59 research outputs found
Ultrasonic attenuation in magnetic fields for superconducting states with line nodes in Sr2RuO4
We calculate the ultrasonic attenuation in magnetic fields for
superconducting states with line nodes vertical or horizontal relative to the
RuO_2 planes. This theory, which is valid for fields near Hc2 and not too low
temperatures, takes into account the effects of supercurrent flow and Andreev
scattering by the Abrikosov vortex lattice. For rotating in-plane field
H(theta) the attenuation alpha(theta)exhibits variations of fourfold symmetry
in the rotation angle theta. In the case of vertical nodes, the transverse T100
sound mode yields the weakest(linear)H and T dependence of alpha, while the
longitudinal L100 mode yields a stronger (quadratic) H and T dependence. This
is in strong contrast to the case of horizontal line nodes where alpha is the
same for the T100 and L100 modes (apart from a shift of pi/4 in field
direction) and is roughly a quadratic function of H and T. Thus we conclude
that measurements of alpha in in-plane magnetic fields for different in-plane
sound modes may be an important tool for probing the nodal structure of the gap
in Sr_2RuO_4.Comment: 5 pages, 6 figures, replaced in non-preprint form, to appear in Phys.
Rev.
Asymmetric double-well potential for single atom interferometry
We consider the evolution of a single-atom wavefunction in a time-dependent
double-well interferometer in the presence of a spatially asymmetric potential.
We examine a case where a single trapping potential is split into an asymmetric
double well and then recombined again. The interferometer involves a
measurement of the first excited state population as a sensitive measure of the
asymmetric potential. Based on a two-mode approximation a Bloch vector model
provides a simple and satisfactory description of the dynamical evolution. We
discuss the roles of adiabaticity and asymmetry in the double-well
interferometer. The Bloch model allows us to account for the effects of
asymmetry on the excited state population throughout the interferometric
process and to choose the appropriate splitting, holding and recombination
periods in order to maximize the output signal. We also compare the outcomes of
the Bloch vector model with the results of numerical simulations of the
multi-state time-dependent Schroedinger equation.Comment: 9 pages, 6 figure
Field dependence of the vortex structure in chiral p-wave superconductors
To investigate the different vortex structure between two chiral pairing p_x
+(-) i p_y, we calculate the pair potential, the internal field, the local
density of states, and free energy in the vortex lattice state based on the
quasiclassical Eilenberger theory, and analyze the magnetic field dependence.
The induced opposite chiral component of the pair potential plays an important
role in the vortex structure. It also produces H^{1/2}-behavior of the
zero-energy density of states at higher field. These results are helpful when
we understand the vortex states in Sr2RuO4.Comment: 11 pages, 10 figures, to be published in Phys. Rev.
Electron waves in chemically substituted graphene
We present exact analytical and numerical results for the electronic spectra
and the Friedel oscillations around a substitutional impurity atom in a
graphene lattice. A chemical dopant in graphene introduces changes in the
on-site potential as well as in the hopping amplitude. We employ a T-matrix
formalism and find that disorder in the hopping introduces additional
interference terms around the impurity that can be understood in terms of
bound, semi-bound, and unbound processes for the Dirac electrons. These
interference effects can be detected by scanning tunneling microscopy.Comment: 4 pages, 7 figure
Reduction of Pauli paramagnetic pair-breaking effect in antiferromagnetic superconductors
Antiferromagnetic superconductors in a magnetic field are studied. We examine
a mechanism which significantly reduces the Pauli paramagnetic pair-breaking
effect. The mechanism is realized even in the presence of the orbital
pair-breaking effect. We illustrate it using a three-dimensional model with an
intercalated magnetic subsystem. The upper critical field is calculated for
various parameters. It is shown that the upper critical field can reach several
times the pure Pauli paramagnetic limit. The possible relevance to the large
upper critical field observed in the heavy fermion antiferromagnetic
superconductor CePt_3Si discovered recently is briefly discussed. We try to
understand the large upper critical field in the compound CePt_3Si and
field-induced superconductivity in the compound CePb_3 within a unified
framework.Comment: 5 pages, 2 figures, revtex4, minor correction
Neutron scattering in a d_{x^2-y^2}-wave superconductor with strong impurity scattering and Coulomb correlations
We calculate the spin susceptibility at and below T_c for a d_{x^2-y^2}-wave
superconductor with resonant impurity scattering and Coulomb correlations. Both
the impurity scattering and the Coulomb correlations act to maintain peaks in
the spin susceptibility, as a function of momentum, at the Brillouin zone edge.
These peaks would otherwise be suppressed by the superconducting gap. The
predicted amount of suppression of the spin susceptibility in the
superconducting state compared to the normal state is in qualitative agreement
with results from recent magnetic neutron scattering experiments on
La_{1.86}Sr_{0.14}CuO_4 for momentum values at the zone edge and along the zone
diagonal. The predicted peak widths in the superconducting state, however, are
narrower than those in the normal state, a narrowing which has not been
observed experimentally.Comment: 24 pages (12 tarred-compressed-uuencoded Postscript figures), REVTeX
3.0 with epsf macros, UCSBTH-94-1
Electronic structure and the minimum conductance of a graphene layer on SiO2 from density-functional methods.
The effect of the SiO substrate on a graphene film is investigated using
realistic but computationally convenient energy-optimized models of the
substrate supporting a layer of graphene. The electronic bands are calculated
using density-functional methods for several model substrates. This provides an
estimate of the substrate-charge effects on the behaviour of the bands near
, as well as a variation of the equilibrium distance of the graphene
sheet. A model of a wavy graphene layer is examined as a possible candidate for
understanding the nature of the minimally conducting states in graphene.Comment: 6 pages, 5 figure
Unconventional Vortices and Phase Transitions in Rapidly Rotating Superfluid ^{3}He
This paper studies vortex-lattice phases of rapidly rotating superfluid ^3He
based on the Ginzburg-Landau free-energy functional. To identify stable phases
in the p-Omega plane (p: pressure; Omega: angular velocity), the functional is
minimized with the Landau-level expansion method using up to 3000 Landau
levels. This system can sustain various exotic vortices by either (i) shifting
vortex cores among different components or (ii) filling in cores with
components not used in the bulk. In addition, the phase near the upper critical
angular velocity Omega_{c2} is neither the A nor B phases, but the polar state
with the smallest superfluid density as already shown by Schopohl. Thus,
multiple phases are anticipated to exist in the p-Omega plane. Six different
phases are found in the present calculation performed over 0.0001 Omega_{c2} <=
Omega <= Omega_{c2}, where Omega_{c2} is of order (1- T/T_c) times 10^{7}
rad/s. It is shown that the double-core vortex experimentally found in the B
phase originates from the conventional hexagonal lattice of the polar state
near Omega_{c2} via (i) a phase composed of interpenetrating polar and
Scharnberg-Klemm sublattices; (ii) the A-phase mixed-twist lattice with polar
cores; (iii) the normal-core lattice found in the isolated-vortex calculation
by Ohmi, Tsuneto, and Fujita; and (iv) the A-phase-core vortex discovered in
another isolated-vortex calculation by Salomaa and Volovik. It is predicted
that the double-core vortex will disappear completely in the experimental p-T
phase diagram to be replaced by the A-phase-core vortex for Omega >~ 10^{3} ~
10^{4} rad/s. C programs to minimize a single-component Ginzburg-Landau
functional are available at {http://phys.sci.hokudai.ac.jp/~kita/index-e.html}.Comment: 13 pages, 9 figure
Electron-Phonon Interaction and Ultrasonic Attenuation in the Ruthenate and Cuprate superconductors
This article derives an electron-phonon interaction suitable for interpreting
ultrasonic attenuation measurements in the ruthenate and cuprate
superconductors. The huge anisotropy found experimentally (Lupien et al., 2001)
in Sr2RuO4 in the normal state is accounted for in terms of the layered
square-lattice structure of Sr2RuO4, and the dominant contribution to the
attenuation in Sr2RuO4 is found to be due to electrons in the gamma band. The
experimental data in the superconducting state is found to be inconsistent with
vertical lines nodes in the gap in either (100) or (110) planes. Also, a
general method, based on the use of symmetry, is developed to allow for the
analysis of ultrasonic attenuation experiments in superconductors in which the
electronic band structure is complicated or not known. Our results, both for
the normal-state anisotropy, and relating to the positions of the gap nodes in
the superconducting state, are different from those obtained from analyses
using a more traditional model for the electron-phonon interaction in terms of
an isotropic electron stress tensor. Also, a brief discussion of the ultrasonic
attenuation in UPt3 is given.Comment: 12 pages. Comments have been added to the original version of this
article showing how, for the ultrasonic attenuation for a hexagonal crystal
(which must be isotropic with respect to rotations about the c axis) our
approach reproduces the results of the traditional isotropic electron stress
tensor mode
Infrared conductivity of a d_{x^2-y^2}-wave superconductor with impurity and spin-fluctuation scattering
Calculations are presented of the in-plane far-infrared conductivity of a
d_{x^2-y^2}-wave superconductor, incorporating elastic scattering due to
impurities and inelastic scattering due to spin fluctuations. The impurity
scattering is modeled by short-range potential scattering with arbitrary phase
shift, while scattering due to spin fluctuations is calculated within a
weak-coupling Hubbard model picture. The conductivity is characterized by a
low-temperature residual Drude feature whose height and weight are controlled
by impurity scattering, as well as a broad peak centered at 4 Delta_0 arising
from clean-limit inelastic processes. Results are in qualitative agreement with
experiment despite missing spectral weight at high energies.Comment: 29 pages (11 tar-compressed-uuencoded Postscript figures), REVTeX 3.0
with epsf macro
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