Quasiparticle lifetimes in metallic quantum-well nanostructures

Abstract

Quasiparticle lifetimes in metals as described by Fermi-liquid theory1 are essential in surface chemistry2 and determine the mean free path of hot carriers3. Relaxation of hot electrons is governed by inelastic electron–electron scattering, which occurs on femtosecond timescales owing to the large scattering phase space competing with screening effects4. Such lifetimes are widely studied by time-resolved two-photon photoemission5,6, which led to understanding of electronic decay at surfaces6,7,8. In contrast, quasiparticle lifetimes of metal bulk5,9,10,11,12 and films11,13,14,15 are not well understood because electronic transport10,16,17 leads to experimental lifetimes shorter than expected theoretically13,15,18. Here, we lift this discrepancy by investigating Pb quantum-well structures on Si(111), a two-dimensional model system19,20,21,22,23,24,25,26,27,28,29. For electronic states confined to the film by the Si bandgap we find quantitative agreement with Fermi-liquid theory and ab initio calculations4,7 for bulk Pb, which we attribute to efficient screening. For states resonant with Si bands, extra decay channels open for electron transfer to Si, resulting in lifetimes shorter than expected for bulk. Thereby we demonstrate that for understanding electronic decay in nanostructures coupling to the environment is essential, and that even for electron confinement to a few ångströms Fermi-liquid theory for bulk can remain valid.