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XVIII INTERNATIONAL SYMPOSIUM “NANOSTRUCTURES: PHYSICS AND TECHNOLOGY”, MINSK, REPUBLIC OF BELARUS, SEPTEMBER, 2020. SPIN RELATED PHENOMENA IN NANOSTRUCTURES

Ballistic Conductance in a Topological 1T '-MoS2 Nanoribbon V. Sverdlova,*, E. A.-M. El-Sayedb,**, H. Kosinab,***, and S. Selberherrb,**** a

Christian Doppler Laboratory for Nonvolatile Magnetoresistive Memory and Logic at Institute for Microelectronics, TU Wien, Austria b Institute for Microelectronics, TU Wien, Austria *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] ****e-mail: [email protected] Received June 23, 2020; revised July 23, 2020; accepted July 27, 2020

Abstract—A MoS2 sheet in its 1T ' phase is a two-dimensional topological insulator. It possesses highly conductive edge states which, due to topological protection, are insensitive to back scattering and are suitable for device channels. A transition between the topological and conventional insulator phases in a wide 1T '-MoS2 sheet is controlled by an electric field orthogonal to the sheet. In order to enhance the current through the channel several narrow nanoribbons are stacked. We evaluate the subbands in a narrow nanoribbon of 1T '-MoS2 by using an effective k ∙ p Hamiltonian. In contrast to a wide channel, a small gap in the spectrum of edge states in a nanoribbon increases with the electric field. It results in a rapid decrease in the nanoribbon conductance with the field, making it potentially suitable for current switching. Keywords: topological insulators, topologically protected edge states, nanoribbons, subbands, k ∙ p Hamiltonian, ballistic conductance DOI: 10.1134/S1063782620120386

1. INTRODUCTION Edge states in two-dimensional (2D) topological insulators (TI) propagate without backscattering, which makes them attractive for designing highly conductive transistor channels [1]. However, possessing robust conductive channels is only one requirement for a good transistor. To make a switch it is necessary to suppress the current through the channel as a function of gate voltage. A standard approach is to restore the traditional band order [2]. Recently it was discovered that the 1T ' phase of MoS2, a well-known 2D material with a high promise for future microelectronic devices [3], is a TI [4]. The inverted band structure is well approximated by parabolas, with the conduction and valence bands having masses of myd((xp)) [4]. The spin-orbit interaction opens a gap at the intersection of the valence and conduction bands, which appears at a finite value of the momentum ky along the quantization axis OY (Fig. 1, solid green). A topologically protected highly conductive edge state must exist within the gap. By applying an electric field Ez along the OZ axis perpendicular to the nanoribbon, the gap at one of the minima can be

reduced, closed (Fig. 1, dotted red), and open again (Fig. 1, dotted–dashed blue) at large electric fields. The gap becomes a direct gap, so no edge states are allowed within the bulk gap. 2. METHODS In orde