Instability of Insulators near Quantum Phase Transitions

Abstract

Thin films of amorphous indium oxide undergo a magnetic field driven superconducting to insulator quantum phase transition. In the insulating phase, the current-voltage characteristics show large current discontinuities due to overheating of electrons. We show that the onset voltage for the discontinuities vanishes as we approach the quantum critical point. As a result, the insulating phase becomes unstable with respect to any applied voltage making it, at least experimentally, immeasurable. We emphasize that unlike previous reports of the absence of linear response near quantum phase transitions, in our system, the departure from equilibrium is discontinuous. Because the conditions for these discontinuities are satisfied in most insulators at low temperatures, and due to the decay of all characteristic energy scales near quantum phase transitions, we believe that this instability is general and should occur in various systems while approaching their quantum critical point. Accounting for this instability is crucial for determining the critical behavior of systems near the transition.

The Critical Current of Disordered Superconductors near T=0

“…The premature triggering of the escape transition is also observed in insulating a:InO samples and was previously interpreted as a result of the high disorder in the samples [31,33,58,59]. As this is a possible explanation for the premature triggering of the LR→HR transition we repeat this explanation below.…”

“…3 of Ref. [58] where this phenomenon is discussed in details. As the self-heating theories [13,31] do not predict the exact values of the transitions it is surprising that the I c 's presented in the main-text are predicted quantitatively from the heat-balance equation.…”

Electron-phonon decoupling in two dimensions

“…3 (b). Let us stress that exact positions of the jumps are random as the boundaries here are simply bounds on the true jumps; where the actual jumps occur depends on the decay mechanisms of the states, as discussed in 13 , 31 . Moreover, we do not estimate numerical values for these boundaries, because in order to obtain an accurate value for the voltage scale, , we would also need to include the effects of localisation into the electron-phonon cooling rate 32 , which goes beyond the aim of this work.…”

Electron-Phonon Decoupling in Two Dimensions

“…3(b). Let us stress that exact positions of the jumps are random as the boundaries here are simply bounds on the true jumps; where the actual jumps occur depends on the decay mechanisms of the states, as discussed in 13,31 . Moreover, we do not estimate numerical values for these boundaries, because in order to obtain an accurate value for the voltage scale, V 0 , we would also need to include the effects of localisation into the electron-phonon cooling rate 32 , which goes beyond the aim of this work.…”