551 research outputs found
Electronic Correlations within Fermionic Lattice Models
We investigate two-site electronic correlations within generalized Hubbard
model, which incorporates the conventional Hubbard model (parameters:
(hopping between nearest neighbours), (Coulomb repulsion (attraction))
supplemented by the intersite Coulomb interactions (parameters:
(parallel spins), (antiparellel spins)) and the hopping of
the intrasite Cooper pairs (parameter: ). As a first step we find the
eigenvalues and eigenvectors of the dimer and we
represent each partial Hamiltonian
() in the second quantization with the use of the Hubbard
and spin operators. Each dimer energy level possesses its own Hamiltonian
describing different two-site interactions which can be active only in the case
when the level will be occupied by the electrons. A typical feature is the
appearence of two generalized interactions ascribed to two different
energy levels which do not vanish even for and their
coupling constants are equal to in this case. The competition between
ferromagnetism, antiferromagnetism and superconductivity (intrasite and
intersite pairings) is also a typical feature of the model because it persists
in the case and . The same types of the
electronic, competitive interactions are scattered between different energy
levels and therefore their thermodynamical activities are dependent on the
occupation of these levels. It qualitatively explains the origin of the phase
diagram of the model. We consider also a real lattice as a set of interacting
dimers to show that the competition between magnetism and superconductivity
seems to be universal for fermonic lattice models.Comment: 12 page
Double non-equivalent chain structure on vicinal Si(557)-Au surface
We study electronic and topographic properties of the vicinal Si(557)-Au
surface using scanning tunneling microscopy and reflection of high energy
electron diffraction technique. STM data reveal double wire structures along
terraces. Moreover behavior of the voltage dependent STM tip - surface distance
is different in different chains. While the one chain shows oscillations of the
distance which are sensitive to the sign of the voltage bias, the oscillations
in the other chain remain unchanged with respect to the positive/negative
biases. This suggests that one wire has metallic character while the other one
- semiconducting. The experimental results are supplemented by theoretical
calculations within tight binding model suggesting that the observed chains are
made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.
Low-temperature transport through a quantum dot between two superconductor leads
We consider a quantum dot coupled to two BCS superconductors with same gap
energies . The transport properties are investigated by means of
infinite- noncrossing approximation. In equilibrium density of states, Kondo
effect shows up as two sharp peaks around the gap bounds. Application of a
finite voltage bias leads these peaks to split, leaving suppressed peaks near
the edges of energy gap of each lead. The clearest signatures of the Kondo
effect in transport are three peaks in the nonlinear differential conductance:
one around zero bias, another two at biases . This result is
consistent with recent experiment. We also predict that with decreasing
temperature, the differential conductances at biases anomalously
increase, while the linear conductance descends.Comment: replaced with revised versio
Can the initial singularity be detected by cosmological tests?
In the present paper we raise the question whether initial cosmological
singularity can be proved from the cosmological tests. The classical general
relativity predict the existence of singularity in the past if only some energy
conditions are satisfied. On the other hand the latest quantum gravity
applications to cosmology suggest of possibility of avoiding the singularity
and replace it with the bounce. The distant type Ia supernovae data are used to
constraints on bouncing evolutional scenario where square of the Hubble
function is given by formulae
, where are density parameters and . We show that the on the
base of the SNIa data standard bouncing models can be ruled out on the
confidence level. If we add the cosmological constant to the standard
bouncing model then we obtain as the best-fit that the parameter
is equal zero which means that the SNIa data do not support the bouncing term
in the model. The bounce term is statistically insignificant the present epoch.
We also demonstrate that BBN offer the possibility of obtaining stringent
constraints of the extra term . The other observational test
methods like CMB and the age of oldest objects in the Universe are used. We
also use the Akaike informative criterion to select a model according to the
goodness of fit and we conclude that this term should be ruled out by Occam's
razor, which makes that the big bang is favored rather then bouncing scenario.Comment: 30 pages, 7 figures improved versio
Electron transport across a quantum wire in the presence of electron leakage to a substrate
We investigate electron transport through a mono-atomic wire which is tunnel
coupled to two electrodes and also to the underlying substrate. The setup is
modeled by a tight-binding Hamiltonian and can be realized with a scanning
tunnel microscope (STM). The transmission of the wire is obtained from the
corresponding Green's function. If the wire is scanned by the contacting STM
tip, the conductance as a function of the tip position exhibits oscillations
which may change significantly upon increasing the number of wire atoms. Our
numerical studies reveal that the conductance depends strongly on whether or
not the substrate electrons are localized. As a further ubiquitous feature, we
observe the formation of charge oscillations.Comment: 7 pages, 7 figure
Classical big-bounce cosmology: dynamical analysis of a homogeneous and irrotational Weyssenhoff fluid
A dynamical analysis of an effective homogeneous and irrotational Weyssenhoff
fluid in general relativity is performed using the 1+3 covariant approach that
enables the dynamics of the fluid to be determined without assuming any
particular form for the space-time metric. The spin contributions to the field
equations produce a bounce that averts an initial singularity, provided that
the spin density exceeds the rate of shear. At later times, when the spin
contribution can be neglected, a Weyssenhoff fluid reduces to a standard
cosmological fluid in general relativity. Numerical solutions for the time
evolution of the generalised scale factor in spatially-curved models are
presented, some of which exhibit eternal oscillatory behaviour without any
singularities. In spatially-flat models, analytical solutions for particular
values of the equation-of-state parameter are derived. Although the scale
factor of a Weyssenhoff fluid generically has a positive temporal curvature
near a bounce, it requires unreasonable fine tuning of the equation-of-state
parameter to produce a sufficiently extended period of inflation to fit the
current observational data.Comment: 34 pages, 18 figure
Dynamical System Approach to Cosmological Models with a Varying Speed of Light
Methods of dynamical systems have been used to study homogeneous and
isotropic cosmological models with a varying speed of light (VSL). We propose
two methods of reduction of dynamics to the form of planar Hamiltonian
dynamical systems for models with a time dependent equation of state. The
solutions are analyzed on two-dimensional phase space in the variables where is a function of a scale factor . Then we show how the
horizon problem may be solved on some evolutional paths. It is shown that the
models with negative curvature overcome the horizon and flatness problems. The
presented method of reduction can be adopted to the analysis of dynamics of the
universe with the general form of the equation of state .
This is demonstrated using as an example the dynamics of VSL models filled with
a non-interacting fluid. We demonstrate a new type of evolution near the
initial singularity caused by a varying speed of light. The singularity-free
oscillating universes are also admitted for positive cosmological constant. We
consider a quantum VSL FRW closed model with radiation and show that the
highest tunnelling rate occurs for a constant velocity of light if and . It is also proved that the considered class of
models is structurally unstable for the case of .Comment: 18 pages, 5 figures, RevTeX4; final version to appear in PR
Electron transport through strongly interacting quantum dot coupled to normal metal and superconductor
We study the electron transport through the quantum dot coupled to the normal
metal and BCS-like superconductor (N - QD - S) in the presence of the Kondo
effect and Andreev scattering. The system is described by the single impurity
Anderson model in the limit of strong on-dot interaction. We use recently
proposed equation of motion technique for Keldysh nonequilibrium Green's
function together with the modified slave boson approach to study the electron
transport. We derive formula for the current which contains various tunneling
processes and apply it to study the transport through the system. We find that
the Andreev conductance is strongly suppressed and there is no zero-bias
(Kondo) anomaly in the differential conductance. We discuss effects of the
particle-hole asymmetry in the electrodes as well as the asymmetry in the
couplings.Comment: Supercond. Sci. Technol. - accepted for publicatio
Microscopic derivation of Ginzburg-Landau equations for coexistent states of superconductivity and magnetism
Ginzburg-Landau (GL) equations for the coexistent states of superconductivity
and magnetism are derived microscopically from the extended Hubbard model with
on-site repulsive and nearest-neighbor attractive interactions. In the derived
GL free energy a cubic term that couples the spin-singlet and spin-triplet
components of superconducting order parameters (SCOP) with magnetization
exists. This term gives rise to a spin-triplet SCOP near the interface between
a spin-singlet superconductor and a ferromagnet, consistent with previous
theoretical studies based on the Bogoliubov de Gennes method and the
quasiclassical Green's function theory. In coexistent states of singlet
superconductivity and antiferromagnetism it leads to the occurrence of
pi-triplet SCOPs.Comment: 18 page
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