82 research outputs found
Interplay between single-particle and two-particle tunneling in normal metal-d-wave superconductor junctions probed by shot noise
We discuss how life-time broadening of quasiparticle states influences
single- and two-particle current transport through zero-energy states at normal
metal/d-wave superconductor junctions. We distinguish between intrinsic
broadening (imaginary part of the energy), which couples the bound
states with the superconducting reservoir, and broadening due to leakage
through the junction barrier, which couples the bound states with the normal
metal reservoir. We show that shot noise is highly sensitive to the mechanism
of broadening, while the conductance is not. In the limit of small but finite
intrinsic broadening, compared to the junction transparency ,
, the low-voltage shot noise at zero frequency and zero
temperature becomes proportional to the magnitude of intrinsic
broadening ( is the maximum d-wave gap).Comment: 6 pages, 4 figures; presented at the SDP2001 conference in Toky
Proximity Effect in Normal Metal - High Tc Superconductor Contacts
We study the proximity effect in good contacts between normal metals and high
Tc (d-wave) superconductors. We present theoretical results for the spatially
dependent order parameter and local density of states, including effects of
impurity scattering in the two sides, s-wave pairing interaction in the normal
metal side (attractive or repulsive), as well as subdominant s-wave paring in
the superconductor side. For the [100] orientation, a real combination d+s of
the order parameters is always found. The spectral signatures of the proximity
effect in the normal metal includes a suppression of the low-energy density of
states and a finite energy peak structure. These features are mainly due to the
impurity self-energies, which dominate over the effects of induced pair
potentials. For the [110] orientation, for moderate transparencies, induction
of a d+is order parameter on the superconductor side, leads to a proximity
induced is order parameter also in the normal metal. The spectral signatures of
this type of proximity effect are potentially useful for probing time-reversal
symmetry breaking at a [110] interface.Comment: 10 pages, 10 figure
Ferromagnetic features on zero-bias conductance peaks in ferromagnet/insulator/superconductor junction
We present a formula for tunneling conductance in ballistic
ferromagnet/ferromagnetic insulator/superconductor junctions where the
superconducting state has opposite spin pairing symmetry. The formula can
involve correctly a ferromagnetism has been induced by effective mass
difference between up- and down-spin electrons. Then, this effective mass
mismatch ferromagnet and standard Stoner ferromagnet have been employed in this
paper. As an application of the formulation, we have studied the tunneling
effect for junctions including spin-triplet p-wave superconductor. The
conductace spectra show a clear difference between two ferromagnets depending
upon the way of normalization of the conductance. Especially, a essential
difference is seen in zero-bias conductance peaks reflecting characteristics of
each ferromagnets. From obtained results, it will be suggested that the
measurements of the tunneling conductance in the junction provide us a useful
information about the mechanism of itinerant ferromagnetism in metal.Comment: 8 pages, 8 figures, references added to the first versio
Phase Crystals
Superconductivity owes its properties to the phase of the electron pair
condensate that breaks the symmetry. In the most traditional ground
state, the phase is uniform and rigid. The normal state can be unstable towards
special inhomogeneous superconducting states: the Abrikosov vortex state, and
the Fulde-Ferrell-Larkin-Ovchinnikov state. Here we show that the phase-uniform
superconducting state can go into a fundamentally different and more ordered
non-uniform ground state, that we denote as a phase crystal. The new state
breaks translational invariance through formation of a spatially periodic
modulation of the phase, manifested by unusual superflow patterns and
circulating currents, that also break time-reversal symmetry. We list the
general conditions needed for realization of phase crystals. Using microscopic
theory we then derive an analytic expression for the superfluid density tensor
for the case of a non-uniform environment in a semi-infinite superconductor. We
demonstrate how the surface quasiparticle states enter the superfluid density
and identify phase crystallization as the main player in several previous
numerical observations in unconventional superconductors, and predict existence
of a similar phenomenon in superconductor-ferromagnetic structures. This
analytic approach provides a new unifying aspect for the exploration of
boundary-induced quasiparticles and collective excitations in superconductors.
More generally, we trace the origin of phase crystallization to non-local
properties of the gradient energy, which implies existence of similar
pattern-forming instabilities in many other contexts.Comment: 8 pages, 4 figure
Dynamical effects of an unconventional current-phase relation in YBCO dc-SQUIDs
The predominant d-wave pairing symmetry in high temperature superconductors
allows for a variety of current-phase relations in Josephson junctions, which
is to a certain degree fabrication controlled. In this letter we report on
direct experimental observations of the effects of a non-sinusoidal
current-phase dependence in YBCO dc-SQUIDs, which agree with the theoretical
description of the system.Comment: 4 pages, 4 ps figures, to apprear in Phys. Rev. Let
Odd-frequency pairing in normal metal/superconductor junctions
We study the induced odd-frequency pairing states in ballistic normal
metal/superconductor (N/S) junctions where a superconductor has even-frequency
symmetry in the bulk and a normal metal layer has an arbitrary length. Using
the quasiclassical Green's function formalism, we demonstrate that, quite
generally, the pair amplitude in the junction has an admixture of an
odd-frequency component due to the breakdown of translational invariance near
the N/S interface where the pair potential acquires spatial dependence. If a
superconductor has even-parity pair potential (spin-singlet s-wave state), the
odd-frequency pairing component with odd-parity is induced near the N/S
interface, while in the case of odd-parity pair potential (spin-triplet
-wave or spin-singlet -wave) the odd-frequency component with
even-parity is generated. We show that in conventional s-wave junctions, the
amplitude of the odd-frequency pairing state is enhanced at energies
corresponding to the peaks in the local density of states (LDOS). In - and
-wave junctions, the amplitude of the odd-frequency component on the S
side of the N/S interface is enhanced at zero energy where the midgap Andreev
resonant state (MARS) appears due to the sign change of the pair potential. The
odd-frequency component extends into the N region and exceeds the
even-frequency component at energies corresponding to the LDOS peak positions,
including the MARS.Comment: 27 pages, 12 figure
- …