9,074 research outputs found
Wormhole Effect in a Strong Topological Insulator
An infinitely thin solenoid carrying magnetic flux Phi (a `Dirac string')
inserted into an ordinary band insulator has no significant effect on the
spectrum of electrons. In a strong topological insulator, remarkably, such a
solenoid carries protected gapless one-dimensional fermionic modes when
Phi=hc/2e. These modes are spin-filtered and represent a distinct bulk
manifestation of the topologically non-trivial insulator. We establish this
`wormhole' effect by both general qualitative considerations and by numerical
calculations within a minimal lattice model. We also discuss the possibility of
experimental observation of a closely related effect in artificially engineered
nanostructures.Comment: 4 pages, 3 figures. For related work and info visit
http://www.physics.ubc.ca/~fran
Topological Anderson Insulator in Three Dimensions
Disorder, ubiquitously present in solids, is normally detrimental to the
stability of ordered states of matter. In this letter we demonstrate that not
only is the physics of a strong topological insulator robust to disorder but,
remarkably, under certain conditions disorder can become fundamentally
responsible for its existence. We show that disorder, when sufficiently strong,
can transform an ordinary metal with strong spin-orbit coupling into a strong
topological `Anderson' insulator, a new topological phase of quantum matter in
three dimensions.Comment: 5 pages, 2 figures. For related work and info visit
http://www.physics.ubc.ca/~franz
Temperature evolution and bifurcations of metastable states in mean-field spin glasses, with connections with structural glasses
The correlations of the free-energy landscape of mean-field spin glasses at
different temperatures are investigated, concentrating on models with a first
order freezing transition. Using a ``potential function'' we follow the
metastable states of the model in temperature, and discuss the possibility of
level crossing (which we do not find) and multifurcation (which we find). The
dynamics at a given temperature starting from an equilibrium configuration at a
different temperature is also discussed. In presence of multifurcation, we find
that the equilibrium is never achieved, leading to aging behaviour at slower
energy levels than usual aging. The relevance of the observed mechanisms for
real structural glasses is discussed, and some numerical simulations of a soft
sphere model of glass are presented.Comment: 16 pages, LaTeX, 10 figures (12 postscript files
Glassy Mean-Field Dynamics of the Backgammon model
In this paper we present an exact study of the relaxation dynamics of the
backgammon model. This is a model of a gas of particles in a discrete space
which presents glassy phenomena as a result of {\it entropy barriers} in
configuration space. The model is simple enough to allow for a complete
analytical treatment of the dynamics in infinite dimensions. We first derive a
closed equation describing the evolution of the occupation number
probabilities, then we generalize the analysis to the study the autocorrelation
function. We also consider possible variants of the model which allow to study
the effect of energy barriers.Comment: 21 pages, revtex, 4 uuencoded figure
On the origin of ultrametricity
In this paper we show that in systems where the probability distribution of
the the overlap is non trivial in the infinity volume limit, the property of
ultrametricity can be proved in general starting from two very simple and
natural assumptions: each replica is equivalent to the others (replica
equivalence or stochastic stability) and all the mutual information about a
pair of equilibrium configurations is encoded in their mutual distance or
overlap (separability or overlap equivalence).Comment: 13 pages, 1 figur
TDC Chip and Readout Driver Developments for COMPASS and LHC-Experiments
A new TDC-chip is under development for the COMPASS experiment at CERN. The
ASIC, which exploits the 0.6 micrometer CMOS sea-of-gate technology, will allow
high resolution time measurements with digitization of 75 ps, and an
unprecedented degree of flexibility accompanied by high rate capability and low
power consumption. Preliminary specifications of this new TDC chip are
presented.
Furthermore a FPGA based readout-driver and buffer-module as an interface
between the front-end of the COMPASS detector systems and an optical S-LINK is
in development. The same module serves also as remote fan-out for the COMPASS
trigger distribution and time synchronization system. This readout-driver
monitors the trigger and data flow to and from front-ends. In addition, a
specific data buffer structure and sophisticated data flow control is used to
pursue local pre-event building. At start-up the module controls all necessary
front-end initializations.Comment: 5 pages, 4 figure
From Isovists via Mental Representations to Behaviour: First Steps Toward Closing the Causal Chain
This study addresses the interrelations between human wayfinding performance, the mental representation of routes, and the geometrical layout of path intersections. The virtual reality based empirical experiment consisted of a route learning and reproduction task and two choice reaction tasks measuring the acquired knowledge of route decision points. In order to relate the recorded behavioural data to the geometry of the environment, a specific adaptation of isovist-based spatial analysis was developed that accounts for directional bias in human spatial perception and representation. Taken together, the applied analyses provided conclusive evidence for correspondences between geometrical properties of environments as captured by isovists and their mental representation
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
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