8,801 research outputs found
Quantum states in a magnetic anti-dot
We study a new system in which electrons in two dimensions are confined by a
non homogeneous magnetic field. The system consists of a heterostructure with
on top of it a superconducting disk. We show that in this system electrons can
be confined into a dot region. This magnetic anti-dot has the interesting
property that the filling of the dot is a discrete function of the magnetic
field. The circulating electron current inside and outside the anti-dot can be
in opposite direction for certain bound states. And those states exhibit a
diamagnetic to paramagnetic transition with increasing magnetic field. The
absorption spectrum consists of many peaks, some of which violate Kohn's
theorem, and which is due to the coupling of the center of mass motion with the
other degrees of freedom.Comment: 6 pages, 12 ps figure
Identification of Piecewise Linear Models of Complex Dynamical Systems
The paper addresses the realization and identification problem or a subclass
of piecewise-affine hybrid systems. The paper provides necessary and sufficient
conditions for existence of a realization, a characterization of minimality,
and an identification algorithm for this subclass of hybrid systems. The
considered system class and the identification problem are motivated by
applications in systems biology
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
Electronic properties of bilayer phosphorene quantum dots in the presence of perpendicular electric and magnetic fields
Using the tight-binding approach, we investigate the electronic properties of
bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular
electric and magnetic fields. Since BLP consists of two coupled phosphorene
layers, it is of interest to examine the layer-dependent electronic properties
of BLP QDs, such as the electronic distributions over the two layers and the
so-produced layer-polarization features, and to see how these properties are
affected by the magnetic field and the bias potential. We find that in the
absence of a bias potential only edge states are layer-polarized while the bulk
states are not, and the layer-polarization degree (LPD) of the unbiased edge
states increases with increasing magnetic field. However, in the presence of a
bias potential both the edge and bulk states are layer-polarized, and the LPD
of the bulk (edge) states depends strongly (weakly) on the interplay of the
bias potential and the interlayer coupling. At high magnetic fields, applying a
bias potential renders the bulk electrons in a BLP QD to be mainly distributed
over the top or bottom layer, resulting in layer-polarized bulk Landau levels
(LLs). In the presence of a large bias potential that can drive a
semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different
magnetic-field dependences, i.e., the zeroth LLs exhibit a linear-like
dependence on the magnetic field while the other LLs exhibit a square-root-like
dependence.Comment: 11 pages, 6 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
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