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Title: Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO 3 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. By 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 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A 1g phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.

Authors:
 [1];  [2];  [1];  [3];  [1];  [1];  [1];  [1];  [4];  [1]
  1. SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States)
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  4. SLAC National Accelerator Lab., Menlo Park, CA (United States)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1183694
Report Number(s):
SLAC-PUB-16301
Journal ID: ISSN 1530-6984
DOE Contract Number:
AC02-76SF00515
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nano Letters; Journal Volume: 15; Journal Issue: 6
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; MATSCI; ultrathin films; time-resolved photoemission; coherent phonons; high-temperature superconductivity

Citation Formats

Yang, Shuolong, Sobota, Jonathan A., Leuenberger, Dominik, Kemper, Alexander F., Lee, James J., Schmitt, Felix T., Li, Wei, Moore, Rob G., Kirchmann, Patrick S., and Shen, Zhi -Xun. Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO3 films. United States: N. p., 2015. Web. doi:10.1021/acs.nanolett.5b01274.
Yang, Shuolong, Sobota, Jonathan A., Leuenberger, Dominik, Kemper, Alexander F., Lee, James J., Schmitt, Felix T., Li, Wei, Moore, Rob G., Kirchmann, Patrick S., & Shen, Zhi -Xun. Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO3 films. United States. doi:10.1021/acs.nanolett.5b01274.
Yang, Shuolong, Sobota, Jonathan A., Leuenberger, Dominik, Kemper, Alexander F., Lee, James J., Schmitt, Felix T., Li, Wei, Moore, Rob G., Kirchmann, Patrick S., and Shen, Zhi -Xun. Mon . "Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO3 films". United States. doi:10.1021/acs.nanolett.5b01274. https://www.osti.gov/servlets/purl/1183694.
@article{osti_1183694,
title = {Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO3 films},
author = {Yang, Shuolong and Sobota, Jonathan A. and Leuenberger, Dominik and Kemper, Alexander F. and Lee, James J. and Schmitt, Felix T. and Li, Wei and Moore, Rob G. and Kirchmann, Patrick S. and Shen, Zhi -Xun},
abstractNote = {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. Results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.},
doi = {10.1021/acs.nanolett.5b01274},
journal = {Nano Letters},
number = 6,
volume = 15,
place = {United States},
year = {Mon Jun 01 00:00:00 EDT 2015},
month = {Mon Jun 01 00:00:00 EDT 2015}
}