Robust quantum Griffiths singularity above 1.5 K in nitride thin films

Quantum Griffiths singularity (QGS), which is closely correlated with the quenched disorder, is characterized by the divergence of the dynamical critical exponent when approaching zero temperature and the applicability of activated scaling. Typically such a phenomenon is rather rare and only observed in extremely low temperatures. Here we report the experimental observation of a robust QGS in NbN thin films, which survives in a rather high temperature range. The electrical transport properties were measured under the magnetic field up to 12 T. The field induced superconductor-metal transitions were observed with the continuously changed transition points with the decrease of temperature. The dynamical critical exponent based on the conventional power-law scaling reveals a divergent trend when approaching the low-temperature limit. Moreover, the temperature and field dependence of sheet resistance can be described by the activated scaling analysis in the temperature region up to 1.6 K $\ensuremath{\le}T\ensuremath{\le}$ 4.0 K. We argue that the robustness of QGS in the present system originates from the pronounced Pauli paramagnetic effect due to the strong coupling between magnetic field and the spin degree of freedom of the superconducting electrons.