2,219 research outputs found
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors
We calculate the c-axis infrared conductivity in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO-planes including a phenomenological d-wave pseudogap of
amplitude . For temperatures decreasing below a temperature , a gap for develops in in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for . We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.Comment: to be published in Phys. Rev. B (November 1, 1999
Fermi surface topology and vortex state in MgB2
Based on a detailed modeling of the Fermi surface topology of MgB2 we
calculated the anisotropy of the upper critical field Bc2 within the two gap
model. The sigma-band is modeled as a distorted cylinder and the pi-band as a
half-torus, with parameters determined from bandstructure calculations. Our
results show that the unusual strong temperature dependence of the Bc2
anisotropy, that has been observed recently, can be understood due to the small
c-axis dispersion of the cylindrical Fermi surface sheets and the small
interband pairing interaction as obtained from bandstructure calculations. We
calculate the magnetic field dependence of the density of states within the
vortex state for field in c-axis direction and compare with recent measurements
of the specific heat on MgB2 single crystals.Comment: 2 pages, 2 figure
Transfer-matrix approach to multiband Josephson junctions
We study the influence of multiple bands on the properties of Josephson
junctions. In particular we focus on the two gap superconductor magnesium
diboride. We present a formalism to describe tunneling at a point contact
between two MgB2 electrodes generalizing the transfer-matrix approach to
multiple bands. A simple model is presented to determine the effective hopping
amplitudes between the different energy bands as a function of the
misorientation angle of the electrodes. We calculate the critical current and
the current-voltage characteristics for N-I-S and S-I-S contacts with different
orientation for junctions with both high and low transparency. We find that
interband tunneling processes become increasingly important with increasing
misorientation angle. This is reflected in certain features in the differential
tunneling conductance in both the tunneling limit as well as for multiple
Andreev reflections.Comment: 13 pages, 8 figure
Strong surface contribution to the Nonlinear Meissner Effect
We demonstrate that in a d-wave superconductor the bulk nonlinear Meissner
effect is dominated by a surface effect due to Andreev bound states at low
temperatures. The contribution of this surface effect to the nonlinear response
coefficient follows a 1/T^3 law with opposite sign compared to the bulk 1/T
behavior. The cross-over from bulk dominated behavior to surface dominated
behavior occurs at a temperature of T/T_c ~ 1/sqrt(kappa). We present an
approximate analytical calculation, which supports our numerical calculations
and provides a qualitative understanding of the effect. The effect can be
probed by intermodulation distortion experiments.Comment: 4 pages, 3 figure
High temperature superconductivity in dimer array systems
Superconductivity in the Hubbard model is studied on a series of lattices in
which dimers are coupled in various types of arrays. Using fluctuation exchange
method and solving the linearized Eliashberg equation, the transition
temperature of these systems is estimated to be much higher than that of
the Hubbard model on a simple square lattice, which is a model for the high
cuprates. We conclude that these `dimer array' systems can generally
exhibit superconductivity with very high . Not only -electron systems,
but also -electron systems may provide various stages for realizing the
present mechanism.Comment: 4 pages, 9 figure
Anisotropic s-wave superconductivity: comparison with experiments on MgB2 single crystals
The recently discovered superconductivity in MgB2 has captured world
attention due to its simple crystal structure and relatively high
superconducting transition temperature Tc=39K. It appears to be generally
accepted that it is phonon-mediated s-wave BCS-like superconductivity.
Surprisingly, the strongly temperature dependent anisotropy of the upper
critical field, observed experimentally in magnesium diboride single crystals,
is still lacking a consistent theoretical explanation. We propose a simple
single-gap anisotropic s-wave order parameter in order to compare its
implications with the prediction of a multi-gap isotropic s-wave model. The
quasiparticle density of states, thermodynamic properties, NMR spin-lattice
relaxation rate, optical conductivity, and Hc2 anisotropy have been analyzed
within this anisotropic s-wave model. We show that the present model can
capture many aspects of the unusual superconducting properties of MgB2
compound, though more experimental data appear to be necessary from single
crystal MgB2.Comment: 7 pages, 6 figures, some minor changes, to appear in Europhys. Let
Nonlinear microwave response of MgB2
We calculate the intrinsic nonlinear microwave response of the two gap
superconductor MgB2 in the clean and dirty limits. Due to the small value of
the pi band gap, the nonlinear response at low temperatures is larger than for
a single gap Bardeen-Cooper-Schrieffer (BCS) s-wave superconductor with a
transition temperature of 40 K. Comparing this result with the intrinsic
nonlinear d-wave response of YBa2Cu3O7 (YBCO) we find a comparable response at
temperatures around 20 K. Due to its two gap nature, impurity scattering in
MgB2 can be used to reduce the nonlinear response if the scattering rate in the
pi band is made larger than the one in the sigma band.Comment: 4 pages, 4 figure
Anisotropic critical fields of MgB2 single crystals
The recently discovered superconductivity in MgB2 has created the world
sensation. In spite of the relatively high superconducting transition
temperature Tc=39K, the superconductivity is understood in terms of rare two
gap superconductor with energy gaps attached to the sigma- and pi-band.
However, this simple model cannot describe the temperature dependent anisotropy
in H_c2 or the temperature dependence of the anisotropic magnetic penetration
depth. Here we propose a model with two anisotropic energy gaps with different
shapes. Indeed the present model describes a number of pecularities of MgB2
which have been revealed only recently through single crystal MgB2.Comment: 4 pages, 1 figure, to appear in Acta Physica Polonica B, proceedings
of the International Conference on Strongly Correlated Electron Systems,
SCES2002, Krakow, Polan
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