432 research outputs found
Vortex matter in mesoscopic superconductors
Superconducting mesoscopic devices in magnetic fields present novel
properties which can only be accounted for by both the quantum confinement of
the Cooper pairs and by the interaction between the magnetic-field-induced
vortices. Sub-micrometer disks, much the same as their semiconductor
counterparts known as quantum dots, are being subject to experimental
investigation by measuring their conducting properties and, more recently,
their magnetization by using state-of-the-art ballistic Hall magnetometry. In
this work I review the main results obtained in these two types of experiments
as well as the current theoretical developments which are contributing to our
understanding of the superconducting condensate in these systems.Comment: 16 pages, 4 figures. Invited presentation at the 13th International
Conference on High Magnetic Fields in Semiconductor Physics to appear in
Physica
Electronic structure and transport properties of atomic NiO spinvalves
Ab-initio quantum transport calculations show that short NiO chains suspended
in Ni nanocontacts present a very strong spin-polarization of the conductance.
The generalized gradient approximation we use here predicts a similiar
polarization of the conductance as the one previously computed with non-local
exchange, confirming the robustness of the result. Their use as nanoscopic
spinvalves is proposed.Comment: 2 pages, 1 figure; accepted in JMMM (Proceedings of ICM'06, Kyoto
Electronic transport and vibrational modes in the smallest molecular bridge: H2 in Pt nanocontacts
We present a state-of-the-art first-principles analysis of electronic
transport in a Pt nanocontact in the presence of H2 which has been recently
reported by Smit et al. in Nature 419, 906 (2002). Our results indicate that at
the last stages of the breaking of the Pt nanocontact two basic forms of bridge
involving H can appear. Our claim is, in contrast to Smit et al.'s, that the
main conductance histogram peak at G approx 2e^2/h is not due to molecular H2,
but to a complex Pt2H2 where the H2 molecule dissociates. A first-principles
vibrational analysis that compares favorably with the experimental one also
supports our claim .Comment: 5 pages, 3 figure
Robust grid adaptation for efficient uncertainty quantification
In the recent past, adjoint methods have been successfully applied in error estimation of integral outputs (functionals) of the numerical solution of partial differential equations. The adjoint solution can also be used as a grid adaptation indicator, with the objective of optimally targeting and reducing the numerical error in the functional of interest below a prespecified threshold. In situations where we seek to quantify the effect of aleatory uncertainties on statistical moments of the output functional, it becomes necessary to evaluate the functional accurately at multiple sample points in probability space. If the numerical accuracy of these sample evaluations is not uniform, variations in the numerical error can affect the evaluation of the statistical moments. Although it is possible to independently adapt the meshes to obtain more accurate solutions at each sample point in stochastic space, such a procedure can be both cumbersome and computationally expensive. To improve the efficiency of this process, a new robust grid adaptation technique is proposed that is aimed at minimizing the numerical error over a range of variations of the uncertain parameters of interest about a nominal state. Using this approach, it is possible to generate computational grids that are insensitive to small variations of the uncertain parameters that can both locally and globally change the solution and, as a result, the error distribution. This is in contrast with classical adjoint techniques, which seek to adapt the grid with the aim of minimizing numerical errors for a specific flow condition (and geometry). It is demonstrated that flow computations on these robust grids result in low numerical errors under the expected range of variations of the uncertain input parameters. The effectiveness of this strategy is demonstrated in problems involving the Poisson equation and the Euler equations at transonic and supersonic/hypersonic speeds
Correlation Effects on Transport Through Few-Electrons Systems
We study lateral tunneling through a quantum box including electron-electron
interactions in the presence of a magnetic field which breaks single particle
degeneracies. The conductance at zero temperature as a function of the Fermi
energy in the leads consists of a set of peaks related to changing by one the
electron occupancy in the box. We find that the position and heights of the
peaks are controlled by many-body effects. We compute the conductance up to 8
electrons for several cases where correlation effects dominate. In the range of
intermediate fields spin selection rules quench some peaks. At low and high
fields the behavior of the conductance as a function of the number of electrons
is very different due to big changes in the many-body ground state
wavefunctions.Comment: 9 pages, 2 postscript figures, Latex 3.1
Dependence of the vortex configuration on the geometry of mesoscopic flat samples
The influence of the geometry of a thin superconducting sample on the
penetration of the magnetic field lines and the arrangement of vortices are
investigated theoretically. We compare superconducting disks, squares and
triangles with the same surface area having nonzero thickness. The coupled
nonlinear Ginzburg-Landau equations are solved self-consistently and the
important demagnetization effects are taken into account. We calculate and
compare quantities like the free energy, the magnetization, the Cooper-pair
density, the magnetic field distribution and the superconducting current
density for the three geometries. For given vorticity the vortex lattice is
different for the three geometries, i.e. it tries to adapt to the geometry of
the sample. This also influences the stability range of the different vortex
states. For certain magnetic field ranges we found a coexistence of a giant
vortex placed in the center and single vortices toward the corners of the
sample. Also the H-T phase diagram is obtained.Comment: 9 pages, 17 figures (submitted to Phys. Rev. B
Canted phase in double quantum dots
We perform a Hartree-Fock calculation in order to describe the ground state
of a vertical double quantum dot in the absence of magnetic fields parallel to
the growth direction. Intra- and interdot exchange interactions determine the
singlet or triplet character of the system as the tunneling is tuned. At finite
Zeeman splittings due to in-plane magnetic fields, we observe the continuous
quantum phase transition from ferromagnetic to symmetric phase through a canted
antiferromagnetic state. The latter is obtained even at zero Zeeman energy for
an odd electron number.Comment: 5 pages, 3 figure
Superconducting properties of mesoscopic cylinders with enhanced surface superconductivity
The superconducting state of an infinitely long superconducting cylinder
surrounded by a medium which enhances its superconductivity near the boundary
is studied within the nonlinear Ginzburg-Landau theory. This enhancement can be
due to the proximity of another superconductor or due to surface treatment.
Quantities like the free energy, the magnetization and the Cooper-pair density
are calculated. Phase diagrams are obtained to investigate how the critical
field and the critical temperature depend on this surface enhancement for
different values of the Ginzburg-Landau parameter \kappa. Increasing the
superconductivity near the surface leads to higher critical fields and critical
temperatures. For small cylinder diameters only giant vortex states nucleate,
while for larger cylinders multivortices can nucleate. The stability of these
multivortex states also depends on the surface enhancement. For type-I
superconductors we found the remarkable result that for a range of values of
the surface extrapolation length the superconductor can transit from the
Meissner state into superconducting states with vorticity L > 1. Such a
behaviour is not found for the case of large \kappa, i.e. type-II
superconductivity.Comment: submitted to Phys. Rev.
Revealing hidden clonal complexity in Mycobacterium tuberculosis infection by qualitative and quantitative improvement of sampling
AbstractThe analysis of microevolution events, its functional relevance and impact on molecular epidemiology strategies, constitutes one of the most challenging aspects of the study of clonal complexity in infection by Mycobacterium tuberculosis. In this study, we retrospectively evaluated whether two improved sampling schemes could provide access to the clonal complexity that is undetected by the current standards (analysis of one isolate from one sputum). We evaluated in 48 patients the analysis by mycobacterial interspersed repetitive unit–variable number tandem repeat of M. tuberculosis isolates cultured from bronchial aspirate (BAS) or bronchoalveolar lavage (BAL) and, in another 16 cases, the analysis of a higher number of isolates from independent sputum samples. Analysis of the isolates from BAS/BAL specimens revealed clonal complexity in a very high proportion of cases (5/48); in most of these cases, complexity was not detected when the isolates from sputum samples were analysed. Systematic analysis of isolates from multiple sputum samples also improved the detection of clonal complexity. We found coexisting clonal variants in two of 16 cases that would have gone undetected in the analysis of the isolate from a single sputum specimen. Our results suggest that analysis of isolates from BAS/BAL specimens is highly efficient for recording the true clonal composition of M. tuberculosis in the lungs. When these samples are not available, we recommend increasing the number of isolates from independent sputum specimens, because they might not harbour the same pool of bacteria. Our data suggest that the degree of clonal complexity in tuberculosis has been underestimated because of the deficiencies inherent in a simplified procedure
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