511 research outputs found
Hidden Symmetries of Electronic Transport in a Disordered One-Dimensional Lattice
Correlated, or extended, impurities play an important role in the transport
properties of dirty metals. Here, we examine, in the framework of a
tight-binding lattice, the transmission of a single electron through an array
of correlated impurities. In particular we show that particles transmit through
an impurity array in identical fashion, regardless of the direction of
transversal. The demonstration of this fact is straightforward in the continuum
limit, but requires a detailed proof for the discrete lattice. We also briefly
demonstrate and discuss the time evolution of these scattering states, to
delineate regions (in time and space) where the aforementioned symmetry is
violated
Topological phase transitions in small mesoscopic chiral p-wave superconductors
Spin-triplet chiral p-wave superconductivity is typically described by a
two-component order parameter, and as such is prone to unique emergent effects
when compared to the standard single-component superconductors. Here we present
the equilibrium phase diagram for small mesoscopic chiral p-wave
superconducting disks in the presence of magnetic field, obtained by solving
the microscopic Bogoliubov-de Gennes equations self-consistently. In the
ultra-small limit, the cylindrically-symmetric giant-vortex states are the
ground state of the system. However, with increasing sample size, the
cylindrical symmetry is broken as the two components of the order parameter
segregate into domains, and the number of fragmented domain walls between them
characterizes the resulting states. Such domain walls are topological defects
unique for the p-wave order, and constitute a dominant phase in the mesoscopic
regime. Moreover, we find two possible types of domain walls, identified by
their chirality-dependent interaction with the edge states
DC conductivity of twisted bilayer graphene: Angle-dependent transport properties and effects of disorder
The in-plane DC conductivity of twisted bilayer graphene (TBLG) is calculated
using an expansion of the real-space Kubo-Bastin conductivity in terms of
Chebyshev polynomials. We investigate within a tight-binding (TB) approach the
transport properties as a function of rotation angle, applied perpendicular
electric field and vacancy disorder. We find that for high-angle twists, the
two layers are effectively decoupled, and the minimum conductivity at the Dirac
point corresponds to double the value observed in monolayer graphene. This
remains valid even in the presence of vacancies, hinting that chiral symmetry
is still preserved. On the contrary, for low twist angles, the conductivity at
the Dirac point depends on the twist angle and is not protected in the presence
of disorder. Furthermore, for low angles and in the presence of an applied
electric field, we find that the chiral boundary states emerging between AB and
BA regions contribute to the DC conductivity, despite the appearance of
strongly localized states in the AA regions. The results agree with recent
conductivity experiments on twisted bilayer graphene
Quantum mechanics of spin transfer in coupled electron-spin chains
The manner in which spin-polarized electrons interact with a magnetized thin
film is currently described by a semi-classical approach. This in turn provides
our present understanding of the spin transfer, or spin torque phenomenon.
However, spin is an intrinsically quantum mechanical quantity. Here, we make
the first strides towards a fully quantum mechanical description of spin
transfer through spin currents interacting with a Heisenberg-coupled spin
chain. Because of quantum entanglement, this requires a formalism based on the
density matrix approach. Our description illustrates how individual spins in
the chain time-evolve as a result of spin transfer.Comment: 4 pages, 3 (colour) figure
Disordered graphene Josephson junctions
A tight-binding approach based on the Chebyshev-Bogoliubov-de Gennes method
is used to describe disordered single-layer graphene Josephson junctions.
Scattering by vacancies, ripples or charged impurities is included. We compute
the Josephson current and investigate the nature of multiple Andreev
reflections, which induce bound states appearing as peaks in the density of
states for energies below the superconducting gap. In the presence of single
atom vacancies, we observe a strong suppression of the supercurrent that is a
consequence of strong inter-valley scattering. Although lattice deformations
should not induce inter-valley scattering, we find that the supercurrent is
still suppressed, which is due to the presence of pseudo-magnetic barriers. For
charged impurities, we consider two cases depending on whether the average
doping is zero, i.e. existence of electron-hole puddles, or finite. In both
cases, short range impurities strongly affect the supercurrent, similar to the
vacancies scenario
Tight-binding study of bilayer graphene Josephson junctions
Using highly efficient simulations of the tight-binding Bogoliubov-de Gennes
model we solved self-consistently for the pair correlation and the Josephson
current in a Superconducting-Bilayer graphene-Superconducting Josephson
junction. Different doping levels for the non-superconducting link are
considered in the short and long junction regime. Self-consistent results for
the pair correlation and superconducting current resemble those reported
previously for single layer graphene except in the Dirac point where remarkable
differences in the proximity effect are found as well as a suppression of the
superconducting current in long junction regime. Inversion symmetry is broken
by considering a potential difference between the layers and we found that the
supercurrent can be switched if junction length is larger than the Fermi
length
Partially unzipped carbon nanotubes as magnetic field sensors
The conductance, , through graphene nanoribbons (GNR) connected to a
partially unzipped carbon nanotube (CNT) is studied in the presence of an
external magnetic field applied parallel to the long axis of the tube by means
of non-equilibrium Green's function technique. We consider (z)igzag and
(a)rmchair CNTs that are partially unzipped to form aGNR/zCNT/aGNR or
zGNR/aCNT/zGNR junctions. We find that the inclusion of a longitudinal magnetic
field affects the electronic states only in the CNT region, leading to the
suppression of the conductance at low energies. Unlike previous studies, for
the zGNR/aCNT/zGNR junction in zero field, we find a sharp dip in the
conductance as the energy approaches the Dirac point and we attribute this
non-trivial behavior to the peculiar band dispersion of the constituent
subsystems. We demonstrate that both types of junctions can be used as magnetic
field sensors.Comment: final version to appear in Applied Physics Letter
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