Experimental observation of electron-phonon coupling enhancement in Sn nanowires caused by phonon confinement effects

Abstract

Reducing the size of a superconductor below its characteristic length scales can either enhance or suppress its critical temperature (${T}_{c}$). Depending on the bulk value of the electron-phonon coupling strength, electronic and phonon confinement effects will play different roles in the modification of ${T}_{c}$. Experimentally disentangling each contribution has remained a challenge. We have measured both the phonon density of states and ${T}_{c}$ of Sn nanowires with diameters of 18, 35, and 100 nm in order to quantify the effects of phonon confinement on superconductivity. We observe a shift of the phonon frequency towards the low-energy region and an increase in the electron-phonon coupling constant that can account for the measured increase in ${T}_{c}$.