419 research outputs found
The role of intermediate layers in the c-axis conductivity of layered superconductors
A simplified model of c axis transport in the high T_c superconductors is
presented. Expressions are found for the c axis optical conductivity, the d.c.
resistivity, and the c axis penetration depth. Within the framework of this
model, the pseudogap in the optical conductivity arises naturally as a result
of the layered band structure of the high T_c materials. We discuss the
occurence of the pseudogap in terms of three parameters: a band gap Delta_{ps},
a temperature dependent scattering rate Gamma(T), and the strength of the
interlayer coupling t_{perp}. We are also able to find analytic expressions for
the d.c. conductivity and the low temperature penetration depth in terms of
these three parameters. This work is an attempt to present a simple, unified
picture of c axis properties in the high T_c cuprates.Comment: 3 pages, 2 figures, Proceedings of Spectroscopies of Novel
Superconductors '97, To be published in J. Phys. Chem. Solid
Triplanar Model for the Gap and Penetration Depth in YBCO
YBaCuO_7 is a trilayer material with a unit cell consisting of a CuO_2
bilayer with a CuO plane of chains in between. Starting with a model of
isolated planes coupled through a transverse matrix element, we consider the
possibility of intra as well as interplane pairing within a nearly
antiferromagnetic Fermi liquid model. Solutions of a set of three coupled BCS
equations for the gap exhibit orthorhombic symmetry with s- as well as d-wave
contributions. The temperature dependence and a-b in plane anisotropy of the
resulting penetration depth is discussed and compared with experiment.Comment: To appear in Physical Review B1 01Mar97; 12 pages with 10 figures;
RevTeX+eps
Effect of Order-Parameter Suppression on Scattering by Isolated Impurities in Asymmetric Bands
The single-impurity problem in d-wave superconductors with asymmetric bands
is discussed. The effect of local order parameter suppression near the impurity
is to shift the quasiparticle resonance. Contrary to previous work [A. Shnirman
et al., Phys. Rev. B 60, 7517 (1999)] we find that the direction of the shift
is not universally towards the strong scattering limit.Comment: 2 pages, 1 figure, proceedings for M2S-HTSC-V
The geometrically-averaged density of states calculated from the local Green's function as a measure of localization
With the goal of measuring localization in disordered interacting systems, we
examine the finite-size scaling of the geometrically-averaged density of states
calculated from the local Green's function with finite energy resolution. Our
results show that, unlike in a simple energy binning procedure, there is no
limit in which the finite energy resolution is irrelevant.Comment: 2 pages, 1 figure; to be published in the proceedings of SCES '0
Effect of Interband Transitions on the c axis Penetration Depth of Layered Superconductors
The electromagnetic response of a system with two planes per unit cell
involves, in addition to the usual intraband contribution, an added interband
term. These transitions affect the temperature dependence and the magnitude of
the zero temperature c-axis penetration depth. When the interplane hopping is
sufficiently small, the interband transitions dominate the low temperature
behaviour of the penetration depth which then does not reflect the linear
temperature dependence of the intraband term and in comparison becomes quite
flat even for a d-wave gap. It is in this regime that the pseudogap was found
in our previous normal state calculations of the c-axis conductivity, and the
effects are connected.Comment: 8 pages, 5 figure
Effect of bilayer coupling on tunneling conductance of double-layer high T_c cuprates
Physical effects of bilayer coupling on the tunneling spectroscopy of high
T cuprates are investigated. The bilayer coupling separates the bonding
and antibonding bands and leads to a splitting of the coherence peaks in the
tunneling differential conductance. However, the coherence peak of the bonding
band is strongly suppressed and broadened by the particle-hole asymmetry in the
density of states and finite quasiparticle life-time, and is difficult to
resolve by experiments. This gives a qualitative account why the bilayer
splitting of the coherence peaks was not clearly observed in tunneling
measurements of double-layer high-T oxides.Comment: 4 pages, 3 figures, to be published in PR
Low-energy quasiparticle excitations in dirty d-wave superconductors and the Bogoliubov-de Gennes kicked rotator
We investigate the quasiparticle density of states in disordered d-wave
superconductors. By constructing a quantum map describing the quasiparticle
dynamics in such a medium, we explore deviations of the density of states from
its universal form (), and show that additional low-energy
quasiparticle states exist provided (i) the range of the impurity potential is
much larger than the Fermi wavelength [allowing to use recently developed
semiclassical methods]; (ii) classical trajectories exist along which the
pair-potential changes sign; and (iii) the diffractive scattering length is
longer than the superconducting coherence length. In the classically chaotic
regime, universal random matrix theory behavior is restored by quantum
dynamical diffraction which shifts the low energy states away from zero energy,
and the quasiparticle density of states exhibits a linear pseudogap below an
energy threshold .Comment: 4 pages, 3 figures, RevTe
Density of states "width parity" effect in d-wave superconducting quantum wires
We calculate the density of states (DOS) in a clean mesoscopic d-wave
superconducting quantum wire, i.e. a sample of infinite length but finite width
. For open boundary conditions, the DOS at zero energy is found to be zero
if is even, and nonzero if is odd. At finite chemical potential, all
chains are gapped but the qualtitative differences between even and odd
remain.Comment: 7 pages, 8 figures, new figures and extended discussio
Microwave conductivity of YBaCuO including inelastic scattering
The fluctuation spectrum responsible for the inelastic scattering in
YBaCuO which was recently determined from consideration of the
in-plane optical conductivity in the infrared, is used to calculate the
temperature dependence of the microwave conductivity at several measured
frequencies. Reasonable overall agreement can only be achieved if, in addition,
some impurity scattering is included within a model potential intermediate
between weak (Born) and strong (unitary) limit.Comment: 15 pages, 5 figures accepted for publication in Phys. Rev.
Magnetic field effects on the density of states of orthorhombic superconductors
The quasiparticle density of states in a two-dimensional d-wave
superconductor depends on the orientation of the in-plane external magnetic
field H. This is because. in the region of the gap nodes, the Doppler shift due
to the circulating supercurrents around a vortex depend on the direction of H.
For a tetragonal system the induced pattern is four-fold symmetric and, at zero
energy, the density of states exhibits minima along the node directions. But
YBa_2C_3O_{6.95} is orthorhombic because of the chains and the pattern becomes
two-fold symmetric with the position of the minima occuring when H is oriented
along the Fermi velocity at a node on the Fermi surface. The effect of impurity
scattering in the Born and unitary limit is discussed.Comment: 24 pages, 11 Figure
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