32 research outputs found
Crossed Andreev reflection at spin-active interfaces
With the aid of the quasiclassical Eilenberger formalism we develop a theory
of non-local electron transport across three-terminal ballistic
normal-superconducting-normal (NSN) devices with spin-active NS interfaces. The
phenomenon of crossed Andreev reflection (CAR) is known to play the key role in
such transport. We demonstrate that CAR is highly sensitive to electron spins
and yields a rich variety of properties of non-local conductance which we
describe non-perturbatively at arbitrary voltages, temperature, spin-dependent
interface transmissions and their polarizations. Our results can be applied to
multi-terminal hybrid structures with normal, ferromagnetic and half-metallic
electrodes and can be directly tested in future experiments.Comment: 11 pages, 7 figures; figures 6 and 7 are corrected; version published
in Phys. Rev.
Self-consistent microscopic calculations for non-local transport through nanoscale superconductors
We implement self-consistent microscopic calculations in order to describe
out-of-equilibrium non-local transport in normal metal-superconductor-normal
metal hybrid structures in the presence of a magnetic field and for arbitrary
interface transparencies. A four terminal setup simulating usual experimental
situations is described by means of a tight-binding model. We present results
for the self-consistent order parameter and current profiles within the sample.
These profiles illustrate a crossover from a quasi-equilibrium to a strong
non-equilibrium situation when increasing the interface transparencies and the
applied voltages. We analyze in detail the behavior of the non-local
conductance in these two different regimes. While in quasi-equilibrium
conditions this can be expressed as the difference between elastic cotunneling
and crossed Andreev transmission coefficients, in a general situation
additional contributions due to the voltage dependence of the self-consistent
order parameter have to be taken into account. The present results provide a
first step towards a self-consistent theory of non-local transport including
non-equilibrium effects and describe qualitatively a recent experiment [Phys.
Rev. Lett. 97, 237003 (2006)].Comment: 12 pages, 14 figures, 2 figures correcte
Josephson current and Andreev states in superconductor-half metal-superconductor heterostructures
We develop a detailed microscopic theory describing dc Josephson effect and
Andreev bound states in superconducting junctions with a half-metal. In such
systems the supercurrent is caused by triplet pairing states emerging due to
spin-flip scattering at the interfaces between superconducting electrodes and
the half-metal. For sufficiently clean metals we provide a detailed
non-perturbative description of the Josephson current at arbitrary
transmissions and spin-flip scattering parameters for both interfaces. Our
analysis demonstrates that the behavior of both the Josephson current and
Andreev bound states crucially depends on the strength of spin-flip scattering
showing a rich variety of features which can be tested in future experiments.Comment: 12 pages, 7 figures; version published in Phys. Rev.
Nonlocal Andreev reflection at high transmissions
We analyze non-local effects in electron transport across three-terminal
normal-superconducting-normal (NSN) structures. Subgap electrons entering
S-electrode from one N-metal may form Cooper pairs with their counterparts
penetrating from another N-metal. This phenomenon of crossed Andreev reflection
-- combined with normal scattering at SN interfaces -- yields two different
contributions to non-local conductance which we evaluate non-perturbatively at
arbitrary interface transmissions. Both these contributions reach their maximum
values at fully transmitting interfaces and demonstrate interesting features
which can be tested in future experiments.Comment: 4 pages, 4 figure
Non-local electron transport and cross-resistance peak in NSN heterostructures
We develop a microscopic theory describing the peak in the temperature
dependence of the non-local resistance of three-terminal NSN devices. This peak
emerges at sufficiently high temperatures as a result of a competition between
quasiparticle/charge imbalance and subgap (Andreev) contributions to the
conductance matrix. Both the height and the shape of this peak demonstrate the
power law dependence on the superconductor thickness in contrast to the
zero-temperature non-local resistance which decays (roughly) exponentially with
increasing . A similar behavior was observed in recent experiments.Comment: 4 pages, 3 figure
Micromechanical field-effect transistor terahertz detectors with optical interferometric readout
We investigate the response of the micromechanical field-effect transistors
(MMFETs) to the impinging terahertz (THz) signals. The MMFET uses the
microcantilevers MC as a mechanically floating gate and the movable mirror of
the Michelson optical interferometer. The MC mechanical oscillations are
transformed into optical signals and the MMFET operates as the detector of THz
radiation with the optical output. The combination of the mechanical and
plasmonic resonances in the MMFET with the optical amplification enables an
effective THz detection.Comment: 8 pages, 3 figure
Triplet superconductivity in a ferromagnetic vortex
We argue that odd-frequency triplet superconductivity can be conveniently
realized in hybrid superconductor-ferromagnet (SF) structures with a
ferromagnetic vortex. We demonstrate that due to proximity-induced long-range
triplet pairing such SFS junctions can sustain appreciable supercurrent which
can be directly measured in experiments.Comment: 4 pages, 3 figure
Experimental and theoretical analysis of the upper critical field in FSF trilayers
The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve
cores is studied experimentally and theoretically in geometries perpendicular
and parallel to the heterostructure surface. The series of samples with
variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are
prepared in a single run, utilizing a wedge deposition technique. The critical
field H_{c2} is measured in the temperature range K and for magnetic
fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of
the superconducting transition temperature versus F-layers thickness, induced
by an external magnetic field, has been observed for the first time. In order
to properly interpret the experimental data, we develop a quasiclassical
theory, enabling one to evaluate the temperature dependence of the critical
field and the superconducting transition temperature for an arbitrary set of
the system parameters. A fairly good agreement between our experimental data
and theoretical predictions is demonstrated for all samples, using a single set
of fit parameters. This confirms adequacy of the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual
superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59}
spin-valve core trilayers.Comment: 16 pages, 7 figures; published versio
Minigap, Parity Effect and Persistent Currents in SNS Nanorings
We have evaluated a proximity-induced minigap in the density of states (DOS)
of SNS junctions and SNS nanorings at an arbitrary concentration of
non-magnetic impurities. We have demonstrated that an isotropic energy minigap
in the electron spectrum opens up already at arbitrarily weak disorder, while
angle resolved DOS at higher energies can remain strongly anisotropic. The
minigap value can be tuned by passing a supercurrent through an
SNS junction or by applying a magnetic flux to an SNS ring. A
non-monotonous dependence of on has been found at weak
disorder. We have also studied persistent currents in isolated SNS nanorings.
For odd number of electrons in the ring we have found a non-trivial
current-phase (current-flux) relation which -- at relatively high disorder --
may lead to a -junction state and spontaneous currents in the ground state
of the system.Comment: 7 pages, 8 figure
Effect of Impurity Scattering on the Nonlinear Microwave Response in High-Tc Superconductors
We theoretically investigate intermodulation distortion in high-Tc
superconductors. We study the effect of nonmagnetic impurities on the real and
imaginary parts of nonlinear conductivity. The nonlinear conductivity is
proportional to the inverse of temperature owing to the dependence of the
damping effect on energy, which arises from the phase shift deviating from the
unitary limit. It is shown that the final-states interaction makes the real
part predominant over the imaginary part. These effects have not been included
in previous theories based on the two-fluid model, enabling a consistent
explanation for the experiments with the rf and dc fields