1,071 research outputs found
Charge current in ferromagnet-superconductor junction with pairing state of broken time-reversal symmetry
We calculate the tunneling conductance spectra of a ferromagnetic
metal/insulator/superconductor using the Blonder-Tinkham-Klapwijk (BTK)
formulation. Two possible states for the superconductor are considered with the
time reversal symmetry () broken, i.e., , or
. In both cases the tunneling conductance within the gap
is suppressed with the increase of the exchange interaction due to the
suppression of the Andreev reflection. In the -wave case the
peaks that exist when the ferromagnet is a normal metal in the amplitude of the
s-wave component due to the bound state formation are reduced symmetrically,
with the increase of the exchange field, while in the
-wave case the residual density of states within the gap
develops a dip around E=0 with the increase of the exchange field. These
results would be useful to discriminate between -broken pairing states
near the surface in high- superconductorsComment: 17 pages with 11 figure
Two regimes for effects of surface disorder on the zero-bias conductance peak of tunnel junctions involving d-wave superconductors
Impurity-induced quasiparticle bound states on a pair-breaking surface of a
d-wave superconductor are theoretically described, taking into account
hybridization of impurity- and surface-induced Andreev states. Further a theory
for effects of surface disorder (of thin impurity surface layer) on the
low-bias conductance of tunnel junctions is developed. We find a threshold
for surface impurity concentration , which separates the two regimes
for surface impurity effects on the zero-bias conductance peak (ZBCP). Below
the threshold, surface impurities do not broaden the ZBCP, but effectively
reduce its weight and generate impurity bands. For low impurity bands can
be, in principle, resolved experimentally, being centered at energies of bound
states induced by an isolated impurity on the surface. For larger
impurity bands are distorted, move to lower energies and, beginning with the
threshold concentration , become centered at zero energy. With
increasing above the threshold, the ZBCP is quickly destroyed in the case
of strong scatterers, while it is gradually suppressed and broaden in the
presence of weak impurity potentials. More realistic cases, taking into account
additional broadening, not related to the surface disorder, are also
considered.Comment: 9 pages, 7 figure
Transport properties of ferromagnet/d-wave superconductor/ferromagnet double junctions
We investigate transport properties of a trilayer made of a d-wave
superconductor connected to two ferromagnetic electrodes. Using Keldysh
formalism we show that crossed Andreev reflection and elastic cotunneling exist
also with d-wave superconductors. Their properties are controlled by the
existence of zero energy states due to the anisotropy of the d-wave pair
potential.Comment: 16 pages, 4 figures, revised versio
Mechanisms of Spontaneous Current Generation in an Inhomogeneous d-Wave Superconductor
A boundary between two d-wave superconductors or an s-wave and a d-wave
superconductor generally breaks time-reversal symmetry and can generate
spontaneous currents due to proximity effect. On the other hand, surfaces and
interfaces in d-wave superconductors can produce localized current-carrying
states by supporting the T-breaking combination of dominant and subdominant
order parameters. We investigate spontaneous currents in the presence of both
mechanisms and show that at low temperature, counter-intuitively, the
subdominant coupling decreases the amplitude of the spontaneous current due to
proximity effect. Superscreening of spontaneous currents is demonstrated to be
present in any d-d (but not s-d) junction and surface with d+id' order
parameter symmetry. We show that this supercreening is the result of
contributions from the local magnetic moment of the condensate to the
spontaneous current.Comment: 4 pages, 5 figures, RevTe
Anomalous proximity effect in d-wave superconductors
The anomalous proximity effect between a d-wave superconductor and a surface
layer with small electronic mean free path is studied theoretically in the
framework of the Eilenberger equations. The angular and spatial structure of
the pair potential and the quasiclassical propagators in the interface region
is calculated selfconsistently. The variation of the spatially-resolved
quasiparticle density of states from the bulk to the surface is studied. It is
shown that the isotropic gapless superconducting state is induced in the
disordered layer.Comment: 6 pages, 5 postscript figures. Submitted to Phys.Rev.
Local density of states for the corner geometry interface of d-wave superconductors, within the extended Hubbard model
The spatial variations of the order parameter, and the local density of
states (LDOS) on the corner of s-wave or -wave superconductors, as
well as in superconductor-insulator-normal metal interfaces, are calculated
self consistently using the Bogoliubov-deGennes formalism within the two
dimensional extended Hubbard model. The exact diagonalization method is used.
Due to the suppression of the dominant d-wave order parameter, the extended
s-wave order parameter is induced near the surface, that alternates its sign
for the topmost sites at adjacent edges of the lattice and decays to zero in
the bulk. The presence of surface roughness results into the appearance of the
zero band conduction peak (ZBCP) near the corner surface which lacks from the
predictions of the quasiclassical theory.Comment: 13 pages with 17 figure
Caveolin-1 expression and cavin stability regulate caveolae dynamics in adipocyte lipid store fluctuation
Adipocytes specialized in the storage of energy as fat are among the most caveolae-enriched cell types. Loss of caveolae produces lipodystrophic diabetes in humans, which cannot be reversed by endothelial rescue of caveolin expression in mice, indicating major importance of adipocyte caveolae. However, how caveolae participate in fat cell functions is poorly understood. We investigated dynamic conditions of lipid store fluctuations and demonstrate reciprocal regulation of caveolae density and fat cell lipid droplet storage. We identified caveolin-1 expression as a crucial step in adipose cell lines and in mice to raise the density of caveolae, to increase adipocyte ability to accommodate larger lipid droplets, and to promote cell expansion by increased glucose utilization. In human subjects enrolled in a trial of 8 weeks of overfeeding to promote fattening, adipocyte expansion response correlated with initial caveolin-1 expression. Conversely, lipid mobilization in cultured adipocytes to induce lipid droplet shrinkage led to biphasic response of cavin-1 with ultimate loss of expression of cavin-1 and -3 and EHD2 by protein degradation, coincident with caveolae disassembly. We have identified the key steps in cavin/caveolin interplay regulating adipocyte caveolae dynamics. Our data establish that caveolae participate in a unique cell response connected to lipid store fluctuation, suggesting lipid-induced mechanotension in adipocytes
Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors
We have performed a detailed study of the tunneling spectra of bicrystal
grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and
NCCO. In all experiments the tunneling direction was along the CuO planes. With
the exception of NCCO, for all materials a pronounced zero bias conductance
peak was observed which decreases with increasing temperature and disappears at
the critical temperature. These results can be explained by the presence of a
dominating d-wave symmetry of the order parameter resulting in the formation of
zero energy Andreev bound states at surfaces and interfaces of HTS. The absence
of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair
potential in this material. The observed nonlinear shift of spectral weight to
finite energies by applying a magnetic field is in qualitative agreement with
recent theoretical predictions.Comment: 4 pages, 4 figures, to be published in Phys. Rev.
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