Superconductivity in infinite layer nickelate films using aluminium and in-situ synthesis*

The addition of the nickelates to the list of non-conventional superconductors represents an opportunity to understand the physics of these materials and develop new superconductors. An additional twist has been added through the recent discovery of Europium as a hole dopant to the Ni t2g bands in Nd_1-xEu_xNiO₂. This material is fabricated by first growing a perovskite parent material, Nd_1-xEu_xNiO₃ using oxide molecular beam epitaxy and subsequently reducing it by depositing Al metal in ultra high vacuum. This process results in superconducting films with a Tc as high as 21 K. Measurements of x-ray absorption spectroscopy as a function of doping concentration x, point to a novel aliovalent hole doping mechanism, where the Eu 4f bands lie near the Fermi level and leading to the coexistence of Eu³⁺ and Eu²⁺ oxidation states. We present x-ray spectroscopic measurements to understand the microscopic mechanism of this doping. The role of magnetism in this material is explored using a combination of high-field (35 T) magneto transport measurements, where the upper critical field is found to exceed the Pauli limit. Through this combination of synthesis and characterization we highlight the unique properties of this new Eu-based superconductor.