162 research outputs found
Pseudo-spin-dependent scattering in carbon nanotubes
The breaking of symmetry is the ground on which many physical phenomena are
explained. This is important in particular for bipartite lattice structure as
graphene and carbon nanotubes, where particle-hole and pseudo-spin are relevant
symmetries. Here we investigate the role played by the defect-induced breaking
of these symmetries in the electronic scattering properties of armchair
single-walled carbon nanotubes. From Fourier transform of the local density of
states we show that the active electron scattering channels depend on the
conservation of the pseudo-spin. Further, we show that the lack of
particle-hole symmetry is responsible for the pseudo-spin selection rules
observed in several experiments. This symmetry breaking arises from the lattice
reconstruction appearing at defect sites. Our analysis gives an intuitive way
to understand the scattering properties of carbon nanotubes, and can be
employed for newly interpret several experiments on this subject. Further, it
can be used to design devices such as pseudo-spin filter by opportune defect
engineering
Proposal for an on-demand source of polarized electrons into the edges of a topological insulator
We propose a device that allows for the emission of pairs of spin-polarized
electrons into the edge-states of a two dimensional topological insulator.
Charge and spin emission is achieved using a periodically driven quantum dot
weakly coupled to the edge states of the host topological insulator. We present
calculations of the emitted time-dependent charge and spin currents of such a
dynamical scatterer using the Floquet scattering matrix approach. Experimental
signatures of spin-polarized two-particle emission can be found in noise
measurements. Here a new form of noise suppression, named
--antibunching, is introduced. Additionally, we propose a set-up
in which entanglement of the emitted electrons is generated. This entanglement
is based on a post-selection procedure and becomes manifest in a violation of a
Clauser-Horne-Shimony-Holt inequality.Comment: 10 pages + 7 figure
Signatures of spin-related phases in transport through regular polygons
We address the subject of transport in one-dimensional ballistic polygon
loops subject to Rashba spin-orbit coupling. We identify the role played by the
polygon vertices in the accumulation of spin-related phases by studying
interference effects as a function of the spin-orbit coupling strength. We find
that the vertices act as strong spin-scattering centers that hinder the
developing of Aharovov-Casher and Berry phases. In particular, we show that the
oscillation frequency of interference pattern can be doubled by modifying the
shape of the loop from a square to a circle.Comment: 4 pages, 4 figures. To appear in Phys. Rev.
Rashba spin-orbit interaction in graphene armchair nanoribbons
We study graphene nanoribbons (GNRs) with armchair edges in the presence of
Rashba spin-orbit interaction (RSOI). We impose the boundary conditions on the
tight binding Hamiltonians for bulk graphene with RSOI by means of a sine
transform and study in detail the influence of RSOI on the spectra and the spin
polarization. We show that the spin polarization perpendicular to the GNR
changes sign when reversing the momentum along the GNR if the bands are coupled
by strong RSOI. Furthermore, we derive a linearized approximation to the RSOI
Hamiltonian and find that only the neighboring modes of an energy band have to
be taken into account in order to achieve a good approximation for the same
band. Due to their experimental availability and various proposals for
engineering appropriate RSOI, GNRs with armchair edges are a promising
candidate for possible spintronics applications.Comment: added journal reference, small updates, 9 pages, 8 figure
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
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
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
- …