The realization of two-dimensional electronic gases (2DEGs) in oxide-based heterostructures has led to important discoveries [1,2] about superconductivity in low dimensions, such as the observation of pairing interactions without superconductivity [3] and density-of-states features resembling the pseudogap in cuprates [4]. Consequently, this 2DEG has emerged as a model system to study the physics of Cooper pair formation in two dimensions and to gain useful insights about complex problems, e.g., the phase diagram of high temperature superconductors.

In this talk, we will focus on: i) the facile realization of a 2DEG in AlOx/SrTiO3 using a method devised in our group that is more accessible to researchers than conventional hetero-epitaxial techniques, and ii) the phenomenon of gate-tunable superconductivity in two dimensions.

A conducting 2DEG can be realized on the bare surface of SrTiO3 by the creation of oxygen vacancies [5]. The electrons populate the bands arising from Ti 3d orbitals. In a recent work (Rodel et al., Advanced Materials, 28:1976–1980, 2016), we have shown that the deposition in ultra-high vacuum of a thin layer of metallic Al on SrTiO3 leads to the creation of the same 2DEG due to the withdrawal of oxygen atoms from the surface by the reducing agent Al (which turns into insulating AlOx). The resulting heterostructure, AlOx/SrTiO3, is suitable for transport experiments because the layer of AlOx protects the 2DEG in ambient conditions against the percolation of oxygen from air. From transport experiments, we have determined that the 2DEG is superconducting with a critical temperature of 320 mK. The critical parameters (temperature and field) are tunable with the gate voltage, leading to a 'superconducting dome' in the phase diagram. Phase fluctuations of the order parameter play a dominant role in the phenomenon of superconductivity in two dimensions. By continuously varying the carrier density, we have been able to tune the 2DEG across different regimes of electronic interactions that influence the phenomenon of Cooper pair condensation. We will present our results on this topic.

References:

[1] Reyren et al., Science 317, 1196 (2007)

[2] Caviglia et al., Nature 456, 624 (2008)

[3] Cheng et al., Nature 521, 196 (2015)

[4] Richter et al., Nature 502, 528 (2013)

[5] Santander-Syro et al., Nature 469, 189 (2011)