A Universal Critical Density Underlying the Physics of Electrons at the LaAlO3/SrTiO3 Interface

TL;DR

This study identifies a universal Lifshitz transition at a specific electron density in LaAlO3/SrTiO3 interfaces, linking orbital changes to diverse phenomena like magnetism and superconductivity, regardless of sample variations.

Contribution

It reveals a universal critical density associated with a Lifshitz transition between d-orbitals, unifying the understanding of transport, magnetic, and superconducting properties in this system.

Findings

Critical density is independent of LaAlO3 thickness and mobility.

Transition involves a change between d-orbitals of different symmetries.

Maximum superconducting temperature coincides with the critical transition.

Abstract

The two-dimensional electron system formed at the interface between the insulating oxides LaAlO3 and SrTiO3 exhibits ferromagnetism, superconductivity, and a wide range of unique magnetotransport properties. A key challenge is to find a unified microscopic mechanism that underlies these emergent phenomena. Here we show that a universal Lifshitz transition between d-orbitals lies at the core of the observed transport phenomena in this system. Our measurements find a critical electronic density at which the transport switches from single to multiple carriers. This density has a universal value, independent of the LaAlO3 thickness and electron mobility. The characteristics of the transition, its universality, and its compatibility with spectroscopic measurements establish it as a transition between d-orbitals of different symmetries. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed universal transition to a range of reported magnetotransport properties. Interestingly, we also find that the maximum of the superconducting transition temperature occurs at the same critical transition, indicating a possible connection between the two phenomena. Our observations demonstrate that orbital degeneracies play an important role in the fascinating behavior observed so far in these oxides.