Reliable Contrast Inversion Detection in Charge Density Waves via Scanning Tunneling Spectroscopy

Scanning Tunneling Microscopy and Spectroscopy (STM/STS) is the only technique capable of detecting one of the hallmarks of the Charge Density Wave (CDW) state: The inversion of the imaging contrast at opposite bias polarities (Contrast Inversion – CI). However, STM/STS faces a well-known limitation: the so-called set-point effect, which can potentially introduce artifacts in the measurements. The renewed interest in CDWs, spurred by the discovery of CDW states in Kagome materials, multiband CDW gaps, and CDW gaps away from the Fermi level, calls for an assessment of the limits of the application of STM/STS to the detection of CI.

We compare several STS imaging modes in TiSe₂, a CDW material with a CDW gap shifted away from Fermi level. Our results show that only constant height imaging reveals a single CI at the gap edges associated with the CDW ground state, while the other modes including constant current imaging and dynamic conductance maps reveal either additional unrelated CIs or none. This identifies the most reliable scheme to identify and measure the amplitude and position of the CDW gap, which is often hard to detect in tunneling spectra. These experimental findings can be qualitatively reproduced using a simple model based on the 1D Bardeen equation for the tunnel current, offering practical insights for future studies.