Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO$_{3}$ Films

TL;DR

This study investigates how the frequency of a specific phonon mode in ultrathin FeSe films on SrTiO$_{3}$ varies with film thickness, revealing a substrate-induced phonon softening effect through femtosecond spectroscopy.

Contribution

It provides the first detailed measurement of thickness-dependent coherent phonon frequencies in FeSe/SrTiO$_{3}$ films, highlighting the impact of substrate-induced strain on phonon behavior.

Findings

Phonon frequency increases with film thickness from 5.00 to 5.25 THz.

The observed mode is identified as the Se A$_\textrm{1g}$ phonon.

Substrate-induced lattice strain causes phonon softening in ultrathin films.

Abstract

Ultrathin FeSe films grown on SrTiO$_{3}$ substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. Using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO$_{3}$ films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1 unit cell, 3 unit cell, and 60 unit cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00(2) to 5.25(2) THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A$_\textrm{1g}$ phonon. The dominant mechanism for the phonon softening in 1 unit cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.