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 ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>T</mi> <mi>c</mi> <mi>onset</mi></msubsup> <annotation>$T_{\rm{c}}^{{\rm{onset}}}$</annotation></semantics> </math> = 18.8 K, <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msubsup><mi>T</mi> <mi>c</mi> <mi>zero</mi></msubsup> <annotation>$T_{\rm{c}}^{{\rm{zero}}}$</annotation></semantics> </math> = 16.5 K). The upper critical fields are anisotropic and violate the estimated Bardeen-Cooper-Schrieffer (BCS) Pauli limit ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow><msub><mi>H</mi> <mrow><mi>Pauli</mi> <mo>,</mo> <mi>μ</mi> <mo>=</mo> <mn>1</mn> <msub><mi>μ</mi> <mi>B</mi></msub> </mrow> </msub> <mo>=</mo> <mn>1.86</mn> <mo>×</mo> <mrow></mrow> <msub><mi>T</mi> <mrow><mi>c</mi> <mo>,</mo> <mi>H</mi> <mo>=</mo> <mn>0</mn></mrow> </msub> </mrow> <annotation>${H}_{\mathrm{Pauli},\mu =1{\mu}_{B}}=1.86\ensuremath{\times{}}{T}_{\mathrm{c},H=0}$</annotation></semantics> </math> ) 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.