126 research outputs found
Spin imbalance in hybrid superconducting structures with spin-active interfaces
We consider a heterostructure consisting of a normal metal and a
superconductor separated by a spin-active interface. At finite-bias voltages,
spin-filtering and spin-mixing effects at the interface allow for an induced
magnetization (spin imbalance) on the superconducting side of the junction,
which relaxes to zero in the bulk. Such interfaces are also known to host a
pair of in-gap Andreev bound states which were recently observed
experimentally. We show that these states are responsible for the dominant
contribution to the induced spin imbalance close to the interface. Motivated by
recent experiments on spin-charge density separation in superconducting
aluminum wires, we propose an alternative way to observe spin imbalance without
applying an external magnetic field. We also suggest that the peculiar
dependence of the spin imbalance on the applied bias voltage permits an
indirect bound-state spectroscopy.Comment: 7 pages, 4 figure
Spin-polarized currents and noise in NS junctions with Yu-Shiba-Rusinov impurities
Conventional superconductors disordered by magnetic impurities demonstrate
physical properties drastically different from their pristine counterparts. In
our previous work [Phys. Rev. B 92, 245430 (2015)] we explored spectral and
thermodynamic properties of such systems for two extreme cases: completely
random and ferromagnetically aligned impurity magnetic moments. Here we
consider transport properties of these systems, and show that they have a
potential to be used in superconducting spintronic devices. Each magnetic
impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum,
residing at sub-gap energies. Provided the YSR states form metallic bands, we
demonstrate that the tunneling current carried by these states can be highly
spin-polarized when the impurities are ferromagnetically ordered. The spin
polarization can be switched by simply tuning the bias voltage. Moreover, even
when the impurity spins are completely uncorrelated, one can still achieve
almost 100% spin polarization of the current, if the tunnel interface is
spin-active. We compute electric current and noise, varying parameters of the
interface between tunneling and fully transparent regimes, and analyze the
relative role of single-particle and Andreev reflection processes.Comment: 14 pages, 10 figure
Spectral properties of superconductors with ferromagnetically ordered magnetic impurities
We present a comprehensive theoretical study of thermodynamic properties of
superconductors with a dilute concentration of magnetic impurities, with focus
on how the properties of the superconducting host change if the magnetic
moments of the impurities order ferromagnetically. Scattering off the magnetic
impurities leads to the formation of a band of Yu-Shiba-Rusinov states within
the superconducting energy gap that drastically influences superconductivity.
In the magnetically ordered system, the magnetization displays a sudden drop as
function of impurity density or magnetic moment amplitude. The drop occurs as
the spin-polarized impurity band crosses the Fermi level and is associated with
a quantum phase transition first put forward by Sakurai for the single impurity
case. Taking into account that the background magnetic field created by the
ordered impurity moments enters as a Zeeman shift, we find that the
superconducting phase transition changes from second order to first order for
high enough impurity concentration.Comment: 16 pages, 13 figure
Superconducting Pairing Amplitude and Local Density of States in Presence of Repulsive Centers
We study the properties of superconductor in presence of a finite
concentration of repulsive centers. The superconductor is described by the
negative Hubbard model while repulsive centers are treated as randomly
distributed impurities with repulsive interaction. Analyzing the paring
potential and local density of states at impurity sites we find a wide range of
the system parameters where the - like state could possibly be realized.
Comparison of our results to the single repulsive center case is also given.Comment: 2 pages, 2 figures, Physica B in pres
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
Low-Temperature Thermal Conductivity of Superconductors With Gap Nodes
We report a detailed analytic and numerical study of electronic thermal
conductivity in d-wave superconductors. We compare theory of the cross over at
low temperatures from T-dependence to T^3-dependence for increasing temperature
with recent experiments on YBCO in zero magnetic field for temperatures from
0.04K to 0.4K by Hill et al., Phys. Rev. Lett. 92, 027001 (2004). Transport
theory, including impurity scattering and inelastic scattering within strong
coupling superconductivity, can consistently fit the temperature dependence of
the data in the lower half of the temperature regime. We discuss the conditions
under which we expect power-law dependences over wide temperature intervals.Comment: 4 pages, 3 figure
Transport through vertical graphene contacts under intense laser fields
We theoretically study the electronic and transport properties of two
graphene layers vertically coupled by an insulating layer under the influence
of a time-periodic external light field. The non-adiabatic driving induces
excitations of electrons and a redistribution of the occupied states which is
manifested in the opening of gaps in the quasienergy spectrum of graphene. When
a voltage is applied between the top and bottom graphene layers, the
photo-induced nonequilibrium occupation modifies the transport properties of
the contact. We investigate the electronic and transport properties of the
contact by using the nonequilibrium Green's function formalism. To illustrate
the behavior of the differential conductance of the vertical contact under the
light illumination, we consider two cases. First, we assume that both the
bottom and top layers consist of graphene and second we consider a finite mass
term in the bottom layer. We obtain that the differential conductance is
strongly suppressed due to the opening of gaps in the quasienergy spectrum in
graphene. Additionally, the conductance shows features corresponding to the
tunneling of photoexcited electrons at energies of the van Hove singularity for
both the top and bottom layers. In the case of a finite mass term in the bottom
layer, the differential conductance can be directly related to the tunneling of
photoexcited electrons
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
0-pi Transitions in a Superconductor/Chiral Ferromagnet/Superconductor Junction induced by a Homogeneous Cycloidal Spiral
We study the pi phase in a superconductor-ferromagnet-superconductor
Josephson junction, with a ferromagnet showing a cycloidal spiral spin
modulation with in-plane propagation vector. Our results reveal a high
sensitivity of the junction to the spiral order and indicate the presence of
0-pi quantum phase transitions as function of the spiral wave vector. We find
that the chiral magnetic order introduces chiral superconducting triplet pairs
that strongly influence the physics in such Josephson junctions, with potential
applications in nanoelectronics and spintronics.Comment: 4 pages, 4 figures; the derivation part has been reorganized + added
note and new references, published versio
- …