Nernst effect beyond the coherence critical field of a nanoscale granular superconductor

Abstract

We report measurements of the Nernst effect and of the magneto-resistance of granular aluminum films near the metal to insulator transition. These films show sharp transitions as a function of temperature and magnetic field. At low temperatures the Nernst signal displays a sharp peak at a field where more than 90% of the normal state resistance has been restored, suggesting a transition involving entropy transport after superconducting coherence has been destroyed. At temperatures higher than the critical temperature the fluctuation paraconductivity scales with the Nernst signal, in agreement with a description in terms of fluctuations of the order parameter.

Optical signatures of the superconducting Goldstone mode in granular aluminum: Experiments and theory

“…The distribution of J ij is ultimately determined by the experimental distribution of grain sizes. From experiments 38,39 we know that for the range of resistances of interest the distribution of sizes is close to a log normal distribution with an average diameter 2 nm and variance of 0.5 nm. This is the distribution that we will use in the rest of the paper.…”

“…1. Following the experimental results of Ref [38][39][40] ,. we assume that the distribution of grain sizes is log-normal with a typical radius of 1nm.…”

“…As a result, a Josephson array of SC nano-grains can be highly inhomogeneous even when the SC transition occurs very far from the SIT. Unlike inhomogeneities induced by impurities it is possible to know experimentally 14,[38][39][40] , with relative accuracy, the grain size, its distribution, and the density of grains, that control the resistivity of the material and the inhomogeneity of its SC properties. An additional advantage of the nanograins, as compared to strongly-disordered superconductors and arrays made of artificial Josephson junctions, is that in this case the capacitive charging energy of each grain dominates over the junction capacitance.…”

“…The typical grain size, a few nanometers, and its shape, spherical, is optimal 44 to observe strong shell effects and consequently strong inhomogeneities in the array even in the low-resistivity region, where the average coupling among grains is strong enough to give a dc conductivity consistent with metallic behavior. An indirect proof of the emerging inhomogeneity is provided by the experimentally observed enhancement of the critical temperature 14,38,39,45 with respect to bulk Al. Indeed, it has been argued 44 that the local enhancement of the superconducting gap due to shell effects can increase, within a percolative scheme, the critical temperature of the array.…”

Granular superconductors for high kinetic inductance and low loss quantum devices

“…High sheet resistance is reached by reducing inter-grain coupling rather than by atomic scale disorder. The phase diagram of T c vs resistivity ρ n has the well known "dome" shape, first rising to reach a maximum value which depends on the temperature of the substrate during film growth (respectively 2.3 K at room temperature [21][22][23][24] , 3 nm grain size 22 , and 3.2 K at 100 K 20,[25][26][27][28][29] , 2 nm grain size with a narrower distribution 29,30 ) and thereafter decreasing as the metal-to-insulator transition is approached.…”