Electronic excitations in the Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$

TL;DR

This study combines experimental and computational methods to characterize high-energy electronic excitations in SrCu2(BO3)2, a model frustrated quantum magnet, revealing key energy scales of d-d and charge-transfer excitations.

Contribution

It provides a comprehensive analysis of local and interband electronic excitations in SrCu2(BO3)2 using RIXS, optical spectroscopy, and calculations, offering benchmarks for theoretical models.

Findings

Cu L3-edge RIXS identifies localized d-d excitations between 1.8 and 2.4 eV.

Optical spectroscopy detects charge-transfer excitations near 1.2-1.6 eV and around 4.5 eV.

Results define energy scales crucial for understanding magnetic interactions in SrCu2(BO3)2.

Abstract

SrCu2(BO3)2 (SCBO) is a paradigmatic realization of the Shastry-Sutherland model, hosting geometrically frustrated spin dimers and a variety of quantum magnetic phases and phenomena. Although its magnetic properties have been extensively studied, the high-energy electronic excitations that determine the crystal-field environment and Cu-O hybridization have remained largely unexplored. Here we combine Cu L3-edge resonant inelastic x-ray scattering (RIXS), broadband optical spectroscopy, and electronic-structure calculations to determine the relevant local and interband excitation energy scales in SCBO. RIXS resolves a well-defined manifold of localized Cu2+ d-d excitations between 1.8 and 2.4 eV, whose energies and polarization dependence are well reproduced by multireference quantum-chemistry calculations. In contrast, optical spectroscopy identifies charge-transfer excitations with an absorption onset near 1.2-1.6 eV and a broader higher-energy structure around 4.5 eV, which are qualitatively captured by DFT+U calculations. Taken together, these results define the characteristic energy scales of d-d and CT excitations, offering quantitative benchmarks for computational frameworks and providing essential input for refining superexchange-based magnetic models of this prototypical frustrated quantum antiferromagnet.