We developed the model of the internal phonon bottleneck to describe the energy exchange between the acoustically soft ultrathin metal film and acoustically rigid substrate. Discriminating phonons in the film into two groups, escaping and nonescaping, we show that electrons and nonescaping phonons may form a unified subsystem, which is cooled down only due to interactions with escaping phonons, either due to direct phonon conversion or indirect sequential interaction with an electronic system. Using an amplitude-modulated absorption of the sub-THz radiation technique, we studied electron-phonon relaxation in ultrathin disordered films of tungsten silicide. We found an experimental proof of the internal phonon bottleneck. The experiment and simulation based on the proposed model agree well, resulting in ${\tau }_{\mathrm{e}-\mathrm{ph}}\sim 140–190$ ps at $T_{\mathrm{C}}=3.4\mathrm{K}$, supporting the results of earlier measurements by independent techniques.

• Received 9 July 2017
• Revised 5 March 2018

DOI:https://doi.org/10.1103/PhysRevB.97.184512