Resonant X-ray Scattering Study for Rare Earth Tetraborides: GdB4 and TmB4

Abstract

Resonant x-ray scattering is a process of absorption and re-emission of xray photons during transitions of electrons between well-defined core states and unoccupied states. Therefore, it is sensitive to unoccupied states and, by tuning x-ray photon energy to absorption edges of a specific ion, we can probe unoccupied states of the corresponding ions. Strongly intertwined interaction among spin, orbital and charge degrees of freedom is the essence to induce emergent materials properties, and the investigation to understand their interplay has been actively pursued. In this work, resonant X-ray scattering studies were performed for GdB4 and TmB4 which show differences in many physical characters. Spin- and orbital- degrees of freedom were explored by utilizing the advantage of resonant X-ray scattering experiments. For GdB4 temperature dependence of anisotropic tensor susceptibility (ATS) scattering intensities were measured near L-edges of Gd to investigate how spin ordering affects ATS. ATS scattering intensities for L2- and L3-edges show diferent temperature dependence below TN. At L3-edge the intensities reect spin order parameter while intensities at L2-edge do not show noticeable change. The difference is explained in terms of spin-orbit coupling and isotropic distribution of spin-polarized 5d states of Gd ions. Above TN, ATS scattering intensities demonstrate that thermal motions enhance charge distribution anisotorpy of Gd 5d states in paramagnetic phase. By providing experimental evidence to reveal spin ordering in 4f and 5d orbitals of the magnetic Tm ions with orbital-selectivity, further spectroscopic application of resonant x-ray scattering was demonstrated in TmB4, which introduced a route to understand fundamental mechanism of spin polarization. The enhanced sensitivity of resonant X-ray scattering to various electric multipole transitions expects to find strong applications in studying various spin and polar ordering in materials to include high-order moments.