Phonon heat capacity and self-heating normal domains in NbTiN nanostrips

TL;DR

This study investigates the thermal properties of NbTiN nanostrips, revealing a reduced phonon heat capacity and unique heat flow behavior, which are crucial for understanding thermal management in superconducting devices.

Contribution

It demonstrates that the phonon heat capacity in NbTiN nanostrips is lower than expected and links this to grain size effects on phonon spectra, advancing understanding of thermal transport in these materials.

Findings

Reduced phonon heat capacity compared to 3D Debye model

Heat flow proportional to (Te^p - TB^p) with p~3

Grain size influences phonon spectrum and thermal behavior

Abstract

Self-heating normal domains in thin superconducting NbTiN nanostrips were characterized via steady-state hysteretic current-voltage characteristics measured at different substrate temperatures. The temperature dependence and the magnitude of the current, which sustains a domain in equilibrium at different voltages, can only be explained with a phonon heat capacity noticeably less than expected for 3-d Debye phonons. This reduced heat capacity coincides with the value obtained earlier from magnetoconductance and photoresponse studies of the same films. The rate of heat flow from electrons at a temperature Te to phonons in the substrate at a temperature TB is proportional to (T_e^p - T_B^p) with the exponent p~3, which differs from the exponents for heat flows mediated by the electron-phonon interaction or by escaping of 3-d Debye phonons via the film/substrate interface. We attribute both findings to the effect of the mean grain size on the phonon spectrum of thin granular NbTiN films. Our findings are significant for understanding the thermal transport in superconducting devices exploiting thin granular films.