Selective Orbital Imaging of Excited States with X-Ray Spectroscopy: The Example of α-MnS

Abstract

Herein we show that nonresonant inelastic x-ray scattering involving anscore level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of thescore hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states in a straightforward manner, thereby facilitating the identification of their orbital character. In addition, thescore hole does not add an extra orbital angular momentum component to the multiplet structure so that the well-established Sugano-Tanabe-Kamimura diagrams can be used for the analysis of the spectra. Forα-MnS we observe the spherical charge density corresponding to its high-spin3d5(A16) ground state configuration and we were able to selectively image its excited states and identify them ast2g(T25) andeg(E5) with an energy splitting10Dqof 0.78 eV.

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Plain Language Summary

Transition-metal compounds have a wide variety of extraordinary properties, including metal-to-insulator transitions, electrical resistance that is influenced by external magnetic fields, and even superconductivity. To understand these rich phenomena, researchers look to the wealth of possible electronic states. In particular, one would like to know which electron orbitals are occupied in the ground state and which are available for excitations. While a variety of techniques exist to probe these states, the complex nature of correlated materials makes analysis of the data far from straightforward. Here, we demonstrate that it is possible in a spectroscopic experiment to make a direct visual image of these excited states and, by doing so, identify their orbital character.

We carry out experiments onα-MnS, a rock-salt-type antiferromagnetic insulator with orbital degrees of freedom that are present in its excited states. Building off earlier work, we use x-ray scattering to map out the cores-shell orbital. The spherical charge distribution of this “s-core hole” allows us to image the excited state by measuring the orientational dependence of the spectral intensities.

This result constitutes considerable progress in understanding the rich behavior of transition-metal compounds, since we no longer need complex calculations. Instead, we now can literally see from the image what the excited states are.