Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO₃ films

Journal Article · Mon Jun 01 00:00:00 EDT 2015 · Nano Letters

DOI:https://doi.org/10.1021/acs.nanolett.5b01274· OSTI ID:1183694

Yang, Shuolong ^[1]; Sobota, Jonathan A. ^[2]; Leuenberger, Dominik ^[1]; Kemper, Alexander F. ^[3]; Lee, James J. ^[1]; Schmitt, Felix T. ^[1]; Li, Wei ^[1]; Moore, Rob G. ^[1]; Kirchmann, Patrick S. ^[4]; Shen, Zhi -Xun ^[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)

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 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.

• Research Organization: SLAC National Accelerator Lab., Menlo Park, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: AC02-76SF00515
• OSTI ID: 1183694
• Report Number(s): SLAC-PUB-16301
• Journal Information: Nano Letters, Vol. 15, Issue 6; ISSN 1530-6984
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
MATSCI
ultrathin films
time-resolved photoemission
coherent phonons
high-temperature superconductivity