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

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

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

Current-voltage characteristics were measured in the mixed state of Y 1 Ba 2 Cu 3 O 72d superconducting films in the regime where flux flow becomes unstable and the differential conductivity dj͞dE becomes negative. Under conditions where its negative slope is steep, the j͑E͒ curve develops a pronounced staircaselike pattern. We attribute the steps in j͑E͒ to the formation of a dynamical phase consisting of the successive nucleation of quantized distortions in the local vortex velocity and flux distribution within the moving flux matter. DOI: 10.1103/PhysRevLett.87.177001 PACS numbers: 74.40. +k, 74.60.Ge, 74.72.Bk In a type II superconductor, a magnetic field H above the lower critical value H c1 introduces flux vortices containing an elementary quantum of flux F o h͞2e, and interactions between the vortices tend to align them into a uniform lattice A transport current exerts a Lorentz driving force F L j 3 F o on the vortices and the motion is opposed by a viscous drag F d 2hv f , where h is the coefficient of viscosity. If we assume that pinning forces F p are negligible (because F L ¿ F p ), then the steady state motion reflects a balance between driving (F L ), drag (F d ), and elastic forces (F e ) on each vortex. For a perfectly uniform distribution, the net elastic force on each vortex vanishes, resulting in free flux flow A different scenario prevails at ultrahigh dissipation levels and electric fields sufficient to alter the electronic distribution function and/or the electronic temperature. Here j͑E͒ becomes nonlinear and can develop an unstable region with negative differential conductivity (NDC) (region "C" i

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