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

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• 作者：Shuolong Yang ; Jonathan A. Sobota ; Dominik Leuenberger ; Alexander F. Kemper ; James J. Lee ; Felix T. Schmitt ; Wei Li ; Rob G. Moore ; Patrick S. Kirchmann ; Zhi-Xun Shen
• 刊名：Nano Letters
• 出版年：2015
• 出版时间：June 10, 2015
• 年：2015
• 卷：15
• 期：6
• 页码：4150-4154
• 全文大小：350K
• ISSN：1530-6992

文摘

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鈥損robe 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 卤 0.02 to 5.25 卤 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. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.