Electrostatic tuning of a quantum phase transition

Quantum phase transitions are transitions between the ground states of physical systems traversed by varying a parameter of the Hamiltonian. We have been examining the properties of ultrathin quench-condensed films of amorphous Bi, which exhibit a number of quantum phase transitions tuned by disorder, perpendicular and parallel magnetic fields and charge density. We have developed a procedure in which the substrate for film growth serves as the gate insulator in a field effect transistor configuration. This permits electrostatic tuning of a transition between insulating and superconducting behaviour. Finite size scaling of the conductivity has been carried out yielding critical exponents that suggest that the insulator-superconductor transition belongs to the universality class of the 3D XY model. This electric field-induced superconductivity can in turn be destroyed by magnetic fields and the data can also be scaled in with essentially the same results. These findings are quite different from those obtained in the study of substantially thicker films of indium oxide and titanium nitride.