Nernst effect measurements of epitaxial Y0.95Ca0.05Ba2(Cu1-xZnx)3Oy and Y0.9Ca0.1Ba2Cu3Oy superconducting films

Nernst response, viscosity and mobile entropy in vortex liquids

In a liquid of superconducting vortices, a longitudinal thermal gradient generates a transverse electric field. This Nernst signal peaks at an intermediate temperature and magnetic field, presumably where the entropy difference between the vortex core and the superfluid environment is largest. There is a puzzling similarity of the amplitude of this peak across many different superconductors. This peak can be assimilated to a minimum in the viscosity to entropy density ratio of the vortex liquid. Expressed in units ofℏkB, this minimum is one order of magnitude larger than what is seen in common liquids. Moreover, the entropy stocked in the vortex core isnotidentical to the entropy bound to a moving magnetic flux line. Due to a steady exchange of normal quasi-particles, entropy can leak from the vortex core. A slowly moving vortex will be peeled off its entropy within a distance of the order of a superconducting coherence length, provided that theΔEFratio is sufficiently large.

Superconducting fluctuations in organic molecular metals enhanced by Mott criticality

Unconventional superconductivity typically occurs in materials in which a small change of a parameter such as bandwidth or doping leads to antiferromagnetic or Mott insulating phases. As such competing phases are approached, the properties of the superconductor often become increasingly exotic. For example, in organic superconductors and underdoped high-T_c cuprate superconductors a fluctuating superconducting state persists to temperatures significantly above T_c. By studying alloys of quasi-two-dimensional organic molecular metals in the κ-(BEDT-TTF)₂X family, we reveal how the Nernst effect, a sensitive probe of superconducting phase fluctuations, evolves in the regime of extreme Mott criticality. We find strong evidence that, as the phase diagram is traversed through superconductivity towards the Mott state, the temperature scale for superconducting fluctuations increases dramatically, eventually approaching the temperature at which quasiparticles become identifiable at all.

Reversal of nonlocal vortex motion in the regime of strong nonequilibrium

We investigate nonlocal vortex motion in weakly pinning a-NbGe nanostructures, which is driven by a transport current I and remotely detected as a nonlocal voltage V{nl}. At a high I of a given polarity, V{nl} changes sign dramatically. This is followed by V{nl} becoming even in I, with the opposite sign at low and high temperatures T. These findings can be explained by a Nernst-like effect resulting from local electron overheating (low T), and a magnetization enhancement due to a nonequilibrium quasiparticle distribution that leads to a gap enhancement near the vortex core (high T).