Structural and magnetic phase stability and the surface states of gd-doped topological insulator Bi2Te3

Introducing magnetism into a topological insulator can profoundly alter its electronic structure, leading to exotic phenomena. We present density-functional theory calculations on Gd substitution in the topological insulator Bi 2 Te 3 surface. The surface state and energy gap are investigated as a function of Gd position, concentration, and magnetic ordering. A single Gd prefers to be near the surface, lifting the degeneracy of the topological surface state (TSS) and opening an energy gap. For Gd dimers, intra-quintuple-layer (intra-QL) dimers prefer antiferromagnetic (AFM) ordering due to the superexchange interaction, whereas inter-QL dimers reveal degenerate ferromagnetic and AFM ordering. The AFM intra-atomic-layer dimers retain the Dirac point of TSS, indicating preservation of time reversal symmetry, whereas other Gd dimers open the TSS gap. Our results imply that Gd doping sensitively tunes the electronic structure of topological insulators, depending on its distribution and magnetic coupling, providing a route to manipulate the TSS • Magnetic Gd doping lifts Bi 2 Te 3 topological surface state degeneracy and opens a gap. • Gd dimer formation prefers antiferromagnetic ordering near the surface. • Orbital hybridization and superexchange interaction are crucial for magnetic ordering and gap opening.