Field effect nanostructures in superconducting oxide 2DEG – nano-SO2DEG

2DEG at oxide interfaces have electronics properties that are radically different from those encountered in classical semiconducting hetero-structures, making them particularly interesting to realize experiments at the frontier between mesoscopic physics and condensed matter. In particular, the possibility to have both two-dimensional superconductivity and Rashba SOC gathered together in the 2DEG is an unconventional situation that has not equivalent in other electronic system. In order to fully explore this very peculiar regime it is crucial to fabricate nanostructure where the 2DEG can be manipulated with local gates. The lateral dimensions of these devices must be comparable to the characteristics lengths involved in the physical phenomenon of interest (i. e. superconducting coherence length, Thouless length, Rashba SOC length...). We fabricate LaAlO3/SrTiO3 -based field-effect devices using tow different techniques, the amorphous LaAlO3 template and the ion-irradiation method.

We showed that the superconductivity can be electrostatically modulated over a wide range both by a top-gate voltage and a back-gate. A superconductor-to-insulator quantum phase transition is induced when the quantum well is strongly depleted. By analyzing the magnetotransport measurements, the presence of strong spin-orbit coupling that could be controlled with the top-gate and the back-gate voltages was demonstrated. The spin-splitting energy on the order of a few meV was found to increase linearly with the interfacial electric field in agreement with the Rashba mechanism. These results, published recently, correspond to the first objective of the project (S. Hurand et al. Sci. Rep. (2015), S. Hurand et al. Appl. Phys. Lett. submitted)

Our main objective is to evidence the role of Rashba spin orbit coupling on the Andreev states in a Superconducting-Normal-Superconducting junction. We will make mesoscopic devices where two superconducting pads will be defined in an oxide 2DEG, and connected through a normal region with strong spin orbit coupling. Linear (nanowire) or annular (ring) geometry will be investigated. We will study the Josephson current through the whole structure, and look for specific signatures of anomalous Current Phase Relation related to the spin orbit coupling in the non-superconducting part. The relevant parameters such as the geometry or the spin orbit coupling strength will be controlled with a set of several local gates. The results will be compared to recent theoretical predictions. The main challenge regarding this objective is to manage to have the relevant energies in these nanostructures (SOC Rashba, Zeeman and Thouless energies) falling in the right range to observe clear signatures.

1. Biscaras, J. et al. Limit of the electrostatic doping in two-dimensional electron gases of LaXO3(X=Al,Ti)/SrTiO3. Sci. Rep. 4, 6788 (2014).
2. Hurand, S. et al. Field-effect control of superconductivity and Rashba spinorbit coupling in top-gated LaAlO3 \SrTiO 3 devices. Sci. Rep. 5, 12751, (2015).

Two-dimensional electron gases (2DEGs) based on conventional semiconducting heterostructures have played a crucial role both in fundamental science and technology. The recent progress in the fabrication of high quality epitaxial interfaces between transition metal oxides gives a unique opportunity to engineer similar artificial structures where new interesting phenomenon can take place. The discovery of two-dimensional superconducting electron gases in SrTiO3 based oxide heterostructures has attracted much attention in this context.
The aim of this project is to make nanostructures on two-dimensional electron gases at the LaXO3/SrTiO3 interface (X=Ti or Al) and to study the coupling between superconductivity and strong Rashba Spin-Orbit Coupling (SOC). The fabrication and operation of these devices rely on the possibility to control the properties of the 2DEG at the nanoscale with electric fields imposed by means of local metallic gates. The present proposal grounds on earlier results obtained over the passed two years in the field of oxide interfaces. The expertise of the team, made of young researchers, both in condensed matter and mesoscopic physics is a key asset to complete successfully this project.

Two main issues will be addressed in this project:

The first one is to study nanostructures where the 2DEG is confined in a Quantum Point Contact (QPC) geometry thanks to local top gates. In the normal state, we will investigate the electronic transport in a restricted number of channels and attempt to reach the regime of quantification of the conductance. In the superconducting state, we will study the electric field tunable Josephson current, which is carried out by Andreev bound states localized at the QPC. At finite voltage, dc transport and shot noise measurements in the regime of Multiple Andreev Reflection should reveal important information on the 2DEG conduction such as the number of conduction channels involved and their associated transmission probabilities.

The second and main one is to study mesoscopic nanodevices involving superconducting parts coupled by a normal region of strong Rashba SOC. The latter one is expected to strongly affect the Andreev states by lifting the twofold spin degeneracy in the normal region. As a result, the Josephson current through the structure must display an anomalous current phase relation due to the SOC in the non-superconducting part. Following theoretical proposals, experiments will be performed both on devices with a linear geometry (nanowire) and on ones with an annular (ring) geometry. The direct observation of Rashba SOC spin-dependent Andreev states will be a milestone on the road to observe two major predictions in solid state physics : Majorana fermions on one hand and the Aharonov-Casher effect on the other one.