E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S

S C I E N T I F I C H I G H L I G H T S

1 0 2 H I G H L I G H T S 2 0 2 2 I

character of the involved states, as mentioned above. Therefore, the modulation pattern also allows to answer the question on the ground state: Ba3InIr2O9 resides on the large-hopping side, as sketched in Figure 92b.

In conclusion, RIXS interferometry is a powerful tool to unravel the electronic structure and to determine the character and symmetry of electronic states.

PRINCIPAL PUBLICATION AND AUTHORS

Quasimolecular electronic structure of the spin-liquid candidate Ba3InIr2O9, A. Revelli (a), M. Moretti Sala (b,c), G. Monaco (d), M. Magnaterra (a), J. Attig (e), L. Peterlini (e), T. Dey (a,f,g), A.A. Tsirlin (g), P. Gegenwart (g), T. Fröhlich (a), M. Braden (a), C. Grams (a), J. Hemberger (a), P. Becker (h), P.H.M. van Loosdrecht (a), D.I. Khomskii (a), J. van den Brink (i,j), M. Hermanns (k,l), M. Grüninger (a), Phys. Rev. B 106, 155107 (2022); https:/doi.org/10.1103/PhysRevB.106.155107 (a) Institute of Physics II, University of Cologne (Germany) (b) ESRF (c) Dipartimento di Fisica, Politecnico di Milano (Italy) (d) Dipartimento di Fisica e Astronomia Galileo Galilei , Università di Padova (Italy) (e) Institute for Theoretical Physics, University of Cologne (Germany) (f) Department of Physics, IIT (ISM) Dhanbad, Jharkhand (India) (g) Experimental Physics VI, Center for Electronic Correlations and Magnetism, University of Augsburg (Germany) (h) Crystallography Section, Institute of Geology and Mineralogy, University of Cologne (Germany) (i) Institute for Theoretical Solid State Physics, IFW Dresden (Germany) (j) Institute for Theoretical Physics and Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden (Germany) (k) Department of Physics, Stockholm University, AlbaNova University Center (Sweden) (l) Nordita, KTH Royal Institute of Technology and Stockholm University (Sweden)

REFERENCES

[1] A. Revelli et al., Sci. Adv. 5, eaav4020 (2019). [2] A. Revelli et al., Phys. Rev. Res. 2, 043094 (2020).

A new charge reordering in magnetite

A new charge reordering in magnetite was discovered using core-level X-ray spectroscopy. This reordering takes place in three steps and maps the temperature- dependence of the electrical conductivity and magnetism up to the Curie-temperature.

Magnetite (Fe3O4) is one of the most abundant Fe- bearing minerals on Earth, with many applications in paleomagnetism, medicine, data recording and engineering. It is a prototypical magnetic material, used for the first compass. Understanding the ordering of the charges in magnetite is one of the oldest questions in solid state physics, as magnetite exhibits a first-order metal- to-insulator transition at TV~123 K [1]. Fe ions reside in two interstitial sites in Fe3O4: (i) Fe2+ and Fe3+ in octahedral sites and (ii) Fe3+ in tetrahedral sites. Several decades of research have revealed that an intricate charge and orbital ordering takes place below the metal-to-insulator transition temperature, where the Fe ions at the octahedral sites form a network of Fe3+-Fe2+-Fe3+ linear molecules (called

trimerons) [2]. Short-range structural fluctuations linked to the presence of trimerons have been shown to exist in the high-temperature metallic-like phase [3]. However, the role of these short-range trimerons and the mechanism behind their collapse remains an unsolved puzzle. This motivated the design of a new experiment to address these questions.

Fluorescence-detected X-ray absorption spectroscopy with high energy resolution was used at beamline ID26 to investigate the short-range structural fluctuations and charge ordering in magnetite. Here, a 1s core- electron was resonantly excited to the empty 3d orbitals of Fe, providing an element- and site-selective probe of the Fe ions in Fe3O4. A new charge reordering was found to occur in the metallic-like phase, where the extra electron of the Fe2+ ion at the octahedral sites is transferred progressively to the tetrahedral sites with increasing temperature (Figure 94). This implies that the octahedral sublattice becomes hole-doped and, hence, the Fe3+-Fe2+-Fe3+ trimerons are altered. The level of hole- doping could be continuously (and reversibly) tuned by controlling the temperature, and three regimes of self- doping were identified that describe the temperature-

Concerning magnetic excitations, a stunning example is the demonstration of their nearest-neighbour character in proximate Kitaev spin liquids [2]. In the present case, RIXS interferometry establishes Ba3InIr2O9 as a spin-orbit- entangled cluster Mott insulator and reveals the precise character of the quasimolecular ground state. Such cluster Mott insulators may open up a new route to realising unconventional magnetic phases of matter.