台湾光子源软X射线AGM-AGS RIXS光束线的研制
Development of the Soft X-ray AGM-AGS RIXS Beamline at Taiwan Photon Source
摘要：
本文报导了一条高分辨率、高效率的软X射线共振非弹性X射线散射光束线的研制。该光束线采用了主动光栅单色仪（AGM）和主动光栅光谱仪（AGS）的光学设计，实现了光栅色散的能量补偿原理。主动光栅可以用来减小离焦、彗差和高阶像差，以及减小由热变形和光学抛光引起的斜率误差。AGS安装在一个可旋转的花岗岩平台上，可以在很宽的范围内进行动量分辨的RIXS测量。文中简要介绍了为这条光束线研制的几种高精度仪器。从该AGM-AGS光束线获得的最佳能量分辨率为12.4 meV（530 eV），分辨率为42，000，而入射软X射线的带宽保持在0.5 eV。为了证明高分辨率RIXS的科学影响，我们提出了一个例子的动量分辨RIXS测量高温超导铜氧化物，La $_{2-x}$ Sr $_x$ CuO $_4$。测量揭示了超导铜氧化物中的A $_{1g}$顶端氧声子，为研究这些声子与电荷密度波之间的耦合提供了新的机会。
英文摘要：
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.
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