Persistent negative THz photoconductivity in the vertical topological p-n junction Sb₂Te₃/Bi₂Te₃

A thin-film heterostructure of Sb₂Te₃ and Bi₂Te₃ can function as a vertical topological p-n junction with a layer-dependent Fermi level. The spatial separation of the intrinsically p and n doped topological surface states makes this structure a promising candidate for realizing a topological exciton condensate. Here we perform infrared pump-THz probe measurements on epitaxial Sb₂Te₃/Bi₂Te₃ heterostructures to track the real and imaginary parts of the optical conductivity in response to photoexcitation. After the initial free carrier excitation and decay on the order of a few ps, a persistent negative photoconductivity (NPC) is observed over a time > 1 ns. The NPC shows a strong temperature dependence and is only present below 100 K. We will discuss various sources of the NPC in terms of possible exciton formation, lattice heating or scattering between the topological surface states and localized defect states.