James M. Knight

Weak measurement of photon polarization

Abstract An optical procedure for measuring the recently introduced weak value of a photon polarization variable is suggested and some of its experimental aspects are discussed. The deflection of a beam passing through a birefringent prism is considered. It is shown that if the beam also passes through preselection and postselection polarization filters, it may have a deflection which is greater than the deflection of the beam without the filters. The magnitude of the deflection is give by weak measurement theory.

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(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 successive nucleation of quantized distortions in the local vortex velocity and flux distribution within the moving flux matter.

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.

Superconductivity and oscillatory magnetoresistance at a topological-insulator/chalcogenide interface

The Bi2Te3/FeTe heterostructure intersects several phenomena and key classes of materials in condensed matter physics: topological insulators, superconductivity, magnetism, and the physics of interfaces. While neither the topological insulator (Bi2Te3) nor the iron chalcogenide (FeTe) are themselves superconductors, superconductivity forms in a thin 7nm interfacial layer between the two. The restricted dimensionality and the extraordinarily conductive normal state, possibly sourced by the topologically protected surface states, have led to the observation of novel phenomena such as the Likharev vortex explosion and transitions in behavior resulting from the interplay between current induced depairing and the Berezinski-Kosterlitz-Thouless regime. Bi2Te3/FeTe also displays the anomalous oscillatory magnetoresistance phenomenon, which we had previously observed in cuprates.

High-Field Flux Dynamics In Disordered Two-Band Superconductivity

The mixed state response of disordered magnesium diboride has a con- spicous absence of the usual flux-density (B) dependent broadening of conductance- temperature and voltage-current curves. This B independence implies a vortex vis- cosity that rises linearly with B. We show that this this anomalous viscosity function can arise from the quasiparticle sea of the quenched weaker band and the unusually long electric field penetration depth that delocalizes the vortex electric fields.

Hot-Electron Instability in Superconductors

High flux velocities in a superconductor can distort the quasiparticle distribution function and elevate the electronic temperature. Close to Tc, a non-thermal distribution function shrinks the vortex core producing the well-known Larkin-Ovchinnikov flux instability. In the present work we consider the opposite limit of low temperatures, where electron-electron scattering is more rapid than electron-phonon, resulting in an electronic temperature rise with a thermal-like distribution function. This produces a different kind of flux instability, due to a reduction in condensate and expansion of the vortex core. Measurements in YBCO films confirm the distinct predictions ofthis mechanism.