629 research outputs found
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 arises from and
order MAR processes, while in the tunnel limit the step at
is known to result from 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 of the ferromagnet manifests itself as an additional
conductance peak at 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 () and spin-down ()
electrons. Due to the presence of Andreev bound states in the
system the positions of subgap electric current steps are split at temperature with respect to the
nonmagnetic result . It is found that under the condition
the spin current also manifests subgap
structure, but only for odd values of . The split steps corresponding to
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
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
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
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