Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (∆R/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ∆R/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe. , tremendous experimental and theoretical effort has been devoted to exploring their characteristics. These Fe-based pnictide compounds exhibit a very interesting phase diagram, with antiferromagnetism (or spin-density wave) at low doping and superconductivity at intermediate doping [2]. The simultaneous presence of magnetism and superconductivity in the phase diagram implies that magnetism plays an important role in the superconductivity mechanism. The existence of precursor superconductivity above T c which competes with the spin-density wave order [3], and a pseudogaplike feature with onset around 200 K [4] were observed on underdoped (Ba, K)Fe 2 As 2 and nearly optimally doped SmFeAsO 0.8 F 0.2 , respectively. Additionally, a coherent lattice oscillation was also found in Co-doped BaFe 2 As 2 using time-resolved pump-probe reflectivity with 40 fs time resolution [5]. Among various FeSCs, the iron chalcogenide FeSe [6] stands out due to its structure simplicity, which consists of iron-chalcogenide layers stacking one by another with the same Fe +2 charge state as the iron pnictides. This so-called "11" system is so simple that it could be the key structure to understanding the origin of high-T c superconductivity [7]. There has been considerable concern over the interplay between electronic structure, phonons, magnetism, and superconductivity in 11-type FeSe. Therefore, further studies of their quasiparticle dynamics are indispensable to understanding the high-T c mechanism in FeSCs. Here we report the first time-resolved femtosecond spectroscopy study of FeSe single crystals to elucidate the electronic structure and the quasiparticle (QP) dynamics.In this study, FeSe single crystals were grown in evacuated quartz ampoules using a KCl/AlCl 3 flux [8]. The crystalline structure of the samples was examined by xray diffraction. The superconducting transition temperature T c of the FeSe single crystal was determined to be 8.8 K by the middle point of the resistive transition. The femtosecond spectroscopy measurement was performed using a dual-color pump-probe system (for light source, the repetition rate: 5.2 MHz, the wavelength: 800 nm, and the pulse duration: 100 fs) and an avalanche photodetector with the standard lock-in te...
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