94 research outputs found
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
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
A dissipative environment may improve the quantum annealing performances of the ferromagnetic p-spin model
We investigate the quantum annealing of the ferromagnetic -spin model in
a dissipative environment ( and ). This model, in the large limit, codifies the Grover's algorithm for searching in an unsorted
database. The dissipative environment is described by a phonon bath in thermal
equilibrium at finite temperature. The dynamics is studied in the framework of
a Lindblad master equation for the reduced density matrix describing only the
spins. Exploiting the symmetries of our model Hamiltonian, we can describe many
spins and extrapolate expected trends for large , and . While at weak
system bath coupling the dissipative environment has detrimental effects on the
annealing results, we show that in the intermediate coupling regime, the phonon
bath seems to speed up the annealing at low temperatures. This improvement in
the performance is likely not due to thermal fluctuation but rather arises from
a correlated spin-bath state and persists even at zero temperature. This result
may pave the way to a new scenario in which, by appropriately engineering the
system-bath coupling, one may optimize quantum annealing performances below
either the purely quantum or classical limit.Comment: 9 Pag, 5 Fig, Submitte
Anomalous Josephson effect in S/SO/F/S heterostructures
We study the anomalous Josephson effect, as well as the dependence on the
direction of the critical Josephson current, in an S/N/S junction, where the
normal part is realized by alternating spin-orbit coupled and ferromagnetic
layers. We show that to observe these effects it is sufficient to break spin
rotation and time reversal symmetry in spatially separated regions of the
junction. Moreover, we discuss how to further improve these effects by
engineering multilayers structures with more that one couple of alternating
layers.Comment: 10 pages, 8 figure
Structural relaxation and low energy properties of Twisted Bilayer Graphene
The structural and electronic properties of twisted bilayer graphene are investigated from first principles and tight binding approach as a function of the twist angle (ranging from the first "magic" angle to , with the former corresponding to the largest unit cell, comprising 11164 carbon atoms). By properly taking into account the long-range van der Waals interaction, we provide the patterns for the atomic displacements (with respect to the ideal twisted bilayer). The out-of-plane relaxation shows an oscillating ("buckling") behavior, very evident for the smallest angles, with the atoms around the AA stacking regions interested by the largest displacements. The out-of-plane displacements are accompanied by a significant in-plane relaxation, showing a vortex-like pattern, where the vorticity (intended as curl of the displacement field) is reverted when moving from the top to the bottom plane and viceversa. Overall, the atomic relaxation results in the shrinking of the AA stacking regions in favor of the more energetically favorable AB/BA stacking domains. The measured flat bands emerging at the first magic angle can be accurately described only if the atomic relaxations are taken into account. Quite importantly, the experimental gaps separating the flat band manifold from the higher and lower energy bands cannot be reproduced if only in-plane or only out-of-plane relaxations are considered. The stability of the relaxed bilayer at the first magic angle is estimated to be of the order of 0.5-0.9 meV per atom (or 7-10 K). Our calculations shed light on the importance of an accurate description of the vdW interaction and of the resulting atomic relaxation to envisage the electronic structure of this really peculiar kind of vdW bilayers
Spin-orbit coupling in a Quantum Dot at high magnetic field
We describe the simultaneous effects of the spin-orbit (SO) perturbation and
a magnetic field on a disk shaped quantum dot (QD). {As it is known the}
combination of electrostatic forces among the electrons confined in the QD
and the Pauli principle can induce a spin polarization when (applied in the
direction orthogonal to the QD) is above a threshold value. In the presence of
an electric field parallel to , coupled to the spin by a Rashba term,
we demonstrate that a symmetry breaking takes place: we can observe it by
analyzing the splitting of the levels belonging to an unperturbed multiplet. We
also discuss the competitive effects of the magnetic field, the SO perturbation
and the electron electron interaction, in order to define the hierarchy of the
states belonging to a multiplet. We demonstrate how this hierarchy depends on
the QD's size. We show the spin texture due to the combined effects of the
Rashba effect and the interaction responsible for the polarization.Comment: 8 pages, 3 figures, PACS: 73.21.La,71.15.Mb,75.75.+
Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport.
The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-Ï€ transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3Â K, and the first evidence of an incomplete 0-Ï€ transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations
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