Noriaki Takagi

Charge density wave (CDW) resulting from a periodic distortion in the lattice creates orders beyond the original lattice. In 2 H − NbSe 2 , one of the layered transition metal dichalcogenides, the 3 × 3 charge order appears in two-dimensional (2D) layers. Although CDW can be observed by different techniques, the spatial distribution within a 2D layer has never been systematically visualized. Here, by using scanning tunneling microscopy and density functional theory, we monitored the evolution of CDW along the c axis and found a strong relation with the orbital charges. The results show that the appearance of the CDW varies while tuning the tunneling current and undergoes a transition from Se orbital charges to Nb orbital charges. The calculation of orbital charge distributions shows that both CDW intensity modulation and the transition are strongly intertwined with the distribution of Se 4 p orbitals and Nb 4 d orbitals.

We have investigated the image potential states (IPSs) for the unreconstructed Ir ( 001 ) − ( 1 × 1 ) and the reconstructed Ir ( 001 ) − ( 5 × 1 ) surfaces by using two-photon photoemission spectroscopy. We reveal that the Ir ( 001 ) − ( 5 × 1 ) reconstructed surface covered by Xe adatoms exhibits a band gap of 100 meV, which arises from the new periodic potential created by the reconstruction, whereas no band gap was observed on the Ir ( 001 ) − ( 1 × 1 ) surface within the measured momentum range. The experimental results are compared with the theoretical ones obtained within density functional theory for both the clean and Xe-covered Ir ( 001 ) − ( 5 × 1 ) reconstructed surfaces. The planar averaged charge density distributions of the IPS for both the surfaces show that the Xe adsorption does not significantly alter the positions of charge density maxima, which rationalizes why the band gap arising from the substrate superlattice potential does not change significantly upon Xe adsorption. The agreement between the experimentally observed band gap for the IPS on the Xe-covered surface and the theoretical calculations highlights the robustness of IPS behavior under Xe adsorption. Published by the American Physical Society 2025

Multiphoton photoemission provides a means to investigate unoccupied electronic states via nonlinear light-matter interactions. In this work, we employ five-photon photoemission spectroscopy to identify, for the first time, the image-potential state (IPS) on the Ir(111) surface. Distinct from commonly studied noble metals such as Cu and Ag, the Ir(111) electronic structure leads to a strong sensitivity to excitation energy: a reduction from 1.57 eV to 1.49 eV significantly diminishes the signal. The theoretical analysis attributes this effect to the d-band proximity to the Fermi level, which influences the initial-state population and transition probabilities governing the multiphoton excitation pathways.

Topological materials have become promising materials for next-generation devices by utilizing their exotic electronic states. Their exotic states caused by spin-orbital coupling usually locate on the surfaces or at the edges. Scanning tunneling spectroscopy (STS) is a powerful tool to reveal the local electronic structures of condensed matters. Therefore, STS provides us with an almost perfect method to access the exotic states of topological materials. In this topical review, we report the current investigations by several methods based on the STS technique for layered topological material from transition metal dichalcogenide Weyl semimetals (WTe ₂ and MoTe ₂ ) to two dimensional topological insulators ( layered bismuth and silicene ). The electronic characteristics of these layered topological materials are experimentally identified.