Universität zu Köln

Gresista, Lasse ORCID: 0000-0003-0185-7391 (2025). Exploring Quantum Magnetism in Moiré, Maple-Leaf, and Pyrochlore Materials via Functional Renormalization. PhD thesis, Universität zu Köln.

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Abstract

Magnets have fascinated people for centuries and profoundly shaped society—from compass needles that guided early explorers to permanent magnets in electric motors that power modern technology. In conventional magnetic materials, known as ferromagnets, the spins of the electrons align collectively, producing a stable magnetic field. Depending on the atomic ingredients and crystal structure, however, the interactions between electron spins can favor very different, and sometimes competing, patterns of alignment. When this competition is particularly strong, theory predicts that the quantum nature of electrons can give rise to even more unusual forms of magnetism, in which spins fail to order altogether, even at absolute zero. Instead, they remain in a fluctuating, highly entangled state known as a quantum spin liquid. Despite decades of theoretical and experimental effort—driven by the exotic nature of these phases, their potential relevance for quantum computing, and their possible connection to high-temperature superconductivity—an unambiguous realization of a quantum spin liquid in real materials remains elusive. In this thesis, we numerically investigate three classes of magnetic materials that are considered promising candidates for hosting quantum spin liquid behavior: moiré materials, maple-leaf magnets, and pyrochlore rare-earth oxides. Using effective theoretical models in the form of frustrated spin Hamiltonians, we study how quantum fluctuations modify their ground-state phase diagrams—by stabilizing new ordered phases, shifting phase boundaries, or, in the most favorable scenario, suppressing classical magnetic order altogether in favor of quantum spin liquid behavior. To systematically assess the impact of quantum effects, we employ and extend the pseudo-fermion functional renormalization group—a particularly versatile many-body method for describing quantum fluctuations in frustrated magnets—and complement it with classical and semiclassical approaches.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Gresista, Lasse lasse.gresista@gmail.com https://orcid.org/0000-0003-0185-7391 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-796059
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Frustrated magnetism English Quantum spin liquids English Pyrochlore rare-earth oxides English Maple-leaf materials English Moiré materials English Pseudo-fermion Functional renormalization group UNSPECIFIED Luttinger-Tisza UNSPECIFIED Cluster-Mean-Field-Approximation UNSPECIFIED Semiclassical Monte Carlo UNSPECIFIED
Date of oral exam:	28 November 2025
Referee:	Name Academic Title Trebst, Simon Prof. Dr. Rizzi, Matteo Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/79605