Critical flux pinning and enhanced upper-critical-field in magnesium diboride films

We have conducted pulsed transport measurements on c-axis oriented magnesium diboride films over the entire relevant ranges of magnetic field 0 \alt H \alt H_{c2} (where \hcu is the upper critical field) and current density 0 \alt j \alt j_{d} (where $j_{d}$ is the depairing current density). The intrinsic disorder of the films combined with the large coherence length and three-dimensionality, compared to cuprate superconductors, results in a six-fold enhancement of $H_{c2}$ and raises the depinning current density $j_{c}$ to within an order of magnitude of $j_{d}$. The current-voltage response is highly non-linear at all fields, resulting from a combination of depinning and pair-breaking, and has no trace of an Ohmic free-flux-flow regime. Keywords: pair, breaking, depairing, superconductor, superconductivity, flux, fluxon, vortex, mgb

The ballistic acceleration of a supercurrent in a superconductor

One of the most primitive but elusive current-voltage (I-V) responses of a superconductor is when its supercurrent grows steadily after a voltage is first applied. The present work employed a measurement system that could simultaneously track and correlate I(t) and V(t) with sub-nanosecond timing accuracy, resulting in the first clear time-domain measurement of this transient phase where the quantum system displays a Newtonian like response. The technique opens doors for the controlled investigation of other time dependent transport phenomena in condensed-matter systems.Comment: 4 pages, 3 figure

Steps in the Negative-Differential-Conductivity Regime of a Superconductor

Current-voltage characteristics were measured in the mixed state of Y1Ba2Cu3O(7-delta) superconducting films in the regime where flux flow becomes unstable and the differential conductivity dj/dE becomes negative. Under conditions where its negative slope is steep, the j(E) curve develops a pronounced staircase like pattern. We attribute the steps in j(E) to the formation of a dynamical phase consisting of the succesive nucleation of quantized distortions in the local vortex velocity and flux distribution within the moving flux matter.Comment: 5 pages, 3 figure

Current concepts of the etiology of urolithiasis in man and animals

Call number: LD2668 .R4 1963 K9

Energy Relaxation at a Hot-Electron Vortex Instability

At high dissipation levels, vortex motion in a superconducting film has been observed to become unstable at a certain critical vortex velocity v*. At substrate temperatures substantially below Tc, the observed behavior can be accounted for by a model in which the electrons reach an elevated temperature relative to the phonons and the substrate. Here we examine the underlying assumptions concerning energy flow and relaxation times in this model. A calculation of the rate of energy transfer from the electron gas to the lattice finds that at the instability, the electronic temperature reaches a very high value close to the critical temperature. Our calculated energy relaxation times are consistent with those deduced from the experiments. We also estimate the phonon mean free path and assess its effect on the flow of energy in the film.Comment: 8 pages, 7 figure