Influence of spin filtering and spin mixing on the subgap structure of I-V characteristics in superconducting quantum point contact

The effect of spin filtering and spin mixing on the dc electric current for voltage biased magnetic quantum point contact with superconducting leads is theoretically studied. The I-V characteristics are calculated for the whole range of spin filtering and spin mixing parameters. It is found that with increasing of spin filtering the subharmonic step structure of the dc electric current, typical for low-transparency junction and junction without considerable spin filtering qualitatively changes. In the lower voltage region and for small enough spin mixing the peak structure arises. When spin mixing increases the peak subgap structure evolves to the step structure. The voltages where subharmonic gap features are located are found to be sensitive to the value of spin filtering. The positions of peaks and steps are calculated analytically and the evolution of the subgap structure from well-known tunnel limit to the large spin filtering case is explained in terms of multiple Andreev reflection (MAR) processes. In particular, it is found that for large spin filtering the subgap feature at $eV_k$ arises from $2k^{\rm th}$ and $(2k\pm 1)^{\rm th}$ order MAR processes, while in the tunnel limit the step at $eV_n$ is known to result from $n^{\rm th}$ order MAR process.Comment: 9 pages, 3 figure

Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions

The behavior of dc subgap current in magnetic quantum point contact is discussed for the case of low-transparency junction with different tunnel probabilities for spin-up ($D_\uparrow$) and spin-down ($D_\downarrow$) electrons. Due to the presence of Andreev bound states $\pm \epsilon_0$ in the system the positions of subgap electric current steps $eV_n = (\Delta \pm \epsilon_0)/n$ are split at temperature $T \neq 0$ with respect to the nonmagnetic result $eV_n=2\Delta/n$. It is found that under the condition $D_\uparrow \neq D_\downarrow$ the spin current also manifests subgap structure, but only for odd values of $n$. The split steps corresponding to $n=1,2$ in subgap electric and spin currents are analytically calculated and the following steps are described qualitatively.Comment: 4 pages, 1 figure, minor stylistic changes, journal-ref adde

The asymmetric single-impurity Anderson model - the modified perturbation theory

We investigate the single-impurity Anderson model by means of the recently introduced modified perturbation theory. This approximation scheme yields reasonable results away from the symmetric case. The agreement with exactly known results for the symmetric case is checked, and results for the non-symmetric case are presented. With decreasing conduction band occupation, the breakdown of the screening of the local moment is observed. In the crossover regime between Kondo limit and mixed-valence regime, an enhanced zero-temperature susceptibility is found.Comment: 7 pages, 7 figures, to appear in Physica

Dynamical Coulomb blockade of multiple Andreev reflections

We analyze the dynamical Coulomb blockade of multiple Andreev reflections (MAR) in a superconducting quantum point contact coupled to a macroscopic impedance. We find that at very low transmission the blockade scales as $n^2$ with $n = {Int}(2\Delta/eV)$, where $V$ is the bias voltage and $\Delta$ is the superconducting gap, as it would correspond to the occurrence of "shots" of charge $ne$. For higher transmission the blockade is reduced both due to Pauli principle and to elastic renormalization of the MAR probability, and for certain voltage regions it may even become an "antiblockade", i.e. the current is enhanced due to the coupling with the electromagnetic environment.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let

Nonequilibrium Transport in Superconductor/Ferromagnet/Superconductor Diffusive Junctions: Interplay between Proximity Effect and Ferromagnetism

The theory of the I-V characteristics in diffusive superconductor/weak ferromagnet/superconductor (SFS) junction is developed. We show that the exchange field $h$ of the ferromagnet manifests itself as an additional conductance peak at $eV \sim \Delta+h$ in the phase-coherent regime, when the Thouless energy is of the order of superconducting order parameter. The excess current exhibits non-monotonous dependence on the exchange field and non-trivial temperature behavior, which is strongly influenced by the temperature dependence of the exchange field.Comment: 5 pages, 2 figure

Sobre el origen de las razas caprinas españolas

Aún hoy, transcurridos más de 50 años desde que Aparicio Sánchez estableciera el origen de las razas caprinas españolas, su hipótesis está llena de contenido y actualidad. Las tres formas prehistóricas propuestas por este autor como troncos originarios, la Capra Prisca, la Capra Aegagrus y otra forma desconocida de procedencia africana son asumidas, así como las variedades Alpina y Pirenaica de la cabra Aegagrus anteriormente reconocida por Sanson (1903) y que fueron desarrolladas por este autor para el ámbito español. En este trabajo, matizamos las convergencias y divergencias con los autores antes mencionados, se reconoce al Íbice africano como posible forma prehistórica que intervino en la formación del sustrato a partir del cual se formaron nuestras razas y añadimos los posibles orígenes de razas actuales que no estaban caracterizadas ni reconocidas hace 50 años (Verata, Retinta Cacereña, Florida, Canaria, etc), si bien aún, no es posible determinar los orígenes de algunas de ellas ante los escasos conocimientos que se tienen de sus caracteres étnicos

The breed concept. A epistemological approach

It is realized an approximation of the knowledge of a breed"s notion, revising its causality and the aspects that affect its conceptualization. For this, the breed’s taxonomical position and its relation with the other levels is studied, the principal anthropogenic and natural mechanisms are analyzed which, from the primitive wild substrates, generated the domestic breeds which are understood today. Lastly, when considering the processes that led to formation, their acceptance and the definitions that the different authors have made, it is considered as the best definition the following: The breeds are populations that are distinguished by a group of exteriorly visible characters, which are genetically determined and that have differentiated from others of the same species throughout the historical process, taking into account that these have originated and localized in a determined area with a common environment.Realizamos una aproximación al conocimiento de la noción de raza, revisando su causalidad y los aspectos que afectan a su conceptualización. Para ello, se estudia la posición taxonómica de la raza y su relación con los otros niveles, se analizan los principales mecanismos antropógenos y naturales que, desde los substratos primitivos salvajes, generaron a las razas domésticas tal como hoy día se entienden. Por último, ante la observación de los procesos que llevaron a la formación y al reconocimiento de las mismas y ante las definiciones que los diferentes autores han dado, se considera como la definición más adecuada la siguiente: Las razas son poblaciones que se distinguen por un conjunto de caracteres visibles exteriormente, que están determinados genéticamente y que se han diferenciado de otras de la misma especie a lo largo de proceso histórico, teniendo en cuenta que se han originado y localizado en un área determinada con un ambiente común

Influence of the Carthusian strain in the andalusian horses

In this work the evolution of the Carthusian influence has been analysed in 33.486 Andalusian horses (P.R.E.) registered in the Stud Book from its opening in the year 1912 until 1990. It has been able to verify that the maximum number of the Carthusian predecessors, in a given generation level is reflected, in the form of greater percentage of influence, in three subsequent generations. The level of Carthusian influence in the set of the P.R.E. horses population has been increased in the successive generation intervals, being produced the four increases more meaningful in successive decades, from the years 30 until the years 70, with a total increase of the 55.28 percent of influence. In summary, more than 70 percent of the P.R.E. reproducers posses some Carthusian forefather between their registered antecedent in the Stud Book, being included in the level of the 80-100 percent of Carthusian blood the greater group of P.R.E. reproducers.En este trabajo se ha analizado la evolución de la influencia Cartujana en los 33.486 caballos de P.R.E. inscritos en el Stud Book hasta el año 1990. Se ha podido constatar que el máximo número de antecesores Cartujanos en un determinado nivel generacional se refleja, en forma de mayor porcentaje de influencia, en tres generaciones posteriores. El nivel de influencia Cartujana en el conjunto de la población de caballos de P.R.E. se ha visto incrementada en los sucesivos intervalos generacionales, produciéndose los cuatro incrementos más significativos en décadas sucesivas, desde los años 30 hasta los años 70, con un incremento total del 55,28 p.100 de influencia. En suma, más del 70 p.100 de la población de reproductores del P.R.E., posee algún antepasado Cartujano entre sus antecesores inscritos en el Libro genealógico, incluyéndose en el nivel del 80-100 p.100 de sangre Cartujana el mayor grupo de animales reproductores de la población del P.R.E

Genetic conservation index in the Carthusian strain of the andalusian horses

In this work we have analysed the evolution of the effective number of the population base or founding of the Carthusian strain of the Spanish Pure Breed Horse (P.R.E.). The Stud Book has supplied the necessary information to constitute the data base formed with the set of Carthusian horses (2673 animals registered up to 1991). Concerning the base or founding population of the Carthusian strain, being 220 the number of animals that integrates the foundational group. At present and due to the selection policy of the individuals more significants, as well as to the type of line inbreeding employed within a strain with a low effective number of reproducers, only exist effective genetic representation of 22 antecedent (11 sires and 11 mares). The mean Genetic Conservation Index, seems have arrived to a stabilisation phase after a gradual increasing throughout the successive generations analysed. The interval 1980-1990 is emphasised, then takes a value of 14.23 the one which can be considered a good conservation level of those genetic lines that they were constituted by the group of founding ancestors.En este trabajo se ha analizado la evolución del número efectivo de la población base o fundadora de la estirpe Cartujana del caballo de P.R.E. El Libro Genealógico de la raza ha suministrado la información necesaria para constituir la base de datos formada por los 2.673 caballos Cartujanos inscritos hasta el año 1991. En cuanto a la población base o fundadora de la estirpe Cartujana, está constituida por aquellos antecesores que cierran los pedigríes de los animales que forman la estirpe. Según nuestros resultados es de 220 el número de animales que integra el grupo fundacional. En la actualidad y debido a la política de utilización exhaustiva de los individuos más emblemáticos y al tipo de endocría en línea empleada dentro de una estirpe con un bajo número efectivo de reproductores, sólo existe representación genética efectiva de 22 antecesores (11 sementales y 11 yeguas). El Índice de Conservación Genético medio parece haber llegado a una fase de estabilización después de un paulatino incremento a lo largo de los sucesivos periodos generacionales, siendo en el último intervalo analizado (1980-1990) de 14,23