Coupled Sublattice Melting and Charge-Order Transition in Two Dimensions

TL;DR

This study reveals a complex, multi-step melting process in a two-dimensional charge-ordered alloy, involving sequential disordering of sublattices and charge order collapse, combining experimental and theoretical insights.

Contribution

It uncovers a novel, multi-step melting transition in a 2D charge-ordered alloy, providing detailed atomistic understanding of the process.

Findings

Sequential short-range and long-range disordering of K sublattice.

Collapse of charge order in Sn lattice during melting.

Experimental and theoretical analysis elucidates atomistic melting mechanisms.

Abstract

Two-dimensional melting is one of the most fascinating and poorly understood phase transitions in nature. Theoretical investigations often point to a two-step melting scenario involving unbinding of topological defects at two distinct temperatures. Here we report on a novel melting transition of a charge-ordered K-Sn alloy monolayer on a silicon substrate. Melting starts with short-range positional fluctuations in the K sublattice while maintaining long-range order, followed by longer-range K diffusion over small domains, and ultimately resulting in a molten sublattice. Concomitantly, the charge-order of the Sn host lattice collapses in a multi-step process with both displacive and order-disorder transition characteristics. Our combined experimental and theoretical analysis provides a rare insight into the atomistic processes of a multi-step melting transition of a two-dimensional materials system.