4,362 research outputs found
Impurity scattering in unconventional density waves: non-crossing approximation for arbitrary scattering rate
We present a detailed theoretical study on the thermodynamic properties of
impure quasi-one dimensional unconventional charge-, and spin-density waves in
the framework of mean-field theory. The impurities are of the ordinary
non-magnetic type. Making use of the full self-energy that takes into account
all ladder-, and rainbow-type diagrams, we are able to calculate the relevant
low temperature quantities for arbitrary impurity concentration and scattering
rates. These are the density of states, specific heat and the shift in the
chemical potential. Our results therefore cover the whole parameter space: they
include both the self-consistent Born and the resonant unitary limits, and most
importantly give exact results in between.Comment: 11 pages, 8 figure
Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field
We show that the superconducting transition temperature T_c(H) of a very thin
highly disordered film with strong spin-orbital scattering can be increased by
parallel magnetic field H. This effect is due to polarization of magnetic
impurity spins which reduces the full exchange scattering rate of electrons;
the largest effect is predicted for spin-1/2 impurities. Moreover, for some
range of magnetic impurity concentrations the phenomenon of {\it
superconductivity induced by magnetic field} is predicted: superconducting
transition temperature T_c(H) is found to be nonzero in the range of magnetic
fields .Comment: 4 pages, 2 figure
Fermi-liquid effects in the gapless state of marginally thin superconducting films
We present low temperature tunneling density-of-states measurements in Al
films in high parallel magnetic fields. The thickness range of the films, t=6-9
nm, was chosen so that the orbital and Zeeman contributions to their parallel
critical fields were comparable. In this quasi-spin paramagnetically limited
configuration, the field produces a significant suppression of the gap, and at
high fields the gapless state is reached. By comparing measured and calculated
tunneling spectra we are able to extract the value of the antisymmetric
Fermi-liquid parameter G^0 and thereby deduce the quasiparticle density
dependence of the effective parameter G^0_{eff} across the gapless state.Comment: 6 pages, 4 figure
Interface superconductivity in LaNdSrCuO/LaSrCuO bilayers
We identify a distinct superconducting phase at the interface of a
LaNdSrCuO (LNSCO)/LaSrCuO
(LSCO) epitaxial bilayer system using ac screening measurements. A model based
on inter-diffusion of quasiparticles and condensate at the interface yields a
thickness of 25 nm for the interfacial layer. Two-dimensional
superconductivity of the interface layer appears to be governed by
Kosterlitz-Thouless-Berezinskii transition. A parallel magnetic field
suppresses the superconducting transition temperature of this layer with a pair
breaking parameter varying as
Interplay of paramagnetic, orbital and impurity effects on the phase transition of a normal metal to superconducting state
We derive the generalized Ginzburg-Landau free energy functional for
conventional and unconventional singlet superconductors in the presence of
paramagnetic, orbital and impurity effects. Within the mean field theory, we
determine the criterion for appearence of the non uniform
(Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state, with vortex lattice
structure and additional modulation along the magnetic field. We also discuss
the possible change of the order of transition from normal to superconducting
state. We find that the superconducting phase diagram is very sensitive to
geometrical effects such as the nature of the order parameter and the shape of
the Fermi surface. In particular, we obtain the qualitative phase diagrams for
three-dimensional isotropic s-wave superconductors and in quasi two-dimensional
d-wave superconductors under magnetic field perpendicular to the conducting
layers.
In addition, we determine the criterion for instability toward non uniform
superconducting state in s-wave superconductors in the dirty limit.Comment: 15 pages, 4 figure
Glucose metabolism and oscillatory behavior of pancreatic islets
A variety of oscillations are observed in pancreatic islets.We establish a
model, incorporating two oscillatory systems of different time scales: One is
the well-known bursting model in pancreatic beta-cells and the other is the
glucose-insulin feedback model which considers direct and indirect feedback of
secreted insulin. These two are coupled to interact with each other in the
combined model, and two basic assumptions are made on the basis of biological
observations: The conductance g_{K(ATP)} for the ATP-dependent potassium
current is a decreasing function of the glucose concentration whereas the
insulin secretion rate is given by a function of the intracellular calcium
concentration. Obtained via extensive numerical simulations are complex
oscillations including clusters of bursts, slow and fast calcium oscillations,
and so on. We also consider how the intracellular glucose concentration depends
upon the extracellular glucose concentration, and examine the inhibitory
effects of insulin.Comment: 11 pages, 16 figure
Discrete transverse superconducting modes in nano-cylinders
Spatial variation in the superconducting order parameter becomes significant
when the system is confined at dimensions well below the typical
superconducting coherence length. Motivated by recent experimental success in
growing single-crystal metallic nanorods, we study quantum confinement effects
on superconductivity in a cylindrical nanowire in the clean limit. For large
diameters, where the transverse level spacing is smaller than superconducting
order parameter, the usual approximations of Ginzburg-Landau theory are
recovered. However, under external magnetic field the order parameter develops
a spatial variation much stronger than that predicted by Ginzburg-Landau
theory, and gapless superconductivity is obtained above a certain field
strength. At small diameters, the discrete nature of the transverse modes
produces significant spatial variations in the order parameter with increased
average magnitude and multiple shoulders in the magnetic response.Comment: 10 pages, 8 figure
Sound propagation in density wave conductors and the effect of long-range Coulomb interaction
We study theoretically the sound propagation in charge- and spin-density
waves in the hydrodynamic regime. First, making use of the method of comoving
frame, we construct the stress tensor appropriate for quasi-one dimensional
systems within tight-binding approximation. Taking into account the screening
effect of the long-range Coulomb interaction, we find that the increase of the
sound velocity below the critical temperature is about two orders of magnitude
less for longitudinal sound than for transverse one. It is shown that only the
transverse sound wave with displacement vector parallel to the chain direction
couples to the phason of the density wave, therefore we expect significant
electromechanical effect only in this case.Comment: revtex, 14 pages (in preprint form), submitted to PR
Theory of fluctuations in a two-band superconductor
A theory of fluctuations in two-band superconductor MgB is developed.
Since the standard Ginzburg-Landau (GL) approach fails in description of its
properties, we generalize it basing on the microscopic theory of a two-band
superconductor. Calculating the microscopic fluctuation propagator, we build up
the nonlocal two-band GL functional and the corresponding time-dependent GL
equations. This allows us to calculate the main fluctuation observables such as
fluctuation specific heat and conductivity.Comment: 11 pages, 2 figures, subm. to Phys. Rev.
Mesoscopic variations of local density of states in disordered superconductors
We explore correlations of inhomogeneous local density of states (LDoS) for
impure superconductors with different symmetries of the order parameter (s-wave
and d-wave) and different types of scatterers (elastic and magnetic
impurities). It turns out that the LDoS correlation function of superconductor
always slowly decreases with distance up to the phase-breaking length
and its long-range spatial behavior is determined only by the
dimensionality, as in normal metals. On the other hand, the energy dependence
of this correlation function is sensitive to symmetry of the order parameter
and nature of scatterers. Only in the simplest case of s-wave superconductor
with elastic scatterers the inhomogeneous LDoS is directly connected to the
corresponding characteristics of normal metal.Comment: 16 pages, 5 figures, accepted to Phys. Rev.
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