Pressure-Induced Insulator-to-Metal Transition Provides Evidence for Negative-$U$ Centers in Large-Gap Disordered Insulators

TL;DR

This study demonstrates that pressure can induce an insulator-to-metal transition in large-gap disordered insulators by reversing negative-U centers, revealing new insights into electron-phonon interactions and conduction mechanisms.

Contribution

It provides direct evidence for negative-U centers causing insulator-metal transitions in amorphous oxides, a novel insight into disordered insulator physics.

Findings

Resistance drops by 10^9 times under pressure

Transition occurs with minimal strain (~10^-5)

Reversal of electron-phonon interaction destabilizes trapped electrons

Abstract

Attractive negative-$U$ interactions between electrons facilitated by strong electron-phonon interaction are common in highly polarizable and disordered materials such as amorphous chalcogenides, but there is no direct evidence for them in large-band-gap insulators. Here we report how such negative-$U$ centers are responsible for widespread insulator-to-metal transitions in amorphous HfO$_2$ and Al$_2$O$_3$ thin films with a 10$^9$-fold resistance drop. Triggered by a static hydraulic pressure or a 0.1 ps impulse of magnetic pressure, the transition can proceed at such low pressure that there is very little overall deformation (strain~10$^{-5}$). Absent a significant energy change overall, the transition is attributed to the reversal of localized electron-phonon interaction: By reversing the sign of $U$, trapped electrons are destabilized and released, thus clearing conduction paths previously blocked by charged traps. The results also suggest that Mott insulators when disordered may become Anderson insulators with strong electron-phonon interactions regulating incipient conduction paths, a novel finding of technological significance for electronic devices.