Magnetoconductance and photoresponse properties of disordered NbTiN films

TL;DR

This study investigates phonon properties and electron-phonon interactions in disordered NbTiN films using magnetoconductance and photoresponse techniques, revealing reduced sound velocities and altered heat capacities compared to bulk materials.

Contribution

It provides new insights into phonon dynamics and electron-phonon coupling in disordered NbTiN films, with systematic comparisons to other superconducting films.

Findings

Electron-phonon scattering rate varies as T^{3.45} from 12 to 20 K.

Sound velocity is reduced by about 50% in disordered films compared to bulk.

Phonon heat capacities are increased but less than Debye model predictions.

Abstract

We report on the study of phonon properties and electron-phonon coupling in thin NbTiN films, which are intensively exploited in superconducting devices. Studied NbTiN films with thicknesses less than 10~nm are disordered with respect to electron transport, the Ioffe-Regel parameter of $k_F l_e = 2.5-3.0$ ($k_F$ is the Fermi wave vector and $l_e$ is the electron mean free path), and inelastic electron-phonon interaction, the product $q_T l_e \ll 1$ ($q_T$ is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derived the inelastic electron-phonon scattering rate $1/\tau_{e-ph}$ and determined sound velocities and phonon heat capacities. In the temperature range from 12 to 20~K, the scattering rate varies with temperature as $1/\tau_{e-ph}\propto T^{3.45\pm0.05}$; its value extrapolated to 10~K amounts to approximately 16~ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we found a systematic reduction of the sound velocity in all these films by about 50\% as compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology.