Unconventional superconductivity in Nd_1-xEu_xNiO₂ thin films

The infinite-layer nickelates present a new class of superconductors that have intriguing similarities to the copper-based high T_c superconductors, providing a platform to study unconventional superconductivity. We investigate the upper critical magnetic field and superconducting gap of superconducting Nd_1-xEu_xNiO₂ (NENO) thin films with x=0.2 to 0.35, synthesized using Al reduction technique.¹ Magnetoresistance measurements reveal magnetic field enhanced superconductivity, which is the result of interaction between magnetic Eu ions and superconducting states in the Ni d_x2-y2 orbital. The upper critical magnetic fields, H_c2, for in-plane and out-of-plane magnetic fields are anisotropic and violate the weak-coupling Pauli limit H_c2,P=1.86T_c. Terahertz spectroscopy confirms a large gap-to-Tc ratio 2∆/k_BT_c, consistent with the Pauli limit violation. We interpret these results as evidence of strong electron-electron coupling and a large superconducting energy gap of NENO.



[1] Wei, W., Vu, D., et al. Sci Adv 9, 27 (2023).