Isotropic Quantum Griffiths Singularity in Nd_{0.8}Sr_{0.2}NiO_{2} Infinite-Layer Superconducting Thin Films

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Phenomena

Background:

• Unconventional superconductors like infinite layer nickelates are crucial for understanding exotic electronic states.
• Magnetic field-induced superconductor-metal transitions (SMT) provide a unique platform to study quantum criticality.
• Quantum Griffiths singularity (QGS) is a theoretical concept that has been experimentally challenging to confirm.

Purpose of the Study:

• To investigate the emergence of QGS in Nd_{0.8}Sr_{0.2}NiO_{2} infinite layer superconducting thin films.
• To characterize the nature (isotropic or anisotropic) of the SMT and associated QGS under magnetic fields.
• To explore the interplay between QGS and Kondo scattering in this unconventional superconductor.

Main Methods:

• Fabrication of Nd_{0.8}Sr_{0.2}NiO_{2} infinite layer superconducting thin films.
• Measurement of isothermal magnetoresistance curves under in-plane and perpendicular magnetic fields.
• Scaling analysis of magnetoresistance data to extract critical exponents.
• Analysis of resistance-temperature (R(T)) relations at different temperature ranges.

Main Results:

• Observed isotropic superconductor-metal transition (SMT) features under both in-plane and perpendicular magnetic fields.
• Identified QGS characteristics through scaling analysis of magnetoresistance, showing divergent effective dynamic critical exponents near the zero-temperature critical point (B_{c}^{*}).
• Quantum fluctuations associated with QGS quantitatively explain the upturn of the upper critical field around zero temperature.
• Observed Kondo scattering signatures (lnT and T^{2} dependence of resistance) in the metallic state at higher magnetic fields.

Conclusions:

• The Nd_{0.8}Sr_{0.2}NiO_{2} thin films exhibit an isotropic QGS associated with the magnetic field-induced SMT.
• Quantum fluctuations play a significant role in the phase boundary of the SMT.
• The interplay between isotropic QGS and Kondo scattering suggests the importance of rare regions and provides insights into the exotic superconductivity mechanism in this material.