147 research outputs found
Superconductive proximity in a Topological Insulator slab and excitations bound to an axial vortex
We consider the proximity effect in a Topological Insulator sandwiched
between two conventional superconductors, by comparing s-wave spin singlet
superconducting pairing correlations and odd-parity triplet pairing
correlations with zero spin component orthogonal to the slab ("polar " phase).
A superconducting gap opens in the Dirac dispersion of the surface states
existing at the interfaces. An axial vortex is included, piercing the slab
along the normal to the interfaces with the superconductors. It is known that,
when proximity is s-wave, quasiparticles in the gap are Majorana Bound States,
localized at opposite interfaces. We report the full expression for the quantum
field associated to the midgap neutral fermions, as derived in the two-orbital
band model for the TI. When proximity involves odd-parity pairing, midgap modes
are charged Surface Andreev Bound States, and they originate from interfacial
circular states of definite chirality, centered at the vortex singularity and
decaying in the TI film with oscillations. When the chemical potential is moved
away from midgap, extended states along the vortex axis are also allowed. Their
orbital structure depends on the symmetry of the bulk band from where the
quasiparticle level splits off.Comment: 13 pages no figures, accepted for publication in Phys. Rev.
Quantum interference of electrons in a ring: tuning of the geometrical phase
We calculate the oscillations of the DC conductance across a mesoscopic ring,
simultaneously tuned by applied magnetic and electric fields orthogonal to the
ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit
coupling. They result from mixing of the dynamical phase, including the Zeeman
spin splitting, and of geometric phases. By changing the applied fields, the
geometric phase contribution to the conductance oscillations can be tuned from
the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a
realistic device, we also include nonzero backscattering at the connection
between ring and contacts, and a random phase for electron wavefunction,
accounting for dephasing due to disorder.Comment: 4 pages, 3 figures, minor change
Coherent response of a low T_c Josephson junction to an ultrafast laser pulse
By irradiating with a single ultrafast laser pulse a superconducting
electrode of a Josephson junction it is possible to drive the quasiparticles
(qp's) distribution strongly out of equilibrium. The behavior of the Josephson
device can, thus, be modified on a fast time scale, shorter than the qp's
relaxation time. This could be very useful, in that it allows fast control of
Josephson charge qubits and, in general, of all Josephson devices. If the
energy released to the top layer contact of the junction is of the order
of , the coherence is not degradated, because the perturbation is
very fast. Within the framework of the quasiclassical Keldysh Green's function
theory, we find that the order parameter of decreases. We study the
perturbed dynamics of the junction, when the current bias is close to the
critical current, by integrating numerically its classical equation of motion.
The optical ultrafast pulse can produce switchings of the junction from the
Josephson state to the voltage state. The switches can be controlled by tuning
the laser light intensity and the pulse duration of the Josephson junction.Comment: 17 pages, 5 figure
Advantages of using YBCO-Nanowire-YBCO heterostructures in the search for Majorana Fermions
We propose an alternative platform to observe Majorana bound states in solid
state systems. High critical temperature cuprate superconductors can induce
superconductivity, by proximity effect, in quasi one dimensional nanowires with
strong spin orbit coupling. They favor a wider and more robust range of
conditions to stabilize Majorana fermions due to the large gap values, and
offer novel functionalities in the design of the experiments determined by
different dispersion for Andreev bound states as a function of the phase
difference.Comment: 4 Pages, 3 figures, submission date 30-Apr-201
Spin Hall effect in a two-dimensional electron gas in the presence of a magnetic field
We study the spin Hall effect of a two-dimensional electron gas in the
presence of a magnetic field and both the Rashba and Dresselhaus spin-orbit
interactions. We show that the value of the spin Hall conductivity, which is
finite only if the Zeeman spin splitting is taken into account, may be tuned by
varying the ratio of the in-plane and out-of-plane components of the applied
magnetic field. We identify the origin of this behavior with the different role
played by the interplay of spin-orbit and Zeeman couplings for in-plane and
out-of-plane magnetic field components.Comment: 5 pages, 5 figures, submitte
a review
In this review article we describe spin-dependent transport in materials with
spin–orbit interaction of Rashba type. We mainly focus on semiconductor
heterostructures, however we consider topological insulators, graphene and
hybrid structures involving superconductors as well. We start from the Rashba
Hamiltonian in a two dimensional electron gas and then describe transport
properties of two- and quasi-one-dimensional systems. The problem of spin
current generation and interference effects in mesoscopic devices is described
in detail. We address also the role of Rashba interaction on localisation
effects in lattices with nontrivial topology, as well as on the Ahronov–Casher
effect in ring structures. A brief section, in the end, describes also some
related topics including the spin-Hall effect, the transition from weak
localisation to weak anti localisation and the physics of Majorana fermions in
hybrid heterostructures involving Rashba materials in the presence of
superconductivity
Thermal transport driven by charge imbalance in graphene in magnetic field, close to the charge neutrality point at low temperature: Non local resistance
Graphene grown epitaxially on SiC, close to the charge neutrality point
(CNP), in an orthogonal magnetic field shows an ambipolar behavior of the
transverse resistance accompanied by a puzzling longitudinal magnetoresistance.
When injecting a transverse current at one end of the Hall bar, a sizeable non
local transverse magnetoresistance is measured at low temperature. While Zeeman
spin effect seems not to be able to justify these phenomena, some dissipation
involving edge states at the boundaries could explain the order of magnitude of
the non local transverse magnetoresistance, but not the asymmetry when the
orientation of the orthogonal magnetic field is reversed. As a possible
contribution to the explanation of the measured non local magnetoresistance
which is odd in the magnetic field, we derive a hydrodynamic approach to
transport in this system, which involves particle and hole Dirac carriers, in
the form of charge and energy currents. We find that thermal diffusion can take
place on a large distance scale, thanks to long recombination times, provided a
non insulating bulk of the Hall bar is assumed, as recent models seem to
suggest in order to explain the appearance of the longitudinal resistance. In
presence of the local source, some leakage of carriers from the edges generates
an imbalance of carriers of opposite sign, which are separated in space by the
magnetic field and diffuse along the Hall bar generating a non local transverse
voltage.Comment: 25 pages, 12 figure
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