that this photo-induced
spin-dependent separation can be maintained for a desired period of time.Comment: shortened, revised version 2 article published at Appl. Phys. Let
Challenging cosmic ray propagation with antiprotons. Evidence for a "fresh" nuclei component?
Recent measurements of the cosmic ray (CR) antiproton flux have been shown to
challenge existing CR propagation models. It was shown that the reacceleration
models designed to match secondary to primary nuclei ratios (e.g.,
boron/carbon) produce too few antiprotons. Matching both the secondary to
primary nuclei ratio and the antiproton flux requires artificial breaks in the
diffusion coefficient and the primary injection spectrum suggesting the need
for other approaches.
In the present paper we discuss one possibility to overcome these
difficulties. Using the measured antiproton flux AND B/C ratio to fix the
diffusion coefficient, we show that the spectra of primary nuclei as measured
in the heliosphere may contain a fresh local "unprocessed" component at low
energies perhaps associated with the Local Bubble, thus decreasing the measured
secondary to primary nuclei ratio. The independent evidence for SN activity in
the solar vicinity in the last few Myr supports this idea. The model reproduces
antiprotons, B/C ratio, and elemental abundances up to Ni (Z<=28). Calculated
isotopic distributions of Be and B are in perfect agreement with CR data. The
abundances of three "radioactive clock" isotopes in CR, 10Be, 26Al, 36Cl, are
all consistent and indicate a halo size z_h~4 kpc based on the most accurate
data taken by the ACE spacecraft.Comment: To be published in The Astrophysical Journal, v.586, 2003 April 1;
final version: 19 pages, 24 ps-figures, emulateapj5.sty (modified),
natbib.sty, aastex.cls. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
The Glacier Complexes of the Mountain Massifs of the North-West of Inner Asia and their Dynamics
The subject of this paper is
the glaciation of the mountain massifs
Mongun-Taiga, Tavan-Boghd-Ola, Turgeni-
Nuru, and Harhira-Nuru. The glaciation is
represented mostly by small forms that
sometimes form a single complex of domeshaped
peaks. According to the authors,
the modern glaciated area of the mountain
massifs is 21.2 km2 (Tavan-Boghd-Ola),
20.3 km2 (Mongun-Taiga), 42 km2 (Turgeni-
Nuru), and 33.1 km2 (Harhira-Nuru).
The area of the glaciers has been shrinking
since the mid 1960’s. In 1995–2008, the rate
of reduction of the glaciers’ area has grown
considerably: valley glaciers were rapidly
degrading and splitting; accumulation
of morainic material in the lower parts
of the glaciers accelerated. Small glaciers
transformed into snowfields and rock
glaciers. There has been also a degradation
of the highest parts of the glaciers and the
collapse of the glacial complexes with a
single zone of accumulation into isolated
from each other glaciers. Reduced snow
cover area has led to a rise in the firn
line and the disintegration of a common
accumulation area of the glacial complex.
In the of the Mongun-Taiga massif, in 1995–
2008, the firn line rose by 200–300 m. The
reduction of the glaciers significantly lagged
behind the change in the position of the
accumulation area boundary. In the past two
years, there has been a significant recovery
of the glaciers that could eventually lead to
their slower degradation or stabilization of
the glaciers in the study area
Superconductivity in two-band systems with variable charge carrier density. The case of MgB2
The theory of thermodynamic properties of two-band superconductor with
reduced density charge carriers is developed on the base of phonon
superconducting mechanism with strong electron-phonon interaction. This theory
is adapted to describe the behavior of critical temperature Tc, energy gaps
Delta1, Delta2, and the relative jump of electron specific heat (Cs - Cn)/Cn in
the point T = Tc along with the variation of charge carrier density in the
compound MgB2 when substitutional impurities with different valence are
introduced into the system. It is shown, that according to the filling
mechanism of energy bands which overlap on Fermi surface, the quantities Tc,
Delta1, Delta2 decrease when this compound is doped with electrons and remain
constant or weakly change when the system is doped with holes. The theory
qualitatively agrees with the experimental data. Also is shown that the
consideration of inter- and intraband scattering of electrons on impurity
potential improves this agreement.Comment: 19 pages, 6 figures, 1 table. to be published in JETP (first number
2007
Two-band superconductors: Hidden criticality deep in the superconducting state
We show that two-band superconductors harbor hidden criticality deep in the
superconducting state, stemming from the critical temperature of the weaker
band taken as an independent system. For sufficiently small interband coupling
the coherence length of the weaker band exhibits a remarkable
deviation from the conventional monotonic increase with temperature, namely, a
pronounced peak close to the hidden critical point. The magnitude of the peak
scales proportionally to \gamma^(-\mu), with the Landau critical exponent \mu =
1/3, the same as found for the mean-field critical behavior with respect to the
source field in ferromagnets and ferroelectrics. Here reported hidden
criticality of multi-band superconductors can be experimentally observed by,
e.g., imaging of the variations of the vortex core in a broader temperature
range. Similar effects are expected for the superconducting multilayers.Comment: 6 pages, 2 figures, Supplementary material included. Accepted for
publication in PR
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