229 research outputs found
Normal zone in -coated conductors
We consider the distribution of an electric field in YBCO-coated conductors
for a situation in which the DC transport current is forced into the copper
stabilizer due to a weak link -- a section of the superconducting film with a
critical current less than the transport current. The electric field in the
metal substrate is also discussed. The results are compared with recent
experiments on normal zone propagation in coated conductors for which the
substrate and stabilizer are insulated from each other. The potential
difference between the substrate and stabilizer, and the electric field in the
substrate outside the normal zone can be accounted for by a large screening
length in the substrate, comparable to the length of the sample. During a
quench, the electric field inside the interface between YBCO and stabilizer, as
well as in the buffer layer, can be several orders of magnitude greater than
the longitudinal macroscopic electric field inside the normal zone. We
speculate on the possibility of using possible microscopic electric discharges
caused by this large (kV/cm) electric field as a means to detect a
quench.Comment: 8 pages, 4 figure
Electronic structure of d-wave superconducting quantum wires
We present analytical and numerical results for the electronic spectra of
wires of a d-wave superconductor on a square lattice. The spectra of Andreev
and other quasiparticle states, as well as the spatial and particle-hole
structures of their wave functions, depend on interference effects caused by
the presence of the surfaces and are qualitatively different for half-filled
wires with even or odd number of chains. For half-filled wires with an odd
number of chains N at (110) orientation, spectra consist of N doubly degenerate
branches. By contrast, for even N wires, these levels are split, and all
quasiparticle states, even the ones lying above the maximal gap, have the
characteristic properties of Andreev bound states. These Andreev states above
the gap can be interpreted as a consequence of an infinite sequence of Andreev
reflections experienced by quasiparticles along their trajectories bounded by
the surfaces of the wire. Our microscopic results for the local density of
states display atomic-scale Friedel oscillations due to the presence of the
surfaces, which should be observable by scanning tunneling microscopy. For
narrow wires the self-consistent treatment of the order parameter is found to
play a crucial role. In particular, we find that for small wire widths the
finite geometry may drive strong fluctuations or even stablilize exotic
quasi-1D pair states with spin triplet character.Comment: 21 pages, 20 figures. Slightly modified version as published in PR
Influence of impurity-scattering on tunneling conductance in d-wave superconductors with broken time reversal symmetry
Effects of impurity scattering on tunneling conductance in dirty
normal-metal/insulator/superconductor junctions are studied based on the Kubo
formula and the recursive Green function method. The zero-bias conductance peak
(ZBCP) is a consequence of the unconventional pairing symmetry in
superconductors. The impurity scattering in normal metals suppresses the
amplitude of the ZBCP. The degree of the suppression agrees well with results
of the quasiclassical Green function theory. When superconductors have
+is-wave pairing symmetry, the time-reversal symmetry is broken in
superconductors and the ZBCP splits into two peaks. The random impurity
scattering reduces the height of the two splitting peaks. The position of the
splitting peaks, however, almost remains unchanged even in the presence of the
strong impurity scattering. Thus the two splitting peaks never merge into a
single ZBCP.Comment: 12 pages, 5 figures, using jpsj2.cls and overcite.st
A phenomenological theory of zero-energy Andreev resonant states
A conceptual consideration is given to a zero-energy state (ZES) at the
surface of unconventional superconductors. The reflection coefficients in
normal-metal / superconductor (NS) junctions are calculated based on a
phenomenological description of the reflection processes of a quasiparticle.
The phenomenological theory reveals the importance of the sign change in the
pair potential for the formation of the ZES. The ZES is observed as the
zero-bias conductance peak (ZBCP) in the differential conductance of NS
junctions. The split of the ZBCP due to broken time-reversal symmetry states is
naturally understood in the present theory. We also discuss effects of external
magnetic fields on the ZBCP.Comment: 12 page
Spin polarized tunneling in ferromagnet/unconventional superconductor junctions
We study tunneling in ferromagnet/unconventional superconductor (F/S)
junctions. We include the effects of spin polarization, interfacial resistance,
and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev
reflection (AR) at the F/S interface, governing tunneling at low bias voltage,
is strongly modified by these parameters. The conductance exhibits a very wide
variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of
minor error
Zero-bias conductance peak splitting due to multiband effect in tunneling spectroscopy
We study how the multiplicity of the Fermi surface affects the zero-bias peak
in conductance spectra of tunneling spectroscopy. As case studies, we consider
models for organic superconductors -(BEDT-TTF)Cu(NCS) and
(TMTSF)ClO. We find that multiplicity of the Fermi surfaces can lead to
a splitting of the zero-bias conductance peak (ZBCP). We propose that the
presence/absence of the ZBCP splitting is used as a probe to distinguish the
pairing symmetry in -(BEDT-TTF)Cu(NCS).Comment: 7 pages, 7 figure
Discrete-Lattice Model for Surface Bound States and Tunneling in d-Wave Superconductors
Surface bound states in a discrete-lattice model of a cuprate
superconductor are shown to be, in general, coherent superpositions of an
incoming excitation and more than one outgoing excitation, and a simple
graphical construction based on a surface Brillouin zone is developed to
describe their nature. In addition, a momentum-dependent lifetime contribution
to the width of these bound states as observed in tunneling experiments is
derived and elucidated in physical terms.Comment: 4 pages, 1 figure, revte
Long-range nonlocal flow of vortices in narrow superconducting channels
We report a new nonlocal effect in vortex matter, where an electric current
confined to a small region of a long and sufficiently narrow superconducting
wire causes vortex flow at distances hundreds of inter-vortex separations away.
The observed remote traffic of vortices is attributed to a very efficient
transfer of a local strain through the one-dimensional vortex lattice, even in
the presence of disorder. We also observe mesoscopic fluctuations in the
nonlocal vortex flow, which arise due to "traffic jams" when vortex
arrangements do not match a local geometry of a superconducting channel.Comment: a slightly longer version of a tentatively accepted PR
Josephson effect in d-wave superconductor junctions in a lattice model
Josephson current between two d-wave superconductors is calculated by using a
lattice model. Here we consider two types of junctions, , the parallel
junction and the mirror-type junction. The maximum Josephson current
shows a wide variety of temperature () dependence depending on the
misorientation angles and the types of junctions. When the misorientation
angles are not zero, the Josephson current shows the low-temperature anomaly
because of a zero energy state (ZES) at the interfaces. In the case of
mirror-type junctions, has a non monotonic temperature dependence. These
results are consistent with the previous results based on the quasiclassical
theory. [Y. Tanaka and S. Kashiwaya: Phys. Rev. B \textbf{56} (1997) 892.] On
the other hand, we find that the ZES disappears in several junctions because of
the Freidel oscillations of the wave function, which is peculiar to the lattice
model. In such junctions, the temperature dependence of is close to the
Ambegaokar-Baratoff relation.Comment: 17 pages, 10 figures, using jpsj2.cls and oversite.st
Localized surface states in HTSC: Alternative mechanism of zero-bias conductance peaks
It is shown that the quasiparticle states localized in the vicinity of
surface imperfections of atomic size can be responsible for the zero-bias
tunneling conductance peaks in high-Tc superconductors. The contribution from
these states can be easily separated from other mechanisms using their
qualitatively different response on an external magnetic field.Comment: REVTeX, 4 pages, 2 figs; to be published in PR
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