Vincent Humbert : Giant Photoconductance and (Photo-)Superconductivity in Nickelates

The ability to optically manipulate the electronic ground state of complex oxides and their heterostructures offers transformative opportunities for both fundamental research and technological applications^1,2, while the recent discovery of superconductivity in nickelates has established these compounds as a new platform for exploring the interplay between correlated electronic states and unconventional superconductivity in complex oxides. While sharing properties with cuprates, their fabrication is inherently linked with a unique structural change tuned by a topotactic transformation through chemical reduction³. We recently focused on nickelates with our collaborator, probing different ways to reduce this complex oxide^4,5 to study the associated electronic states in both the non-superconducting and superconducting films.

In this seminar, I will start by presenting light-induced effects in partially reduced NdNiO₂ and show that near-ultraviolet illumination can instantaneously generate a volatile, high-mobility two-dimensional electron gas at the interface with SrTiO₃⁶. By integrating transport measurements, STEM-EELS observations, and density functional theory, we demonstrate that this dramatic effect arises from structural and electronic reconstructions at the NdNiO₂//SrTiO₃ interface, combined with a built-in interfacial electric field that enables photogenerated carriers to occupy the Ti 3dₓᵧ band.
           I will then discuss recent results on fully reduced Pr_0.8Sr_0.2NiO₂ thin films on the vortex liquid-to-glass transition in the superconducting mixed state⁷. In particular, we found that films away from optimal doping show a purely two-dimensional transition, revealing that superconductivity is intrinsic to individual NiO₂ planes, as supported by scaling theory and a coherence length along the c-axis shorter than the interplane spacing. This is reminiscent of observations in cuprates, where a crossover from 3D to purely 2D vortex glass is driven by a change of oxygen content⁸. Finally, I will briefly discuss persistent photoconductance and photosuperconductivity effects in Pr_0.8Sr_0.2NiO₂ and compare them to results in other superconducting oxides measured in our group.

1.    ElHage, R. et al. Bimodal ionic photomemristor based on a high-temperature oxide superconductor/semiconductor junction. Nat. Commun. 14, (2023).
2.    ElHage, R. et al. Disentangling photodoping, photoconductivity, and photosuperconductivity in the cuprates. Physical Review Letters 132, 066001 (2024).
3.    Li, D. et al. Superconductivity in an infinite-layer nickelate. Nature 572, 624–627 (2019).
4.    Humbert, V. et al. An oxygen vacancy memristor ruled by electron correlations. Advanced Science 9, 2201753 (2022).
5.    Zhang, D. et al. Accessible synthesis of superconducting nickelates via topotactic reduction induced by aluminum sputter deposition. Commun. Mater. (2025).
6.    Sanchez-Manzano, D. et al. Giant photoconductance at infinite-layer nickelate/SrTiO3 interfaces via an optically induced high-mobility electron gas. Nat. Mater. 25, 49–57 (2026).
7.    Sanchez-Manzano, D. et al. Purely two-dimensional vortex matter in infinite-layer nickelates. arXiv preprint arXiv:2410.14341 (2024).
8.    Sefrioui, Z. et al. Crossover from a three-dimensional to purely two-dimensional vortex-glass transition in deoxygenated YBa2Cu3O7−δ thin films. Phys. Rev. B 60, 15423 (1999).