1,578 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
Effect of Impurity Scattering on the Nonlinear Microwave Response in High-Tc Superconductors
We theoretically investigate intermodulation distortion in high-Tc
superconductors. We study the effect of nonmagnetic impurities on the real and
imaginary parts of nonlinear conductivity. The nonlinear conductivity is
proportional to the inverse of temperature owing to the dependence of the
damping effect on energy, which arises from the phase shift deviating from the
unitary limit. It is shown that the final-states interaction makes the real
part predominant over the imaginary part. These effects have not been included
in previous theories based on the two-fluid model, enabling a consistent
explanation for the experiments with the rf and dc fields
Effect of Surface Andreev Bound States on the Bean-Livingston Barrier in d-Wave Superconductors
We study the influence of surface Andreev bound states in d-wave
superconductors on the Bean-Livingston surface barrier for entry of a vortex
line into a strongly type-II superconductor. Starting from Eilenberger theory
we derive a generalization of London theory to incorporate the anomalous
surface currents arising from the Andreev bound states. This allows us to find
an analytical expression for the modification of the Bean-Livingston barrier in
terms of a single parameter describing the influence of the Andreev bound
states. We find that the field of first vortex entry is significantly enhanced.
Also, the depinning field for vortices near the surface is renormalized. Both
effects are temperature dependent and depend on the orientation of the surface
relative to the d-wave gap function.Comment: 4 pages, 3 figures; minor changes; accepted for publication in Phys.
Rev. Lett
Electronic theory for superconductivity in SrRuO: triplet pairing due to spin-fluctuation exchange
Using a two-dimensional Hubbard Hamiltonian for the three electronic bands
crossing the Fermi level in SrRuO we calculate the band structure and
spin susceptibility in quantitative agreement with
nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS)
experiments. The susceptibility has two peaks at {\bf Q}
due to the nesting Fermi surface properties and at {\bf q}
due to the tendency towards ferromagnetism. Applying spin-fluctuation exchange
theory as in layered cuprates we determine from ,
electronic dispersions, and Fermi surface topology that superconductivity in
SrRuO consists of triplet pairing. Combining the Fermi surface topology
and the results for we can exclude and wave
symmetry for the superconducting order parameter. Furthermore, within our
analysis and approximations we find that -wave symmetry is slightly favored
over p-wave symmetry due to the nesting properties of the Fermi surface.Comment: 5 pages, 5 figures, misprints correcte
Topological Qubit Design and Leakage
We examine how best to design qubits for use in topological quantum
computation. These qubits are topological Hilbert spaces associated with small
groups of anyons. Op- erations are performed on these by exchanging the anyons.
One might argue that, in order to have as many simple single qubit operations
as possible, the number of anyons per group should be maximized. However, we
show that there is a maximal number of particles per qubit, namely 4, and more
generally a maximal number of particles for qudits of dimension d. We also look
at the possibility of having topological qubits for which one can perform
two-qubit gates without leakage into non-computational states. It turns out
that the requirement that all two-qubit gates are leakage free is very
restrictive and this property can only be realized for two-qubit systems
related to Ising-like anyon models, which do not allow for universal quantum
computation by braiding. Our results follow directly from the representation
theory of braid groups which means they are valid for all anyon models. We also
make some remarks on generalizations to other exchange groups.Comment: 13 pages, 3 figure
Dynamical Chiral Symmetry Breaking in Unquenched
We investigate dynamical chiral symmetry breaking in unquenched
using the coupled set of Dyson--Schwinger equations for the fermion and photon
propagators. For the fermion-photon interaction we employ an ansatz which
satisfies its Ward--Green--Takahashi identity. We present self-consistent
analytical solutions in the infrared as well as numerical results for all
momenta. In Landau gauge, we find a phase transition at a critical number of
flavours of . In the chirally symmetric phase the
infrared behaviour of the propagators is described by power laws with
interrelated exponents. For and we find small values for the
chiral condensate in accordance with bounds from recent lattice calculations.
We investigate the Dyson--Schwinger equations in other linear covariant gauges
as well. A comparison of their solutions to the accordingly transformed Landau
gauge solutions shows that the quenched solutions are approximately gauge
covariant, but reveals a significant amount of violation of gauge covariance
for the unquenched solutions.Comment: 33 pages, 8 figures, reference added, version to be published in
Phys. Rev.
Shear wave reflection seismic yields subsurface dissolution and subrosion patterns: application to the Ghor Al-Haditha sinkhole site, Dead Sea, Jordan
Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development.
For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor Al-Haditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area.
The most surprising result of the survey is the absence of evidence of a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200 m depth. Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings
S and D Wave Mixing in High Superconductors
For a tight binding model with nearest neighbour attraction and a small
orthorhombic distortion, we find a phase diagram for the gap at zero
temperature which includes three distinct regions as a function of filling. In
the first, the gap is a mixture of mainly -wave with a smaller extended
-wave part. This is followed by a region in which there is a rapid increase
in the -wave part accompanied by a rapid increase in relative phase between
and from 0 to . Finally, there is a region of dominant with a
mixture of and zero phase. In the mixed region with a finite phase, the
-wave part of the gap can show a sudden increase with decreasing temperature
accompanied with a rapid increase in phase which shows many of the
characteristics measured in the angular resolved photoemission experiments of
Ma {\em et al.} in Comment: 12 pages, RevTeX 3.0, 3 PostScript figures uuencoded and compresse
Microwave Absorption of Surface-State Electrons on Liquid He
We have investigated the intersubband transitions of surface state electrons
(SSE) on liquid He induced by microwave radiation at temperatures from 1.1
K down to 0.01 K. Above 0.4 K, the transition linewidth is proportional to the
density of He vapor atoms. This proportionality is explained well by Ando's
theory, in which the linewidth is determined by the electron - vapor atom
scattering. However, the linewidth is larger than the calculation by a factor
of 2.1. This discrepancy strongly suggests that the theory underestimates the
electron - vapor atom scattering rate. At lower temperatures, the absorption
spectrum splits into several peaks. The multiple peak structure is partly
attributed to the spatial inhomogeneity of the static holding electric field
perpendicular to the electron sheet.Comment: 15 pages, 7 figures, submitted to J. Phys. Soc. Jp
Umklapp scattering from spin fluctuations in Copper-Oxides
The -dependent electronic momentum relaxation rate due to Umklapp
scattering from antiferromagnetic spin fluctuations is studied within a
renormalized mean-field approach to an extended model appropriate to
YBaCuO and other cuprates. Transport coefficients are
calculated in a relaxation time approximation. We compare these results with
those obtained with the phenomenological assumption that all scattering
processes dissipate momentum. We show that the latter, which violates momentum
conservation, leads to quite different magnitudes and temperature dependences
of resistivities and Hall coefficients.Comment: replaced by LaTeX file (due to problems with PostScript
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