Single crystal growth, transport and scanning tunneling microscopy and spectroscopy of FeSe1−xSx

Abstract

Single crystals of sulfur-substituted iron selenide, FeSe_1−xS_x, were grown within eutectics of molten halides, AlCl₃/KCl, AlCl₃/KCl/NaCl or AlCl₃/KBr, under permanent temperature gradient. The innovative “ampoule in ampoule” design of a crystallization vessel allows obtaining mm-sized plate-like single crystals with a sulfur content up to x ∼ 0.19. The sharp anomalies in the physical properties indicate the superconducting and nematic phase transitions in FeSe_0.96 at T_C = 8.4 K and T_N = 90 K, respectively. Scanning tunneling microscopy reveals the presence of dumbbell defects associated with Fe vacancies and dark defects at the chalcogen site associated with S within the FeSe_1−xS_x series of compounds. Scanning tunneling spectroscopy shows the presence of two different superconducting gaps at both hole and electron pockets of the Fermi surface for low S content levels. As a function of sulfur content, T_C follows the conventional dome-shaped curve while T_N decreases with x. The overall appearance of the T–x phase diagram of FeSe_1−xS_x suggests the importance of nematic fluctuations for the formation of the superconducting state in these compounds.