Multiple topological phases in tetradymite compounds M2 Te2 P (M= Ti, Zr, or Hf)

Since the advent of time-reversal-invariant topological insulators, the generalization of topology concepts has led to the discovery of a wide range of topological states. These topological states may undergo topological phase transitions (TPTs) accompanied by discontinuous changes in topological invariants. In this Letter, by combining time- and angle-resolved photoemission spectroscopy measurements with first-principles calculations, we have demonstrated the existence of distinct TPTs in the unoccupied states above the Fermi level in the M2Te2P (M=Ti, Zr, or Hf) family. These TPTs arise from the inversion of the transition metal d and Te p orbitals due to the enhancement of spin-orbit coupling and variation in crystal fields, as the transition metal changes from 3d to 4d and then to 5d. Eventually, dual topological states are achieved in the 5d system Hf2Te2P, where a type-III Dirac semimetal state coexists with a strong topological insulator state. Finally, we reveal a rapid relaxation process in the nonequilibrium population of the Dirac surface state due to the presence of an additional interband scattering channel in the bulk.