Point-contact spectroscopy of the relaxation dynamics of two-level systems upon structural changes in Ni–Nb glasses

Abstract

The point-contact spectra (the energy dependence of the second derivatives of the current–voltage characteristics) of bimetal contacts of the amorphous alloy Ni59Nb41 with silver are investigated in the frequency range 103–5×109 Hz. At low energies the spectral feature due to the scattering of conduction electrons on two-level tunneling systems in point contacts of the quenched alloy is clearly observed in the microwave region. This confirms the conclusion reached previously by the authors for homocontacts [O. P. Balkashin et al., Solid State Commun. 118, 623 (2001)] that rapidly relaxing two-level systems with a characteristic relaxation frequency Γ>1011 Hz are present in metallic glasses. After a high-temperature annealing of the amorphous alloy (for 1 hour at T=800 °C) and its crystallization, the intensity of the spectral feature in the region of low voltages across the contact increases substantially. A sharp frequency dispersion of the amplitude of this feature is observed; it vanishes completely for measurements at frequencies below ∼5×109 Hz. The data attest to the formation of qualitatively new, slowly relaxing two-level systems in the annealed Ni–Nb amorphous alloys. An estimate of the characteristic relaxation frequency of the two-level systems from the frequency dispersion of the measured signal gives a value Γ/2π≈0.7×109 Hz. The possible causes of the formation of two-level systems in point contacts are discussed.