Synopsis: Overheated electrons in disordered films

Giant jumps in the current-voltage characteristics of disordered films could be the first evidence that electron transport in insulators can occur in the absence of phonons.
Synopsis figure
Illustration: M. Ovadia et al. Phys. Rev. Lett. 102, 176802 (2009)

Disordered thin films of YSi, InO, and TiN in a magnetic field display unusual current-voltage characteristics: when certain values of a voltage are applied across the sample, the current plummets by as many as five orders of magnitude. Some researchers have proposed that this drop in current indicates a state of matter termed a “superinsulator,” an interpretation that is presently a matter of lively debate.

In a paper appearing in Physical Review Letters, Boris Altshuler and Igor Aleiner of Columbia University and Vladimir Kravtsov (ICTP in Trieste, Italy) and Igor Lerner (University of Birmingham, UK) are suggesting an alternative theory to describe transport in these disordered, insulating thin films. To explain the abrupt jump in resistance with applied voltage, the authors assume that the electron-phonon interactions are sufficiently weak that the electrons can be out of thermal equilibrium with phonons. In addition, they assume that only the temperature of the electrons determines the resistance of the films. The latter assumption is quite unorthodox since transport in insulators is usually mediated by phonons. Remarkably, a rather general theory based on these assumptions predicts two stable states: one where overheated electrons have a resistance significantly lower compared to a second “cold” state. This bistablity is consistent with the current-voltage measurements of InO films that are presented in a companion paper in Physical Review Letters by Maoz Ovadia, Benjamin Sacépé, and Dan Shahar of the Weizmann Institute in Israel.

If the authors’ interpretation is fully confirmed, these experiments on disordered films would be the discovery of insulators where electron transport occurs without phonons. – Ashot Melikyan


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