"Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO3 Films"

Shuolong Yang: Jonathan A. Sobota, Dominik Leuenberger, Alexander F. Kemper, James J. Lee, Felix T. Schmitt, Wei Li, Rob G. Moore, Patrick S. Kirchmann, and Zhi-Xun Shen; Nano Letters, 06/01/15.

Additional Authors: Jonathan A. Sobota, Dominik Leuenberger, Alexander F. Kemper, James J. Lee, Felix T. Schmitt, Wei Li, Rob G. Moore, Patrick S. Kirchmann, and Zhi-Xun Shen

Abstract:

Ultrathin FeSe films grown on SrTiO3 substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO3 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 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A1g 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.