Ultrafast dynamics in (Na0.32K0.68)0.95Fe1.75Se2 superconductors by using dual-color pump-probe spectroscopy
P. C. Cheng1*, C. W. Luo1,2, Y. J. Chu1, W. Y. Tzeng1, J.-Y. Lin3, K. H. Wu1, J. Y. Juang1, T. Kobayashi1,4, Alexey Golovanov5, D. A. Chareev6, Alexander Vasiliev5
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei, Taiwan
3Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan
4Department of Applied Physics and Chemistry and Institute for Laser Science, University of Electro-Communications, Tokyo, Japan
5Low Temperature Physics and Superconductivity Department, Moscow State University, Moscow, Russian Federation
6Institute of Experimental Mineralogy, Chernogolovka, Russian Federation
* presenting author:Po Chung Cheng, email:pccheng1115@gmail.com
We present the ultrafast pump-probe spectroscopy for (Na0.32K0.68)0.95Fe1.75Se2 single crystal. The photon energy of optical pumping is 3.1 eV. Then we detected the time evolution of transient reflectivity changes ΔR/R in picosecond timescale by 1.55-eV femtosecond pulses. The amplitude of ΔR/R rose dramatically at Tc indicated a superconducting gap opening with the magnitude of 12.9 meV. However, the unexpected non-divergent relaxation time at Tc implied that intrinsic phase separation in a mesoscopic scale between the superconducting and insulating block antiferromagnetic phases. On the other hand, the disordering process of antiferromagnetic spin could be observed even below Tc, which is also an evidence for phase separation. Finally, we found the spin order in insulating regions can affect the photoexcited quasiparticles in superconducting phase regions due to proximity effect, which further provides an aspect of pairing mechanism in alkali metal iron selenide superconductors.

Keywords: iron base superconductor, ultrafast dynamics