Experimental studies on granular Al thin films

Abstract

Content of this chapter are experimental studies on the transport- and dynamical conductivity of superconducting granular Al, i.e. films composed of nanoscaled grains of aluminum coupled into a macroscopic array. We will start our discussion with a short introduction to superconductivity in confined geometries at the nanoscale governed by the interplay of phase-number uncertainty and the shell effect. Section 3.2 will provide both an introduction to the superconductivity of granular Al with emphasis on the well-known yet enigmatic superconducting dome in the phase diagram and a broader context aiming for a unified picture of granular and unconventional superconductors by virtue of a common fundamental interplay of superconducting energy scales. In Sec. 3.3 we will start with a discussion of the resistive transitions and -fluctuations of various samples with different resistivity, and afterwards focus on dynamical conductivity $\widehat{\sigma }\left( v \right)$ and, in Sec. 3.4, identify the energy gap ∆ and the superfluid stiffiness J as underlying energy scales shaping the superconducting dome in terms of an amplitude-phase crossover with a pseudogap feature discussed as natural consequence thereof in Sec. 3.4.2. In the following Sec. 3.5 we will closely examine a low-energy absorption evident in several samples, which we identify as the collectiive excitation of the phase field, the supeconducting Goldstone mode, within a nearly parameter free microscopic model.

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