Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO$_3$/SrTiO$_3$ interfaces

TL;DR

This study demonstrates large capacitance enhancement and negative electronic compressibility at LaAlO₃/SrTiO₃ interfaces, revealing tunable electron densities and overscreening effects crucial for oxide electronic device applications.

Contribution

It introduces a method to control and deplete the interface electron system using top gates and reports the first observation of negative compressibility at this oxide interface.

Findings

Capacitance exceeds geometric limits at low densities

Complete depletion of the metallic interface with small voltages

Evidence of overscreening and negative compressibility

Abstract

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.