130 research outputs found
Green function theory of dirty two-band superconductivity
We study the effects of random nonmagnetic impurities on the superconducting
transition temperature in a two-band superconductor, where we assume the
equal-time spin-singlet s-wave pair potential in each conduction band and the
hybridization between the two bands as well as the band asymmetry. In the clean
limit, the phase of hybridization determines the stability of two states:
called and . The interband impurity scatterings decrease
of the two states exactly in the same manner when the Hamiltonian preserves
time-reversal symmetry. We find that a superconductor with larger hybridization
shows more moderate suppression of . This effect can be explained by the
presence of odd-frequency Cooper pairs which are generated by the band
hybridization in the clean limit and are broken by impurities.Comment: 11 pages, 2 figure
Properties of rough interfaces in superconductors with d-wave pairing
Theoretical model of a rough interface in a superconductor with d-wave symmetry of the order parameter is proposed. The surface roughness is introduced by means of a surface layer with small electronic mean free path. The proximity effect between such a layer and a bulk d-wave superconductor is studied theoretically in the framework of the quasiclassical Eilenberger theory. It is shown that as a result of strong scattering in the interlayer the d-wave component of the order parameter near the interface is reduced while the s-wave component localized near the interface is generated. Angular and spatial structure of the pair potential and the electronic density of states near the interface is calculated. The interplay of the zero-energy (midgap) and finite-energy bound states leads to peculiarities in the energy dependence of the angle-averaged density of states. We argue that the model is relevant for the description of rough interfaces in high Tc superconductors. In the framework of the present approach we calculate the Josephson critical current for several types of junctions with rough interface
Josephson Effect due to Odd-Frequency Pairs in Diffusive Half Metals
Motivated by a recent experiment [Keizer et al., Nature (London) 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions using the recursive Green function method. The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs. As a consequence, the local density of states in a half metal has a large peak at the Fermi energy. Therefore the odd-frequency pairs can be detected experimentally by using the scanning tunneling spectroscopy
Dirty two-band superconductivity with interband pairing order
We study theoretically the effects of random nonmagnetic impurities on the
superconducting transition temperature in a two-band superconductor
characterized by an equal-time s-wave interband pairing order parameter. The
Fermi-Dirac statistics of electrons allows a spin-triplet s-wave pairing order
as well as a spin-singlet s-wave order parameter due to the two-band degree of
freedom. In a spin-singlet superconductor, is insensitive to the impurity
concentration when we estimate the self-energy due to the random impurity
potential within the Born approximation. On the other hand in a spin-triplet
superconductor, decreases with the increase of the impurity
concentration. We conclude that Cooper pairs belonging to odd-band-parity
symmetry class are fragile under the random impurity potential even though they
have s-wave pairing symmetry.Comment: 7 pages, 2 figures embedde
Electron transport in a ferromagnet-superconductor junction on graphene
In a usual ferromagnet connected with a superconductor, the exchange potential suppresses the superconducting pairing correlation. We show that this common knowledge does not hold in a ferromagnetsuperconductor junction on a graphene. When the chemical potential of a graphene is close to the conical point of energy dispersion, the exchange potential rather assists the charge transport through a junction interface. The loose-bottomed electric structure causes this unusual effec
Microscopic theory of tunneling spectroscopy in SrRuO
We study the surface Andreev bound state (ABS) of superconducting
SrRuO, which is a candidate material for the realization of the chiral
-wave superconducting state. In order to clarify the role of chiral edge
modes as ABSs, the surface density of states and the tunneling conductance is
calculated in the normal metal/SrRuO junction within the framework of
recursive Green's function method, while taking into account the orbital
degrees of freedom (including Spin-Orbit interactions) with realistic material
parameters. In SrRuO, there are two bands and
originating from quasi-one-dimensional orbitals and and a
two-dimensional band originating from orbital. We discuss
about the contributions of various electronic bands to LDOS and the influence
of atomic spin-orbit interaction (SOI). In the light of our calculations,
quasi-one-dimensional model with dominant pair potentials in and
bands is consistent with conductance measurements in
Au/SrRuO junctions.Comment: to be published in J. Phys. Soc. Jpn., 10 pages, 12 figure
Tunability of Andreev levels via spin-orbit coupling in Zeeman-split Josephson junctions
We study Andreev reflection and Andreev levels in Zeeman-split
superconductor/Rashba wire/Zeeman-split superconductor junctions by solving the
Bogoliubov de-Gennes equation. We theoretically demonstrate that the Andreev
levels can be controlled by tuning either the strength of Rashba
spin-orbit interaction or the relative direction of the Rashba spin-orbit
interaction and the Zeeman field. In particular, it is found that the magnitude
of the band splitting is tunable by the strength of the Rashba spin-orbit
interaction and the rength of the wire, which can be interpreted by a spin
precession in the Rashba wire. We also find that if the Zeeman field in the
superconductor has the component parallel to the direction of the junction, the
- curve becomes asymmetric with respect to the
superconducting phase difference . Whereas the Andreev reflection
processes associated with each pseudospin band are sensitive to the relative
orientation of the spin-orbit field and the exchange field, the total electric
conductance interestingly remains invariant.Comment: 10 pages, 8 figure
Effects of the phase coherence on the local density of states in superconducting proximity structures
We theoretically study the local density of states in superconducting
proximity structure where two superconducting terminals are attached to a side
surface of a normal-metal wire. Using the quasiclassical Green's function
method, the energy spectrum is obtained for both of spin-singlet -wave and
spin-triplet -wave junctions. In both of the cases, the decay length of the
proximity effect at the zero temperature is limited by a depairing effect due
to inelastic scatterings. In addition to the depairing effect, in -wave
junctions, the decay length depends sensitively on the transparency at the
junction interfaces, which is a unique property to odd-parity superconductors
where the anomalous proximity effect occurs.Comment: 11 pages, 9 figure
Josephson effect in two-band superconductors
We study theoretically the Josephson effect between two time-reversal
two-band superconductors, where we assume the equal-time spin-singlet -wave
pair potential in each conduction band. %as well as the band asymmetry and the
band hybridization in the normal state. The superconducting phase at the first
band and that at the second band characterize a
two-band superconducting state. We consider a Josephson junction where an
insulating barrier separates two such two-band superconductors. By applying the
tunnel Hamiltonian description, the Josephson current is calculated in terms of
the anomalous Green's function on either side of the junction. We find that the
Josephson current consists of three components which depend on three types of
phase differences across the junction: the phase difference at the first band
, the phase difference at the second band ,
and the difference at the center-of-mass phase .
A Cooper pairs generated by the band hybridization carries the last current
component. In some cases, the current-phase relationship deviates from the
sinusoidal function as a result of time-reversal symmetry breaking down.Comment: 6 page, 2 figure
Josephson current in a normal-metal nanowire coupled to superconductor/ferromagnet/superconductor junction
We consider superconducting nanowire proximity coupled to superconductor /
ferromagnet / superconductor junction, where the magnetization penetrates into
superconducting segment in nanowire decaying as with site index and the decay length . We tune chemical
potential and spin-orbit coupling so that topological superconducting regime
hosting Majorana fermion is realized for long . We find that when
becomes shorter, zero energy state at the interface between superconductor and
ferromagnet splits into two away from zero energy. Accordingly, the behavior of
Josephson current is drastically changed due to this "zero mode-non-zero mode
crossover". By tuning the model parameters, we find an almost second-harmonic
current-phase relation, , with phase difference . Based
on the analysis of Andreev bound state (ABS), we clarify that current-phase
relation is determined by coupling of the states within the energy gap. We find
that the emergence of crossing points of ABS is a key ingredient to generate
dependence in current-phase relation. We further study both the
energy and dependence of pair amplitudes in the ferromagnetic region.
For long , odd-frequency spin-triplet -wave component is dominant. The
magnitude of the odd-frequency pair amplitude is enhanced at the energy level
of ABS.Comment: 13 pages, 17 figure
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