Development of the Soft X-ray AGM-AGS RIXS Beamline at Taiwan Photon Source

TL;DR

This paper details the development of a high-resolution soft X-ray RIXS beamline at Taiwan Photon Source, utilizing active gratings for improved energy resolution and demonstrating its scientific application in studying high-temperature superconductors.

Contribution

The paper introduces a novel AGM-AGS beamline design with active gratings that enhance resolution and reduce aberrations for soft X-ray RIXS measurements.

Findings

Achieved an energy resolution of 12.4 meV at 530 eV

Demonstrated momentum-resolved RIXS on cuprates

Revealed phonon features in superconducting materials

Abstract

We report on the development of a high-resolution and highly efficient beamline for soft-X-ray resonant inelastic X-ray scattering (RIXS) located at Taiwan Photon Source. This beamline adopts an optical design that uses an active grating monochromator (AGM) and an active grating spectrometer (AGS) to implement the energy compensation principle of grating dispersion. Active gratings are utilized to diminish defocus, coma and higher-order aberrations as well as to decrease the slope errors caused by thermal deformation and optical polishing. The AGS is mounted on a rotatable granite platform to enable momentum-resolved RIXS measurements with scattering angle over a wide range. Several high-precision instruments developed in house for this beamline are briefly described. The best energy resolution obtained from this AGM-AGS beamline was 12.4 meV at 530 eV, achieving a resolving power 42,000, while the bandwidth of the incident soft X-rays was kept at 0.5 eV. To demonstrate the scientific impacts of high-resolution RIXS, we present an example of momentum-resolved RIXS measurements on a high-temperature superconducting cuprate, La$_{2-x}$Sr$_x$CuO$_4$. The measurements reveal the A$_{1g}$ apical oxygen phonons in superconducting cuprates, opening a new opportunity to investigate the coupling between these phonons and charge density waves.