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

Resonant tunneling through a small quantum dot coupled to superconducting leads

We address the problem of non-linear transport through discrete electronic levels in a small quantum dot coupled to superconducting electrodes. In our approach the low temperature I-V characteristics can be calculated including all multiple quasi-particle and Andreev processes. The limit of very weak coupling to the leads and large charging energies is briefly analyzed comparing the calculated lineshapes of the I-V curves with recent experimental results. When the coupling to the leads increases and Coulomb blockade effects can be neglected, the combination of multiple Andreev processes and resonant transmission gives rise to a rich subgap structure which largely differs from the one found in the more studied S-N-S systems. We show how multiple processes can be included within a simple sequential tunneling picture qualitatively explaining the subgap structure. We suggest an experimental set-up where the predicted effects could be observed.Comment: 11 pages, 4 postscript figures, to be published in Phys. Rev. B (rapid communications

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

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

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

Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot

Tunneling spectroscopy of a Nb coupled carbon nanotube quantum dot reveals the formation of pairs of Andreev bound states (ABS) within the superconducting gap. A weak replica of the lower ABS is found, which is generated by quasi-particle tunnelling from the ABS to the Al tunnel probe. An inversion of the ABS-dispersion is observed at elevated temperatures, which signals the thermal occupation of the upper ABS. Our experimental findings are well supported by model calculations based on the superconducting Anderson model.Comment: 6 pages, 7 figure

Evaluation of cell culture in microfluidic chips for application in monoclonal antibody production

Microfluidic chips are useful devices for cell culture that allow cell growth under highly controlled conditions, as is required for production of therapeutic recombinant proteins. To understand the optimal conditions for growth of cells amenable of recombinant protein expression in these devices,we culturedHEK-293T cells under different microfluidic experimental conditions. The cells were cultured in polymethyl methacrylate (PMMA) and polydi-methylsiloxane (PDMS)microdevices, in the absence or presence of the cell adhesion agent poly-D-lysine. Different microchannel geometries and thicknesses, as well as the influence of the flow rate have also been tested, showing their great influence in cell adhesion and growth. Results show that the presence of poly-D-lysine improves the adhesion and viability of the cells in continuous or discontinuous flow. Moreover, the optimal adhesion of cells was observed in the corners of themicrochannels, as well as in wide channels possibly due to the decrease in the flow rate in these areas. These studies provide insight into the optimal architecture of microchannels for long-term culture of adherent cells in order to use microfluidics devices as bioreactors for monoclonal antibodies production.Fil: Peñaherrera Pazmiño, Ana Belén. Universidad Tecnológica Nacional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Payés, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; ArgentinaFil: Sierra Rodero, Marina. Universidad Tecnológica Nacional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Vega, M.. Universidad Tecnológica Nacional; ArgentinaFil: Rosero, G.. Universidad Tecnológica Nacional; Argentina. Universidad de Buenos Aires; ArgentinaFil: Lerner, Betiana. Universidad Tecnológica Nacional; Argentina. Universidad de Buenos Aires; ArgentinaFil: Helguera, Gustavo Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; ArgentinaFil: Pérez, M. S.. Universidad Tecnológica Nacional; Argentina. Universidad de Buenos Aires; Argentin

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