Whatever happened to Mott?

In the case of randomly placed ion damage tracks in a bulk superconductor, theories suggest that increasing the applied field beyond the "saturation field" H_s should result in a single, sharp depinning transition (analogous to the Mott-Hubbard metal-insulating transition) accompanied by an abrupt decrease in magnetic hysteresis and critical current density J_c, and increased dissipation by an interstitial flux line (IFL) liquid phase. However, in the case of thin film patterned with an ordered lattice of artificial pinning centers (APC), anomalies in hysteresis and dissipation are observed at multiple "matching fields" nH_n=1<H_s, where n≤s is a positive integer. Unexpected matching anomalies observed at "supermatching fields" (n>s), signal the existence of several ordered IFL lattice phases, rather than a simple IFL liquid phase. The relationship of such multiple anomalies to a possible Mott transition is an open question. We discuss the effects of temperature, applied DC field and field/film-plane angle on the mobility and non-linear dynamics of flux lines in a patterned thin film. We show how sensitive AC susceptibility measurements performed at variable electromagnetic drive and frequency address fundamental questions concerning the existence of the Mott transition and the conditions for melting of the IFL lattice and depinning of multiquantum fluxoids contained at the APC; and we review the serious experimental difficulties in distinguishing equilibrium versus dynamic depinning phenomena.