Berry's phase observed in the ordered state of Fe(Se,S)

Among iron-based superconductors, FeSe offers a unique platform in that it exhibits a nematically ordered phase without long-range magnetic ordering. Several experiments have shown that the low-temperature Fermi surface of FeSe consists only of very small, shallow pockets~[1-3]. Tuning the ground state via isoelectronic chemical substitution provides an ideal way to solve the puzzles regarding the nematic ordering in this material. Here, by using ultra-high magnetic fields up to $\sim$\,90\,T, we report observations of Shubnikov-de Haas (SdH) oscillations in isoelectronically substituted Fe(Se,S). For the smallest pocket of $\sim0.2$\% of the Brillouin-zone, we observe non-zero $\pi$ Berry's phase shift in the SdH oscillations. Our results indicate presence of Dirac cone, which would be a key to understand the mechanism of the nematic ordering in this system. \\ ~[1] S. Kasahara {\it et al}., Proc.\,Natl.\,Acad.\,Sci. U.\,S.,A. {\bf 111}, 16309 (2014). \\ ~[2] T. Terashima {\it et al}., Phys. Rev. B {\bf 90}, 144517 (2014). \\ ~[3] M.W. Watson {\it et al}., Phys. Rev. Lett. {\bf 115}, 027006 (2015).