4,545 research outputs found
Peak effect in twinned superconductors
A sharp maximum in the critical current as a function of temperature
just below the melting point of the Abrikosov flux lattice has recently been
observed in both low and high temperature superconductors. This peak effect is
strongest in twinned crystals for fields aligned with the twin planes. We
propose that this peak signals the breakdown of the collective pinning regime
and the crossover to strong pinning of single vortices on the twin boundaries.
This crossover is very sharp and can account for the steep drop of the
differential resistivity observed in experiments.Comment: 4 pages, revtex 3.0, no figure
Plasticity in current-driven vortex lattices
We present a theoretical analysis of recent experiments on current-driven
vortex dynamics in the Corbino disk geometry. This geometry introduces
controlled spatial gradients in the driving force and allows the study of the
onset of plasticity and tearing in clean vortex lattices. We describe plastic
slip in terms of the stress-driven unbinding of dislocation pairs, which in
turn contribute to the relaxation of the shear, yielding a nonlinear response.
The steady state density of free dislocations induced by the applied stress is
calculated as a function of the applied current and temperature. A criterion
for the onset of plasticity at a radial location in the disk yields a
temperature-dependent critical current that is in qualitative agreement with
experiments.Comment: 11 pages, 4 figure
Synthesis, variable temperature NMR investigations and solid state characterisation of novel octafluorofluorene compounds
The preparation of a number of new 9-substituted octafluorofluorene derivatives, solution NMR studies, and the first examples of solid state structures of octafluorofluorenes [1,2,3,4,5,6,7,8-octafluorofluorene, C13H2F8, 1; 1,2,3,4,5,6,7,8-octafluoro-9-(pentafluoro)phenylfluorene, C19HF13, 8; 1,1′,2,2′,3,3′,4,4′,5,5′,6,6′,7,7′,8,8′-hexadecafluoro-9,9′-bifluorenyl, C26H2F16, 11] are reported. Variable temperature 19F NMR investigations have been performed on the 9-aryl substituted compounds 1,2,3,4,5,6,7,8-octafluoro-9-(pentafluoro)phenyl-9-hydroxyfluorene, C19HF13O, 4, 1,2,3,4,5,6,7,8-octafluoro-9-(nonafluoro-4′-biphenylyl)-9-hydroxyfluorene, C25HF17O, 5, and 8, and the energetic barriers to rotation of the aryl have been determined. A lower rotational barrier is observed for compound 4 with respect to compound 8, while 5 does not show fluxional behaviour below 338 K. The results of the variable temperature experiments performed on 8 have been rationalized by 2D NMR studies, and compared to the solid state data resulting from the X-ray structural analysis
Supersolidity in electron-hole bilayers with a large density imbalance
We consider an electron-hole bilayer in the limit of extreme density
imbalance, where we have a single particle in one layer interacting
attractively with a Fermi liquid in the other parallel layer. Using an
appropriate variational wave function for the dressed exciton, we provide
strong evidence for the existence of the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) phase in electron-hole bilayers with a large density imbalance.
Furthermore, within this unusual limit of FFLO, we find that a dilute gas of
minority particles forms excitons that condense into a two-dimensional
"supersolid".Comment: 6 pages, 2 figure
Driven depinning of strongly disordered media and anisotropic mean-field limits
Extended systems driven through strong disorder are modeled generically using
coarse-grained degrees of freedom that interact elastically in the directions
parallel to the driving force and that slip along at least one of the
directions transverse to the motion. A realization of such a model is a
collection of elastic channels with transverse viscous couplings. In the
infinite range limit this model has a tricritical point separating a region
where the depinning is continuous, in the universality class of elastic
depinning, from a region where depinning is hysteretic. Many of the collective
transport models discussed in the literature are special cases of the generic
model.Comment: 4 pages, 2 figure
Magnetic anomalies of steel drums: a review of the literature and research results of the INGV
The detection and evaluation of the status of disposal sites that contain
hazardous waste materials is becoming an increasingly important element
in environmental investigations. Close cooperation between the Istituto
Nazionale di Geofisica e Vulcanologia (INGV; National Institute of Volcanology
and Geophysics) in Rome and the Italian environmental police
has resulted in numerous underground investigations of different buried
materials. Among the geophysical investigation tools, magnetometry is the
most effective, rapid and precise of all of the geophysical methods for localizing
buried steel drums. Analysis of magnetic map anomalies can provide
a variety of information about buried materials, including extension,
distribution and depth, with processing of the acquired magnetic data.
This information is also very useful in case of excavations that are aimed
at the recovery of hazardous waste. This study determines the most relevant
analyses reported in the literature, with modeling of magnetometric
methods for environmental applications both theoretically and experimentally.
Some studies and research results achieved by the INGV in relation
to magnetic anomalies produced by buried steel drums are also
reported, as found in field operations and as achieved from test sites
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