William P. Comaskey | Searcharxiv

arXiv2025

How Can We Engineer Electronic Transitions Through Twisting and Stacking in TMDC Bilayers and Heterostructures? A First-Principles Approach

Layered two-dimensional (2D) materials exhibit unique properties, expanding opportunities in material design. We investigate MX$_2$ transition metal dichalcogenides (TMDCs) (M = Mo, W; X = S, Se, Te) in homo- and heterobilayers with different stacking and twist angles. Twisted bilayers introduce Moiré patterns, significantly altering electronic properties. Using first-principles Density Functional Theory (DFT) with range-separated hybrid functionals, we examine 30 MX$_2$ combinations, revealing how stacking and composition influence stability and band gap energy (E$_g$). Notably, the MoTe$_2$/WSe$_2$ heterostructure with a 60\textdegree~shift maintains a direct band gap, highlighting its potential for applications. Homobilayers under low-strain conditions exhibit diverse stacking-dependent electronic behaviors, where MoS$_2$, WS$_2$, and WSe$_2$ transition between direct and indirect band gaps at specific twist angles. MoS$_2$ can even switch between semiconductor and metallic states. Critical twist angles (17.9\textdegree, 42.1\textdegree, 77.9\textdegree, and 102.1\textdegree) in twisted WS$_2$ and WSe$_2$ bilayers yield symmetric Moiré patterns with tunable band gaps. Our findings emphasize that controlling heterostructures and twist angles is a powerful strategy for engineering electronic properties, offering a pathway for next-generation materials.

cond-mat.mtrl-sci↗

arXiv2022

Spin Current Density Functional Theory of the Quantum Spin-Hall Phase

The spin current density functional theory (SCDFT) is the generalization of the standard DFT to treat a fermionic system embedded in the effective external field produced by the spin-orbit coupling interaction. Even in the absence of a spin polarization, the SCDFT requires the electron-electron potential to depend on the spin currents $\mathbf{J}^x$, $\mathbf{J}^y$ and $\mathbf{J}^z$, which only recently was made possible for practical relativistic quantum-mechanical simulations [Phys. Rev. B {\bf 102}, 235118 (2020)]. Here, we apply the SCDFT to the quantum spin-Hall phase and show how it improves (even qualitatively) the description of its electronic structure relative to the DFT. We study the Bi (001) 2D bilayer and its band insulator to topological insulator phase transition (via $s+p_z \leftrightarrow p_x +ip_y$ band inversion) as a function of mechanical strain. We show that the explicit account of spin currents in the electron-electron potential of the SCDFT is key to the appearance of a Dirac cone at the $Γ$ point in the valence band structure at the onset of the topological phase transition. Finally, the valence band structure of this system is rationalized using a simple first-order $\mathbf{k} \cdot \mathbf{p}$ quasi-degenerate perturbation theory model.

cond-mat.mtrl-sci↗