Lifthitz Transition and Shadow Gap in Li(Fe_1−xCo_x)As Investigated by STM/STS

Recently, APRES experiments reported a superconducting (SC) gap opened on a shallow insulating band in Co-doped LiFeAs (Miao et al. 6:6056, 2015). Theoretically, this particular SC gap is characterized by asymmetric density of states (DOS) and vanishing of SC coherence peak, addressed as shadow gap. Using the scanning tunneling microscopy/spectroscopy (STM/STS), 1% and 3% Co-doped LiFeAs were studied. The Co dopants were atomically resolved. STS results at the Co sites on both samples showed no bound states. Negligible difference between spectra at Co sites and defect-free area was observed, suggesting weak impurity potentials of the Co dopants. Similar to LiFeAs, two SC coherence peaks at Δ_α = 5.6 meV and Δ_β = 2.5 meV were observed in STS spectra of the 1% doped sample, which were the SC gaps of the inner hole band α and the outer hole band β. In the SC state, spectra of 3% Co-doped sample showed a broad peak at E₁ = − 7.2 meV and other two peaks at ± 3.9 meV. Above T_c, the peak at E₁ shifts to − 4.8 meV, and the peaks at ± 3.9 meV vanish. We showed that the distinct difference of STS results in 1% and 3% Co-doped LiFeAs was caused by Lifthitz transition and shadow gap on the shallow band. By fitting the spectra at low energies, we found the anisotropy of the β band was greatly increased when shadow gap opens on the shallow α band.