167 research outputs found
Quasiclassical theory of superconductivity: a multiple interface geometry
The purpose of the paper is to suggest a new method which allows one to study
multiple coherent reflection/transmissions by partially transparent interfaces
(e.g. in multi-layer mesoscopic structures or grain boundaries in high-Tc's) in
the framework of the quasiclassical theory of superconductivity. It is argued
that typically the trajectory of the particle is a simply connected tree (no
loops) with knots, i.e. the points where interface scattering events occur and
ballistic pieces of the trajectory are mixed. A linear boundary condition for
the 2-component trajectory "wave function" which factorizes matrix (retarded)
Green's function, is formulated for an arbitrary interface, specular or
diffusive. To show the usage of the method, the current response to the vector
potential (the total superfluid density rho_s) of a SS' sandwich with the
different signs of the order parameter in S and S', is calculated. In this
model, a few percent of reflection by the SS' interface transforms the
paramagnetic response (rho_s < 0) created by the zero-energy Andreev bound
states near an ideal interface (see Fauchere et al. PRL, 82, 3336 (1999),
cond-mat/9901112), into the usual diamagnetic one (rho_s >0).Comment: Extended abstract submitted to "Electron Transport in Mesoscopic
Systems", Satellite conference to LT22, Goteborg, 12-15 August, 1999. 2 pages
Minor changes + the text height problem fixe
Edge Current due to Majorana Fermions in Superfluid He A- and B-Phases
We propose a method utilizing edge current to observe Majorana fermions in
the surface Andreev bound state for the superfluid He A- and B-phases. The
proposal is based on self-consistent analytic solutions of quasi-classical
Green's function with an edge. The local density of states and edge mass
current in the A-phase or edge spin current in the B-phase can be obtained from
these solutions. The edge current carried by the Majorana fermions is partially
cancelled by quasiparticles (QPs) in the continuum state outside the superfluid
gap. QPs contributing to the edge current in the continuum state are
distributed in energy even away from the superfluid gap. The effect of Majorana
fermions emerges in the depletion of the edge current by temperature within a
low-temperature range. The observations that the reduction in the mass current
is changed by -power in the A-phase and the reduction in the spin current
is changed by -power in the B-phase establish the existence of Majorana
fermions. We also point out another possibility for observing Majorana fermions
by controlling surface roughness.Comment: 13 pages, 4 figures, published versio
Phase-Sensitive Impurity Effects in Vortex Core of Moderately Clean Chiral Superconductors
We study impurity effects in vortex core of two-dimensional moderately clean
su perconductors within the quasiclassical theory. The impurity scattering rate
\G amma(E) of the Andreev bound states in vortex core with +1 vorticity of
p-wav e superconductors with {\mib d}=\hat{\mib z}(p_x+\iu p_y) is suppre
ssed, compared to the normal state scattering rate in the
energ y region \Gamma_{\rm n}^3/E_\delta^2\ll E\ll E_\delta\equiv
|\delta_0|\Delta_\i nfty with scattering phase shift
and the pair-po tential in bulk . Further we
find that for p-wave superconductors with {\mib
d}=\hat{\mib z}(p_x-\iu p_y) is at most {\cal O}(E/\Delta_\i nfty). These
results are in marked contrast to the even-parity case (s,d-wave), where
is known to be proportional to \ln(\Delta_\i
nfty/E) . Parity- and chirality-dependences of impurity effects are attributed
to the Andr eev reflections involved in the impurity-induced scattering between
bound states . Implications for the flux flow conductivity is also discussed.
Novel enhanceme nt of flux flow conductivity is expected to occur at for {\mib d}=\hat{\mib z}(p_x+\iu p_y) and at
for {\mib d}=\hat{\mib z}(p_x-\iu p_y).Comment: 9 pages, No figures, To appear in JPSJ Vol. 69, No. 10 (2000
Numerical Study of Impurity Effects on Quasiparticles within S-wave and Chiral P-wave Vortices
The impurity problems within vortex cores of two-dimensional s-wave and
chiral p-wave superconductors are studied numerically in the framework of the
quasiclassical theory of superconductivity and self-consistent Born
approximation under a trial form of the pair potential. The dispersion and
impurity scattering rate (the inverse of the relaxation time) of the Andreev
bound state localized in vortex cores are deduced from the angular-resoloved
local density of states. The energy dependence of the impurity scattering rates
depends on the pairing symmetry; particularly, in the chiral p-wave vortex core
where chirality and vorticity have opposite sign and hence the total angular
momentum is zero, the impurities are ineffective and the scattering rate is
vanishingly small. Owing to the cancellation of angular momentum between
chirality and vorticity, the chiral p-wave vortex core is similar to locally
realized s-wave region and therefore non-magnetic impurity is harmless as a
consequence of Anderson's theorem. The results of the present study confirm the
previous results of analytical study (J. Phys. Soc. Jpn. {\bf 69} (2000) 3378)
in the Born limit.Comment: 8pages, 9figures, submitted to J. Phys. Soc. Jp
Majorana edge modes of superfluid 3He A-phase in a slab
Motivated by a recent experiment on the superfluid 3He A-phase with a chiral
p-wave pairing confined in a thin slab, we propose designing a concrete
experimental setup for observing the Majorana edge modes that appear around the
circumference edge region. We solve the quasi-classical Eilenberger equation,
which is quantitatively reliable, to evaluate several observables. To derive
the property inherent to the Majorana edge state, the full quantum mechanical
Bogoliubov-de Gennes equation is solved in this setting. On the basis of the
results obtained, we perform decisive experiments to check the Majorana nature.Comment: 5 pages, 5 figure
Analytical Formulation of the Local Density of States around a Vortex Core in Unconventional Superconductors
On the basis of the quasiclassical theory of superconductivity, we obtain a
formula for the local density of states (LDOS) around a vortex core of
superconductors with anisotropic pair-potential and Fermi surface in arbitrary
directions of magnetic fields. Earlier results on the LDOS of d-wave
superconductors and NbSe are naturally interpreted within our theory
geometrically; the region with high intensity of the LDOS observed in numerical
calculations turns out to the enveloping curve of the trajectory of Andreev
bound states. We discuss experimental results on YNiBC within the
quasiclassical theory of superconductivity.Comment: 13 pages, 16 figure
Quasiparticle States near the Surface and the Domain Wall in a p_x\pm i p_y-Wave Superconductor
The electronic states near a surface or a domain wall in the p-wave
superconductor are studied for the order parameter of the form p_x\pm i
p_y-wave, which is a unitary odd-parity state with broken time-reversal
symmetry. This state has been recently suggested as the superconducting state
of Sr_2 Ru O_4. The spatial variation of the order parameter and vector
potential is determined self-consistently within the quasi-classical
approximation. The local density of states at the surface is constant and does
not show any peak-like or gap-like structure within the superconducting energy
gap, in contrast to the case of the d-wave superconductors. The influence of an
external magnetic field is mainly observable in the energy range above the bulk
gap. On the other hand, there is a small energy gap in the local density of
states at the domain wall between domains of the two degenerate p_x+i p_y-wave
and p_x-i p_y-wave states.Comment: 26 pages, 9 figures, to be published in J. Phys. Soc. Jpn. Vol. 68
(1999) No. 3, erratum: to appear in J. Phys. Soc. Jpn. Vol. 68 (1999) No.
SOFC Anode Fabricated by Magnetically Aligning of Ni Particles
Ni particles are aligned by magnetic field during the drying process after screen-printing Ni/8YSZ (yttria-stabilized zirconia) paste. By applying a magnetic field, Ni particles are magnetically polarized, attracted to each other, and align along the magnetic field. It is proposed, that not only tortuosity of Ni but also that of YSZ and of pores is decreased. Symmetrical half cells are fabricated with 15-µm-thick anodes and 200-µm-thick YSZ electrolytes. A current collector made of porous Ni with a thickness of approximately 5 µm was printed on top of each anode. The microstructural changes in the anodes are analyzed by scanning electron microscopy. Impedance measurements are performed at 700°C in H 2 /H 2 O atmospheres containing 10% and 60% H 2 O. The initial polarization resistance was decreased after applying a magnetic field of 100 mT by up to 25%. However, with higher magnetic field, the polarization resistance increases, which might be explained by a pronounced increase of the surface roughness with 30 µm peak-to-valley, causing current constriction
Quasiparticle Bound States and Low-Temperature Peaks of the Conductance of NIS Junctions in d-Wave Superconductors
Quasiparticle states bound to the boundary of anisotropically paired
superconductors, their contributions to the density of states and to the
conductance of NIS junctions are studied both analytically and numerically. For
smooth surfaces and real order parameter we find some general results for the
bound state energies. In particular, we show that under fairly general
conditions quasiparticle states with nonzero energies exist for momentum
directions within a narrow region around the surface normal. The energy
dispersion of the bound states always has an extremum for the direction along
the normal. Along with the zero-bias anomaly due to midgap states, we find, for
quasi two-dimensional materials, additional low-temperature peaks in the
conductance of NIS junctions for voltages determined by the extrema of the
bound state energies. The influence of interface roughness on the conductance
is investigated within the framework of Ovchinnikov's model. We show that
nonzero-bias peaks at low temperatures may give information on the order
parameter in the bulk, even though it is suppressed at the surface.Comment: 14 pages, PostScrip
The Effect of Surfaces on the Tunneling Density of States of an Anisotropically Paired Superconductor
We present calculations of the tunneling density of states in an
anisotropically paired superconductor for two different sample geometries: a
semi-infinite system with a single specular wall, and a slab of finite
thickness and infinite lateral extent. In both cases we are interested in the
effects of surface pair breaking on the tunneling spectrum. We take the stable
bulk phase to be of symmetry. Our calculations are performed
within two different band structure environments: an isotropic cylindrical
Fermi surface with a bulk order parameter of the form ,
and a nontrivial tight-binding Fermi surface with the order parameter structure
coming from an anti-ferromagnetic spin-fluctuation model. In each case we find
additional structures in the energy spectrum coming from the surface layer.
These structures are sensitive to the orientation of the surface with respect
to the crystal lattice, and have their origins in the detailed form of the
momentum and spatial dependence of the order parameter. By means of tunneling
spectroscopy, one can obtain information on both the anisotropy of the energy
gap, |\Delta(\p)|, as well as on the phase of the order parameter,
\Delta(\p) = |\Delta(\p)|e^{i\varphi(\p)}.Comment: 14 pages of revtex text with 11 compressed and encoded figures. To
appear in J. Low Temp. Phys., December, 199
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