981 research outputs found
Observation of thermally activated glassiness and memory dip in a-NbSi insulating thin films
We present electrical conductance measurements on amorphous NbSi insulating
thin films. These films display out-of equilibrium electronic features that are
markedly different from what has been reported so far in disordered insulators.
Like in the most studied systems (indium oxide and granular Al films), a slow
relaxation of the conductance is observed after a quench to liquid helium
temperature which gives rise to the growth of a memory dip in MOSFET devices.
But unlike in these systems, this memory dip and the related conductance
relaxations are still visible up to room temperature, with clear signatures of
a temperature dependent dynamics
Destruction of superconductivity in disordered materials : a dimensional crossover
The disorder-induced Superconductor-to-Insulator Transition in amorphous
NbSi two-dimensional thin films is studied for different niobium
compositions through a variation of the sample thickness . We show that
the critical thickness , separating a superconducting regime from an
insulating one, increases strongly with diminishing , thus attaining values
of over 100 {\AA}. The corresponding phase diagram in the plane is
inferred and related to the three-dimensional situation. The two-dimensional
Superconductor-to-Insulator Transition well connects with the three-dimensional
Superconductor-to-Metal Transition
Multidimensional solitons in a low-dimensional periodic potential
Using the variational approximation(VA) and direct simulations, we find
stable 2D and 3D solitons in the self-attractive Gross-Pitaevskii equation
(GPE) with a potential which is uniform in one direction () and periodic in
the others (but the quasi-1D potentials cannot stabilize 3D solitons). The
family of solitons includes single- and multi-peaked ones. The results apply to
Bose-Einstein condensates (BECs) in optical lattices (OLs), and to spatial or
spatiotemporal solitons in layered optical media. This is the first prediction
of {\em mobile} 2D and 3D solitons in BECs, as they keep mobility along .
Head-on collisions of in-phase solitons lead to their fusion into a collapsing
pulse. Solitons colliding in adjacent OL-induced channels may form a bound
state (BS), which then relaxes to a stable asymmetric form. An initially
unstable soliton splits into a three-soliton BS. Localized states in the
self-repulsive GPE with the low-dimensional OL are found too.Comment: 4 pages, 5 figure
Magnetic field-induced quantum superconductor-insulator transition in
A study of magnetic-field tuned superconductor-insulator transitions in
amorphous thin films shows that quantum
superconductor-insulator transitions are characterized by an unambiguous
signature -- a kink in the temperature profile of the critical magnetic field.
Using this criterion, we show that the nature of the magnetic-field tuned
superconductor-insulator transition depends on the orientation of the field
with respect to the film. For perpendicular magnetic field, the transition is
controlled by quantum fluctuations with indications for the existence of a Bose
insulator; while for parallel magnetic field, the transition is classical,
driven by the breaking of Cooper pairs at the temperature dependent critical
field .Comment: 5 pages, 4 figure
Cinétique transcuticulaire et accumulation d'un carbamate nématicide et de ses métabolites : origine de la la sélectivité vis-à-vis des nématodes
Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films
a-Nb(x)Si(1-x) thin films with thicknesses down to 25 {\AA} have been
structurally characterized by TEM (Transmission Electron Microscopy)
measurements. As-deposited or annealed films are shown to be continuous and
homogeneous in composition and thickness, up to an annealing temperature of
500{\deg}C. We have carried out low temperature transport measurements on these
films close to the superconductor-to-insulator transition (SIT), and shown a
qualitative difference between the effect of annealing or composition, and a
reduction of the film thickness on the superconducting properties of a-NbSi.
These results question the pertinence of the sheet resistance R_square as the
relevant parameter to describe the SIT.Comment: 9 pages, 12 figure
A length scale for the superconducting Nernst signal above T in NbSi
We present a study of the Nernst effect in amorphous superconducting thin
films of NbSi. The field dependence of the Nernst coefficient
above T displays two distinct regimes separated by a field scale set by
the Ginzburg-Landau correlation length. A single function , with the
correlation length as its unique argument set either by the zero-field
correlation length (in the low magnetic field limit) or by the magnetic length
(in the opposite limit), describes the Nernst coefficient. We conclude that the
Nernst signal observed on a wide temperature () and field () range is exclusively generated by short-lived Cooper pairs.Comment: 4 pages, 4 figure
Higher-order vortex solitons, multipoles, and supervortices on a square optical lattice
We predict new generic types of vorticity-carrying soliton complexes in a
class of physical systems including an attractive Bose-Einstein condensate in a
square optical lattice (OL) and photonic lattices in photorefractive media. The
patterns include ring-shaped higher-order vortex solitons and supervortices.
Stability diagrams for these patterns, based on direct simulations, are
presented. The vortex ring solitons are stable if the phase difference \Delta
\phi between adjacent solitons in the ring is larger than \pi/2, while the
supervortices are stable in the opposite case, \Delta \phi <\pi /2. A
qualitative explanation to the stability is given.Comment: 9 pages, 4 figure
Matter-wave solitons in radially periodic potentials
We investigate two-dimensional (2D) states of Bose-Einstein condensates (BEC)
with self-attraction or self-repulsion, trapped in an axially symmetric
optical-lattice potential periodic along the radius. Unlike previously studied
2D models with Bessel lattices, no localized states exist in the linear limit
of the present model, hence all localized states are truly nonlinear ones. We
consider the states trapped in the central potential well, and in remote
circular troughs. In both cases, a new species, in the form of \textit{radial
gap solitons}, are found in the repulsive model (the gap soliton trapped in a
circular trough may additionally support stable dark-soliton pairs). In remote
troughs, stable localized states may assume a ring-like shape, or shrink into
strongly localized solitons. The existence of stable annular states, both
azimuthally uniform and weakly modulated ones, is corroborated by simulations
of the corresponding Gross-Pitaevskii equation. Dynamics of strongly localized
solitons circulating in the troughs is also studied. While the solitons with
sufficiently small velocities are stable, fast solitons gradually decay, due to
the leakage of matter into the adjacent trough under the action of the
centrifugal force. Collisions between solitons are investigated too. Head-on
collisions of in-phase solitons lead to the collapse; -out of phase
solitons bounce many times, but eventually merge into a single soliton without
collapsing. The proposed setting may also be realized in terms of spatial
solitons in photonic-crystal fibers with a radial structure.Comment: 16 pages, 23 figure
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