50 research outputs found
Quantum Ratchets at High Temperatures
Using the continued-fraction method we solve the Caldeira-Leggett master
equation in the phase-space (Wigner) representation to study Quantum ratchets.
Broken spatial symmetry, irreversibility and periodic forcing allows for a net
current in these systems. We calculate this current as a function of the force
under adiabatic conditions. Starting from the classical limit we make the
system quantal. In the quantum regime tunnel events and over-barrier wave
reflection phenomena modify the classical result. Finally, using the
phase-space formalism we give some insights about the decoherence in these
systems.Comment: submitted to Physia E (proceedings of conference "Frontiers of
Quantum and Mesoscopic Thermodynamics", Prague 26-29 July 2004
Thermodynamic perturbation theory for dipolar superparamagnets
Thermodynamic perturbation theory is employed to derive analytical
expressions for the equilibrium linear susceptibility and specific heat of
lattices of anisotropic classical spins weakly coupled by the dipole-dipole
interaction. The calculation is carried out to the second order in the coupling
constant over the temperature, while the single-spin anisotropy is treated
exactly. The temperature range of applicability of the results is, for weak
anisotropy (A/kT << 1), similar to that of ordinary high-temperature
expansions, but for moderately and strongly anisotropic spins (A/kT > 1) it can
extend down to the temperatures where the superparamagnetic blocking takes
place (A/kT \sim 25), provided only the interaction strength is weak enough.
Besides, taking exactly the anisotropy into account, the results describe as
particular cases the effects of the interactions on isotropic (A = 0) as well
as strongly anisotropic (A \to \infty) systems (discrete orientation model and
plane rotators).Comment: 15 pages, 3 figure
Sentinel-3 MWR Microwave Radiometer – Our contribution to the success of the Copernicus programme
Revista oficial de la Asociación Española de Teledetección[EN] The MWR builds, together with the SRAL altimeter, the S3 topography mission. The MWR, developed by EADS CASA Espacio as prime contractor, provides information for tropospheric path correction of SRAL measurements. MWR data can also be used for determining surface emissivity and soil moisture over land, surface energy budget investigations and ice characterization. The MWR instrument is a Noise Injection Radiometer (NIR), working at two frequencies (23.8/36.5 GHz), embarking a dual frequency horn antenna pointing to the cold sky for embedded autonomous calibration.[ES] El radiómetro MWR, desarrollado por EADS CASA Espacio, constituye la misión topográfica de Sentinel-3 (S3) junto al radar altímetro SRAL. Su función es la entrega de información auxiliar para la corrección troposférica de las medidas del SRAL. Sus datos también pueden servir para caracterizar magnitudes físicas del suelo y la criosfera, así como para la evaluación de balances energéticos. El MWR es un radiómetro de inyección de ruido (NIR) que funciona a dos frecuencias (23,8 y 36,5 GHz) e incorpora una bocina para la obtención de la temperatura de brillo del cielo como medio de calibración autónoma.Los autores querrían agradecer el apoyo y la contribución por parte de los clientes del MWR (ESA y TAS-F), así como de los principales subcontratistas, TAS-E, CRISA, TAS-I, CDE, RYMSA, COBHAM, HVS, ABSL, APCO, IABG e ITD.Palacios, M.; Bergadà, M.; González, R.; Gamonal, A.; García-Fernández, J.; Camacho, Y. (2014). El radiómetro de microondas MWR de Sentinel-3: Nuestra contribución al éxito de Copernicus. Revista de Teledetección. (41):79-88. doi:10.4995/raet.2014.2294.SWORD798841Aguirre, M., Berruti, B., Bezy, J.L., Drinkwater, M., Heliere, F., Klein, U., Mavrocordatos, C., Silvestrin, P., Greco, B., Benveniste, J., 2007. Sentinel-3. The Ocean and Medium-Resolution Land Mission for GMES Operational Services. ESA bulletin, 131, 24-29.Guijarro, J., Auriol, A., Costes, M., Jayles, C., Vincent, P., 2000. MWR & DORIS-Supporting Envisat Radar Altimetry Mission, ESA bulletin, 104, 41-46.L'Abbate, M., Bombaci, O., Alberti, G., Femenias, P., 2003. ENVISAT-1 MWR, in flight functional and performance verification. En: Proceedings of Envisat Validation Workshop, Frascati (Italy).Xie, Y., Chen, J., Liu, D., Lv, C., Liu, K., & Miao, J. (2013). DEVELOPMENT AND CALIBRATION OF A K-BAND GROUND-BASED HYPERSPECTRAL MICROWAVE RADIOMETER FOR WATER VAPOR MEASUREMENTS. Progress In Electromagnetics Research, 140, 415-438. doi:10.2528/pier1305070
Thermally assisted magnetization reversal in the presence of a spin-transfer torque
We propose a generalized stochastic Landau-Lifshitz equation and its
corresponding Fokker-Planck equation for the magnetization dynamics in the
presence of spin transfer torques. Since the spin transfer torque can pump a
magnetic energy into the magnetic system, the equilibrium temperature of the
magnetic system is ill-defined. We introduce an effective temperature based on
a stationary solution of the Fokker-Planck equation. In the limit of high
energy barriers, the law of thermal agitation is derived. We find that the
N\'{e}el-Brown relaxation formula remains valid as long as we replace the
temperature by an effective one that is linearly dependent of the spin torque.
We carry out the numerical integration of the stochastic Landau-Lifshitz
equation to support our theory. Our results agree with existing experimental
data.Comment: 5 figure
Peculiarities of the stochastic motion in antiferromagnetic nanoparticles
Antiferromagnetic (AFM) materials are widely used in spintronic devices as
passive elements (for stabilization of ferromangetic layers) and as active
elements (for information coding). In both cases switching between the
different AFM states depends in a great extent from the environmental noise. In
the present paper we derive the stochastic Langevin equations for an AFM vector
and corresponding Fokker-Planck equation for distribution function in the phase
space of generalised coordinate and momentum. Thermal noise is modeled by a
random delta-correlated magnetic field that interacts with the dynamic
magnetisation of AFM particle. We analyse in details a particular case of the
collinear compensated AFM in the presence of spin-polarised current. The energy
distribution function for normal modes in the vicinity of two equilibrium
states (static and stationary) in sub- and super-critical regimes is found. It
is shown that the noise-induced dynamics of AFM vector has pecuilarities
compared to that of magnetisation vector in ferromagnets.Comment: Submitted to EPJ ST, presented at the 4-th Conference on Statistical
Physics, Lviv, Ukraine, 201
Langevin Simulation of Thermally Activated Magnetization Reversal in Nanoscale Pillars
Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model
incorporating thermal fluctuations and dipole-dipole interactions (calculated
by the Fast Multipole Method) are presented for systems composed of nanoscale
iron pillars of dimension 9 nm x 9 nm x 150 nm. Hysteresis loops generated
under sinusoidally varying fields are obtained, while the coercive field is
estimated to be 1979 14 Oe using linear field sweeps at T=0 K. Thermal
effects are essential to the relaxation of magnetization trapped in a
metastable orientation, such as happens after a rapid reversal of an external
magnetic field less than the coercive value. The distribution of switching
times is compared to a simple analytic theory that describes reversal with
nucleation at the ends of the nanomagnets. Results are also presented for
arrays of nanomagnets oriented perpendicular to a flat substrate. Even at a
separation of 300 nm, where the field from neighboring pillars is only 1
Oe, the interactions have a significant effect on the switching of the magnets.Comment: 19 pages RevTeX, including 12 figures, clarified discussion of
numerical technique
Evolution of the use of corticosteroids for the treatment of hospitalised COVID-19 patients in Spain between March and November 2020: SEMI-COVID national registry
Objectives: Since the results of the RECOVERY trial, WHO recommendations about the use of corticosteroids (CTs) in COVID-19 have changed. The aim of the study is to analyse the evolutive use of CTs in Spain during the pandemic to assess the potential influence of new recommendations. Material and methods: A retrospective, descriptive, and observational study was conducted on adults hospitalised due to COVID-19 in Spain who were included in the SEMI-COVID- 19 Registry from March to November 2020. Results: CTs were used in 6053 (36.21%) of the included patients. The patients were older (mean (SD)) (69.6 (14.6) vs. 66.0 (16.8) years; p < 0.001), with hypertension (57.0% vs. 47.7%; p < 0.001), obesity (26.4% vs. 19.3%; p < 0.0001), and multimorbidity prevalence (20.6% vs. 16.1%; p < 0.001). These patients had higher values (mean (95% CI)) of C-reactive protein (CRP) (86 (32.7-160) vs. 49.3 (16-109) mg/dL; p < 0.001), ferritin (791 (393-1534) vs. 470 (236- 996) µg/dL; p < 0.001), D dimer (750 (430-1400) vs. 617 (345-1180) µg/dL; p < 0.001), and lower Sp02/Fi02 (266 (91.1) vs. 301 (101); p < 0.001). Since June 2020, there was an increment in the use of CTs (March vs. September; p < 0.001). Overall, 20% did not receive steroids, and 40% received less than 200 mg accumulated prednisone equivalent dose (APED). Severe patients are treated with higher doses. The mortality benefit was observed in patients with oxygen saturation </=90%. Conclusions: Patients with greater comorbidity, severity, and inflammatory markers were those treated with CTs. In severe patients, there is a trend towards the use of higher doses. The mortality benefit was observed in patients with oxygen saturation </=90%
Epidemiological trends of HIV/HCV coinfection in Spain, 2015-2019
Altres ajuts: Spanish AIDS Research Network; European Funding for Regional Development (FEDER).Objectives: We assessed the prevalence of anti-hepatitis C virus (HCV) antibodies and active HCV infection (HCV-RNA-positive) in people living with HIV (PLWH) in Spain in 2019 and compared the results with those of four similar studies performed during 2015-2018. Methods: The study was performed in 41 centres. Sample size was estimated for an accuracy of 1%. Patients were selected by random sampling with proportional allocation. Results: The reference population comprised 41 973 PLWH, and the sample size was 1325. HCV serostatus was known in 1316 PLWH (99.3%), of whom 376 (28.6%) were HCV antibody (Ab)-positive (78.7% were prior injection drug users); 29 were HCV-RNA-positive (2.2%). Of the 29 HCV-RNA-positive PLWH, infection was chronic in 24, it was acute/recent in one, and it was of unknown duration in four. Cirrhosis was present in 71 (5.4%) PLWH overall, three (10.3%) HCV-RNA-positive patients and 68 (23.4%) of those who cleared HCV after anti-HCV therapy (p = 0.04). The prevalence of anti-HCV antibodies decreased steadily from 37.7% in 2015 to 28.6% in 2019 (p < 0.001); the prevalence of active HCV infection decreased from 22.1% in 2015 to 2.2% in 2019 (p < 0.001). Uptake of anti-HCV treatment increased from 53.9% in 2015 to 95.0% in 2019 (p < 0.001). Conclusions: In Spain, the prevalence of active HCV infection among PLWH at the end of 2019 was 2.2%, i.e. 90.0% lower than in 2015. Increased exposure to DAAs was probably the main reason for this sharp reduction. Despite the high coverage of treatment with direct-acting antiviral agents, HCV-related cirrhosis remains significant in this population