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

Subsurface impurities buried in 1T-TaS2 and 1T-TaSe2 were 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 similar to 2 nm, while in the insulating phase spherically oscillating structures were frequently found (Friedel oscillation). The dimensions of oscillation, being similar to 15 nm in period and similar to 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.