Moir\'e Modulated Lattice Strain and Thickness-Dependent Lattice Expansion in Epitaxial Ultrathin Films of PdTe$_2$

TL;DR

This study demonstrates how Moiré patterns in epitaxial PdTe₂ ultrathin films induce lattice strain and expansion, significantly affecting electronic properties and enabling potential quantum material engineering.

Contribution

It reveals the relationship between Moiré pattern periodicity, lattice strain, and electronic band structure in PdTe₂ ultrathin films, combining experimental STM observations with simulations and DFT calculations.

Findings

Moiré pattern periodicity increases as film thickness decreases

Lattice strain up to ~2.9% affects the band gap at the Γ point

Strain can close and reopen the band gap, causing band inversion

Abstract

We report the epitaxial growth of PdTe$_2$ ultrathin films on topological insulator Bi$_2$Se$_3$. A prominent Moir\'e pattern was observed in STM measurements. The Moir\'e periodicity increases as film thickness decreases, indicating a lattice expansion of epitaxial PdTe$_2$ thin films with lower thicknesses. In addition, our simulations based on Moir\'e Metrology reveal uniaxial lattice strains at the edge of PdTe$_2$ domains, and anisotropic strain distributions throughout the Moir\'e supercell with a net change in lattice strain up to ~2.9%. Our DFT calculations show that this strain effect leads to a narrowing of the band gap at $\Gamma$ point near the Fermi level. Under a strain of ~2.8%, the band gap at $\Gamma$ closes completely. Further increasing the lattice strain makes the band gap reopen and the order of conduction band and valence bands inverted in energy. The results offer a proof of concept for constructing quantum grids of topological materials under the modulation of Moir\'e potentials.