Intrinsic Negative-U Centers in Freestanding LaAlO3/SrTiO3 Micro-membranes

TL;DR

This paper demonstrates that freestanding LaAlO3/SrTiO3 micro-membranes naturally host negative-U centers, which coexist with superconductivity and are stable across a wide temperature range, revealing intrinsic interfacial phenomena.

Contribution

It introduces the discovery of intrinsic negative-U centers in freestanding LAO/STO membranes, highlighting their stability and potential impact on interfacial superconductivity.

Findings

Negative-U centers are observed in freestanding LAO/STO membranes.

Transport data aligns with negative-U Anderson model calculations.

Characteristic center sizes are estimated to be 20-80 nm.

Abstract

The LaAlO3/SrTiO3 (LAO/STO) interface hosts a rich range of electronic phenomena, including unconventional electron pairing that in quantum dots gives rise to a negative effective charging energy U. Here, we show freestanding LAO/STO micro-membranes naturally hosting negative-U centers, where lateral confinement arises intrinsically, rather than from engineered nanostructures. These centers coexist with gate-tunable superconductivity and can remain stable upon thermal cycling from millikelvin temperatures to room temperature. Transport is in excellent agreement with calculations based on a negative-U Anderson model, and electrostatic simulations indicate characteristic center sizes of 20-80 nm. Our findings suggest that negative-U centers may arise from the intrinsic interfacial inhomogeneity typical of LAO/STO, and should therefore be considered a general feature of the LAO/STO interface. This could have important consequences for the microwave response of interfacial superconducting devices.