226 research outputs found
Dissipative charge transport in diffusive superconducting double-barrier junctions
We solve the coherent multiple Andreev reflection (MAR) problem and calculate
current-voltage characteristics (IVCs) for Josephson SINIS junctions, where S
are local-equilibrium superconducting reservoirs, I denotes tunnel barriers,
and N is a short diffusive normal wire, the length of which is much smaller
than the coherence length, and the resistance is much smaller than the
resistance of the tunnel barriers. The charge transport regime in such
junctions qualitatively depends on a characteristic value \gamma = \Delta
\tau_d of relative phase shifts between the electrons and retro-reflected holes
accumulated during the dwell time \tau_d. In the limit of small electron-hole
dephasing \gamma << 1, our solution recovers a known formula for a short
mesoscopic connector extended to the MAR regime. At large dephasing, the
subharmonic gap structure in the IVC scales with 1/ \gamma, which thus plays
the role of an effective tunneling parameter. In this limit, the even gap
subharmonics are resonantly enhanced, and the IVC exhibits portions with
negative differential resistance.Comment: 8 pages, 3 figures, typos corrected, to be published in Phys. Rev.
Subgap current in superconducting tunnel junctions with diffusive electrodes
We calculate the subgap current in planar superconducting tunnel junctions
with thin-film diffusive leads. It is found that the subharmonic gap structure
of the tunnel current scales with an effective tunneling transparency which may
exceed the junction transparency by up to two orders of magnitude depending on
the junction geometry and the ratio between the coherence length and the
elastic scattering length. These results provide an alternative explanation of
anomalously high values of the subgap current in tunnelling experiments often
ascribed to imperfection of the insulating layer. We also discuss the effect of
finite lifetime of quasiparticles as the possible origin of additional
enhancement of multiparticle tunnel currents.Comment: 4 pages, 4 figures, to be published in Phys. Rev.
Electron cooling by diffusive normal metal - superconductor tunnel junctions
We investigate heat and charge transport in NN'IS tunnel junctions in the
diffusive limit. Here N and S are massive normal and superconducting electrodes
(reservoirs), N' is a normal metal strip, and I is an insulator. The flow of
electric current in such structures at subgap bias is accompanied by heat
transfer from the normal metal into the superconductor, which enables
refrigeration of electrons in the normal metal. We show that the two-particle
current due to Andreev reflection generates Joule heating, which is deposited
in the N electrode and dominates over the single-particle cooling at low enough
temperatures. This results in the existence of a limiting temperature for
refrigeration. We consider different geometries of the contact: one-dimensional
and planar, which is commonly used in the experiments. We also discuss the
applicability of our results to a double-barrier SINIS microcooler.Comment: 9 pages, 4 figures, submitted to Phys. Rev.
Nonequilibrium effects in tunnel Josephson junctions
We study nonequilibrium effects in current transport through voltage biased
tunnel junction with long diffusive superconducting leads at low applied
voltage, , and finite temperatures. Due to a small value of the
Josephson frequency, the quasiparticle spectrum adiabatically follows the time
evolution of the superconducting phase difference, which results in the
formation of oscillating bound states in the vicinity of the tunnel junction
(Andreev band). The quasiparticles trapped by the Andreev band generate higher
even harmonics of the Josephson ac current, and also, in the presence of
inelastic scattering, a non-equilibrium dc current, which may considerably
exceed the dc quasiparticle current given by the tunnel model. The distribution
of travelling quasiparticles also deviates from the equilibrium due to the
spectrum oscillations, which results in an additional contribution to the dc
current, proportional to .Comment: 11 pages, 7 figures, to be published in Phys. Rev.
Josephson -junctions based on structures with complex normal/ferromagnet bilayer
We demonstrate that Josephson devices with nontrivial phase difference in the ground state can be realized in structures composed
from longitudinally oriented normal metal (N) and ferromagnet (F) films in the
weak link region. Oscillatory coupling across F-layer makes the first harmonic
in the current-phase relation relatively small, while coupling across N-layer
provides negative sign of the second harmonic. To derive quantitative criteria
for a -junction, we have solved two-dimensional boundary-value problem
in the frame of Usadel equations for overlap and ramp geometries of S-NF-S
structures. Our numerical estimates show that -junctions can be
fabricated using up-to-date technology.Comment: 14 pages, 9 figure
Anomalous proximity effect in d-wave superconductors
The anomalous proximity effect between a d-wave superconductor and a surface
layer with small electronic mean free path is studied theoretically in the
framework of the Eilenberger equations. The angular and spatial structure of
the pair potential and the quasiclassical propagators in the interface region
is calculated selfconsistently. The variation of the spatially-resolved
quasiparticle density of states from the bulk to the surface is studied. It is
shown that the isotropic gapless superconducting state is induced in the
disordered layer.Comment: 6 pages, 5 postscript figures. Submitted to Phys.Rev.
Josephson current in a superconductor-ferromagnet junction with two non-collinear magnetic domains
We study the Josephson effect in a
superconductor--ferromagnet--superconductor (SFS) junction with ferromagnetic
domains of non-collinear magnetization. As a model for our study we consider a
diffusive junction with two ferromagnetic domains along the junction. The
superconductor is assumed to be close to the critical temperature , and
the linearized Usadel equations predict a sinusoidal current-phase relation. We
find analytically the critical current as a function of domain lengths and of
the angle between the orientations of their magnetizations. As a function of
those parameters, the junction may undergo transitions between 0 and
phases. We find that the presence of domains reduces the range of junction
lengths at which the phase is observed. For the junction with two domains
of the same length, the phase totally disappears as soon as the
misorientation angle exceeds . We further comment on possible
implication of our results for experimentally observable 0-- transitions
in SFS junctions.Comment: 9 pages, 4 figures, minor changes, references adde
Broadening of hot-spot response spectrum of superconducting NbN nanowire single-photon detector with reduced nitrogen content
The spectral detection efficiency and the dark count rate of superconducting
nanowire single-photon detectors (SNSPD) has been studied systematically on
detectors made from thin NbN films with different chemical compositions.
Reduction of the nitrogen content in the 4 nm thick NbN films results in a more
than two orders of magnitude decrease of the dark count rates and in a red
shift of the cut-off wavelength of the hot-spot SNSPD response. The observed
phenomena are explained by an improvement of uniformity of NbN films that has
been confirmed by a decrease of resistivity and an increase of the ratio of the
measured critical current to the depairing current. The latter factor is
considered as the most crucial for both the cut-off wavelength and the dark
count rates of SNSPD. Based on our results we propose a set of criteria for
material properties to optimize SNSPD in the infrared spectral region.Comment: 15 pages, 6 figure
Proximity fingerprint of s+- superconductivity
We suggest a straightforward and unambiguous test to identify possible
opposite signs of superconducting order parameter in different bands proposed
for iron-based superconductors (s+- state). We consider proximity effect in a
weakly coupled sandwich composed of a s+- superconductor and thin layer of
s-wave superconductor. In such system the s-wave order parameter is coupled
differently with different s+- gaps and it typically aligns with one of these
gaps. This forces the other s+- gap to be anti-aligned with the s-wave gap. In
such situation the aligned band induces a peak in the s-wave density of states
(DoS), while the anti-aligned band induces a dip. Observation of such
contact-induced negative feature in the s-wave DoS would provide a definite
proof for s+- superconductivity.Comment: 4 pages, one figur
- …