Direct observation of the nearly commensurate charge density wave in 1T-TaS₂ by non-contact AFM

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In recent years, there has been growing interest in transition-metal dichalcogenides (TMDs) due to their remarkable physical properties and potential applications of nanodevice materials. In 1T-TaS₂, a member of the TMD family, Ta atoms are aligned in the so-called “Star of David” pattern, caused from the periodic modulation of electronic charge density: charge density wave (CDW). At room temperature, 1T-TaS₂ is in the nearly commensurate CDW (NC-CDW) phase, whereas in the commensurate CDW (C-CDW) phase below 180 K [1].

We demonstrate direct observations of the CDW order in 1T-TaS₂ by non-contact atomic force microscopy (NC-AFM) as in Fig. 1. Although there are a lot of previous studies detecting the CDW orders by X-ray diffraction (XRD) and scanning tunnel microscopy (STM), observation by NC-AFM has not been reported. XRD can detect lattice distortion accompanied to the CDW phases; STM image indicates distribution of electronic states. In contrast, AFM can detect force between the tip and the sample including the electrostatic force, so it enables a possible direct visualization of the charge density distribution of the CDW.

Single crystals of 1T-TaS₂ used in this experiment were grown with the chemical vapor transport method and were cleaved under ultra-high vacuum conditions. AFM measurements were also performed in an ultra-high vacuum chamber at room temperature. The CDW triangle lattice structure was visualized through the attractive interaction between tip and sample. The period of the observed CDW order is approximately 1 nm, which is consistent with the previous results of STM measurements. In this presentation, we will discuss why the CDW can be observed in NC-AFM.

References:

[1] R. E. Thomson, B. Burk, A. Zettl, and John Clarke. Phys. Rev. B 49, 16899 (1994).