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.
@article{arxiv.2601.13366,
title = {Intrinsic Negative-U Centers in Freestanding LaAlO3/SrTiO3 Micro-membranes},
author = {Giulia Meucci and Pinelopi O. Konstantinopoulou and Thies Jansen and Gunjan Nagda and Damon J. Carrad and Emiliano Di Gennaro and Yu Chen and Nicola Manca and Nicolas Bergeal and Manuel Bibes and Alexei Kalaboukhov and Marco Salluzzo and Roberta Citro and Felix Trier and Nini Pryds and Fabio Miletto Granozio and Alessia Sambri and Thomas S. Jespersen},
journal= {arXiv preprint arXiv:2601.13366},
year = {2026}
}