Two-way twisting of a confined monolayer: orientational ordering within the van der Waals gap between graphene and its crystalline substrate

TL;DR

This paper investigates the orientational ordering of alkali atom monolayers confined between graphene and an iridium surface, revealing a two-way twist behavior as density increases, with implications for nanotribology and intercalation materials.

Contribution

It introduces the concept of two-way twisting in confined monolayers and provides experimental evidence using STM and electron diffraction.

Findings

Discovery of two-way twist behavior in alkali monolayers

Experimental validation with STM and electron diffraction

Relevance to nanotribology and intercalation phenomena

Abstract

Two-dimensional confinement of lattices produces a variety of order and disorder phenomena. When the confining walls have atomic granularity, unique structural phases are expected, of relevance in nanotribology, porous materials or intercalation compounds where \textit{e.g.} electronic states can emerge accordingly. The interlayer's own order is frustrated by the competing interactions exerted by the two confining surfaces. We revisit the concept of orientational ordering, introduced by Novaco and McTague to describe the twist of incommensurate monolayers on crystalline surfaces. We predict a two-way twist of the monolayer as its density increases. We discover such a behavior in alkali atom monolayers (sodium, cesium) confined between graphene and an iridium surface, using scanning tunneling microscopy and electron diffraction.