Visualizing the Charge Density Wave Transition in 2H-NbSe2 in Real Space

TL;DR

This study uses atomic-resolution STM to visualize the charge density wave transition in 2H-NbSe2, revealing defect-induced nanoscale order and emphasizing electron-lattice interactions over Fermi surface effects.

Contribution

It provides the first real-space visualization of the CDW transition in NbSe2, highlighting the role of defects and energy-independent wavelength in the transition mechanism.

Findings

Static CDW order appears near defects at temperatures above Tcdw.

Nanoscale CDW patches grow and coalesce as temperature decreases.

Energy gap at 0.7 eV below Fermi level indicates electron-lattice interactions dominate.

Abstract

We report the direct observation in real space of the charge density wave (CDW) phase transition in pristine 2H-NbSe2 using atomic-resolution scanning tunneling microscopy (STM). We find that static CDW order is established in nanoscale regions in the vicinity of defects at temperatures that are several times the bulk transition temperature Tcdw. On lowering the temperature, the correlation length of these patches increases steadily until CDW order is established in all of space, demonstrating the crucial role played by defects in the physics of the transition region. The nanoscale CDW order has an energy and temperature-independent wavelength. Spectroscopic imaging measurements of the real-space phase of the CDW indicate that an energy gap in NbSe2 occurs at 0.7eV below the Fermi energy in the CDW phase, suggesting that strong electron-lattice interactions and not Fermi surface physics is the dominant cause for CDW formation in NbSe2.