Observation of anomalous temperature dependence of spectrum on small Fermi surfaces in a BiS$_2$-based superconductor*

We have performed an angle-resolved photoemission spectroscopy study of the BiS2-based superconductor Nd(O,F)BiS$_2$. Two small electron-like Fermi surfaces around X ($\pi$, 0) are observed, which enclose 2.4\% and 1.1\% of the Brillouin zone area, respectively, corresponding to an electron doping of 7\% per Bi site. The low- energy spectrum consists of a weakly-dispersing broad hump and a dispersive branch, which follows well the calculated band dispersion. This hump is drastically suppressed with increasing temperature, while the dispersive branch is essentially unaffected. The anomalous thermal effect indicates a highly interacting electronic state, in which the superconducting pairing develops