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

January 13, 2021

A team lead by scientists from the MPI CPfS developed a novel experimental method that provides direct images of excited states in a transition metal compound without the need for complex calculations. This constitutes a major step for the understanding of the electronic structure and the rich physics of these materials.

Transition metal compounds have a wide variety of extraordinary properties, including metal-to-insulator and spin-state transitions, colossal magneto resistance, superconductivity, multiferroicity, and bad metal behavior.  To understand these rich phenomena, one needs to look to the wealth of possible electronic states. In particular, one would like to know which electron orbitals are involved. 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, a team of scientists from MPI-CPfS in Dresden, University of Cologne, Heidelberg University, the German Synchrotron in Hamburg (DESY) and the Taiwanese National Synchrotron Radiation Research Center in Hsinchu, demonstrate that it is possible in a spectroscopic experiment to make a direct visual image of the excited states and, by doing so, identify their orbital character [1].

Fig. 1. Top: The angular dependence of the integrated intensity of the 3sà3d spectra (black dots) gives a visual image of the spherical charge distribution of the high-spin Mn 3d5 ion. Bottom: The angular dependence of each s-NIXS peak (orange and blue circles) follows the angular dependence of the corresponding orbitals, here eg and t2g. The distance in energy between the peaks in the spectra is a direct measure of the crystal field splitting 10Dq.

The team carried out experiments on α-MnS, a rock salt type antiferromagnetic insulator with orbital degrees of freedom that are present in its excited states. The newly developed spectroscopic technique is s-NIXS, where large momentum transfers q are used in the scattering of x-rays to access the quadrupolar 3sà3d transition. Spectra were collected for a wide range of q-directions. The directional dependence of the energy-integrated intensity of the spectra provides a spatial image of the orbital occupation in the ground state, as was discovered in an earlier work [2]. What is new here is the study of the directional dependence of the spectral features. It turns out the technique yields also a spatial image of the excited states. This simplifies considerably the identification of the so-called multiplet states which typically dominate the electronic structure of transition metal compounds.

In the α-MnS experiment, the directional dependence of the energy-integrated intensity of the spectra yields a spherical charge distribution, revealing the high-spin 3d5 ground state of the Mn ion. As far as the spectral features are concerned, two peaks can be seen in the spectra, which can be assigned to g and t2g orbitals on the basis of their angular shapes (Fig. 1).

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[1] A. Amorese, B. Leedahl, M. Sundermann, H. Gretarsson, Z. Hu, H.-J. Lin, C. T. Chen, M. Schmidt, H. Borrmann, Yu. Grin, A. Severing, M. W. Haverkort, and L. H. Tjeng, Phys. Rev. X 11, 011002 (2021), https://doi.org/10.1103/PhysRevX.11.011002 

[2] H. Yavaş, M. Sundermann, K. Chen, A. Amorese, A. Severing, H. Gretarsson, M.W. Haverkort, L.H. Tjeng, Nature Physics 15, 559 (2019), https://doi.org/10.1038/s41567-019-0471-2.

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