Nonequilibrium Carrier Dynamics in FeSe_0.8Te_0.2

We report on the nonequilibrium dynamics of photoinduced collective excitations in FeSe_0.8Te_0.2 films in a superconducting (SC) state. Epitaxial films were grown on CaF₂ single-crystal substrates by pulsed laser deposition. The SC properties were tuned by varying the strain level by altering the film thickness of the epitaxial film. It was found that increased strain in film can depress SC when the film thickness riches a threshold minimum, while the increased thickness can result in static domain structure via generation of the dislocation network, but with the introduction of new grain boundaries or microcracks. This fact was observed in angle-resolved light scattering measurements of the surface morphology within a broad temperature range, down to 7 K. It was also found that the surface domains undergo temperature-dependent reorganization when FeSe_0.8Te_0.2 film undergoes a solid-to-solid structural phase transition. Ultrafast time-resolved measurements of reflectivity were performed with 35-fs laser pulse excitation, 1 kHz repetition rate, the central wavelength of 800 nm, and orthogonal polarization for pump and probe pulses. The FeSe_0.8Te_0.2 film was maintained in its SC state at 7.5 K, while excitation fluence was tuned from up to ~10 mJ/cm², sufficiently below the damage threshold. The obtained reflectance traces show three distinct regions attributed to different dynamical processes in the material within the 50 ps time scale: (i) drop of the reflectivity signal within 200 fs, (ii) its subsequent rise within 1 ps, and (iii) much longer process of tens picoseconds. First two processes are associated with collective excitations and photoexcitation of quasi-particles (QP) in FeSe_0.8Te_0.2 . A gradual increase of the excitation energy results in a gradual growth of QP density. Moreover, the QPs decay rate slows down as excitation fluence increases. The transient measurements manifest a gradual time-shift of the specific signal minimum on the reflectivity diagram, since the QPs decay rate slows down as excitation fluence increases. The characteristic time of the process (ii) shows a linear increase with excitation fluence, while the process (i) is more complex. The process (iii) is associated with phonon-phonon scattering demonstrating a decrease of its characteristic time as excitation increasing. This could be a signature of photoinduced structural transformation or a hidden phase in the film. A strong acoustic phonon generation starts at ~ 100 ps, right after the process (iii) completes. This reveals new features of photoinduced dynamics of superconducting FeSe_0.8Te_0.2 in high excitation regime.