Emergence of Van Hove singularities in (Sb₂Te₃)_m(Sb₂)_n Misfit Materials

Misfit layered materials, naturally occurring heterostructures consisting of distinct van der Waals bonded (“2D”) layers, have emerged as promising candidates for realizing novel quantum states, with implications for both classical and quantum computing. Of particular interest are (Sb₂Te₃)_m(Sb₂)_n superlattices, composed of m quintuple layers of Sb₂Te₃ and n bilayers of Sb₂, where insertion of Sb₂ layers into Sb₂Te₃ is predicted to lower the bandgap, leading to a semi-metallic state.¹ Furthermore, it has been suggested that adjustment of the m:n ratio will alter the weak topological indices, enabling dislocations to host topologically protected modes. Although the transition from semiconductor to semimetal has been demonstrated for single Sb₂ layers inserted into semiconducting Sb₂Te₃, the electronic states have yet to be investigated for higher-order members (n ≥ 2). Here, we report on an experimental-computational study of (Sb₂Te₃)₁(Sb₂)_n (n = 0, 2, 3, 4). Angle-resolved photoemission spectroscopy reveals a saddle point near M, which gradually approaches the Fermi level as n increases. Corresponding Van Hove singularities in the density of states are identified using scanning tunneling spectroscopy in conjunction with self-consistent GW theory.

¹Phys. Rev. B 91, 201101 (2015)