Recently, two-dimensional imaging methods have been widely implemented in the photoelectron spectroscopy apparatus. In this special issue, we highlight the recent progress in the wide range of topics from topological insulator, superconductor to organic material. The key concept here is the interrelation between the real space and the momentum space. It is well recognized that our knowledge of surface and interface has been largely embodied based on the observation using various types of microscopic methods : not to mention, optical microscope, electron microscope, probe microscope, and others. On the other hand, the minute investigation in the momentum space (“k-space”) provides us with the detailed information on the band structure of the material, which governs the fundamental properties such as conductivity, photon absorption/emission, and various electronic excitations. Thus, investigation of the “both world” would undoubtedly give us more fundamental understanding of the behavior of both “localized electron” and “itinerant electron”, and further ramifications. We hope you enjoy an unprecedented landscape of real space and momentum space herein.

We review our recent study on the electronic states of Te(0001) using high-spatial-resolution angle-resolved photoemission spectroscopy (ARPES) with micro-focused synchrotron light. The results demonstrate the formation of in-gap states confined to the edges, and support topologically nontrivial character of Te helix chain.

Two-dimensional (2D) crystals exhibit unique electronic structures depending on the number of layers, differing from their bulk counterparts. Recent advances allow the fabrication of van der Waals (vdW) heterostructures, stacked from 2D crystals with tunable configurations, such as twist angles and material combinations, opening a vast landscape of unexplored phenomena. However, determining atomic coordinates in these microscale samples, crucial for band calculations, remains challenging. To address this, we developed a sample preparation process compatible with complex vdW heterostructures for angle-resolved photoemission spectroscopy, enabling direct observation of their electronic structures. This revealed layer-number-dependent band structures and spin-splitting in few-layer WTe₂, demonstrating odd-even layer-number effects. Moreover, 180°-twisted stacking of monolayers of T-polytype transition metal dichalcogenides was shown to break spatial inversion symmetry, unlocking new electronic functionalities and broadening the potential for discovering unique properties in 2D materials.

Electronic processes at interfaces embedded in organic electronic devices are essential for their functionalities. Surface science researches, whose one predominant concept is the usage of single crystal surfaces of the materials as substrates for “well-defined” interfaces, have disclosed crucial insights into such interfaces. However, studies targeting the electronic states of organic molecular single crystal surfaces and well-defined interfaces formed on these are quite limited so far mainly due to some technical hurdles for the application of photoelectron spectroscopy measurements on bulk molecular solid-state samples. This article is devoted to outlining experimental requirements and know-how for applying photoelectron spectroscopy measurements to molecular single crystal samples, taking our recent research using microscopic X-ray photoelectron spectroscopy as examples.

Electron-phonon interaction is one of the most fundamental interactions. Anharmonic vibrational modes interact with the conductive electrons and provide fascinating phenomena such as unusual superconductivity or high thermoelectric properties. BaIr₂Ge₇, a cage-structured compound with encapsulated Ba atoms, shows superconductivity at T_c＝2.5 K. Although specific heat measurements suggest the presence of anharmonic phonon modes, the atomic displacement parameter (ADP) is unusually small. Thus, the existence of anharmonic phonon modes and their relationship with the electronic state has not been clarified. In this study, we performed micro-focused angle-resolved photoemission spectroscopy (ARPES) on BaIr₂Ge₇ to explore the existence of the anharmonic phonon mode through the temperature-dependent spectra. The presence of two domains was identified by scanning the electronic states of the cleavage surface. The anharmonic phonon modes, extracted from the temperature-dependent photoemission linewidth, provide insight into its relationship with electrical resistivity, specific heat, and ADP.

We review our recent angle-resolved photoemission spectroscopy (ARPES) study utilizing high-resolution micro-ARPES to visualize and analyze the spatial inhomogeneity of superconducting gaps at the micro-scale in high-T_c cuprate superconductors. By combining micro-ARPES with advanced data analysis, we mapped and quantified physical parameters, revealing correlations likely linked to local doping inhomogeneity. This approach demonstrates the potential of micro-ARPES for combinatorial analysis and material design.