Isotropic Quantum Griffiths Singularity in Nd0.8Sr0.2NiO2 Infinite-Layer Superconducting Thin Films

This work reports on the emergence of quantum Griffiths singularity (QGS) associated with the magnetic field induced superconductor-metal transition (SMT) in unconventional Nd0.8Sr0.2NiO2 infinite layer superconducting thin films. The system manifests isotropic SMT features under both in- plane and perpendicular magnetic fields. Importantly, after scaling analysis of the isothermal magnetoresistance curves, the obtained effective dynamic critical exponents demonstrate divergent behavior when approaching the zero-temperature critical point B-c*, identifying the QGS characteristics. Moreover, the quantum fluctuation associated with the QGS can quantitatively explain the upturn of the upper critical field around zero temperature for both the in-plane and perpendicular magnetic fields in the phase boundary of SMT. These properties indicate that the QGS in the Nd0.8Sr0.2NiO2 superconducting thin film is isotropic. Moreover, a higher magnetic field gives rise to a metallic state with the resistance-temperature relation R ( T ) exhibiting lnT T dependence among the 2-10 K range and T-2 dependence of resistance below 1.5 K, which is significant evidence of Kondo scattering. The interplay between isotropic QGS and Kondo scattering in the unconventional Nd(0.8)Sr(0.2)NiO(2)superconductor can illustrate the important role of rare region in QGS and help to uncover the exotic superconductivity mechanism in this system.