At the interface between insulating oxides such as SrTiO3/LaAlO3 or LaTiO3/SrTiO3, a superconducting two-dimensional electron gas (2DEG) has been discovered [1-3], whose carrier density can be tuned by applying a gate voltage. The unique possibility of modulating the superfluid density easily and continuously opens new perspectives to tackle fundamental issues in condensed matter physics, such as the Superconductor to Insulator Quantum Phase Transition (QPT) in a two-dimensional system.
Using two different external parameters, the magnetic field and the electric field, we explored the phase diagram of the 2DEG. Studying the magnetic-field driven QPT, we first evidence that the system can be described as a disordered array of superconducting puddles, and show that the critical behavior belongs to the (2+1)D XY universality class in the clean limit at the local scale, and to the dirty limit at the scale of the array, in agreement with the Harris criteria [4].
The electric-field driven QPT reveals an anomalous critical behavior, that we attribute to density driven superconducting fluctuations, different from the regular amplitude and phase fluctuations in a superconductor. We propose a scenario where the observed inhomogeneities origin from an intrinsic electronic phase separation, driven by the non-rigidity of the bands upon filling and Rashba Spin-Orbit coupling [5,6]. Superconductivity being intimately related to the electronic density, the dynamics in the Cooper pair channel is dominated by nearly critical dynamical density fluctuations.
References
[1] N. Reyren et al, Science 317, 1196 (2007)
[2] J. Biscaras et al , Nature Communications 1, 89 (2010)
[3] J. Biscaras et al, Physical Review Letters 109, 246808 (2012)
[4] J. Biscaras et al, Nature Materials 12, 542 (2013)
[5] N. Scopigno et al, Physical Review Letters 116, 026804 (2016)
[6] S. Hurand et al, arXiv 1506.06874v1
Laboratoire de Physique et d’Etude des Matériaux – ESPCI-CNRS-UPMC, PSL Research University, 75005 Paris