Kubo formula for Floquet states and photoconductivity oscillations in a 2D electron gas

The recent discovery of the microwave induced vanishing resistance states in a two dimensional electron system (2DES) is an unexpected and surprising phenomena. In these experiments the magnetoresistance of a high mobility 2DES under the influence of microwave radiation of frequency $\omega$ at moderate values of the magnetic field, exhibits strong oscillations with zero-resistance states (ZRS) governed by the ratio $\omega /\omega_c$, where $\omega_c$ is the cyclotron frequency. In this work we present a model for the photoconductivity of a two dimensional electron system (2DES) subjected to a magnetic field. The model includes the microwave and Landau contributions in a non-perturbative exact way, impurity scattering effects are treated perturbatively. In our model, the Landau-Floquet states act coherently with respect to the oscillating field of the impurities, that in turn induces transitions between these levels. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. We study the effects of both short-range and long-range disorder on the photoconductivity. Our calculation yields a magnetoresistance oscillatory behavior with the correct period and phase. It is found that, in agreement with experiment, negative dissipation can only be induced in very high mobility samples. We analyze the dependence of the results on the microwave power and polarization. For high-intensity radiation multi-photon processes take place predicting new negative-resistance states centered at $\omega / \omega_c=1/2$, and $\omega / \omega_c= 3/2$.Comment: Final version, accepted for publication in Phys. Rev.

Symmetry breaking as the origin of zero-differential resistance states of a 2DEG in strong magnetic fields

Zero resistance differential states have been observed in two-dimensional electron gases (2DEG) subject to a magnetic field and a strong dc current. In a recent work we presented a model to describe the nonlinear transport regime of this phenomenon. From the analysis of the differential resistivity and the longitudinal voltage we predicted the formation of negative differential resistivity states, although these states are known to be unstable. Based on our model, we derive an analytical approximated expression for the Voltage-Current characteristics, that captures the main elements of the problem. The result allow us to construct an energy functional for the system. In the zero temperature limit, the system presents a quantum phase transition, with the control parameter given by the magnetic field. It is noted that above a threshold value ($B>B_{th}$), the symmetry is spontaneously broken. At sufficiently high magnetic field and low temperature the model predicts a phase with a non-vanishing permanent current; this is a novel phase that has not been observed so far.Comment: 6 pages, 2 figure

Simulación de acelerogramas compatibles con espectros de energía sísmica

En este artículo se presenta un estudio sobre el espectro de energía inducida en las estructuras sometidas a acciones sísmicas. En primer lugar se discuten las variables que influyen en dicho espectro, con base en las relaciones que guarda el mismo con otras medidas de la acción sísmica. En segundo lugar, se presenta un procedimiento que permite estimar un espectro suavizado de diseño, basado en la regresión sobre los registros de las dos componentes horizontales de 56 sismos diferentes. Finalmente, se describe un método para simular acelerogramas compatibles con un espectro de energía prescrito y se presenta un ejemplo que ilustra la utilidad del método para la evaluación del daño sísmico.A study on the spectrum of seismic energy input in structures is presented. The factors that determine this spectrum are first dicussed, upon the basis of the relationships linking it with other characteristics of earthquake excitation. A procedure for estimating a desigp smoothed spectrum based upon a regression analysis of the two horizontal components of 56 strong motion records is then presented. Finally, a method for simulating accelerograms compatible with . a specified energy-input spectrum is described, together with an example which shows the applicability of the method to damage evaluation purposes.Peer Reviewe

Análisis y evaluación de la condición física en estudiantes de educación primaria de un medio rural y urbano

El objetivo de este trabajo ha sido analizar y evaluar la condición física de escolares estudiantes de educación primaria para conocer la diferencia que existe entre los alumnos que viven en un medio rural y los que viven en un medio urbano. Para ello se seleccionaron 253 estudiantes de segundo y tercer ciclo de educación primaria. La muestra fue clasificada según el tipo de colegio en dos grupos, urbano y rural. A todos los estudiantes se les realizó una valoración antropométrica (talla, masa, IMC, ratio CC), evaluación del salto vertical (CMJ), flexibilidad (test Sit and Reach), fuerza (dinamometría manual) y resistencia (test course na­vette). Los resultados muestran diferencias a nivel antropométrico donde los alumnos del colegio rural han presentado mayores índices de sobrepeso. En cambio, los resultados obtenidos en la evaluación de la condición física muestran como en las pruebas de fuerza, resistencia y flexibilidad los alumnos del colegio rural obtienen mayores resultados que los alumnos del colegio urbano, mientras que en el salto vertical los alumnos rurales presentan mayores valores

Quantum Fluids in Thermodynamic Geometry

In this work, the Thermodynamic Geometry (TG) of quantum fluids (QF) is analyzed. We present results for two models. The first one is a quantum hard-sphere fluid (QHS) whose Helmholtz free energy is obtained from Path Integrals Monte Carlo simulations (PIMC). It is found that due to quantum contributions in the thermodynamic potential, the anomaly found in TG for the classical hard-sphere fluid related to the sign of the scalar curvature, is now avoided in a considerable region of the thermodynamic space. The second model is a semi-classical square-well fluid (QSW), described by a quantum hard-sphere repulsive interaction coupled with a classical attractive square-well contribution. Behavior of the semi-classical curvature scalar as a function of the thermal de Broglie wavelength $\lambda_B$ is analyzed for several attractive-potential ranges, and description of the semi-classical R-Widom lines defined by the maxima of the curvature scalar, are also obtained and compared with classical results for different square-well ranges.Comment: 9 pages, 8 figure

Anàlisi i avaluació de la condició física en estudiants d’educació primària de medis rural i urbà

L’objectiu d’aquest treball ha estat analitzar i avaluar la condició física d’escolars estudiants d’educació primària per conèixer la diferència que hi ha entre els alumnes que viuen en un medi rural i els que viuen en un medi urbà. Amb aquesta finalitat es van seleccionar 253 estudiants de segon i tercer cicle d’educació primària. La mostra va ser classificada segons el tipus de col·legi en dos grups, urbà i rural. A tots els estudiants es va fer una valoració antropomètrica (talla, massa, IMC, ràtio CC), avaluació del salt vertical (CMJ), flexibilitat (prova de flexió de tronc d’assegut), força (dinamometria manual) i resistència (test cursa de llançadora). Els resultats mostren diferències antropomètriques on els alumnes del col·legi rural han presentat majors índexs de sobreprès. En canvi, els resultats obtinguts en l’avaluació de la condició física mostren com en les proves de força, resistència i flexibilitat els alumnes del col·legi rural obtenen resultats superiors que els alumnes del col·legi urbà, mentre que en el salt vertical els alumnes rurals presenten valors més alts

A consistency test of white dwarf and main sequence ages: NGC 6791

NGC 6791 is an open cluster that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (∼8 Gyr) and the age derived from the cut-off of the white dwarf luminosity function (∼6 Gyr) were found to be significantly different. Here we demonstrate that the origin of this age discrepancy lies in an incorrect evaluation of the white dwarf cooling ages, and we show that when the relevant physical separation processes are included in the calculation of white dwarf sequences both ages are coincident.Facultad de Ciencias Astronómicas y Geofísica

A consistency test of white dwarf and main sequence ages: NGC 6791

NGC 6791 is an open cluster that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (∼8 Gyr) and the age derived from the cut-off of the white dwarf luminosity function (∼6 Gyr) were found to be significantly different. Here we demonstrate that the origin of this age discrepancy lies in an incorrect evaluation of the white dwarf cooling ages, and we show that when the relevant physical separation processes are included in the calculation of white dwarf sequences both ages are coincident.Facultad de Ciencias Astronómicas y Geofísica