Universität zu Köln

Magnaterra, Marco ORCID: 0000-0002-8063-3590 (2025). RIXS interference in dimers, trimers, and tetramers: bond-directional excitations and quasimolecular wavefunctions. PhD thesis, Universität zu Köln.

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Abstract

In the quest to realize exotic quantum states of matter, systems with strongly correlated electrons are of particular interest. Such systems may, e.g., form a Mott insulator, with localized magnetic moments. The interactions between these moments depend on the interplay of charge, lattice, orbital, and spin degrees of freedom. Of particular interest are cases where strong spin-orbit coupling leads to spin-orbit entangled j=1/2 magnetic moments, as realized for Ir⁴⁺ ions within an octahedral ligand cage. These moments can give rise to exotic magnetism. In Kitaev materials with, e.g., a honeycomb lattice of edge-sharing IrO₆ octahedra, theory predicts that exchange interactions between j=1/2 moments predominantly have bond-directional nearest-neighbor character, leading to exchange frustration. Ideally, this results in a quantum spin liquid ground state. Direct evidence for quantum spin liquids and bond-directional interactions remains an open experimental challenge. Another route to realizing exotic quantum states is to search for unusual magnetic moments. We investigate these in cluster Mott insulators, where electrons are delocalized over a cluster, such as a dimer, trimer, or tetramer, forming localized quasimolecular magnetic moments. To probe the excitations in these systems, we use Resonant Inelastic X-ray Scattering (RIXS). This technique allows us to directly measure the energies of spin and orbital excitations and their dependence on exchanged momentum. When the electrons are delocalized over a cluster, like a dimer, trimer, or tetramer, the RIXS intensity exhibits a characteristic periodic modulation as a function of the exchanged momentum. This modulation arises from interference between the RIXS amplitudes from different sites within the cluster. We show that this modulation contains valuable information about bond-directional excitations in Kitaev materials and the quasimolecular wavefunctions in cluster Mott insulators. Bond-directional magnetic interactions imply the existence of bond-directional magnetic excitations, spin excitations that render crystallographically equivalent directions magnetically inequivalent. We show how to observe the bond-directional character of the magnetic excitations using RIXS at the Ir L₃ edge in the Kitaev materials Na₂IrO₃ and α-Li₂IrO₃. We identify a low-energy spin-conserving excitation (at 10 meV and 15 meV, respectively) and a high-energy spin-flip excitation (at 45 meV and 35 meV, respectively). By exploiting the polarization and exchanged momentum dependence of the RIXS process, we simultaneously determine the bond direction and the spin operator involved in a given excitation, thus proving the bond-directional character of the magnetic excitations. In cluster Mott insulators, the quasimolecular character can be probed via the modulation of the RIXS intensity. We study Ba₄NbIr₃O₁₂, which contains Ir₃O₁₂ trimers, using RIXS at the Ir L₃ edge. This work represents the first RIXS experimental determination of the quasimolecular electronic structure in a quasimolecular trimer cluster compound. We show that the modulation reveals information about the symmetry of the quasimolecular wavefunctions. The lacunar spinel GaTa₄Se₈ hosts spin-orbit-entangled quasimolecular wavefunctions on Ta₄ tetrahedra. We study the RIXS modulation at the Ta L₃ edge and find that it is sensitive to the wavefunction composition, particularly to the mixing between bonding and antibonding states. This mixing affects the quasimolecular magnetic moment and renormalizes the effective spin-orbit coupling. Accurate modeling was crucial for correct peak assignment and for understanding the tetrahedral quasimolecular magnetic moment.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Magnaterra, Marco magnaterra@ph2.uni-koeln.de orcid.org/0000-0002-8063-3590 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-787093
Date:	26 May 2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language RIXS interferometry English Resonant Inelastic X-ray Scattering English Kitaev materials English bond-directional excitations English cluster Mott insulators English quasimolecular wavefunctions English quasimolecular orbitals English strongly correlated electrons English quantum spin liquids English
Date of oral exam:	29 July 2025
Referee:	Name Academic Title Grüninger, Markus Prof. Dr. Haverkort, Maurits Prof. Dr. Hess, Christian Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78709