Ultrafast Dynamics in Iron Chalcogenides (Na0.32K0.68)0.95Fe1.75Se2 Single Crystal by Using Dual-Color Femtosecond Spectroscopy
P. C. Cheng1*, C. W. Luo1, Y. R. Chu1, W. Y. Tzeng1, J.-Y. Lin2, K. H. Wu1, J. Y. Juang1, T. Kobayashi1,3, Alexey Golovanov4, Alexander Vasiliev4
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan
3Department of Applied Physics and Chemistry and Institute for Laser Science, University of Electro-Communications, Tokyo, Japan
4Low Temperature Physics and Superconductivity Department, Moscow State University, Moscow, Russian Federation
* presenting author:鄭博仲, email:nervtw@yam.com
In this study, we report focus on single-crystalline (Na0.32K0.68)0.95Fe1.75Se2. The magnetic properties were obtained by temperature-dependence magnetic moment M(T) and the onset of superconducting transition temperature is about 31 K. The femtosecond spectroscopy measurement was performed using a dual-color pump-probe system and the fluences of the pump beam and probe beam are 30.5 and 3.2 µJ/cm2, respectively. The pumping energy is 3.1 eV, and transient reflectivity ΔR/R was measured by 1.55 eV probe pulses. The pump pulses excited carriers and modified charge distribution caused a swift rising at zero time delay. By electron-electron and electron-phonon interactions, the photoexcited qusiparticles (QPs) released their energy and simultaneously emitted longitudinal-optical (LO) phonons. Then, energy spatially transited in crystal through anharmonic interactions between LO phonons and longitudinal-acoustic (LA) phonons. The chain reaction could be recognized in transient reflectivity measurement and phenomenologically described by exponential decay processes and damped oscillation.

Keywords: iron-base superconductor, ultrafast dynamics