Nematicity in FeSe single crystals probed by pump-probe spectroscopy
C. W. Luo1,2*, P. C. Cheng1, S.-H. Wang3, J.-C. Chiang3, J-Y Lin3, K. H. Wu1, J. Y. Juang1, D. A. Chareev4, O. S. Volkova5, A. N. 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
4Institute of Experimental Mineralogy, Chernogolovka, Moscow, Russian Federation
5Low Temperature Physics and Superconductivity Department, Moscow State University, Moscow, Russian Federation
* presenting author:Chih Wei Luo,
The anisotropic quasiparticle dynamics in FeSe single crystals have been studied by polarized pump-probe spectroscopy. Two distinguishable relaxation components were unambiguously observed in transient reflectivity changes. The orientation-dependent fast component with the timescale of 0.1-1.5 ps associated with the electronic structure clearly shows two-fold symmetry, which further reveals the gap opening along ky below the temperature of structure phase transition (Ts) and the electronic nematicity can persist up to 200 K. For the slow component with the timescale of 8-25 ps, it is assigned to the energy relaxation through spin sub-system and also shows a two-fold symmetry below Ts. However, this two-fold symmetry is dramatically weakened above Ts and surprisingly remains till 230 K. Consequently, the high-temperature nematic fluctuations in FeSe may be driven by the order parameters which associated with both charge (orbital) and spin sub-systems.

Keywords: FeSe, Nematicity, pump-probe spectroscopy