RF injection scanning tunneling spectroscopy of a superconducting NbSe₂ surface

NbSe₂ is a transition metal dichalcogenide with a two-dimensional nature, showing superconducting (SC) and charge density properties. Moreover, a 1T phase can be made in the film, which has a different property from the bulk dominant 2 H phase, in which Mott insulator behavior is actively discussed. We observed the surface with STM at 400 mK and injected RF signal (1 GHz and 15 Hz) at the tunneling junction. We detected two quasi-particle (QP) states at the end of the SC gap, which split with the increase of the RF power specified by the electric field at the tunneling junction, V_AC. A previous STM experiment with 65 GHz RF on a vanadium surface observed multiple replicas of the QP peak. However, our experimental result using 1 GHz RF shows a widening of QP features with two enhanced peaks shifted by ~ ± eV_AC from the original QP positions. The behavior was well reproduced by a simulation using a well-known Tien-Gordon model, whose results indicate that the disappearance of multiple peaks is due to the low frequency of the RF signal. In addition, two enhanced peaks at ~ ± eV_AC are deduced from the Bessel function behavior. The energy shift from the original peak linearly changes with eV_AC. We apply this technique to examine the property change at the domain boundary of the 2 H and 1T phase of the NbSe₂ surface. We found the superconducting gap decreases when we move the tip from the 2 H domain into the 1T domain. Moreover, the injection of RF splits a QP peak into two enhanced peaks, whose energy separation is linear with the electric field at the RF generator for both phases. However, the linear energy separation with V_AC shows different coefficients between the 2 H and 1T phases. We conclude that the different coefficient is due to the change of actual V_Ac on the two domains originating from a different dielectric constant and shielding efficiency for the electric field of RF.