High-frequency behavior of metallic glass NixNb1−x point-contacts

Electron - two level system interaction in Ni59Nb41 metallic glass point contacts is studied by means of conventional and high-frequency point-contact spectroscopy, at frequencies of 3.6 kHz, 0.78 GHz and 64 GHz. The amplitude of the zero bias anomaly exhibits only a 25% decrease under microwave irradiation of 64 GHz. This implies that elastic electron interaction with highly-asymmetric slow two-level fluctuators explains only a small part in the anomaly, and the main contribution is due to fast nonmagnetic Kondo-like scattering. Point contacts of Ni44Nb56 in the superconducting state were also studied. The value of the energy gap was evaluated as Δ0∼0.4 meV. Under microwave irradiation with increasing power a gradual decrease of the critical current was observed.