84 research outputs found
Topological Phases for Fermionic Cold Atoms on the Lieb Lattice
We investigate the properties of the Lieb lattice, i.e a face-centered square
lattice, subjected to external gauge fields. We show that an Abelian gauge
field leads to a peculiar quantum Hall effect, which is a consequence of the
single Dirac cone and the flat band characterizing the energy spectrum. Then we
explore the effects of an intrinsic spin-orbit term - a non-Abelian gauge field
- and demonstrate the occurrence of the quantum spin Hall effect in this model.
Besides, we obtain the relativistic Hamiltonian describing the Lieb lattice at
low energy and derive the Landau levels in the presence of external Abelian and
non-Abelian gauge fields. Finally, we describe concrete schemes for realizing
these gauge fields with cold fermionic atoms trapped in an optical Lieb
lattice. In particular, we provide a very efficient method to reproduce the
intrinsic (Kane-Mele) spin-orbit term with assisted-tunneling schemes.
Consequently, our model could be implemented in order to produce a variety of
topological states with cold-atoms.Comment: 12 pages, 9 figure
Adiabatic pumping in the quasi-one-dimensional triangle lattice
We analyze the properties of the quasi-one-dimensional triangle lattice
emphasizing the occurrence of flat bands and band touching via the tuning of
the lattice hopping parameters and on-site energies. The spectral properties of
the infinite system will be compared with the transmission through a finite
piece of the lattice with attached semi-infinite leads. Furthermore, we
investigate the adiabatic pumping properties of such a system: depending on the
transmission through the lattice, this results in nonzero integer charge
transfers or transfers that increase linearly with the lattice size
a review
In this review article we describe spin-dependent transport in materials with
spin–orbit interaction of Rashba type. We mainly focus on semiconductor
heterostructures, however we consider topological insulators, graphene and
hybrid structures involving superconductors as well. We start from the Rashba
Hamiltonian in a two dimensional electron gas and then describe transport
properties of two- and quasi-one-dimensional systems. The problem of spin
current generation and interference effects in mesoscopic devices is described
in detail. We address also the role of Rashba interaction on localisation
effects in lattices with nontrivial topology, as well as on the Ahronov–Casher
effect in ring structures. A brief section, in the end, describes also some
related topics including the spin-Hall effect, the transition from weak
localisation to weak anti localisation and the physics of Majorana fermions in
hybrid heterostructures involving Rashba materials in the presence of
superconductivity
Massless Dirac-Weyl Fermions in a T_3 Optical Lattice
We propose an experimental setup for the observation of quasi-relativistic
massless Fermions. It is based on a T_3 optical lattice, realized by three
pairs of counter-propagating lasers, filled with fermionic cold atoms. We show
that in the long wavelength approximation the T_3 Hamiltonian generalizes the
Dirac-Weyl Hamiltonian for the honeycomb lattice, however, with a larger value
of the pseudo-spin S=1. In addition to the Dirac cones, the spectrum includes a
dispersionless branch of localized states producing a finite jump in the atomic
density. Furthermore, implications for the Landau levels are discussed.Comment: 4 pages, 3 figure
Transport properties of an electron-hole bilayer/superconductor hybrid junction
We investigate the transport properties of a junction consisting of an
electron-hole bilayer in contact with normal and superconducting leads. The
electron-hole bilayer is considered as a semi-metal with two electronic bands.
We assume that in the region between the contacts the system hosts an exciton
condensate described by a BCS-like model with a gap in the
quasiparticle density of states. We first discuss how the subgap electronic
transport through the junction is mainly governed by the interplay between two
kinds of reflection processes at the interfaces: The standard Andreev
reflection at the interface between the superconductor and the exciton
condensate, and a coherent crossed reflection at the
semi-metal/exciton-condensate interface that converts electrons from one layer
into the other. We show that the differential conductance of the junction shows
a minimum at voltages of the order of . Such a minimum can be seen as
a direct hallmark of the existence of the gapped excitonic state
Spin-resolved scattering through spin-orbit nanostructures in graphene
We address the problem of spin-resolved scattering through spin-orbit
nanostructures in graphene, i.e., regions of inhomogeneous spin-orbit coupling
on the nanometer scale. We discuss the phenomenon of spin-double refraction and
its consequences on the spin polarization. Specifically, we study the
transmission properties of a single and a double interface between a normal
region and a region with finite spin-orbit coupling, and analyze the
polarization properties of these systems. Moreover, for the case of a single
interface, we determine the spectrum of edge states localized at the boundary
between the two regions and study their properties
Spin-orbit interaction and snake states in a graphene p-n junction
We study a model of a p-n junction in single-layer graphene in the presence of a perpendicular magnetic field and spin-orbit interactions. By solving the relevant quantum-mechanical problem for a potential step, we determine the exact spectrum of spin-resolved dispersive Landau levels. Close to zero energy, we find a pair of linearly dispersing zero modes, which possess a wave-vector-dependent spin polarization and can be regarded as quantum analogs of spinful snake states. We show that the Rashba spin-orbit interaction, in particular, produces a wave vector shift between the dispersions of these modes with observable interference effects. These effects can in principle provide a way to detect the presence of Rashba spin-orbit interaction and measure its strength. Our results suggest that a graphene p-n junction in the presence of strong spin-orbit interaction could be used as a building block in a spin field-effect transistor
Ground state features of the Frohlich model
Following the ideas behind the Feynman approach, a variational wave function
is proposed for the Fr\"ohlich model. It is shown that it provides, for any
value of the electron-phonon coupling constant, an estimate of the polaron
ground state energy better than the Feynman method based on path integrals. The
mean number of phonons, the average electronic kinetic and interaction
energies, the ground state spectral weight and the electron-lattice correlation
function are calculated and successfully compared with the best available
results.Comment: 6 figure
Electron Scattering in Intrananotube Quantum Dots
Intratube quantum dots showing particle-in-a-box-like states with level
spacings up to 200meV are realized in metallic single-walled carbon nanotubes
by means of low dose medium energy Ar irradiation. Fourier transform scanning
tunneling spectroscopy compared to results of a Fabry-Perot electron resonator
model yields clear signatures for inter- and intra-valley scattering of
electrons confined between consecutive irradiation-induced defects
(inter-defects distance < 10nm). Effects arising from lifting the degeneracy of
the Dirac cones within the first Brillouin zone are also observed
Barrier transmission of Dirac-like pseudospin-one particles
We address the problem of barrier tunneling in the two-dimensional T_3
lattice (dice lattice). In particular we focus on the low-energy,
long-wavelength approximation for the Hamiltonian of the system, where the
lattice can be described by a Dirac-like Hamiltonian associated with a
pseudospin one. The enlarged pseudospin S = 1 (instead of S = 1/2 as for
graphene) leads to an enhanced "super" Klein tunneling through rectangular
electrostatic barriers. Our results are confirmed via numerical investigation
of the tight-binding model of the lattice. For a uniform magnetic field, we
discuss the Landau levels and we investigate the transparency of a rectangular
magnetic barrier. We show that the latter can mainly be described by
semiclassical orbits bending the particle trajectories, qualitatively similar
as it is the case for graphene. This makes it possible to confine particles
with magnetic barriers of sufficient width
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