Abstract
There is an essential need for studying a micro-sized region in sample, especially when the sample properties have spatial variation and/or the sample is finely structured in a micron scale. X rays focused down to a micron scale provide a unique means to study the electronic structure of such samples through absorption and photoemission spectroscopies. To study the electronic structure deeply, sample is often rotated around a certain axis. When the sample rotation is combined with linearly polarized x rays, absorption spectroscopy gives information about orbital and magnetic anisotropy. On the other hand, combining with photoemission spectroscopy, one can perform angle-resolved photoemission spectroscopy (ARPES) and visualize the band structure of a material. The difficulty to perform such measurements is that the x-ray spot on the sample needs to be fixed while rotating the sample. Since it is challenging to fix the sample position in a micron scale during rotation, alternative methods are desired.
To perform x-ray absorption experiments at a fixed sample position but with changing relative angles between crystalline axis and x-ray polarization, we developed a methodology to rotate the angle of linear polarization at SPring-8 BL07LSU. Utilizing a unique segmented cross undulator at BL07LSU, we generated linearly polarized x rays from the interference between left and circular polarized light. By regulating the phase difference between circularly polarized light with opposite helicity, we realized generation of linearly polarized soft x rays at an arbitrary azimuthal angle. This development has allowed us to perform polarization-angle dependent x-ray absorptions study to detect orbital and magnetic anisotropy of h-BN [1] and NiO [2], respectively, at a fixed sample position. We also implemented this polarization angle-rotation scheme in resonant photoemission spectroscopy measurements, and revealed the orbital anisotropy of an Fe-based monolayer magnetic film [3]. Furthermore, we constructed a scanning x ray spectroscopy system equipped with a Wolter mirror that can focus x rays below 1 mm [4]. All combined, electronic anisotropy in a submicron spot could be investigated by x-ray absorption-based spectroscopies. As for ARPES, recent developments on electron analyzer has made it possible to deflect photoelectrons and thereby scan photoemission angles electronically without sample rotation. Therefore, one can acquire band structure and Fermi surface mapping in two-dimensional momentum space for a fixed sample spot. Using this ARPES technique and 10-micron sized beam at MAX IV BLOCH beamline, we visualized spatially varying carrier concentrations and antiferromagnetic states in a copper-oxide high-temperature superconductor [5]. This methodology is useful to evaluate several different physical quantities at various sample positions and correlate them to examine their causal relationships.
[1] Y. Kudo. MH et al., Nucl. Instrum. Methods. Phys. Res. A 1018, 165804 (2021).
[2] Y. Kudo, MH et al., e-J. Surf. Sci. Nanotechnol. 20, 124 (2022).
[3] MH et al., J. Phys.: Condens. Matter 35, 425001 (2023).
[4] H. Ando, MH et al., e-J. Surf. Sci. Nanotechnol. 21, 200 (2023).
[5] M. Miyamoto, MH et al., in preparation.
