Diagnosing critical fluctuations at superconducting transition via nitrogen-vacancy centers in diamond

We demonstrate the use of a thin layer of nitrogen-vacancy (NV) centers in diamond to probe local magnetic fluctuations near phase transition of the high temperature superconductor BSCCO at Tc = 93K. By directly exfoliating the BSCCO flakes onto diamond surface, we characterize the associated static and dynamic magnetic signals at the NV probes as a function of temperature and spatial location. For static magnetic signals, we observe the characteristic Meissner effect below the superconducting transition temperature. For dynamic magnetic noise, notably, we observe that the spin relaxation time, T1, of NV sensors exhibit a substantial drop near the transition temperature, originating from the enhanced magnetic noise due to critical fluctuations in the superconductor. We develop a theoretical model to capture our experimental observations. Our results demonstrate a noninvasive method to probe the rich physics of superconductivity.