Isolated highly localized bands in YBi₂ monolayer caused by 4f orbitals

The novel electronic structures can induce unique physical properties in two-dimensional (2D) materials. In this work, we report isolated highly localized bands (HLB) in [Formula: see text] monolayer by the first-principle calculations within generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). It is found that [Formula: see text] monolayer is an indirect-gap semiconductor using both GGA and GGA+SOC. The calculations reveal that Yb-4f orbitals constitute isolated HLB below the Fermi level at the absence of SOC, and the bands are split into the j  =  7/2 and j  =  5/2 states with SOC. The isolated HLB can lead to a very large Seebeck coefficient and very low electrical conductivity in p-type doping by producing very large effective mass of the carrier. It is proved that isolated HLB have very strong stability against strain, which is very important for practical application. When the onsite Coulomb interaction is added to the Yb-4f orbitals, isolated HLB persist, and only their relative positions in the gap change. These findings open a new window to search for novel electronic structures in 2D materials.