Effect of electron doping in FeTe1-ySey realized by Co and Ni substitution

Angle-resolved photoemission spectroscopy reveals the effects of electron doping, which is realized by Co and Ni substitution for Fe in a FeTe1-ySey (y similar to 0.35) superconductor. The data show consistent band shifts as well as expansion and shrinking of the electron and hole Fermi surface, respectively. Doping of either element leads to a Lifshitz transition realized as a removal of one or two hole pockets. This qualitatively explains the complex behavior of the Hall coefficient observed previously (Bezusyy et al, 2015 Phys. Rev. B 91, 100502), including the change of sign with doping, which takes place only below room temperature. Assuming that Ni substitution should deliver twice as many electrons to the valence band as Co, it appears that such transfer is slightly more effective in the case of Co. Therefore, charge doping cannot account for a much stronger effect of Ni on the superconducting and transport properties (Bezusyy et al, 2015 Phys. Rev. B 91, 100502). Although overall band shifts are roughly proportional to the amount of dopant, clear deviations from the rigid band shift scenario are found. The shape of the electron pockets becomes elliptical only for Ni doping, the effective mass of electron bands increases with doping, and a strong reduction of effective mass is observed for one of the hole bands of the undoped system. The topology of hole and electron pockets for superconducting Fe1.01Te0.67Se0.33 with a critical temperature of 13.6 K indicates a deviation from nesting. Co and Ni doping causes further departure from nesting, which accompanies the reduction of critical temperature.