The Shimada-Ideta group at Hiroshima Synchrotron Radiation Center, Hiroshima University, focuses on researches using angle-resolved photoemission spectroscopy (ARPES) and related techniques to elucidate the electronic structure of strongly correlated-electron systems and so on. The electrons that we focus on exsit in materials and play an important role in their physical properties. For example, in metals that conduct electricity, electrons can move freely in the materials (free electrons), and this is what causes electricity to flow in the material. On the other hand, when the electron density becomes very high, they strongly interact with each other and their interactions with spin and orbital degrees of freedom make it impossible to explain electron motion using band theory, the basic theory of solid state physics. Because the electrons are strongly correlated with each other, such a system is called a "strongly correlated electron system". These materials show a variety of physical properties due to the complex interplay of electron degrees of freedom (charge, spin, and orbital) and crystal structures, and are being actively studied for understanding of their physical properties and industrial applications.

　Superconductivity is a phenomenon in which electrical resistance becomes zero at very low temperatures close to absolute zero (-273°C). This phenomenon was discovered by Kamerlingh Onnes in 1908. It was experimentally proven that superconductivity occurs at very low temperatures in various metals, and the BCS theory was established in which electrons form pairs (Cooper pairs) mediated by phonons. Furthermore, in 1986, it was reported that metal oxides containing copper show superconductivity at about -200°C, and it was found that the temperature at which superconductivity occurs increases to nearly -100°C for metal oxides containing mercury (Hg). This is a remarkable discovery that cannot be explained by the BCS theory and overturns the conventional idea that superconductivity is exhibited only at temperatures near absolute zero.
　However, 35 years have passed since the discovery of high-Tc cuprate superconductors, and we still do not know the origin of superconductivity, i.e., what is the glue of Cooper pairs. In order to provide ideas for new materials with higher superconducting transition temperatures, we need to study the electronic structure in the superconducting or normal state in detail. Therefore, angle-resolved photoemission spectroscopy (ARPES), which allows direct observation of the electronic structure of materials, is a very powerful tool to study the properties of strongly correlated electron systems.

　What is topology anyway? This is a new concept that has emerged from research over the last decade or so; many people first knew about it with the 2016 Nobel Prize in Physics. "Topology" is organized by the mathematical concept. It is often used as an example of how a donut with holes and a mug with a handle are isomorphic, meaning that they are "robust" properties independent of details such as the size and shape of the holes.
　Recently, real materials with such topological properties have been discovered in the surface state. The bulk properties are insulating, but the surface behaves like a metal, with spins moving in one direction (spin current) and the flow unimpeded by impurities or lattice defects. These properties have not been discovered in new materials, but have been demonstrated in materials that have been known for a long time. Only in recent years has the concept of topology been recognized and studied extensively. Basic research for the development of spintronics materials and devices utilizing spin currents has been actively developed.
　Our research group is also studying the electronic structure of these topological insulators.

　Angle-Resolved Photoemission Spectroscopy (ARPES), which we use in our research, is an experimental method established based on the "light quantum hypothesis" proposed by Einstein in 1905. When light is incident on a material, electrons inside the material are ejected as photoelectrons due to the external photoelectric effect. By examining the energy and position of these electrons in detail, the electronic state of the surface, edge, and bulk of the material can be directly investigated (left figure below). ARPES requires a change in sample angle in order to study momentum space. However, in BL-1 and BL-9A at HiSOR, by using the hemispherical electron analyzer with a deflector mode enables us to study tthe electronic structure in momentum space without changing the sample angle, so that even small samples can be observed. For details, please see the Beamline Introduction page.

　This is an experimental method to observe the spin degree of freedom of the electronic states in solids. Although it has been technically difficult to separate the spin into spin-up and spin-down states, a spin-resolving ARPES (SARPES) has recently been developed and tested by VLEED, which was developed by the Spin Group at HiSOR [*]. This technique is very effective in revealing spin-dependent electronic states.
　In recent years, electronic devices such as the flash memory and DRAM (Dynamic Rondom Access Memory) have been the focus of intense research and development worldwide, and their performance has improved significantly. These devices record data using an electron charge stored in a capacitor, and there is a demand for the development of next-generation devices due to read/write speed and volatility issues. Currently, a spintronics device, MRAM (Magnetoresistive Random Access Memory), which controls not only charge but also spin, the internal degree of freedom, is proposed as a new device technology, and attention to electron spin is growing from an application perspective as well. The spin of electrons is also attracting attention from the viewpoint of applications.

[*] T. Okudaet al.,Rev. Sci. Instrum.82, 103302 (2011).

　BL-1 is an experimental beamline that uses highly brilliant and polarization-tunable synchrotron radiation with hν = 23 eV to 300 eV to study the electronic structure of bulk solids and surfaces [**]. In addition to angle-resolved photoemission spectroscopy (ARPES) using the deflector mode, this experimental station can observe spatially resolved electronic states with a beam focused down to 40-50 μm. For polarization dependence (the s and p polarizations), the endstation can be rotated by 90 degrees to allow strictly distinguishable measurements of the orbital symmetry. For additional details, please contact the person on duty (contact information).
[**] K. Shimadaet al., Nuclear Instruments and Methods in Physics Research A467, 504 (2001).

　BL-9A can perform low-energy ARPES of bulk solids and thin films using synchrotron radiation in the ultraviolet region (wavelength: λ~30-200 nm) of hν = 6.5 eV to 40 eV. BL-9A is capable of generating high-brightness synchrotron radiation with high-energy resolution.From October 2022, a hemispherical analyzer (ASTRAIOS 190, SPECS, acquisition angle: ± 20°) and a 6-axis manipulator are installed at the BL-9A endstation (measurable temperature:T~ 10-300 K). Operando measurements are also possible. With these facilities, BL-9A is capable of observing three-dimensional Fermi surfaces in order to understand physical properties, as well as low energy excited states such as changes in the electronic structure of materials at low and high temperatures. For details, please contact the person in charge of the beamline (M. Arita)。