Recovery of the electron–phonon interaction function in superconducting tantalum ballistic contacts

Abstract

The experimentally observed nonlinearities of the current-voltage characteristics (CVCs) of tantalum-based point homo-and hetero-contacts in both normal and superconducting states related to electron-phonon interaction (EPI) were analyzed. It was taken into account that additional nonlinearity of CVCs arising upon contact transition to the superconducting state (superconducting spectral component) is formed not only near the constriction in the region roughly equal to the contact diameter (as is the case for the normal state, and as predicted theoretically for the superconducting state), but also in a markedly larger region that is about the size of the coherence length. In this case, a considerable role in the formation of this superconducting component is played by nonequilibrium phonons with low group velocity, which account for the experimentally observed sharpening of the phonon peaks in the EPI spectra (the second derivatives of the CVCs) during the superconducting transition of the contacts, instead of the theoretically expected peak broadening (spreading), and for the increase in the superconducting contribution to the point contact spectrum in the low and medium energy regions. The high-energy part of the EPI spectrum changes much less significantly during the superconducting transition, which is attributable to suppression of the excess contact current by nonequilibrium quasi-particles. A detailed procedure was proposed for the recovery of the EPI spectral function from the point contact spectrum contribution (the second derivative of the CVC) that arises during the superconducting transition of one or both contacting metals.

Spectroscopy of electron-phonon interaction of superconducting point contacts: experimental aspects

“…A different matter is whether these variations are critically significant. If all the EPI functions reconstructed from the superconducting addition for tantalum homo-and heterocontacts are compared with each other (see review [22]), their variations do not exceed the variations of the EPI functions reconstructed from the normal state. As for 𝑁 𝑏𝑆𝑒 2 , the uncertainty is in this case significantly larger, resulting in variations in the shape of the reconstructed EPI function.…”

“…However, the agreement between theory and experiment demonstrated in Subsection 4.1 is not always the case, even for ballistic superconducting point contacts. Tantalum-based contacts are one such example [22]. Deviations from theoretical predictions are observed for both heterocontacts and homocontacts.…”