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We report electrical transport measurements on the putative heavy fermion superconductor YbRh₂Si₂ [1]. Measurements of the Nyquist noise were made on a high quality single crystal sample, over the temperature range 1K to 0.6 mK. On cooling below 12 mK, there is a clear transition from normal metal into a state in which the resistance decreases with decreasing temperature, initially approximately linearly, towards zero. We propose this to be a superconducting state, with phase fluctuations. The transition to a zero-resistance phase-coherent superconducting state is identified at 3.6 mK. The Nyquist noise was then studied in magnetic fields up to 9 mT, applied in plane, perpendicular to the c-axis. The zero resistance state was observed to be quenched above approximately 6 mT. At 9 mT we observe “re-entrance” of the normal state resistance at the lowest temperature. In the light of this critical field behavior, we discuss the nature of the superconductivity in the system, and the interplay with both electronic and nuclear magnetism. This work opens the prospect of tuning the magnetism by choice of Yb isotope in enriched samples, and studies at ultralow temperatures of electric transport at the field-tuned quantum critical point.
[1] E. Schuberth et al., Science, 351, 495 (2016).