Magnetic response of a metal to an external magnetic field remains independent of the frequency of an ac magnetic field as temperature is reduced. Here, we report the discovery of an anomalous but enormous enhancement of the ac frequency dependent diamagnetic susceptibility below $T\simeq 100$ K in metallic NiSi. In addition, magnetic measurements of NiSi in applied magnetic field depict strong irreversibility in the diamagnetic responses of zero-field cooled and field cooled curves below $T\simeq 55$ K. Even though the metallic behavior of NiSi is primarily manifested to the lowest temperature of $T=1.5$ K, a small downward cusp around $T\simeq 75$ K is also detected in electrical measurement. These observations suggest the existence of a minority superconducting phase in NiSi, which bridges the gap between metal and superconductor. Consequently, this novel intermediate regime provides a new perspective to the development of unconventional superconductors.

• Received 6 June 2016
• Revised 6 November 2016

DOI:https://doi.org/10.1103/PhysRevB.94.184516