Quasiparticle Dynamics and Phonon Softening in FeSe Superconductors

Abstract

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...

Ultrafast dynamics of quasiparticles and coherent acoustic phonons in slightly underdoped (BaK)Fe2As2

“…From high temperature to low temperature, dramatically increases beginning at T c (36 K) and roughly saturates below 25 K. also has a peak at T c . These features were similar to those of quasiparticle relaxation due to the SC gap in other iron-based superconductors26293046. Although iron-based superconductors have multiple bands with different SC gaps, a temperature-dependent one-gap system based on the Rothwarf-Taylor (RT) model was still adequate to estimate the SC gap size for FeSe46, LiFeAs26, Ba 0.6 K 0.4 Fe 2 As 2  30, and SmFeAsO 0.8 F 0.2  29.…”

“…It was reported that the decay rate of the quasiparticles due to the SC gap varies with optical pump fluence55. In our temperature-dependent measurements, the optical pump fluence was 5 μJ/cm 2 below 50 K, which was reasonably low compared with previous experiments on iron-based superconductors252629304655. For enhancement of the signal to noise ratio, the optical pump fluence was 10 μJ/cm 2 for measurements above 50 K because they are in the linear region.…”

Interplay between lattice and spin states degree of freedom in the FeSe superconductor: Dynamic spin state instabilities

“…21 The very well characterized samples were from the same series as in previous investigations. 21,22 The present samples composition is FeSe 0.96 . Raman spectra were measured on freshly cleaved surfaces in a quasi-back scattering configuration using a λ = 532.1 nm solid-state laser.…”

Unveiling the hidden nematicity and spin subsystem in FeSe

“…[7][8][9][10] In this respect, as the structurally simplest Fe-based superconductor, FeSe has unexpectedly emerged in the frontier of Fe-based superconductivity research. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Up to date, few consensuses have been reached on either the electronic structure of FeSe or its nematic and superconducting mechanisms. For example, very recent angle-resolved photoemission spectroscopy (ARPES) has indicated a small Fermi surface (FS) above T s that cannot be reproduced by density functional theory calculations.…”