Large Negative Electronic Compressibility of LaAlO3-SrTiO3 Interfaces with Ultrathin LaAlO3 Layers

TL;DR

This study uses Kelvin probe microscopy to reveal that the LaAlO3-SrTiO3 interface hosts a two-dimensional electron liquid with negative electronic compressibility at certain carrier densities, and observes a metal-insulator transition at lower densities.

Contribution

First direct local measurement of negative electronic compressibility at LaAlO3-SrTiO3 interfaces using Kelvin probe microscopy.

Findings

Electronic compressibility is negative at carrier densities around 10^13/cm^2.

A metal-insulator transition occurs at lower carrier densities.

Local measurements support previous capacitance-based findings.

Abstract

A two-dimensional electron liquid is formed at the n-type interface between SrTiO3 and LaAlO3. Here we report on Kelvin probe microscopy measurements of the electronic compressibility of this electron system. The electronic compressibility is found to be negative for carrier densities of \approx10^13/cm^2. At even smaller densities, a metal-to-insulator transition occurs. These local measurements corroborate earlier measurements of the electronic compressibility of LaAlO3-SrTiO3 interfaces obtained by measuring the capacitance of macroscopic metal-LaAlO3-SrTiO3 capacitors.