Abstract
Single crystals of sulfur-substituted iron selenide, FeSe1-xSx, 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 similar to 0.19. The sharp anomalies in the physical properties indicate the superconducting and nematic phase transitions in FeSe0.96 at T-C = 8.4 K and T-N = 90 K, respectively. Scanning tunneling micros-copy reveals the presence of dumbbell defects associated with Fe vacancies and dark defects at the chal-cogen site associated with S within the FeSe1-xSx 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, TC follows the conventional dome-shaped curve while TN decreases with x. The overall appearance of the T-x phase diagram of FeSe1-xSx suggests the importance of nematic fluctuations for the formation of the superconducting state in these compounds.