Unconventional Superconductivity in Infinite-Layer Nickelates Probed by Controlled Introduction of Disorder

The discovery of superconductivity in thin films of hole-doped infinite-layer (Nd,Sr)NiO₂ marked a significant step in the avenue of superconducting oxides, offering a novel platform for investigating the mechanisms of high-temperature superconductivity and sparking widespread scientific interest. Since then, a variety of rare-earth and dopant combinations have been realized, also extending to the formally undoped parent phase. In parallel, recent advances in synthesis and reduction methods have raised T_c to 35 K in Sm(Eu,Ca,Sr)NiO₂ thin films.

A central question is the symmetry of the superconducting gap structure in these materials. However, probing this has remained an experimentally challenging task due to the restriction of superconducting samples to epitaxial thin films and surface degradation caused during the chemical reduction required to stabilise the Ni¹⁺ valence, which limits the applicability of techniques such as London penetration depth measurements via mutual inductance or tunnel diode oscillator methods, single-particle tunnelling, photoemission spectroscopy, and thermal transport, often employed to investigate superconducting paring symmetry.

To address this challenge, we employ high-energy electron irradiation to controllably introduce disorder in superconducting infinite-layer nickelates thin-films and examine the effect of pair-breaking defects on superconductivity to elucidate the nature of the superconducting gap. Our results reveal a complete suppression of T_c and an increase in normal-state resistivity with added disorder, suggesting the presence of an unconventional sign-changing gap symmetry¹. These observations provide valuable insights into the superconducting order parameter and electronic landscape of infinite-layer nickelates. More recently, we have extended these studies to also explore how these effects depend on choice of rare-earth ion, including Nd-, Pr-, and La-based nickelates.

¹Ranna, et al. Phys. Rev. Lett. 135, 126501 (2025)