Séminaire TQM: Nicolas Bergeal (ESPCI)

Superconducting oxides interfaces

The achievement of high-quality epitaxial interfaces involving transition metal oxides offers
a unique opportunity to design artificial materials that host novel electronic phases. The discovery
of a high mobility two-dimensional electron gas (2-DEG) confined in a quantum well at the
interface between two insulating oxides LaAlO3 and SrTiO3 is perhaps one of the most prominent
examples in the field[1]. Unlike more conventional semiconductor based quantum wells,
conducting electrons at LaAlO3/SrTiO3 fill 3d-bands, which gives a favourable ground for the
emergence of complex electronic phases. In particular, 2D superconductivity [2,3] and strong
Rashba spin orbit coupling [4] have been reported in such interfaces. More recently, the discovery
of a superconducting 2-DEG in (111)-oriented KTaO3-based heterostructures injected new
momentum into the realm of oxide interfaces [5,6]. In this system, the superconducting
Tc can exceed 2K, nearly an order of magnitude higher than that observed in SrTiO3-based
interfaces. Additionally, the increased mass of Ta compared to Ti leads to significantly enhanced
spin-orbit effects, as recently demonstrated [7]. Consequently, KTaO3-based 2-DEGs have the
potential to enable the realization of topological superconducting phases—a concept originally
proposed for SrTiO3-based 2-DEGs but hitherto unattainable due to the limitations of the relevant
energy scales.

A key feature of these electronic systems lies in the possibility to control their carrier density
by electric field effect, which results in gate-tunability of both superconductivity and Rashba spin-
orbit coupling. In this talk, I will review complementary dc and microwave transport measurements
conducted on SrTiO3 and KTaO3-based interfaces employing both back-gate and top-gate
configurations. I will discuss, in particular, gate-induced multigap superconductivity [8,9] and the
role of phase fluctuations within the Berezinskii-Kosterlitz-Thouless model [10]. I will also present
the realization of field effect devices whose physical properties, including superconductivity and
Rashba spin-orbit coupling, can be tuned over a wide range of electrostatic doping, and discuss the
potential of oxides interfaces for the realization of mesoscopic devices [11].

[1] A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004).
[2] A. Caviglia et al., Nature 456, 624–627 (2008).
[3] J. Biscaras et al., Nature Communications 1, 89 (2010).
[4] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010).
[5] C. Liu, et al. Science 371, 716–721(2021).
[6] Chen, Z. et al. Science 372, 721–724 (2021).
[7] Vicente-Arche, L. M. et al. Adv. Mater. 2102102 (2021).
[7] S. Varotto, et al. Nature Commun. 13, 6165 (2022).
[8] G. Singh et al., Nature Mat. 18, 948–954 (2019).
[9] G. Singh et al., Phys. Rev. B 105, 064512 (2022).
[10] Mallik et al. Nature Commun. 13, 4625 (2022).
[11] A.Jouan et al. Nature Elec. 3, 201–206 (2020).