Enhancement of Friedel oscillation in the insulating phases of 1T-TaS₂ and 1T-TaSe₂

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and 1T-TaSe₂were probed by variable temperature UHV-STM. In 1T-TaS₂, which indicates a metal-Mott insulator transition at 180 K, the impurity image varied drastically depending on its electronic structure: in its metallic phase, defects were imaged as less-conducting regions with a lateral size of ∼2 nm, while in the insulating phase spherically oscillating structures were frequently found (Friedel oscillation). The dimensions of oscillation, being ∼15 nm in period and ∼1 nm in amplitude, were substantially larger than those reported in normal metals. On the other hand, the 1T-TaSe₂ crystals examined here showed both metallic and insulating behavior at room temperature, depending on their surface conditions. The enhanced Friedel oscillation was encountered only on the insulating surfaces. These observations are consistent with the theoretical model proposed by Kobayashi, in which low-energy excitations with small wavenumbers play an important role in enhancing the Friedel oscillation.