Evidence for Anisotropic Superconductivity Beyond Pauli Limit in Infinite-Layer Lanthanum Nickelates

Abstract

After being expected to be a promising analog to cuprates for decades, superconductivity has recently been discovered in infinite-layer nickelates, providing new opportunities to explore mechanisms of high-temperature superconductivity. However, in sharp contrast to the single-band and anisotropic superconductivity in cuprates, nickelates exhibit a multi-band electronic structure and an unexpected isotropic superconductivity as reported recently, which challenges the cuprate-like picture in nickelates. Here, it is shown that strong anisotropic magnetotransport behaviors exist in La-based nickelate films with enhanced crystallinity and superconductivity ( $T_c^{\mathrm{onset}}$ = 18.8 K, $T_c^{\mathrm{zero}}$ = 16.5 K). The upper critical fields are anisotropic and violate the estimated Bardeen-Cooper-Schrieffer (BCS) Pauli limit ( $H_{\mathrm{Pauli},\mu =1{\mu }_B}=1.86\times T_{c,H=0}$ ) for in-plane magnetic fields. Moreover, the anisotropic superconductivity is further manifested by the cusp-like peak of the angle-dependent T_c and the vortex motion anisotropy under external magnetic fields.

Keywords: anisotropic superconductivity; high-temperature superconductivity; infinite-layer nickelates; magnetotransports; molecular beam epitaxy; thin films.