Systematic study of electronic states of Ln(O,F)BiS2 by spin- and angle-resolved photoemission spectroscopy

The evolution of fine electronic structures in superconducting $\mathrm{Ln}{\mathrm{O}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}\mathrm{Bi}{\mathrm{S}}_{2}$ (Ln denotes lanthanoid) is systematically investigated with spin- and angle-resolved photoemission spectroscopy. A shape change of the Fermi surface (FS) at the $X$ point from elliptical to rectangular with increasing the electron doping is commonly observed in different lanthanide systems. Besides that, an emergent large Fermi surface around the \cyrchar\CYRG{} point is also observed in all the samples with different doping levels deviating from the theoretical prediction, which might be related to the superconductivity. In addition, we found a humped band around $X$(\ensuremath{\pi},0) in the highly doped samples only, which may be attributed to the ${\mathrm{BiS}}_{2}$ surface reconstruction or surface defects relying on the electron concentration. We also demonstrate the spin-selective excitation of photoelectrons, which indicates the presence of spin-orbital-entangled bulk Rashba splitting in this system. This result may enable the optical tuning of electron spin in a ${\mathrm{BiS}}_{2}$-based system.