235 research outputs found
Transport properties and structures of vortex matter in layered superconductors
In this paper we analyze the structure, phase transitions and some transport
properties of the vortex system when the external magnetic field lies parallel
to the planes in layered superconductors. We show that experimental results for
resistivity are qualitatively consistent with numerical simulations that
describe the melting of a commensurate rotated lattice. However for some
magnetic fields, the structure factor indicates the occurrence of smectic peaks
at an intermediate temperature regime.Comment: 8 pages, 8 eps figure
Spin-orbit induced chirality of Andreev states in Josephson junctions
We study Josephson junctions (JJs) in which the region between the two
superconductors is a multichannel system with Rashba spin-orbit coupling (SOC)
where a barrier or a quantum point contact (QPC) is present. These systems
might present unconventional Josephson effects such as Josephson currents for
zero phase difference or critical currents that \textit{depend on} the current
direction. Here, we discuss how the spin polarizing properties of the system in
the normal state affect the spin characteristic of the Andreev bound states
inside the junction. This results in a strong correlation between the spin of
the Andreev states and the direction in which they transport Cooper pairs.
While the current-phase relation for the JJ at zero magnetic field is
qualitatively unchanged by SOC, in the presence of a weak magnetic field a
strongly anisotropic behavior and the mentioned anomalous Josephson effects
follow. We show that the situation is not restricted to barriers based on
constrictions such as QPCs and should generically arise if in the normal system
the direction of the carrier's spin is linked to its direction of motion.Comment: 19 pages, 9 figures. To appear in PR
Tunable Charge and Spin Seebeck Effects in Magnetic Molecular Junctions
We study the charge and spin Seebeck effects in a spin-1 molecular junction
as a function of temperature (T), applied magnetic field (H), and magnetic
anisotropy (D) using Wilson's numerical renormalization group. A hard-axis
magnetic anisotropy produces a large enhancement of the charge Seebeck
coefficient Sc (\sim k_B/|e|) whose value only depends on the residual
interaction between quasiparticles in the low temperature Fermi-liquid regime.
In the underscreened spin-1 Kondo regime, the high sensitivity of the system to
magnetic fields makes it possible to observe a sizable value for the spin
Seebeck coefficient even for magnetic fields much smaller than the Kondo
temperature. Similar effects can be obtain in C60 junctions where the control
parameter is the gap between a singlet and a triplet molecular state.Comment: 5 pages, 4 figure
Gate induced enhancement of spin-orbit coupling in dilute fluorinated graphene
We analyze the origin of spin-orbit coupling (SOC) in fluorinated graphene
using Density Functional Theory (DFT) and a tight-binding model for the
relevant orbitals. As it turns out, the dominant source of SOC is the atomic
spin-orbit of fluorine adatoms and not the impurity induced SOC based on the
distortion of the graphene plane as in hydrogenated graphene. More
interestingly, our DFT calculations show that SOC is strongly affected by both
the type and concentrations of the graphene's carriers, being enhanced by
electron doping and reduced by hole doping. This effect is due to the charge
transfer to the fluorine adatom and the consequent change in the
fluorine-carbon bonding. Our simple tight-binding model, that includes the SOC
of the orbitals of F and effective parameters based on maximally localized
Wannier functions, is able to account for the effect. The strong enhancement of
the SOC induced by graphene doping opens the possibility to tune the spin
relaxation in this material.Comment: 9 pages, 8 figure
Floquet bound states around defects and adatoms in graphene
Recent studies have focused on laser-induced gaps in graphene which have been
shown to have a topological origin, thereby hosting robust states at the sample
edges. While the focus has remained mainly on these topological chiral edge
states, the Floquet bound states around defects lack a detailed study. In this
paper we present such a study covering large defects of different shape and
also vacancy-like defects and adatoms at the dynamical gap at
( being the photon energy). Our results, based on analytical
calculations as well as numerics for full tight-binding models, show that the
bound states are chiral and appear in a number which grows with the defect
size. Furthermore, while the bound states exist regardless the type of the
defect's edge termination (zigzag, armchair, mixed), the spectrum is strongly
dependent on it. In the case of top adatoms, the bound states quasi-energies
depend on the adatoms energy. The appearance of such bound states might open
the door to the presence of topological effects on the bulk transport
properties of dirty graphene.Comment: 16 pages, 14 figure
Anomalous Josephson Current in Junctions with Spin-Polarizing Quantum Point Contacts
We consider a ballistic Josephson junction with a quantum point contact in a
two-dimensional electron gas with Rashba spin-orbit coupling. The point contact
acts as a spin filter when embedded in a circuit with normal electrodes. We
show that with an in-plane external magnetic field an anomalous supercurrent
appears even for zero phase difference between the superconducting electrodes.
In addition, the external field induces large critical current asymmetries
between the two flow directions, leading to supercurrent rectifying effects.Comment: 4 pages, 4 figures, to appear in PR
Edge channel mixing induced by potential steps in an integer quantum Hall system
We investigate the coherent mixing of co-propagating edge channels in a
quantum Hall bar produced by step potentials. In the case of two edge channels
it is found that, although a single step induces only a few percent mixing, a
series of steps could yield 50% mixing. In addition, a strong mixing is found
when the potential height of a single step allows a different number of edge
channels on the two sides of the step. Charge density probability has been also
calculated even for the case where the step is smoothened.Comment: final version: 7 pages, 6 figure
Diffusion of fluorine adatoms on doped graphene
We calculate the diffusion barrier of fluorine adatoms on doped graphene in
the diluted limit using Density Functional Theory. We found that the barrier
strongly depends on the magnitude and character of the graphene's
doping (): it increases for hole doping () and decreases
for electron doping (). Near the neutrality point the functional
dependence can be approximately by where
meVcm. This effect leads to significant
changes of the diffusion constant with doping even at room temperature and
could also affect the low temperature diffusion dynamics due to the presence of
substrate induced charge puddles. In addition, this might open up the
possibility to engineer the F dynamics on graphene by using local gates.Comment: 4 pages, 4 figure
On the Magnetic Nature of Quantum Point Contacts
We present results for a model that describes a quantum point contact. We
show how electron-electron correlations, within the unrestricted Hartree-Fock
approximation, generate a magnetic moment in the point contact. Having
characterized the magnetic structure of the contact, we map the problem onto a
simple one-channel model and calculate the temperature dependence of the
conductance for different gate voltages. Our results are in good agreement with
experimental results obtained in GaAs devices and support the idea of Kondo
effect in these systems.Comment: 7 pages, 4 figure
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