Experimental review on the disordered-driven superconductor to insulator transition
The disorder-driven superconductor to insulator transition (SIT) is considered to be a prototype of a quantum phase transition at zero temperature. Lately, there has been a renewed interest in this field due to the experimental observations of a number of dramatic features near the SIT of amorphous superconducting materials such as indium oxide and niobium nitride. These novel features included the simple activated temperature dependence of the resistance in the insulating side, a large peak in the magneto resistance, peculiar I-V characteristics and traces of superconductivity at temperatures above Tc. We present experimental results from tunneling spectroscopy, Terahertz spectroscopy and transport measurements that shed light on the physical mechanism governing this phase transition. Our key observations are: 1 - Superconducting gap in the insulating side of the transition. 2 - A possible experimental evidence for collective modes (namely the "Higgs" amplitude mode) in such disordered films close to the SIT. 3 - The possibility to tune the quantum phase transition simply by screening the e-e interactions. These results and their possible consequences will be discussed.