959 research outputs found
Renormalization of the Majorana bound state decay length in a perpendicular magnetic field
Orbital effects of a magnetic field in a proximitized semiconductor nanowire
are studied in the context of the spatial extent of Majorana bound states. We
develop analytical model that explains the impact of concurring effects of
paramagnetic coupling of the nanowire bands via the kinetic energy operator and
spin-orbit interaction on the Majorana modes. We find, that the perpendicular
field, so far considered as to be detrimental to the Majorana fermion
formation, is in fact helpful in establishing the topological zero-energy-modes
in a finite system due to significant decrease in the Majorana decay length
Quantum size effect on the paramagnetic critical field in Pb nanofilms
The quantum size effect on the in-plane paramagnetic critical field in Pb
nanofilms is investigated with the use of the spin-generalized Bogolubov-de
Gennes equations. It is shown that the critical field oscillates as a function
of the nanofilm thickness with the period of ML (even-odd oscillations)
modulated by the beating effect. This phenomena is studied in terms of the
quantization of the electron energy spectra caused by the confinement of the
electron motion in the direction perpendicular to the sample. The calculated
values of critical fields for different nanofilm thicknesses are analyzed in
the context of Clogston-Chandrasekhar limit. The influence of the thermal
effect on the magnetic field induced superconductor to normal metal transition
is also discussed. Furthermore, the thickness-dependence of the electron-phonon
coupling and its influence on the value of the critical magnetic field are
studied.Comment: 9 pages, 9 figure
Fulde-Ferrell state in superconducting core/shell nanowires: role of the orbital effect
The orbital effect on the Fulde-Ferrell (FF) phase is investigated in
superconducting core/shell nanowires subjected to the axial magnetic field. The
confinement in the radial direction results in the quantization of the electron
motion with energies determined by the radial and orbital quantum
numbers. In the external magnetic field the twofold degeneracy with respect to
the orbital magnetic quantum number is lifted which leads to the Fermi wave
vector mismatch between the paired electrons . This mismatch is transfered to the nonzero total
momentum of the Cooper pairs which results in the formation of FF phase
occurring sequentially with increasing magnetic field. By changing the nanowire
radius and the superconducting shell thickness , we discuss the role of
the orbital effect in the FF phase formation in both the nanowire-like () and nanofilm-like () regime. We have found that the
irregular pattern of the FF phase, which appears for the case of the
nanowire-like regime, evolves towards the regular distribution, in which the FF
phase stability regions appear periodically between the BCS state, for the
nanofilm-like geometry. The crossover between these two different phase
diagrams is explained as resulting from the orbital effect and the multigap
character of superconductivity in core/shell nanowires.Comment: 10 pages, 7 figure
Electrically controlled spin-transistor operation in helical magnetic field
A proposal of electrically controlled spin transistor in helical magnetic
field is presented. In the proposed device, the transistor action is driven by
the Landau-Zener transitions that lead to a backscattering of spin polarized
electrons and switching the transistor into the high-resistance state (off
state). The on/off state of the transistor can be controlled by the
all-electric means using Rashba spin-orbit coupling that can be tuned by the
voltages applied to the side electrodes.Comment: 5 pages, 6 figure
Spin filtering effect generated by the inter-subband spin-orbit coupling in the bilayer nanowire with the quantum point contact
The spin filtering effect in the bilayer nanowire with quantum point contact
is investigated theoretically. We demonstrate the new mechanism of the spin
filtering based on the lateral inter-subband spin-orbit coupling, which for the
bilayer nanowires has been reported to be strong. The proposed spin filtering
effect is explained as the joint effect of the Landau-Zener inter-subband
transitions caused by the hybridization of states with opposite spin (due to
the lateral Rashba SO interaction) and the confinement of carriers in the
quantum point contact region.Comment: 14 pages, 11 figure
Influence of the electron density on the thickness-dependent energy gap oscillations in superconducting metallic nanofilms
The thickness-dependent energy gap oscillations in the metallic nanofilms are
investigated by the use of the self-consistent numerical solutions of the
Bogoliubov-de Gennes equations. It is shown, that the oscillations are induced
by the quasi-particle energy quantization triggered by the confinement of
electrons in the direction perpendicular to the sample. We have analyzed, how
the changes in the electron density of states () and the electron-phonon
coupling constant () influence the amplitude of the considered oscillations.
It has been found, that the increase in and the decrease in , can lead
to a significant reduction of the oscillations amplitude. As a result, for the
values of the mentioned parameters corresponding to some of the realistic
situations the thickness-dependent superconducting gap oscillations can be
almost completely suppressed
Probing Andreev reflection reach in semiconductor-superconductor hybrids by Aharonov-Bohm effect
Recent development in fabrication of hybrid nanostructures allows for
creation of quantum interferometers that combine semiconductor and
superconductor materials. We show that in those nanostructures the joint
phenomena of Aharonov-Bohm effect and Andreev reflections can be used to
determine the length on which the electron is retro-reflected as a hole. We
propose to exploit this feature for probing of the quasiparticle coherence
length in semiconductor-superconductor hybrids by a magnetoconductance
measurement
Orbital effect on the in-plane critical field in free-standing superconducting nanofilms
The superconductor to normal metal phase transition induced by the in-plane
magnetic field is studied in free-standing Pb(111) nanofilms. In the considered
structures the energy quantization induced by the confinement leads to the
thickness-dependent oscillations of the critical field (the so-called 'shape
resonances'). In this paper we examine the influence of the orbital effect on
the in-plane critical magnetic field in nanofilms. We demonstrate that the
orbital term suppresses the critical field and reduces the amplitude of the
thickness-dependent critical field oscillations. Moreover, due to the orbital
effect, the slope at becomes finite and decreases with
increasing film thickness in agreement with recent experiments. The temperature
at which the superconductor to normal metal phase transition becomes of
the first order is also analyzed.Comment: 8 pages, 7 figure
Nonlocal Andreev Reflections as a Source of Spin Exchange and Kondo Screening
We report on a novel exchange mechanism, mediating the Kondo screening, in a
correlated double quantum dot structure in the proximity of superconductor,
stemming from nonlocal Andreev reflection processes. We estimate its strength
perturbatively and corroborate analytical predictions with accurate numerical
renormalization group calculations using an effective model for the
superconductor-proximized nanostructure. We demonstrate that
superconductor-induced exchange leads to a two-stage Kondo screening. We
determine the dependence of the Kondo temperature on the coupling to
superconductor and predict a characteristic modification of conventional
low-temperature transport behavior, which can be used to experimentally
distinguish this phenomenon from other Kondo effects.Comment: 10 pages, 6 figures. Version accepted in Phys. Rev. B, before
proof-stage editin
Strong spin Seebeck effect in Kondo T-shaped double quantum dots
We theoretically investigate the thermoelectric and spin thermoelectric
properties of a T-shaped double quantum dot strongly coupled to two
ferromagnetic leads, focusing on transport regime where the system exhibits the
two-stage Kondo effect. We study the dependence of the (spin) Seebeck
coefficient, the corresponding power factor and the figure of merit on
temperature, leads' spin polarization and dot level position. We show that the
thermal conductance fulfills a modified Wiedemann-Franz law. We also
demonstrate that the spin thermopower is enhanced at temperatures corresponding
to the second stage of Kondo screening. Very interestingly, the
spin-thermoelectric response of the system is found to be highly sensitive to
the spin polarization of the leads. In some cases spin polarization of the
order of 1% is sufficient for a strong spin Seebeck effect to occur. This is
explained as a consequence of the interplay between the two-stage Kondo effect
and the exchange field induced in the double quantum dot. All calculations are
performed with the aid of numerical renormalization group technique.Comment: 10 pages, 7 figure
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