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

Strongly nonequilibrium flux flow in the presence of perforating submicron holes

We report on the effects of perforating submicron holes on the vortex dynamics of amorphous Nb0.7Ge0.3 microbridges in the strongly nonequilibrium mixed state, when vortex properties change substantially. In contrast to the weak nonequilibrium - when the presence of holes may result in either an increase (close to Tc) or a decrease (well below Tc) of the dissipation, in the strong nonequilibrium an enhanced dissipation is observed irrespectively of the bath temperature. Close to Tc this enhancement is similar to that in the weak nonequilibrium, but corresponds to vortices shrunk due to the Larkin-Ovchinnikov mechanism. At low temperatures the enhancement is a consequence of a weakening of the flux pinning by the holes in a regime where electron heating dominates the superconducting properties.Comment: 6 pages, 5 figure

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

Transverse voltage in zero external magnetic fields, its scaling and violation of the time reversal symmetry in MgB2

The longitudinal and transverse voltages (resistances) have been measured for MgB$_2$ in zero external magnetic fields. Samples were prepared in the form of thin film and patterned into the usual Hall bar shape. In close vicinity of the critical temperature T$_c$ non-zero transverse resistance has been observed. Its dependence on the transport current has been also studied. New scaling between transverse and longitudinal resistivities has been observed in the form $\rho{_{xy}}\sim d\rho{_{xx}}/dT$. Several models for explanation of the observed transverse resistances and breaking of reciprocity theorem are discussed. One of the most promising explanation is based on the idea of time-reversal symmetry violation