Spatial correlations of charge density wave order across the transition in 2H-NbSe2

TL;DR

This study uses cryogenic 4D-STEM to map local charge density wave (CDW) amplitude correlations in 2H-NbSe2, revealing finite local order above the transition and strain-related effects on correlation length at low temperatures.

Contribution

It introduces a cryogenic 4D-STEM technique to directly measure local CDW amplitude correlations and their temperature dependence in 2H-NbSe2.

Findings

Finite local CDW amplitude exists above the transition temperature.

Correlation length extends to nearly 110 nm below the transition.

Local CDW amplitude anticorrelates with strain fields.

Abstract

Charge density waves (CDWs) involve coupled amplitude and phase degrees of freedom, but direct access to local amplitude correlations remains experimentally challenging. Here, we report cryogenic four-dimensional scanning transmission electron microscopy (4D-STEM) measurements of CDW ordering in a 2H-NbSe2 flake of 24 nm thickness, enabled by liquid helium-based cooling. By mapping the spatial distribution of CDW superlattice intensities at nanometer-scale resolution and analyzing their autocorrelations, we extract the temperature-dependent correlation length associated with the local amplitude of the CDW order parameter, independent of global phase coherence. Our results reveal that a finite local CDW amplitude is already established well above the transition temperature. When the system is cooled below the transition temperature down to 20 K, the correlation length extends to nearly 110 nm, and the local CDW amplitude is found to strongly anticorrelate with the local strain field.