4,346 research outputs found
BCS theory for finite size superconductors
We study finite size effects in superconducting metallic grains and determine
the BCS order parameter and the low energy excitation spectrum in terms of
size, and shape of the grain. Our approach combines the BCS self-consistency
condition, a semiclassical expansion for the spectral density and interaction
matrix elements, and corrections to the BCS mean-field. In chaotic grains
mesoscopic fluctuations of the matrix elements lead to a smooth dependence of
the order parameter on the excitation energy. In the integrable case we observe
shell effects when e.g. a small change in the electron number leads to large
changes in the energy gap.Comment: 4 pages, 2 figures, journal versio
Multi-resonant scatterers in sonic crystals: Locally multi-resonant acoustic metamaterial
An acoustic metamaterial made of a two-dimensional (2D) periodic array of multi-resonant acoustic scatterers is analyzed both experimentally and theoretically. The building blocks consist of a combination of elastic beams of low-density polyethylene foam (LDPF) with cavities of known area. Elastic resonances of the beams and acoustic resonances of the cavities can be excited by sound producing several attenuation peaks in the low frequency range. Due to this behavior the periodic array with long wavelength multi-resonant structural units can be classified as a locally multi-resonant acoustic metamaterial (LMRAM) with strong dispersion of its effective properties. The results presented in this paper could be used to design effective tunable acoustic filters for the low frequency range. (C) 2012 Elsevier Ltd. All rights reserved.This work was supported by MCI Secretaria de Estado de Investigacion (Spanish government) and FEDER funds, under Grants MAT2009-09438 and MTM2009-14483-C02-02. V.R.G. is grateful for the support of "Programa de Contratos Post-Doctorales con Movilidad UPV (CEI-01-11)". A.K. and O.U. are grateful for the support of EPSRC (UK) through research Grant EP/E063136/1.Romero García, V.; Krynkin, A.; García-Raffi, LM.; Umnova, O.; Sánchez Pérez, JV. (2013). Multi-resonant scatterers in sonic crystals: Locally multi-resonant acoustic metamaterial. Journal of Sound and Vibration. 332(1):184-198. doi:10.1016/j.jsv.2012.08.003S184198332
Structural and magnetic properties of amorphous Co-W alloyed nanoparticles
18 páginas, 20 figuras, 4 tablas.-- PACS number(s): 75.75.Fk, 75.50.Kj, 75.30.Gw, 61.46.Df.-- et al.et al.W-capped Co nanoparticles dispersed in an alumina matrix are studied by means of high-resolution transmission electron microscopy, extended x-ray absorption fine structure, SQUID-based magnetic measurements, dc magnetization, ac magnetic susceptibility, and x-ray magnetic circular dichroism. Results show the formation of amorphous Co-W alloy nanoparticles, the magnetic properties of which are modified by the amount of W or Co present in the samples. The average Co magnetic moment depends on the number of W atoms surrounding it. Co-W particles show superparamagnetic behavior and are described as an array of noninteracting particles with random anisotropy axes and an average moment per particle proportional to the particle volume and to the average Co moment for each alloy composition. Values of the magnetic anisotropy constant of the particles are on the order of 106 erg/cm3, higher than that of bulk Co. Evidence of short-range ordering within each amorphous particle is found that provides insight of the origin of their magnetic anisotropy.The financial
support of MICINN-FEDER MAT08/1077 and Aragonese
IMANA project is acknowledged. A.I.F. acknowledges a
CSIC JAE2008-Predoc grant.Peer reviewe
Structural instability of vortices in Bose-Einstein condensates
In this paper we study a gaseous Bose-Einstein condensate (BEC) and show
that: (i) A minimum value of the interaction is needed for the existence of
stable persistent currents. (ii) Vorticity is not a fundamental invariant of
the system, as there exists a conservative mechanism which can destroy a vortex
and change its sign. (iii) This mechanism is suppressed by strong interactions.Comment: 4 pages with 3 figures. Submitted to Phys. Rev. Let
The Bose–Hubbard model with squeezed dissipation
The stationary properties of the Bose–Hubbard model under squeezed dissipation are investigated. The dissipative model does not possess aU (1) symmetry but conserves parity. We find that 〈a j 〉 = 0 always holds, so no symmetry breaking occurs. Without the onsite repulsion, the linear case is known to be critical. At the critical point the system freezes to an EPR state with infinite two mode entanglement. We show here that the correlations are rapidly destroyed whenever the repulsion is switched on. As we increase the latter, the system approaches a thermal state with an effective temperature defined in terms of the squeezing parameter in the dissipators. We characterize this transition by means of a Gutzwiller ansatz and the Gaussian Hartree–Fock–Bogoliubov approximation
Using the new technologies in claudication patients: CReTe: Claudication patients and telematic register
Objetivo: Presentamos una aplicación informática para teléfonos móviles ideada para el control de sesiones
de ejercicio domiciliario en claudicantes. Exponemos también nuestros resultados preliminares.
Material y métodos: La aplicación está dise˜nada para teléfonos móviles con sistema operativo Android,
tecnología de geolocalización y conexión de datos. Está conectada a una base de datos codificada que
preserva el secreto médico. Los datos que se recogen de las sesiones de ejercicio son la fecha, la hora de
inicio y de fin, el tiempo dedicado, el número de las paradas realizadas y la velocidad máxima, la
velocidad media global y la velocidad entre paradas. Durante el mes de enero de 2014 se reclutó a
aquellos pacientes que aceptaran el estudio, poseyeran y entendieran un dispositivo móvil Android con
geolocalización y conexión de datos. Los criterios de exclusión del estudio una vez comenzado fueron
progresión hacia isquemia arterial crítica, enfermedad intercurrente grave, fallecimiento y ausencia de
comunicación con el paciente. Se recogieron las principales morbilidades y los resultados de los
cuestionarios de calidad de vida SF36 y EuroQol 5D. Se midió el tiempo de primera consulta y el tiempo
dedicado por semana y paciente durante un seguimiento de 28 días. Se calculó el cumplimiento de las
sesiones con respecto a las sesiones pactadas con cada paciente, con un mínimo de 2 semanales.
Resultados: Un total de 5 pacientes fueron incluidos, con edad media de 59,3 a˜nos (mediana 52; rango
40-80). De ellos 2 se excluyeron, uno por progresión a isquemia crítica y otro por ausencia de
comunicación. El tiempo medio dedicado por consulta nueva fue de 29,1 min (mediana 27,5; rango
45-20). El cumplimiento de las sesiones fue del 100% en 2 pacientes y del 63% en otro. El tiempo dedicado
en el seguimiento semanal por paciente fue de 1,68 min.
Conclusiones: CReTe es una herramienta aplicable a nuestro medio que precisa de estudios mayores para
demostrar su utilidad.Objective: The preliminary results are presented on the use of a new application for mobile phones designed to control home exercise sessions in claudication.Materials and methods: The application is designed for Android systems with geolocation tech-nology and Internet connection. It is connected to a database encoded to ensure medical confidentiality. The data collected from the exercise sessions were the date, start time and end time, the time spent, the number of stops made, and the maximum speed, average speed and overall speed between stops. During the month of January 2014 patients who accepted and understood the study, and possessed an Android mobile device with geolocation and data connection, were enrolled. The exclusion criteria, 11 the study started, were progression to arterial ischemia, severe intercurrent illness, non-communication with the patient, and death. Major morbidities and results of quality of life questionnaires SF36 and EuroQol 5 D were collec-ted. Time of first consultation and time spent per week per patient during a 28-day follow-up was recorded. Compliance as regards the agreed sessions was calculated for each patient, with a minimum of 2 weeks.Results: The study included 5 patients with a mean age 59.3 years (median 52; range 40-80). Of these, 2 were excluded, one for progression to critical ischemia and another for non-communication. The average time spent in a new consultation was 29.1 minutes (median 27.5, range 45-20). The compliance with the sessions was 100% in 2 patients, and 63% in another. Time spent in the weekly monitoring per patient was 1.68 minutes.Conclusions: CReTe is an applicable tool in our environment that requires further study to demonstrate its usefulness
Dynamics of Symmetry Breaking and Tachyonic Preheating
We reconsider the old problem of the dynamics of spontaneous symmetry
breaking using 3d lattice simulations, and develop a theory of tachyonic
preheating, which occurs due to the spinodal instability of the scalar field.
Tachyonic preheating is so efficient that symmetry breaking typically completes
within a single oscillation of the field distribution as it rolls towards the
minimum of its effective potential. As an application of this theory we
consider preheating in the hybrid inflation scenario, including SUSY-motivated
F-term and D-term inflationary models. We show that preheating in hybrid
inflation is typically tachyonic and the stage of oscillations of a homogeneous
component of the scalar fields driving inflation ends after a single
oscillation. Our results may also be relevant for the theory of the formation
of disoriented chiral condensates in heavy ion collisions.Comment: 7 pages, 6 figures. Higher quality figures and computer generated
movies in gif format illustrating our results can be found at
http://physics.stanford.edu/gfelder/hybri
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