424 research outputs found
Spin fractionalization of an even number of electrons in a Quantum dot
An experiment is proposed of non perturbative tunneling in a Quantum dot
connected to leads in a pillar configuration, which would shed light on the
physics of the mesoscopic Kondo problem. We propose for the first time that
what is coupled to the leads in the case of even number of electrons on the dot
is not just the electrons on the QD, with their own spins, but the very total
spin of the dot macroscopic state itself, that displays features of the Kondo
physics, that leads to fractionalization of the spin in the dot, that is to a
"spinon box".Comment: 4 pages revtex file, 1 figur
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.
Adiabatic Control of the Electron Phase in a Quantum Dot
A Berry phase can be added to the wavefunction of an isolated quantum dot by
adiabatically modulating a nonuniform electric field along a time-cycle. The
dot is tuned close to a three-level degeneracy, which provides a wide range of
possibilities of control. We propose to detect the accumulated phase by
capacitively coupling the dot to a double-path inteferometer. The effective
Hamiltonian for the phase-sensitive coupling is discussed in detail.Comment: 14 pages, 2 .eps 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
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
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
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
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