Abstract
The interface between the two complex oxides LaAlO3 and SrTiO3 has remarkable properties that can be locally reconfigured between conducting and insulating states using a conductive atomic force microscope. Prior investigations of sketched quantum dot devices revealed a phase in which electrons form pairs, implying a strongly attractive electron-electron interaction. Here, we show that these devices with strong electron-electron interactions can exhibit a gate-tunable transition from a pair-tunneling regime to a single-electron (Andreev bound state) tunneling regime where the interactions become repulsive. The electron-electron interaction sign change is associated with a Lifshitz transition where the dxz and dyz bands start to become occupied. This electronically tunable electron-electron interaction, combined with the nanoscale reconfigurability of this system, provides an interesting starting point towards solid-state quantum simulation.
Highlights
Electron-electron interactions lead to many remarkable properties in the solid state, ranging from superconductivity and quantum magnetism to fractionalized excitations [1,2,3], Wigner crystals [4], and a variety of predicted topological phases [5]
The fine details of electron-electron interactions usually depend on carrier density, qualitative details like the interaction sign are usually density independent
While the dome-shaped phase diagram extracted from gate-dependent transport experiments on LAO=STO marks the boundary of superconductivity, it does not reveal details of the underlying nature of the electron-electron interactions
Summary
Electron-electron interactions lead to many remarkable properties in the solid state, ranging from superconductivity and quantum magnetism to fractionalized excitations [1,2,3], Wigner crystals [4], and a variety of predicted topological phases [5]. While the dome-shaped phase diagram extracted from gate-dependent transport experiments on LAO=STO marks the boundary of superconductivity, it does not reveal details of the underlying nature of the electron-electron interactions. The ratio of pairing temperature to Fermi temperature Tp=TF ∼ 0.1–0.8 is much larger than that of conventional BCS superconductors, indicating that the pairing interactions in low-density STO are quite strong and attractive, and are in the BEC-BCS crossover. At high gate voltages (high electron densities on the QD) the transport changes to a conventional single-particle regime. In this regime, the lowenergy excitations of the QD consist of adding or removing a single electron from the dot [Fig. 1(a), bottom right-hand panel]. Coupling the QD to superconducting leads results in the formation of conventional ABS, which are responsible for electron transport in this regime
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