7,705 research outputs found
Extended topological group structure due to average reflection symmetry
We extend the single-particle topological classification of insulators and
superconductors to include systems in which disorder preserves average
reflection symmetry. We show that the topological group structure of bulk
Hamiltonians and topological defects is exponentially extended when this
additional condition is met, and examine some of its physical consequences.
Those include localization-delocalization transitions between topological
phases with the same boundary conductance, as well as gapless topological
defects stabilized by average reflection symmetry.Comment: 8 pages, 5 figures, 1 table; improved section 4 "Extended topological
classification" incl. example of stacked QSH layer
Improved Delay Estimates for a Queueing Model for Random Linear Coding for Unicast
Consider a lossy communication channel for unicast with zero-delay feedback.
For this communication scenario, a simple retransmission scheme is optimum with
respect to delay. An alternative approach is to use random linear coding in
automatic repeat-request (ARQ) mode. We extend the work of Shrader and
Ephremides, by deriving an expression for the delay of random linear coding
over field of infinite size. Simulation results for various field sizes are
also provided.Comment: 5 pages, 3 figures, accepted at the 2009 IEEE International Symposium
on Information Theor
Transport in random quantum dot superlattices
We present a novel model to calculate single-electron states in random
quantum dot superlattices made of wide-gap semiconductors. The source of
disorder comes from the random arrangement of the quantum dots (configurational
disorder) as well as spatial inhomogeneities of their shape (morphological
disorder). Both types of disorder break translational symmetry and prevent the
formation of minibands, as occurs in regimented arrays of quantum dots. The
model correctly describes channel mixing and broadening of allowed energy bands
due to elastic scattering by disorder
Bimodal conductance distribution of Kitaev edge modes in topological superconductors
A two-dimensional superconductor with spin-triplet p-wave pairing supports
chiral or helical Majorana edge modes with a quantized (length -independent)
thermal conductance. Sufficiently strong anisotropy removes both chirality and
helicity, doubling the conductance in the clean system and imposing a
super-Ohmic decay in the presence of disorder. We explain the
absence of localization in the framework of the Kitaev Hamiltonian, contrasting
the edge modes of the two-dimensional system with the one-dimensional Kitaev
chain. While the disordered Kitaev chain has a log-normal conductance
distribution peaked at an exponentially small value, the Kitaev edge has a
bimodal distribution with a second peak near the conductance quantum. Shot
noise provides an alternative, purely electrical method of detection of these
charge-neutral edge modes.Comment: 11 pages, 13 figure
Phase-locked magnetoconductance oscillations as a probe of Majorana edge states
We calculate the Andreev conductance of a superconducting ring interrupted by
a flux-biased Josephson junction, searching for electrical signatures of
circulating edge states. Two-dimensional pair potentials of spin-singlet d-wave
and spin-triplet p-wave symmetry support, respectively, (chiral) Dirac modes
and (chiral or helical) Majorana modes. These produce h/e-periodic
magnetoconductance oscillations of amplitude \simeq (e^{2}/h)N^{-1/2}, measured
via an N-mode point contact at the inner or outer perimeter of the grounded
ring. For Dirac modes the oscillations in the two contacts are independent,
while for an unpaired Majorana mode they are phase locked by a topological
phase transition at the Josephson junction.Comment: 10 pages, 6 figures. New appendix on the gauge invariant
discretization of the Bogoliubov-De Gennes equation. Accepted for publication
in PR
Lagged and instantaneous dynamical influences related to brain structural connectivity
Contemporary neuroimaging methods can shed light on the basis of human neural
and cognitive specializations, with important implications for neuroscience and
medicine. Different MRI acquisitions provide different brain networks at the
macroscale; whilst diffusion-weighted MRI (dMRI) provides a structural
connectivity (SC) coincident with the bundles of parallel fibers between brain
areas, functional MRI (fMRI) accounts for the variations in the
blood-oxygenation-level-dependent T2* signal, providing functional connectivity
(FC).Understanding the precise relation between FC and SC, that is, between
brain dynamics and structure, is still a challenge for neuroscience. To
investigate this problem, we acquired data at rest and built the corresponding
SC (with matrix elements corresponding to the fiber number between brain areas)
to be compared with FC connectivity matrices obtained by 3 different methods:
directed dependencies by an exploratory version of structural equation modeling
(eSEM), linear correlations (C) and partial correlations (PC). We also
considered the possibility of using lagged correlations in time series; so, we
compared a lagged version of eSEM and Granger causality (GC). Our results were
two-fold: firstly, eSEM performance in correlating with SC was comparable to
those obtained from C and PC, but eSEM (not C nor PC) provides information
about directionality of the functional interactions. Second, interactions on a
time scale much smaller than the sampling time, captured by instantaneous
connectivity methods, are much more related to SC than slow directed influences
captured by the lagged analysis. Indeed the performance in correlating with SC
was much worse for GC and for the lagged version of eSEM. We expect these
results to supply further insights to the interplay between SC and functional
patterns, an important issue in the study of brain physiology and function.Comment: Accepted and published in Frontiers in Psychology in its current
form. 27 pages, 1 table, 5 figures, 2 suppl. figure
Georges Canguilhem on sex determination and the normativity of life
Our goal in this paper is to reassess the relationship between norms and life by drawing on the philosophy of Georges Canguilhem, particularly some of his unpublished lectures about teratology and sexual determination. First, we discuss the difficulties Canguilhem identified in the introduction of life and sexuality as objects of philosophical reflection. Second, we reassess Canguilhem’s understanding of normativity as rooted in life and the axiological activity of the living. Third, we analyze how Canguilhem drew from past and contemporary teratology to conceive of the notions of anomaly and abnormality. Finally, we reconstruct Canguilhem’s analysis of a case of hermaphroditism, highlighting how he presented it as evidence that sexual determination is the result of a normative choice. One of the key contributions of the paper to scholarly literature on Canguilhem is a better understanding of his notion of choice, which was considered not the conscious and intentional act of a subject but rather an axiological activity of the living. We conclude by positioning Canguilhem’s concept of normativity and his belief that norms are produced by the living in relation to the naturalist/normativist divide in medicine
Aproximación a un estudio de las reseñas literarias en diversos suplementos culturales españoles.
In this article we have analyzed the most important cultural supplement of the most sold newspapers in our country during three weeks, we have studied the different ways to make literature critic. We wanted to know: what kind of literature is on these publications, how important poetry is nowadays and witches are the most important editorials
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