Infinite-randomness fixed point of the quantum superconductor-metal transitions in amorphous thin films*

The quantum superconductor-metal transition of amorphous indium oxide films has been investigated. As is typical of a field tuned quantum phase transition the crossing of the magnetoresistance isotherms is observed. In contrast with what is found in the case of direct superconductor-insulator transitions in films with lower mobilities, the isotherms do not cross at a single value of magnetic field. Instead the crossing field changes systematically with temperature. Applying a conventional power-law scaling analysis to these films using isotherms near selected crossing fields results in both a temperature and magnetic field dependent exponent product, νz. The temperature dependence of this effective νz value was fit to a theoretically predicted form. Using this fit as a starting point an activated scaling analysis was applied to the data. The critical field was varied until the isotherms collapsed. The ability to fit this transition using activated scaling shows that this transition is well described by an infinite-randomness critical point and quantum Griffiths singularities.