Different response of atomic force microscopy and scanning tunneling microscopy to charge density waves

We have studied the transition metal dichalcogenides 1T–TaS₂ and 1T–TaSe₂ exhibiting charge density waves (CDW) at room temperature by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) with atomic resolution. STM images are dominated by the charge density wave modulation, while the AFM operated with an applied loading in the range of 10⁻⁸–10⁻⁷ N responds only to the atomic surface structure. Several possible explanations for this experimental result are discussed, including differences in what STM and AFM are sensitive to, as well as a possible local pressure dependence of the CDW state.