Ultrafast generation and detection of propagating coherent acoustic phonon wave packets in ultra-thin iron pnictide films

TL;DR

This study demonstrates ultrafast optical techniques to generate and detect coherent acoustic phonon wave packets in ultra-thin iron pnictide films, revealing thickness-dependent mechanical properties and phonon dynamics.

Contribution

It provides the first detailed analysis of propagating acoustic phonons in ultra-thin BaFe extsubscript{2}As extsubscript{2} films, linking film thickness to Young's modulus and phonon behavior.

Findings

Oscillations in differential reflectivity linked to propagating LA phonons.

Young's modulus increases as film thickness decreases.

Phonon frequency approaches 50 GHz in ultra-thin films.

Abstract

We observe pronounced oscillations in differential reflectivity of 9 nm and 60 nm BaFe\textsubscript{2}As\textsubscript{2} (Ba-122) thin films using ultrafast optical spectroscopy. Our studies show that the oscillations result from propagating longitudinal acoustic (LA) phonon wave packets with strong thickness and temperature dependence. Particularly, the experimentally measured oscillation frequency approaches to 50 GHz for the ultra-thin film. Our calculations show that Young's modulus of 9 nm thin film is nearly four times as large as that of 60 nm thin film, consistent with the experiment. The increase in Young's modulus as thickness decrease was attributed to the decrease in parent Ba-122 tetragonality $c/a$ near the film-substrate interface due to material-substrate mismatch effect. %Temperature dependence of LA phonon mode frequency for 9 nm Ba-122 thin film is reported. The temperature-dependent change in LA phonon frequency was attributed to the change in parent Ba-122 othorhombicity $(a-b)/(a+b)$.