Quantum Phase Transitions in low dimensional disordered films

In disordered systems, the electronic ground state is the result of a competition between Coulomb interactions, disorder, which eventually leads to localization of charge carriers, and, when relevant, superconductivity. In this conflict between antagonistic forces, dimensionality plays a special role and determines what ground states are allowed. Indeed, in three-dimensional systems, two distinct quantum phase transitions, the metal-to-insulator transition (MIT) and the superconductor-to-metal transition, separate the three possible ground states. By contrast, in two dimensions, the system can only exhibit a direct superconductor-to-insulator transition (SIT), since metals are theoretically forbidden in the absence of strong electron-electron interactions. One important question is then to understand how the three ground states (superconducting, metallic, and insulating) that are possible in bulk systems evolve when the thickness is reduced.

Thin alloy films provide interesting systems to address this question. I will report on the disorder-induced quantum phase transitions in amorphous Nb_xSi_1-x thin films. The disorder is here tuned by three different parameters: the Nb composition, the thickness and the annealing temperature. Low temperature DC measurements reveal unexpected dissipative states after the destruction of the superconducting long range order. These states then evolve towards an insulating state. Our results provide an insight on the entanglement between the MIT and the SIT.