Dissertations, Theses, and Capstone Projects

Date of Degree

6-2023

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Roman Ya. Kezerashvili

Committee Members

Andrea Alu

Igor V. Bondarev

Godfrey Gumbs

Anatoly Kuklov

Vinod M. Menon

Vladimir I. Tsifrinovich

Subject Categories

Condensed Matter Physics

Keywords

diamagnetic coefficient, magnetoexciton, TMDCs, TMTCs, phosphorene, Xenes

Abstract

This research focuses on the direct and indirect excitons in Rydberg states in monolayers, bilayers, and van der Waals heterostructures composed of 2D semiconductors in the presence of the external magnetic field. In our work, we report binding energies of direct and indirect excitons in Rydberg states, the energy contribution from the magnetic field to the binding energies of magnetoexcitons, and diamagnetic coefficients (DMCs) of magnetoexcitons.

We study isotropic materials: transition metal dichalcogenides, TMDCs (WSe2, WS2, MoSe2, MoS2), and Xenes (silicene, germanene, stanene), and anisotropic materials: phosphorene and transition metal trichalcogenides, TMTCs (TiSe3, TiS3, ZrSe3, ZrS3). For excitons in TMDCs, phosphorene, and TMTCs, we consider freestanding (FS) and encapsulated by hexagonal boron nitride (hBN) monolayers, FS bilayers, and van der Waals heterostructures (vdWHs) when the external magnetic field is perpendicular to the structure. In vdWHs, the top and bottom 2D semiconductor monolayers are separated by the number of hBN layers, N. For excitons in Xenes, we consider FS and hBN-encapsulated monolayers and vdWHs when the external electric and magnetic fields are perpendicular to the structure. Excitons in TMDCs and Xenes structures are studied when the external magnetic field is varied between 0 and 30 T, and for Xenes, the electric field is taken above the critical value which is unique for each material up to the value where a Xene monolayer becomes unstable. Excitons in TMTCs and phosphorene structures are studied when the magnetic field is varied between 0 and 60 T.

In our approach, within the framework of the effective mass approximation, we solve the Schrodinger equation for an interacting electron and hole. For direct excitons in a monolayer, we use the Rytova-Keldysh (RK) potential to describe interactions between an electron and hole. For indirect excitons in a bilayer and vdWH, we use the Rytova-Keldysh and Coulomb potentials to describe interactions between an electron and hole located in two different monolayers. This allows us to investigate the role of the screening in TMDCs, Xenes, TMTCs, and phosphorene.

We show that the energy contribution from the magnetic field to the binding energies and DMCs of direct and indirect magnetoexcitons can be tuned by the magnetic field. For magnetoexcitons in Xenes, the electric field is an additional degree of freedom that can be used to tune binding energies, energy contribution from the magnetic field to the binding energies, and DMCs. Moreover, in vdWHs, we show that varying the number of hBN layers is an additional degree of freedom that can be used to tailor binding energies, energy contribution from the magnetic field to the binding energies, and DMCs. The results reported in this work on excitons in TMDCs vdWHs and in TMTCs, Xenes, and phosphorene structures are the first of their kind. They can be compared with the experimental results when they become available.

Download

Share

COinS