期刊
NANO LETTERS
卷 15, 期 6, 页码 4150-4154出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.nanolett.5b01274
关键词
Ultrathin films; time-resolved photoemission; coherent phonons; high-temperature superconductivity
类别
资金
- U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division [DE-AC02-76SF00515]
- Stanford Graduate Fellowship
- Swiss National Science Foundation [P30022-151328]
- National Science Foundation [PHYS-1066293]
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 pumpprobe 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.
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