Thickness-dependent electron momentum relaxation times in iron films

TL;DR

This study uses terahertz spectroscopy to measure how electron momentum relaxation times in iron films vary with thickness, revealing a distribution of relaxation times rather than a single universal value.

Contribution

It demonstrates that the macroscopic response time in iron films depends on a distribution of microscopic relaxation times influenced by film thickness.

Findings

Response time decreases with thinner films

No single relaxation time explains electron dynamics

Variation in microscopic relaxation times depends on film thickness

Abstract

Terahertz time-domain conductivity measurements in 2 to 100 nm thick iron films resolve the femtosecond time delay between applied electric fields and resulting currents. This current response time decreases from 29 fs for thickest films to 7 fs for the thinnest films. The macroscopic response time is not strictly proportional to the conductivity. This excludes the existence of a single relaxation time universal for all conduction electrons. We must assume a distribution of microscopic momentum relaxation times. The macroscopic response time depends on average and variation of this distribution; the observed deviation between response time and conductivity scaling corresponds to the scaling of the variation. The variation of microscopic relaxation times depends on film thickness because electrons with different relaxation times are affected differently by the confinement since they have different mean free paths.