Negative-U Centers as a Basis of Topological Edge Channels

Abstract

We present the findings of the studies of the silicon sandwich nanostructure that represents the high mobility ultra - narrow silicon quantum well of the p - type (Si - QW), 2 nm, confined by the delta - barriers, 3 nm, heavily doped with boron on the n - type Si (100) surface. The ESR studies show that nanostructured delta - barriers confining the Si - QW consist predominantly of the dipole negative - U centers of boron, which are caused by the reconstruction of the shallow boron acceptors along the <111> crystallographic axis, 2B(0) = B(+) + B(-). The electrically ordered chains of dipole negative - U centers of boron in the delta -barriers appear to give rise to the topological edge states separated vertically, because the value of the longitudinal, Gxx = 4e2/h, and transversal, Gxy = e2/h, conductance measured at extremely low drain-source current indicates the exhibition of the Quantum Spin Hall effect. Besides, the Aharonov - Casher conductance oscillations and the 0.7(2e2/h) - feature obtained are evidence of the interplay of the spontaneous spin polarisation and the Rashba spin - orbit interaction that is attributable to the formation of the topological edge channels. We discuss the phenomenological model of the topological edge channel which can demonstrate the ballistic, Aharonov - Chasher effect or Josephson junction behaviour in dependence on the disorder in the distribution of the negative - U dipole centers in the upper and down delta - barriers.