77 research outputs found
Majorana fermion exchange in strictly one dimensional structures
It is generally thought that adiabatic exchange of two identical particles is
impossible in one spatial dimension. Here we describe a simple protocol that
permits adiabatic exchange of two Majorana fermions in a one-dimensional
topological superconductor wire. The exchange relies on the concept of
"Majorana shuttle" whereby a domain wall in the superconducting order
parameter which hosts a pair of ancillary Majoranas delivers one zero mode
across the wire while the other one tunnels in the opposite direction. The
method requires some tuning of parameters and does not, therefore, enjoy the
full topological protection. The resulting exchange statistics, however,
remains non-Abelian for a wide range of parameters that characterize the
exchange.Comment: 5 pages, 4 figures, supplemental material is include
Quantization and Periodicity of the Axion Action in Topological Insulators
The Lagrangian describing the bulk electromagnetic response of a
three-dimensional strong topological insulator contains a topological `axion'
term of the form '\theta E dot B'. It is often stated (without proof) that the
corresponding action is quantized on periodic space-time and therefore
invariant under '\theta -> \theta +2\pi'. Here we provide a simple, physically
motivated proof of the axion action quantization on the periodic space-time,
assuming only that the vector potential is consistent with single-valuedness of
the electron wavefunctions in the underlying insulator.Comment: 4 pages, 1 figure, version2 (section on axion action quantization of
non-periodic systems added
Stability of Majorana Fermions in Proximity-Coupled Topological Insulator Nanowires
It has been shown previously that a finite-length topological insulator
nanowire, proximity-coupled to an ordinary bulk s-wave superconductor and
subject to a longitudinal applied magnetic field, realizes a one-dimensional
topological superconductor with an unpaired Majorana fermion (MF) localized at
each end of the nanowire. Here, we study the stability of these MFs with
respect to various perturbations that are likely to occur in a physical
realization of the proposed device. We show that the unpaired Majorana fermions
persist in this system for any value of the chemical potential inside the bulk
band gap of order 300 meV in BiSe by computing the Majorana number.
From this calculation, we also show that the unpaired Majorana fermions persist
when the magnetic flux through the nanowire cross-section deviates
significantly from half flux quantum. Lastly, we demonstrate that the unpaired
Majorana fermions persist in strongly disordered wires with fluctuations in the
on-site potential ranging in magnitude up to several times the size of the bulk
band gap. These results suggest this solid-state system should exhibit unpaired
Majorana fermions under accessible conditions likely important for experimental
study or future applications.Comment: 17 pages, 13 figure
Finite-temperature Screening and the Specific Heat of Doped Graphene Sheets
At low energies, electrons in doped graphene sheets are described by a
massless Dirac fermion Hamiltonian. In this work we present a semi-analytical
expression for the dynamical density-density linear-response function of
noninteracting massless Dirac fermions (the so-called "Lindhard" function) at
finite temperature. This result is crucial to describe finite-temperature
screening of interacting massless Dirac fermions within the Random Phase
Approximation. In particular, we use it to make quantitative predictions for
the specific heat and the compressibility of doped graphene sheets. We find
that, at low temperatures, the specific heat has the usual normal-Fermi-liquid
linear-in-temperature behavior, with a slope that is solely controlled by the
renormalized quasiparticle velocity.Comment: 9 pages, 5 figures, Submitted to J. Phys.
Effect of disorder on the ground-state properties of graphene
We calculate the ground-state energy of Dirac electrons in graphene in the
presence of disorder. We take randomly distributed charged impurities at a
fixed distance from the graphene sheet and surface fluctuations (ripples) as
the main scattering mechanisms. Mode-coupling approach to scattering rate and
random-phase approximation for ground-state energy incorporating the many-body
interactions and the disorder effects yields good agreement with experimental
inverse compressibility.Comment: Extended introduction and discussion. To appear in Phys. Rev.
AMoDSim: An Efficient and Modular Simulation Framework for Autonomous Mobility on Demand
Urban transportation of next decade is expected to be disrupted by Autonomous
Mobility on Demand (AMoD): AMoD providers will collect ride requests from users
and will dispatch a fleet of autonomous vehicles to satisfy requests in the
most efficient way. Differently from current ride sharing systems, in which
driver behavior has a clear impact on the system, AMoD systems will be
exclusively determined by the dispatching logic. As a consequence, a recent
interest in the Operations Research and Computer Science communities has
focused on this control logic. The new propositions and methodologies are
generally evaluated via simulation. Unfortunately, there is no simulation
platform that has emerged as reference, with the consequence that each author
uses her own custom-made simulator, applicable only in her specific study, with
no aim of generalization and without public release. This slows down the
progress in the area as researchers cannot build on each other's work and
cannot share, reproduce and verify the results. The goal of this paper is to
present AMoDSim, an open-source simulation platform aimed to fill this gap and
accelerate research in future ride sharing systems
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