Investigation of electronic anisotropies with high purity X-ray polarimetry

The goal of this proposal is the investigation of tiny electronic anisotropies of d and f orbitals in a rare earth cubic compound revealed by polarization analysis of electric quadrupole transitions and dipole-octupole interference at the rare earth L1 edge. We want to detect polarization changes near the L1 absorption edge with a highly sensitive X-ray polarimeter consisting of two polarizer channel-cuts in a crossed setting that is capable to achieve a very high polarization purity up to 10E-10. This technique allows nearly background free measurements of weak electronic anisotropies. It also enables the separation of the two polarization changing effects - X-ray dichroism and X-ray birefringence, whose relative contribution in the X-ray range has hardly been studied. We will investigate at the Gd L1 absorption edge (8376 eV) a model compound with cubic structure, Gd3Ga5O12(GGG), on which several interesting spectroscopic effects have already been discovered.

本研究提案的核心目标为：通过对稀土元素L1边处的电四极跃迁（electric quadrupole transitions）与偶极-八极干涉（dipole-octupole interference）效应开展偏振分析，揭示稀土立方化合物中d轨道与f轨道的微弱电子各向异性。本研究将采用一套高灵敏度X射线偏振仪（X-ray polarimeter），该仪器由两片工作于交叉构型的偏振器通道切光片组成，可实现高达10^-10的极高偏振纯度，以此精准探测L1吸收边附近的偏振变化。该技术可实现近乎无背景的微弱电子各向异性测量，同时能够分离两种偏振改变效应——X射线二向色性（X-ray dichroism）与X射线双折射（X-ray birefringence）；目前学界对这两种效应在X射线波段的相对贡献几乎尚未开展系统研究。本研究将选取具有立方结构的模型化合物钆镓石榴石（Gd3Ga5O12，简称GGG）作为研究对象，在其Gd L1吸收边（8376 eV）处开展实验，该体系此前已被发现存在多项颇具研究价值的光谱学效应。