Anisotropic Upper Critical Field beyond the Pauli Limit in a Nickelate Superconductor: Evidence for a Quantum...

Superconductivity can be destroyed by a magnetic field with an upper bound known as the Pauli limit. Here, we report a large violation of the Pauli limit in both in-plane and perpendicular magnetic fields in La_{0.8}Sr_{0.2}NiO_{2} infinite-layer superconducting thin films. The critical field H_{c2} shows a clear anisotropic behavior and a pronounced upturn at ultralow temperatures, manifesting anisotropic superconductor-metal transition features. Scaling analysis near the T→0 quantum critical point of this transition reveals a quantum Griffiths singularity arising from quenched disorder, hallmarked by a diverging dynamic critical exponent. These quantum Griffiths singularity induced quantum fluctuations provide the origin for the pronounced H_{c2} upturn and represent a key intrinsic mechanism for the robust violation of the Pauli limit. Our findings establish a microscopic mechanism where heavy-fermion mass renormalization suppresses orbital pair breaking, enabling disorder-enhanced quantum fluctuations to preserve superconductivity far beyond the Pauli limit.