231 research outputs found
Cooper pair delocalization in disordered media
We discuss the effect of disorder on the coherent propagation of the bound
state of two attracting particles. It is shown that a result analogous to the
Anderson theorem for dirty superconductors is also valid for the Cooper
problem, namely, that the pair wave function is extended beyond the
single-particle localization length if the latter is large. A physical
justification is given in terms of the Thouless block-scaling picture of
localization. These arguments are supplemented by numerical simulations. With
increasing disorder we find a transition from a regime in which the interaction
delocalizes the pair to a regime in which the interaction enhances
localization.Comment: 5 pages, RevTex with 2 figures include
Reusing the NCBO BioPortal technology for agronomy to build AgroPortal
Many vocabularies and ontologies are produced to represent and annotate agronomic data. By reusing the NCBO BioPortal technology, we have already designed and implemented an advanced prototype ontology repository for the agronomy domain. We plan to turn that prototype into a real service to the community. The AgroPortal project aims at reusing the scientific outcomes and experience of the biomedical domain in the context of plant, agronomic, food, environment (perhaps animal) sciences. We offer an ontology portal which features ontology hosting, search, versioning, visualization, comment, recommendation, enables semantic annotation, as well as storing and exploiting ontology alignments. All of these within a fully semantic web compliant infrastructure. The AgroPortal specifically pays attention to respect the requirements of the agronomic community in terms of ontology formats (e.g., SKOS, trait dictionaries) or supported features. In this paper, we present our prototype as well as preliminary outputs of four driving agronomic use cases. With the experience acquired in the biomedical domain and building atop of an already existing technology, we think that AgroPortal offers a robust and stable reference repository that will become highly valuable for the agronomic domain
Effect of thermal phase fluctuations on the superfluid density of two-dimensional superconducting films
High precision measurements of the complex sheet conductivity of
superconducting Mo77Ge23 thin films have been made from 0.4 K through Tc. A
sharp drop in the inverse sheet inductance, 1/L(T), is observed at a
temperature, Tc, which lies below the mean-field transition temperature, Tco.
Just below Tc, the suppression of 1/L(T) below its mean-field value indicates
that longitudinal phase fluctuations have nearly their full classical
amplitude, but they disappear rapidly as T decreases. We argue that there is a
quantum crossover at about 0.94 Tco, below which classical phase fluctuations
are suppressed.Comment: 14 pages, 3 figures. Subm. to PR
Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition
The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and
one In/InOx film have been measured in zero field using a two-coil mutual
inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to
have three contributions: the inductive superfluid, renormalized by nonvortex
phase fluctuations; conventional vortex-antivortex pairs, whose contribution
turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii
unbinding temperature; and an anomalous contribution. The latter is
predominantly resistive, persists well below the KTB temperature, and is weakly
dependent on frequency down to remarkably low frequencies, at least 100 Hz. It
increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about
half the energy to create a vortex-antivortex pair, indicating that the
frequency dependence is that of individual excitations, rather than critical
behavior.Comment: 10 pages, 10 figs; subm PR
Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit
We report on transport and tunneling measurements performed on ultra-thin
Pb/Ag (strong coupled superconductor/normal metal) multilayers evaporated by
quench condensation. The critical temperature and energy gap of the
heterostructures oscillate with addition of each layer, demonstrating the
validity of the Cooper limit model in the case of multilayers. We observe
excellent agreement with a simple theory for samples with layer thickness
larger than 30\AA . Samples with single layers thinner than 30\AA deviate from
the Cooper limit theory. We suggest that this is due to the "inverse proximity
effect" where the normal metal electrons improve screening in the
superconducting ultrathin layer and thus enhance the critical temperature.Comment: 4 pages, 4 figure
Infrared Studies of the Onset of Conductivity in Ultra-Thin Pb Films
In this paper we report the first experimental measurement of the infrared
conductivity of ultra-thin quenched-condensed Pb films. For dc sheet
resistances such that the ac conductance increases with
frequency but is in disagreement with the predictions of weak localization. We
attribute this behavior to the effects of an inhomogeneous granular structure
of these films, which is manifested at the very small probing scale of infrared
measurements. Our data are consistent with predictions of two-dimensional
percolation theory.Comment: Submitted to Physical Review Letter
Effect of granularity on the insulator-superconductor transition in ultrathin Bi films
We have studied the insulator-superconductor transition (IST) by tuning the
thickness in quench-condensed films. The resistive transitions of the
superconducting films are smooth and can be considered to represent
"homogeneous" films. The observation of an IST very close to the quantum
resistance for pairs, on several substrates supports
this idea. The relevant length scales here are the localization length, and the
coherence length. However, at the transition, the localization length is much
higher than the superconducting coherence length, contrary to expectation for a
"homogeneous" transition. This suggests the invalidity of a purely fermionic
model for the transition. Furthermore, the current-voltage characteristics of
the superconducting films are hysteretic, and show the films to be granular.
The relevant energy scales here are the Josephson coupling energy and the
charging energy. However, Josephson coupling energies () and the charging
energies () at the IST, they are found to obey the relation .
This is again contrary to expectation, for the IST in a granular or
inhomogeneous, system. Hence, a purely bosonic picture of the transition is
also inconsistent with our observations. We conclude that the IST observed in
our experiments may be either an intermediate case between the fermioinc and
bosonic mechanisms, or in a regime of charge and vortex dynamics for which a
quantitative analysis has not yet been done.Comment: accepted in Physical Review
Theoretical Study on Superconductivity in Boron-Doped Diamond
We consider superconductivity in boron (B) doped diamond using a simplified
model for the valence band of diamond. We treat the effects of substitutional
disorder of B ions by the coherent potential approximation (CPA) and those of
the attractive force between holes by the ladder approximation under the
assumption of instantaneous interaction with the Debye cutoff. We thereby
calculate the quasiparticle life time, the evolution of the single-particle
spectra due to doping, and the effect of disorder on the superconducting
critical temperature . We in particular compare our results with those for
supercell calculations to see the role of disorder, which turns out to be of
crucial importance to .Comment: 9 pages, 13 figures, submitted to J. Phys. Soc. Jpn., Errors in
embedded eps figure files have been correcte
Quantum suppression of superconductivity in ultrathin nanowires
We report measurements on ultrathin (<10 nm) nanowires produced by coating
carbon nanotubes with a superconducting amorphous MoGe alloy. We find that
nanowires can be superconducting or insulating depending on their normal state
resistance compared to -- the quantum resistance for
Cooper pairs. If the tunneling of quantum phase slips (QPS) is
prohibited due to strong damping, and so the wires stay superconducting. The
insulating state, observed if , is explained in terms of
proliferation of quantum phase slips and corresponding localization of Cooper
pairs. The observed superconductor-insulator transition is analogous to the
dissipative phase transition which takes place in Josephson Junctions at
(Penttila et al., Phys. Rev. Lett. Vol.82, p.1004, 1999)Comment: 14 pages, 3 figures. Accepted for publication in Natur
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