Probing high-energy electronic excitations in the Shastry-Sutherland compound SrCu2(BO3)2 with resonant inelastic x-ray scattering and optical spectroscopy

The Shastry-Sutherland compound SrCu2(BO3)2 (SCBO) is a paradigmatic low-dimensional quantum spin system, featuring geometrically frustrated interacting spin dimers and a rich landscape of emergent quantum phenomena. While its magnetic properties have been extensively explored, the nature of its electronic excitations - particularly the Cu2+ d–d transitions - remains poorly understood. A detailed characterization of these excitations is essential to determine the crystal field splitting and the degree of Cu 3d–O 2p hybridization, both of which underpin superexchange interactions and magnetic behavior in SCBO. Here, the electronic properties of SCBO are investigated by combining spectroscopic techniques with theoretical approaches for first-principles band structure calculations.Experimentally, we employ L3-edge resonant inelastic x‑ray scattering (RIXS) to directly probe intra-d excitations, supplemented by broadband infrared reflectivity and ellipsometry measurements to resolve charge-transfer (CT) features. Theoretically, multireference quantum chemistry calculations accurately reproduce the energies (1.8–2.4 eV) and symmetries of the observed d–d excitations, while band-structure calculations based on density functional theory elucidate the CT excitation spectrum. These combined approaches define the energy scales for both d–d and CT transitions, validate the computational framework, and provide quantitative microscopic parameters essential for refining superexchange-based magnetic models in this prototypical frustrated quantum antiferromagnet.

沙斯特里-萨瑟兰化合物SrCu₂(BO₃)₂（SCBO）是一类典型的低维量子自旋系统（low-dimensional quantum spin system），其核心特征为存在几何阻挫的相互作用自旋二聚体（geometrically frustrated interacting spin dimers），并展现出丰富的涌现量子现象（emergent quantum phenomena）。尽管其磁性质已被广泛研究，但该体系的电子激发本质——尤其是Cu²+的d-d跃迁（d–d transitions）——仍不甚明晰。对这类电子激发进行详细表征，对于确定晶体场分裂（crystal field splitting）以及Cu 3d-O 2p杂化（Cu 3d–O 2p hybridization）程度至关重要，而这两者均是支撑SCBO中超交换相互作用（superexchange interactions）与磁行为的核心基础。本研究结合光谱技术与第一性原理能带结构计算（first-principles band structure calculations）理论方法，对SCBO的电子性质展开探究。实验层面，我们采用L₃边共振非弹性X射线散射（L3-edge resonant inelastic x‑ray scattering, RIXS）直接探测d内部激发，并辅以宽带红外反射光谱（broadband infrared reflectivity）与椭圆偏振法（ellipsometry）测量，以解析电荷转移（CT, charge-transfer）特征。理论层面，多参考量子化学计算（multireference quantum chemistry calculations）可精准复现观测到的d-d跃迁的能量范围（1.8–2.4 eV）与对称性，而基于密度泛函理论（density functional theory）的能带结构计算则阐明了电荷转移激发谱。上述联合方法明确了d-d跃迁与电荷转移跃迁的能量尺度，验证了计算框架，并为优化这类典型阻挫量子反铁磁体（prototypical frustrated quantum antiferromagnet）中基于超交换相互作用的磁模型提供了不可或缺的定量微观参数。