The hallmark of topology in physics is the presence of gapless Dirac cones emerging at an interface between the topological insulator and a trivial one. Controlling these topological surface states and their chirality remains a big challenge as it would bring many important applications in ultrafast low-consumption electronics, spintronics, valleytronics and quantum computing.
Such topological states are investigated in Pb1-xSnxSe/PbSe heterostructures grown by molecular beam epitaxy. They are localized at topological-to-trivial interfaces formed by stacking Pb1-xSnxSe (topological insulator) and PbSe (trivial insulator) layers. I will focus on two kinds of interfaces: (i) the gradual interface realized by a Sn content gradient; and (ii) the abrupt interface which is atomically thin. These two interfaces yield drastically different topological state properties.
The gradual interface allows for a unique experimental study of a progressive change in band topology over few nanometers. The gapless topological state is observed in magneto-optics and ARPES, and is found to persist independently from the interface thickness. Additional gapped Dirac states localized at the interface are measured in sufficiently thick gradual interfaces, as predicted for a long time by the theory of Volkov and Pankratov[1]. This theory, as well as more recent ones[2], are experimentally demonstrated here.
The abrupt interface, projected on the (001) plane of Pb1-xSnxSe, hosts two degenerated Dirac cones with identical chirality, whose dispersions undergo a Lifshitz transition[3]. This transition is observed and characterized using magneto-optical and ARPES measurements. I will describe the interactions between the two Dirac cones, leading to interesting features like interference patterns of tunneling electrons between Dirac cones, or 1D flat band states at crystalline step edges.
[1] B.Volkov and O.Pankratov, Sov. J. Exp. Theor. Phys. Lett. 42, 178 (1985)
[2] X.Lu and M.O.Goerbig, Phys. Rev. B 102, 155311 (2020)
[3] J.Liu, W.Duan and L.Fu, Phys. Rev. B 88, 241303 (2013)
