Surface pinning of fluctuating charge order: an "extraordinary" surface phase transition

TL;DR

This paper develops a mean-field theory for charge-density wave order in layered materials, highlighting conditions for surface-specific phase transitions and explaining experimental observations of surface versus bulk order differences.

Contribution

It introduces a theoretical framework for understanding surface pinning and extraordinary phase transitions in CDW systems, incorporating Coulomb interactions and screening effects.

Findings

Surface can order at higher temperature than bulk in certain conditions.

Poor screening stabilizes incommensurate charge order at low temperatures.

Experimental surface commensurate CDW observed in NaCCOC aligns with theory.

Abstract

We study the mean-field theory of charge-density wave (CDW) order in a layered system, including the effect of the long-range Coulomb interaction and of screening by uncondensed electrons. We particularly focus on the conditions necessary for an ``extraordinary'' transition, in which the surface orders at a higher temperature, and is more likely to be commensurate, than the bulk. We interpret recent experiments on NaCCOC as indicating the presence of commensurate CDW at the surface that is not present in the bulk. More generally, we show that poor screening of the Coulomb interaction tends to stabilize incommensurate order, possibly explaining why the CDW order in LSCO and NbSe2 remains incommensurate to T -> 0, despite the small magnitude of the incommensurability.