442 research outputs found
Superconducting and charge-density wave instabilities in ultrasmall-radius carbon nanotubes
We perform a detailed analysis of the band structure, phonon dispersion, and
electron-phonon coupling of three types of small-radius carbon nanotubes
(CNTs): (5,0), (6,0), and (5,5) with diameters 3.9, 4.7, and 6.8 \AA
respectively. The large curvature of the (5,0) CNTs makes them metallic with a
large density of states at the Fermi energy. The density of states is also
strongly enhanced for the (6,0) CNTs compared to the results obtained from the
zone-folding method. For the (5,5) CNTs the electron-phonon interaction is
dominated by the in-plane optical phonons, while for the ultrasmall (5,0) and
(6,0) CNTs the main coupling is to the out-of-plane optical phonon modes. We
calculate electron-phonon interaction strengths for all three types of CNTs and
analyze possible instabilities toward superconducting and charge-density wave
phases. For the smallest (5,0) nanotube, in the mean-field approximation and
neglecting Coulomb interactions, we find that the charge-density wave
transition temperature greatly exceeds the superconducting one. When we include
a realistic model of the Coulomb interaction we find that the charge-density
wave is suppressed to very low temperatures, making superconductivity dominant
with the mean-field transition temperature around one K. For the (6,0) nanotube
the charge-density wave dominates even with the inclusion of Coulomb
interactions and we find the mean-field transition temperature to be around
five Kelvin. We find that the larger radius (5,5) nanotube is stable against
superconducting and charge-density wave orders at all realistic temperatures.Comment: 5 pages. 2 figure
Jet-Medium Interactions with Identified Particles
Identified particles have long been of great interest at RHIC in large part
because of the baryon/meson differences observed at intermediate and the
implications for hadronization via quark coalescence. With recent high
statistics data identified particles are also now central to understanding the
details of the jet-medium interactions and energy loss and hadron formation at
intermediate and high . In particular, high identified particle
spectra along with two-particle correlations triggered with direct photons,
neutral pions or electrons from heavy flavor decay with hadrons can provide
information about how medium modifications to jet fragmentation depend on
parton type. I will review recent results with identified particles both in
heavy ion systems and the reference measurements in p+p collisions.Comment: Proceedings for Quark Matter 2009, Knoxville T
Microscopic self-consistent theory of Josephson junctions including dynamical electron correlations
We formulate a fully self-consistent, microscopic model to study the
retardation and correlation effects of the barrier within a Josephson junction.
The junction is described by a series of planes, with electronic correlation
included through a local self energy for each plane. We calculate current-phase
relationships for various junctions, which include non-magnetic impurities in
the barrier region, or an interfacial scattering potential. Our results
indicate that the linear response of the supercurrent to phase across the
barrier region is a good, but not exact indicator of the critical current. Our
calculations of the local density of states show the current-carrying Andreev
bound states and their energy evolution with the phase difference across the
junction.
We calculate the figure of merit for a Josephson junction, which is the
product of the critical current, Ic, and the normal state resistance, R(N), for
junctions with different barrier materials. The normal state resistance is
calculated using the Kubo formula, for a system with zero current flow and no
superconducting order. Semiclassical calculations would predict that these two
quantities are determined by the transmission probabilities of electrons in
such a way that the product is constant for a given superconductor at fixed
temperature. Our self-consistent solutions for different types of barrier
indicate that this is not the case. We suggest some forms of barrier which
could increase the Ic.R(N) product, and hence improve the frequency response of
a Josephson device.Comment: 46 pages, 21 figure
The VMC survey - XV : The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations
Date of Acceptance: 19/03/2015We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t/yr) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t/yr) ≤ 9.4 - while two excesses of clusters are seen at log(t/yr) ~9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ~7.3, but it also hosts some of intermediate ages, log(t/yr) ~9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t/yr) < 9.0, which could have been induced by interactions with the LMC.Peer reviewedFinal Accepted Versio
Phase-slip induced dissipation in an atomic Bose-Hubbard system
Phase slips play a primary role in dissipation across a wide spectrum of
bosonic systems, from determining the critical velocity of superfluid helium to
generating resistance in thin superconducting wires. This subject has also
inspired much technological interest, largely motivated by applications
involving nanoscale superconducting circuit elements, e.g., standards based on
quantum phase-slip junctions. While phase slips caused by thermal fluctuations
at high temperatures are well understood, controversy remains over the role of
phase slips in small-scale superconductors. In solids, problems such as
uncontrolled noise sources and disorder complicate the study and application of
phase slips. Here we show that phase slips can lead to dissipation for a clean
and well-characterized Bose-Hubbard (BH) system by experimentally studying
transport using ultra-cold atoms trapped in an optical lattice. In contrast to
previous work, we explore a low velocity regime described by the 3D BH model
which is not affected by instabilities, and we measure the effect of
temperature on the dissipation strength. We show that the damping rate of
atomic motion-the analogue of electrical resistance in a solid-in the confining
parabolic potential fits well to a model that includes finite damping at zero
temperature. The low-temperature behaviour is consistent with the theory of
quantum tunnelling of phase slips, while at higher temperatures a cross-over
consistent with the transition to thermal activation of phase slips is evident.
Motion-induced features reminiscent of vortices and vortex rings associated
with phase slips are also observed in time-of-flight imaging.Comment: published in Nature 453, 76 (2008
Spectroscopy of thulium and holmium heavily doped tellurite glasses
In this study, we report spectroscopic properties of Tm3þ and Ho3þ codoped tellurite glasses over a wide dopant concentration range in order to assess their potential laser performance under 790 nm diode laser excitation. The impact of Tm3þ and Ho3þ concentrations is investigated to identify specific candidates for fiber laser operation. The emission cross section is calculated and discussed, as well as the gain coefficient of this type of glasses. Energy transfer microparameters and critical ion distances are determined for 3H4, 3F4 (Tm3þ), and 5I7 (Ho3þ) emission levels in the framework of diffusionlimited regime and dipole-dipole interaction. We also report thermal properties of tested glasse
The Role of Dwarf Galaxy Interactions in Shaping the Magellanic System and Implications for Magellanic Irregulars
We present a novel pair of numerical models of the interaction history
between the Large and Small Magellanic Clouds (LMC and SMC, respectively) and
our Milky Way (MW) in light of recent high precision proper motions
(Kallivayalil et al. 2006a,b). Given the new velocities, cosmological
simulations of structure formation favor a scenario where the Magellanic Clouds
(MCs) are currently on their first infall towards our Galaxy (Boylan-Kolchin et
al. 2011, Busha et al. 2011). We illustrate here that the observed irregular
morphology and internal kinematics of the MCs (in gas and stars) are naturally
explained by interactions between the LMC and SMC, rather than gravitational
interactions with the MW. This picture further supports a first infall scenario
(Besla et a. 2007). In particular, we demonstrate that the Magellanic Stream, a
band of HI gas trailing behind the MCs 150 degrees across the sky, can be
accounted for by the action of LMC tides on the SMC before the system was
accreted by the MW. We further demonstrate that the off-center, warped stellar
bar of the LMC and its one-armed spiral, can be naturally explained by a recent
direct collision with the SMC. Such structures are key morphological
characteristics of a class of galaxies referred to as Magellanic Irregulars (de
Vaucouleurs & Freeman 1972), the majority of which are not associated with
massive spiral galaxies. We infer that dwarf-dwarf galaxy interactions are
important drivers for the morphological evolution of Magellanic Irregulars and
can dramatically affect the efficiency of baryon removal from dwarf galaxies
via the formation of extended tidal bridges and tails. Such interactions are
important not only for the evolution of dwarf galaxies but also have direct
consequences for the buildup of baryons in our own MW, as LMC-mass systems are
believed to be the dominant building blocks of MW-type halos.Comment: 33 pages, 21 figures, Accepted for publication in MNRAS, Dec 23 201
STEP: the VST survey of the SMC and the Magellanic Bridge - I : Overview and first results
STEP (the SMC in Time: Evolution of a Prototype interacting late-type dwarf galaxy) is a Guaranteed Time Observation survey being performed at the VST (the ESO VLT Survey Telescope). STEP will image an area of 74 sq. deg. covering the main body of the Small Magellanic Cloud (32 sq. deg.), the Bridge that connects it to the Large Magellanic Cloud (30 sq. deg.) and a small part of the Magellanic Stream (2 sq. deg.). Our g, r, i, Hα photometry is able to resolve individual stars down to magnitudes well below the main-sequence turn-off of the oldest populations. In this first paper, we describe the observing strategy, the photometric techniques and the upcoming data products of the survey. We also present preliminary results for the first two fields for which data acquisition is completed, including some detailed analysis of the two stellar clusters IC 1624 and NGC 419.Peer reviewedFinal Accepted Versio
Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions
The rise of topological phases of matter is strongly connected to their
potential to host Majorana bound states, a powerful ingredient in the search
for a robust, topologically protected, quantum information processing. In order
to produce such states, a method of choice is to induce superconductivity in
topological insulators. The engineering of the interplay between
superconductivity and the electronic properties of a topological insulator is a
challenging task and it is consequently very important to understand the
physics of simple superconducting devices such as Josephson junctions, in which
new topological properties are expected to emerge. In this article, we review
recent experiments investigating topological superconductivity in topological
insulators, using microwave excitation and detection techniques. More
precisely, we have fabricated and studied topological Josephson junctions made
of HgTe weak links in contact with two Al or Nb contacts. In such devices, we
have observed two signatures of the fractional Josephson effect, which is
expected to emerge from topologically-protected gapless Andreev bound states.
We first recall the theoretical background on topological Josephson junctions,
then move to the experimental observations. Then, we assess the topological
origin of the observed features and conclude with an outlook towards more
advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017,
published in "Topological Matter. Springer Series in Solid-State Sciences,
vol 190. Springer
Free Energy of an Inhomogeneous Superconductor: a Wave Function Approach
A new method for calculating the free energy of an inhomogeneous
superconductor is presented. This method is based on the quasiclassical limit
(or Andreev approximation) of the Bogoliubov-de Gennes (or wave function)
formulation of the theory of weakly coupled superconductors. The method is
applicable to any pure bulk superconductor described by a pair potential with
arbitrary spatial dependence, in the presence of supercurrents and external
magnetic field. We find that both the local density of states and the free
energy density of an inhomogeneous superconductor can be expressed in terms of
the diagonal resolvent of the corresponding Andreev Hamiltonian, resolvent
which obeys the so-called Gelfand-Dikii equation. Also, the connection between
the well known Eilenberger equation for the quasiclassical Green's function and
the less known Gelfand-Dikii equation for the diagonal resolvent of the Andreev
Hamiltonian is established. These results are used to construct a general
algorithm for calculating the (gauge invariant) gradient expansion of the free
energy density of an inhomogeneous superconductor at arbitrary temperatures.Comment: REVTeX, 28 page
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