Majorana Zero Modes in Graphene

A clear demonstration of topological superconductivity (TS) and Majorana zero modes remains one of the major pending goal in the field of topological materials. One common strategy to generate TS is through the coupling of an s-wave superconductor to a helical half-metallic system. Numerous proposals for the latter have been put forward in the literature, most of them based on semiconductors or topological insulators with strong spin-orbit coupling. Here we demonstrate an alternative approach for the creation of TS in graphene/superconductor junctions without the need of spin-orbit coupling. Our prediction stems from the helicity of graphene's zero Landau level edge states in the presence of interactions, and on the possibility, experimentally demonstrated, to tune their magnetic properties with in-plane magnetic fields. We show how canted antiferromagnetic ordering in the graphene bulk close to neutrality induces TS along the junction, and gives rise to isolated, topologically protected Majorana bound states at either end. We also discuss possible strategies to detect their presence in graphene Josephson junctions through Fraunhofer pattern anomalies and Andreev spectroscopy. The latter in particular exhibits strong unambiguous signatures of the presence of the Majorana states in the form of universal zero bias anomalies. Remarkable progress has recently been reported in the fabrication of the proposed type of junctions, which offers a promising outlook for Majorana physics in graphene systems.Comment: 14 pages, 8 figures. Included simulations of Andreev spectroscopy and mor

Dielectric Function of Diluted Magnetic Semiconductors in the Infrared Regime

We present a study of the dielectric function of metallic (III,Mn)V diluted magnetic semiconductors in the infrared regime. Our theoretical approach is based on the kinetic exchange model for carrier induced (III,Mn)V ferromagnetism. The dielectric function is calculated within the random phase approximation and, within this metallic regime, we treat disorder effects perturbatively and thermal effects within the mean field approximation. We also discuss the implications of this calculations on carrier concentration measurements from the optical f-sum rule and the analysis of plasmon-phonon coupled modes in Raman spectra.Comment: 6 pages, 6 figures include

Photoassisted sequential resonant tunneling through superlattices

We have analyzed theoretically the photoassisted tunneling current through a superlattice in the presence of an AC potential. For that purpose we have developed a new model to calculate the sequential resonant currrent trhough a superlattice based in the TRansfer Hamiltonian Method. The tunneling current presents new features due to new effective tunneling chanels coming from the photoside bands induced by the AC field. Our theoretical results are in good agreement with the available experimental evidence.Comment: Revtex 3.0 4 pages, 4 figures uuencoded compressed tar-fil

Nonlinear stochastic discrete drift-diffusion theory of charge fluctuations and domain relocation times in semiconductor superlattices

A stochastic discrete drift-diffusion model is proposed to account for the effects of shot noise in weakly coupled, highly doped semiconductor superlattices. Their current-voltage characteristics consist of a number stable multistable branches corresponding to electric field profiles displaying two domains separated by a domain wall. If the initial state corresponds to a voltage on the middle of a stable branch and a sudden voltage is switched so that the final voltage corresponds to the next branch, the domains relocate after a certain delay time. Shot noise causes the distribution of delay times to change from a Gaussian to a first passage time distribution as the final voltage approaches that of the end of the first current branch. These results agree qualitatively with experiments by Rogozia {\it et al} (Phys. Rev. B {\bf 64}, 041308(R) (2001)).Comment: 9 pages, 12 figures, 2 column forma

Finite-frequency counting statistics of electron transport: Markovian Theory

We present a theory of frequency-dependent counting statistics of electron transport through nanostructures within the framework of Markovian quantum master equations. Our method allows the calculation of finite-frequency current cumulants of arbitrary order, as we explicitly show for the second- and third-order cumulants. Our formulae generalize previous zero-frequency expressions in the literature and can be viewed as an extension of MacDonald's formula beyond shot noise. When combined with an appropriate treatment of tunneling, using, e.g. Liouvillian perturbation theory in Laplace space, our method can deal with arbitrary bias voltages and frequencies, as we illustrate with the paradigmatic example of transport through a single resonant level model. We discuss various interesting limits, including the recovery of the fluctuation-dissipation theorem near linear response, as well as some drawbacks inherent of the Markovian description arising from the neglect of quantum fluctuations.Comment: Accepted in New Journal of Physics. Updated tex

Structures and stabilities of Aln+, Aln, and Aln− (n=13–34) clusters

Producción CientíficaPutative global minima of neutral (Aln) and singly charged (Aln+ and Aln−) aluminum clusters with n = 13–34 have been located from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ the generalized gradient approximation of Perdew, Burke, and Ernzerhof to describe exchange-correlation electronic effects. Our results show that icosahedral growth dominates the structures of aluminum clusters for n = 13–22. For n = 23–34, there is a strong competition between decahedral structures, relaxed fragments of a fcc crystalline lattice (some of them including stacking faults), and hexagonal prismatic structures. For such small cluster sizes, there is no evidence yet for a clear establishment of the fcc atomic packing prevalent in bulk aluminum. The global minimum structure for a given number of atoms depends significantly on the cluster charge for most cluster sizes. An explicit comparison is made with previous theoretical results in the range n = 13–30: for n = 19, 22, 24, 25, 26, 29, 30 we locate a lower energy structure than previously reported. Sizes n = 32, 33 are studied here for the first time by an ab initio technique

La ecuación x2/1+ X2/2+...+ x2/n= k mod p

Al estudiar la existencia de soluciones enteras de una ecuación algebraíca con coeficientes enteros es natural utilizar argumentos de divisibilidad. Este enfoque, además de ser estimulante es, en muchos casos, suficiente para determinar el problema

Supporting autonomy in physical education: Perception versus reality.

Esta investigación se centró, por una parte en la percepción de apoyo a la autonomía en las clases de Educación física, que tienen tanto el alumnado como el profesorado, y por otra, en la coherencia entre percepción y la realidad observada en las clases. Se aplicaron dos escalas al profesorado y al alumnado con el objeto de conocer con qué frecuencia se producen comportamientos instructivos favorables al desarrollo de la autonomía, y se observaron y grabaron en video clases impartidas por el profesorado. Los resultados muestran que la frecuencia de los comportamientos instructivos de apoyo a la autonomía durante la intervención de enseñanza es significativamente menor que la percibida por el alumnado y el profesorado. La realidad observada muestra un perfil de profesorado con gran margen de mejora en comportamientos como: calidad de la comunicación, funcionalidad de las tareas, fomento del pensamiento del alumnado, situaciones en las que el alumnado pueda expresar su opinión sobre las tareas y motivación intrínsecaThis investigation, on the one hand it focuses on students´ and teachers´ perception about autonomy support during Physical Education instruction, and on the other hand, in the coherence between perception and reality observed in the classes. Two scales were administered to determine the frequency of instructional behaviors favoring the development of autonomy, and teacher´s classes were observed and videotaped. The results show that the observed frequency of instructional behavior favoring autonomy support during classes is significantly lower than that perceived by students and teachers. The observed reality reveals a teacher profile with room for improvement in behaviors such as: communication quality, task functionality, promotion of students’ thinking, creation of situations in which students can express their opinions of the tasks, and increasing intrinsic motivationEl presente artículo forma parte del estudio realizado con la financiación del proyecto SEJ2007-672687/EDU del Ministerio de Ciencia e Innovació