Nearly hyperuniform, nonhyperuniform, and antihyperuniform density fluctuations in two-dimensional transition metal dichalcogenides with defects

TL;DR

This study investigates how localized defects affect density fluctuations and hyperuniformity in two-dimensional transition metal dichalcogenides, revealing a transition from hyperuniform to antihyperuniform states with increasing defects.

Contribution

It provides the first detailed analysis of defect-induced changes in hyperuniformity in 2D TMDCs using experimental imaging and modeling.

Findings

Low defect concentrations preserve near hyperuniformity.

Increased defects gradually destroy hyperuniformity.

High defect levels lead to antihyperuniform behavior.

Abstract

Hyperuniform many-body systems in $d$-dimensional Euclidean space are characterized by completely suppressed (normalized) infinite-wavelength density fluctuations, and appear to be endowed with novel exotic physical properties. In this work, we analyze the effects of localized defects on the density fluctuations across length scales and on the hyperuniformity property of experimental samples of two-dimensional transition metal dichalcogenides. In particular, we extract atomic coordinates from time series annular dark field-scanning transmission electron microscopy imaging data of 2D tungsten chalcogenides with the 2H structure showing continuous development and evolution of electron-beam induced defects, and construct the corresponding chemical-bonding informed coordination networks between the atoms. We then compute a variety of pair statistics and bond-orientational statistics to characterize the samples. At low defect concentrations, the corresponding materials are nearly hyperuniform. As more defects are introduced, the (approximate) hyperuniformity of the materials is gradually destroyed, and the system becomes non-hyperuniform even when the material still contains a significant amount of crystalline regions. At high defect concentrations, the structures become antihyperuniform with diverging (normalized) large-scale density fluctuations. Overall, the defected materials possess varying degrees of orientation order, and there is apparently no intermediate hexatic phase emerging. We also construct a minimalist structural model and demonstrate that the experimental samples can be essentially viewed as perturbed honeycomb crystals with small correlated displacements and double chalcogen vacancies.