TiSe₂ is a transition-metal dichalcogenide that exhibits a 2a₀ × 2a₀× 2c₀ charge density wave (CDW) at temperatures below 202 K, where a₀ and c₀ are lattice constants. Moreover, superconductivity is induced in TiSe₂ by employing doping or high pressure. Extensive research on TiSe₂ has been conducted and has provided knowledge on the CDW properties and the coexistence or competition of the induced superconductivity with the CDW. However, the driving mechanism for the CDW remains unresolved, though the exciton condensation mechanism and the band-type Jahn–Teller mechanism have been proposed.

One way to address the question of the CDW driving mechanism is to investigate the energy gap of CDW, the CDW gap, which is related to the driving mechanism. Several studies have used scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES) to investigate the CDW gap size for TiSe₂ and the energy at which the CDW gap opens. They also discussed the driving mechanism with reference to information about the CDW gap. However, the estimated gap size and the energy at which the gap opens vary among the previous studies. Thus, a consensus understanding of the CDW gap has not been attained.

In the present study, we investigated the CDW gap in TiSe₂ by performing STM/STS measurements. We observed spatial variation in the tunneling spectrum. In addition, in the STM images, we found that the CDW pattern changed depending on the bias voltage. In the symposium, we will discuss the energy where the CDW gap opens in TiSe₂.