846 research outputs found
Bubbling solutions, entropy enhancement and the fuzzball proposal
In this short note we explain the main idea of the work done in
arXiv:0804.4487[hep-th] and arXiv:0812.2942[hep-th]. We present a family of
black hole microstates, the bubbling solutions. We then explain how supertubes
placed in such backgrounds have their entropy enhanced by the presence of the
background dipole charges. This indicates this could account for a large amount
in the entropy of the three charge black hole.Comment: 2 pages, contribution to the Cargese 2008 proceedings: Theory and
Particle Physics: the LHC perspective and beyon
Regular 3-charge 4D black holes and their microscopic description
The perturbative corrections to Type-IIA String Theory
compactified on a Calabi-Yau three-fold allow the construction of regular
three-charge supersymmetric black holes in four dimensions, whose entropy
scales with the charges as .
We construct an M-theory uplift of these quantum black holes and show that they
can be interpreted as arising from three stacks of M2 branes on a conical
singularity. This in turns allow us relate them via a series of dualities to a
system of D3 branes carrying momentum and thus to give a microscopic
interpretation of their entropy.Comment: 20 pages, LaTe
From Andreev bound states to Majorana fermions in topological wires on superconducting substrates : a story of mutation
We study the proximity effect in a topological nanowire tunnel coupled to an
s-wave superconducting substrate. We use a general Green's function approach
that allows us to study the evolution of the Andreev bound states in the wire
into Majorana fermions. We show that the strength of the tunnel coupling
induces a topological transition in which the Majorana fermionic states can be
destroyed when the coupling is very strong. Moreover, we provide a
phenomenologial study of the effects of disorder in the superconductor on the
formation of Majorana fermions. We note a non-trivial effect of a quasiparticle
broadening term which can take the wire from a topological into a
non-topological phase in certain ranges of parameters. Our results have also
direct consequences for a nanowire coupled to an inhomogenous superconductor
Determining the spin-orbit coupling via spin-polarized spectroscopy of magnetic impurities
We study the spin-resolved spectral properties of the impurity states
associated to the presence of magnetic impurities in two-dimensional, as well
as one-dimensional systems with Rashba spin-orbit coupling. We focus on Shiba
bound states in superconducting materials, as well as on impurity states in
metallic systems. Using a combination of a numerical T-matrix approximation and
a direct analytical calculation of the bound state wave function, we compute
the local density of states (LDOS) together with its Fourier transform (FT). We
find that the FT of the spin-polarized LDOS, a quantity accessible via
spin-polarized STM, allows to accurately extract the strength of the spin-orbit
coupling. Also we confirm that the presence of magnetic impurities is strictly
necessary for such measurement, and that non-spin-polarized experiments cannot
have access to the value of the spin-orbit coupling.Comment: 26 pages, 6 figure
Majorana Fermions in Honeycomb Lattices
We study the formation of Majorana fermions in honeycomb-lattice structures
in the presence of a Zeeman field, Rashba spin-orbit coupling, and in the
proximity of an s-wave superconductor. We show that an exact mapping exists
between an anisotropic hexagonal-lattice nanoribbon at k = 0 and a
one-dimensional chain, for which the existence of Majorana fermions has been
extensively discussed. Consequently we can predict the conditions for the
emergence of Majorana fermions at the edges of such ribbon, and relate the
existence of Majoranas to a band inversion in the bulk band structure. Moreover
we find that similar situations arise in isotropic lattices and we give some
examples which show the formation of Majorana fermions in these structures.Comment: 7 pages, 9 figure
Double-gap superconducting proximity effect in nanotubes
We theoretically explore the possibility of a superconducting proximity
effect in single-walled metallic carbon nanotubes due to the presence of a
superconducting substrate. An unconventional double-gap situation can arise in
the two bands for nanotubes of large radius wherein the tunneling is (almost)
symmetric in the two sublattices. In such a case, a proximity effect can take
place in the symmetric band below a critical experimentally-accessible Coulomb
interaction strength in the nanotube. Furthermore, due to interactions in the
nanotube, the appearance of a BCS gap in this band stabilizes superconductivity
in the other band at lower temperatures. We also discuss the scenario of highly
asymmetric tunneling and show that this case too supports double-gap
superconductivity.Comment: 4 pages, 2 figure
Majorana bound states in open quasi-1D and 2D systems with transverse Rashba coupling
We study the formation of Majorana states in quasi-1D and 2D square lattices
with open boundary conditions, with general anisotropic Rashba coupling, in the
presence of an applied Zeeman field and in the proximity of a superconductor.
For systems in which the length of the system is very large (quasi-1D) we
calculate analytically the exact topological invariant, and we find a rich
phase diagram which is strongly dependent on the width of the system. We
compare our results with previous results based on a few-band approximation. We
also investigate numerically open 2D systems of finite length in both
directions. We use the recently introduced generalized Majorana polarization,
which can locally evaluate the Majorana character of a given state. We find
that the formation of Majoranas depends strongly on the geometry of the system
and if the length and the width are comparable no Majorana states can form,
however, one can show the formation of "quasi-Majorana" states that have a
local Majorana character, but no global Majorana symmetry.Comment: 12 pages, 13 figure
Jarzynski equality for the Jepsen gas
We illustrate the Jarzynski equality on the exactly solvable model of a
one-dimensional ideal gas in uniform expansion or compression. The analytical
results for the probability density of the work performed by the gas
are compared with the results of molecular dynamics simulations for a
two-dimensional dilute gas of hard spheres.Comment: 7 pages, 4 figures, submitted to Europhys. Let
Stochastically perturbed flows: Delayed and interrupted evolution
We present analytical expressions for the time-dependent and stationary
probability distributions corresponding to a stochastically perturbed
one-dimensional flow with critical points, in two physically relevant
situations: delayed evolution, in which the flow alternates with a quiescent
state in which the variate remains frozen at its current value for random
intervals of time; and interrupted evolution, in which the variate is also
re-set in the quiescent state to a random value drawn from a fixed
distribution. In the former case, the effect of the delay upon the first
passage time statistics is analyzed. In the latter case, the conditions under
which an extended stationary distribution can exist as a consequence of the
competition between an attractor in the flow and the random re-setting are
examined. We elucidate the role of the normalization condition in eliminating
the singularities arising from the unstable critical points of the flow, and
present a number of representative examples. A simple formula is obtained for
the stationary distribution and interpreted physically. A similar
interpretation is also given for the known formula for the stationary
distribution in a full-fledged dichotomous flow.Comment: 27 pages; no figures. Submitted to Stochastics and Dynamic
- …