871 research outputs found
Primordial magnetic fields at preheating
Using lattice techniques we investigate the generation of long range
cosmological magnetic fields during a cold electroweak transition. We will show
how magnetic fields arise, during bubble collisions, in the form of magnetic
strings. We conjecture that these magnetic strings originate from the alignment
of magnetic dipoles associated with EW sphaleron-like configurations. We also
discuss the early thermalisation of photons and the turbulent behaviour of the
scalar fields after tachyonic preheating.Comment: 7 pages. Talk presented at Lattice200
Ultra-Sensitive Hot-Electron Nanobolometers for Terahertz Astrophysics
The background-limited spectral imaging of the early Universe requires
spaceborne terahertz (THz) detectors with the sensitivity 2-3 orders of
magnitude better than that of the state-of-the-art bolometers. To realize this
sensitivity without sacrificing operating speed, novel detector designs should
combine an ultrasmall heat capacity of a sensor with its unique thermal
isolation. Quantum effects in thermal transport at nanoscale put strong
limitations on the further improvement of traditional membrane-supported
bolometers. Here we demonstrate an innovative approach by developing
superconducting hot-electron nanobolometers in which the electrons are cooled
only due to a weak electron-phonon interaction. At T<0.1K, the electron-phonon
thermal conductance in these nanodevices becomes less than one percent of the
quantum of thermal conductance. The hot-electron nanobolometers, sufficiently
sensitive for registering single THz photons, are very promising for
submillimeter astronomy and other applications based on quantum calorimetry and
photon counting.Comment: 19 pages, 3 color figure
Three "universal" mesoscopic Josephson effects
1. Introduction
2. Supercurrent from Excitation Spectrum
3. Excitation Spectrum from Scattering Matrix
4. Short-Junction Limit
5. Universal Josephson Effects
5.1 Quantum Point Contact
5.2 Quantum Dot
5.3 Disordered Point Contact (Average supercurrent, Supercurrent
fluctuations)Comment: 21 pages, 2 figures; legacy revie
Construction et classification de certaines solutions algébriques des systèmes de Garnier
22 pagesInternational audienceIn this paper, we classify all (complete) non elementary algebraic solutions of Garnier systems that can be constructed by Kitaev's method: they are deduced from isomonodromic deformations defined by pulling back a given fuchsian equation E by a family of ramified covers. We first introduce orbifold structures associated to a fuchsian equation. This allow to get a refined version of Riemann-Hurwitz formula and then to promtly deduce that E is hypergeometric. Then, we can bound exponents and degree of the pull-back maps and further list all possible ramification cases. This generalizes a result due to C. Doran for the Painleve VI case. We explicitely construct one of these solutions
Full Counting Statistics of Superconductor--Normal-Metal Heterostructures
The article develops a powerful theoretical tool to obtain the full counting
statistics. By a slight extension of the standard Keldysh method we can access
immediately all correlation functions of the current operator. Embedded in a
quantum generalization of the circuit theory of electronic transport, we are
able to study the full counting statistics of a large class of two-terminal
contacts and multi-terminal structures, containing superconductors and normal
metals as elements. The practical use of the method is demonstrated in many
examples.Comment: 35 pages, contribution to "Quantum Noise", ed. by Yu.V. Nazarov and
Ya.M. Blanter, minor changes in text, references adde
Finite temperature phase diagram of a polarised Fermi condensate
The two-component Fermi gas is the simplest fermion system displaying
superfluidity, and as such finds applications ranging from the theory of
superconductivity to QCD. Ultracold atomic gases provide an exceptionally clean
realization of this system, where the interatomic interaction and the atom
species population are both independent, tuneable parameters. This allows one
to investigate the Fermi gas with imbalanced spin populations, which had
previously been experimentally elusive, and this prospect has stimulated much
theoretical activity. Here we show that the finite temperature phase diagram
contains a region of phase separation between the superfluid and normal states
that touches the boundary of second-order superfluid transitions at a
tricritical point, reminiscent of the phase diagram of He-He mixtures.
A variation of interaction strength then results in a line of tricritical
points that terminates at zero temperature on the molecular Bose-Einstein
condensate (BEC) side. On this basis, we argue that tricritical points will
play an important role in the recent experiments on polarised atomic Fermi
gases.Comment: 6 pages, 4 figures. Manuscript extended and figures modified. For
final version, see Nature Physic
Nonlinear Sigma Model for Disordered Media: Replica Trick for Non-Perturbative Results and Interactions
In these lectures, given at the NATO ASI at Windsor (2001), applications of
the replicas nonlinear sigma model to disordered systems are reviewed. A
particular attention is given to two sets of issues. First, obtaining
non-perturbative results in the replica limit is discussed, using as examples
(i) an oscillatory behaviour of the two-level correlation function and (ii)
long-tail asymptotes of different mesoscopic distributions. Second, a new
variant of the sigma model for interacting electrons in disordered normal and
superconducting systems is presented, with demonstrating how to reduce it,
under certain controlled approximations, to known ``phase-only'' actions,
including that of the ``dirty bosons'' model.Comment: 25 pages, Proceedings of the NATO ASI "Field Theory of Strongly
Correlated Fermions and Bosons in Low - Dimensional Disordered Systems",
Windsor, August, 2001; to be published by Kluwe
Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures
Two-dimensional semiconductors - atomic layers of materials with covalent
intra-layer bonding and weak (van der Waals or quadrupole) coupling between the
layers - are a new class of materials with great potential for optoelectronic
applications. Among those, a special position is now being taken by
post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been
found that the band gap in 2D crystals of InSe more than doubles in the
monolayer compared to thick multilayer crystals, while the high mobility of
conduction band electrons is promoted by their light in-plane mass. Here, we
use Raman and PL measurements of encapsulated few layer samples, coupled with
accurate atomic force and transmission electron microscope structural
characterisation to reveal new optical properties of atomically thin GaSe
preserved by hBN encapsulation. The band gaps we observe complement the
spectral range provided by InSe films, so that optical activity of these two
almost lattice-matched PTMC films and their heterostructures densely cover the
spectrum of photons from violet to infrared. We demonstrate the realisation of
the latter by the first observation of interlayer excitonic photoluminescence
in few-layer InSe-GaSe heterostructures. The spatially indirect transition is
direct in k-space and therefore is bright, while its energy can be tuned in a
broad range by the number of layers.Comment: 8 pages 4 figure
Evidence of Majorana fermions in an Al - InAs nanowire topological superconductor
Majorana fermions are the only fermionic particles that are expected to be
their own antiparticles. While elementary particles of the Majorana type were
not identified yet, quasi-particles with Majorana like properties, born from
interacting electrons in the solid, were predicted to exist. Here, we present
thorough experimental studies, backed by numerical simulations, of a system
composed of an aluminum superconductor in proximity to an indium arsenide
nanowire, with the latter possessing strong spin-orbit coupling. An induced 1d
topological superconductor - supporting Majorana fermions at both ends - is
expected to form. We concentrate on the characteristics of a distinct zero bias
conductance peak (ZBP), and its splitting in energy, both appearing only with a
small magnetic field applied along the wire. The ZBP was found to be robustly
tied to the Fermi energy over a wide range of system parameters. While not
providing a definite proof of a Majorana state, the presented data and the
simulations support strongly its existence
High resolution nuclear magnetic resonance spectroscopy of highly-strained quantum dot nanostructures
Much new solid state technology for single-photon sources, detectors,
photovoltaics and quantum computation relies on the fabrication of strained
semiconductor nanostructures. Successful development of these devices depends
strongly on techniques allowing structural analysis on the nanometer scale.
However, commonly used microscopy methods are destructive, leading to the loss
of the important link between the obtained structural information and the
electronic and optical properties of the device. Alternative non-invasive
techniques such as optically detected nuclear magnetic resonance (ODNMR) so far
proved difficult in semiconductor nano-structures due to significant
strain-induced quadrupole broadening of the NMR spectra. Here, we develop new
high sensitivity techniques that move ODNMR to a new regime, allowing high
resolution spectroscopy of as few as 100000 quadrupole nuclear spins. By
applying these techniques to individual strained self-assembled quantum dots,
we measure strain distribution and chemical composition in the volume occupied
by the confined electron. Furthermore, strain-induced spectral broadening is
found to lead to suppression of nuclear spin magnetization fluctuations thus
extending spin coherence times. The new ODNMR methods have potential to be
applied for non-invasive investigations of a wide range of materials beyond
single nano-structures, as well as address the task of understanding and
control of nuclear spins on the nanoscale, one of the central problems in
quantum information processing
- …