Signatures of fractionalization in correlated and topological magnets
The TUM-IAS Hans Fischer Senior Fellowship enabled a highly productive collaboration between the Perkins group (UMN) and the Knolle group (TUM) on dynamical probes of quantum materials, leading to 14 publications. This three-year partnership culminated in the Munich Quantum Matter Days workshop (Oct 27-31, 2025), co-organized by Perkins, Knolle, and Pfleiderer.
Focus Group: Disorder, Topology and Frustration in Spin-Orbit Coupled Quantum Magnets
Prof. Natalia Perkins (University of Minnesota), Alumna Hans Fischer Senior Fellow
Eduard Koller (TUM), Doctoral Candidate
Host: Prof. Johannes Knolle (TUM)
Highlights of collaborative research with the Knolle group (TUM):
Raman circular dichroism as a probe of chiral quantum spin liquids:
We demonstrated that Raman circular dichroism (RCD) provides a powerful probe of chiral quantum spin liquids (QSLs) and directly reflects the topology of their fractionalized excitations. Using the Loudon-Fleury formalism, we showed that the RCD response arises from Berry curvature and quantum metric contributions (Fig. 1) and is equivalent to the result obtained from direct light-matter coupling to emergent spinon bands. Applying this framework to the Kitaev model in a magnetic field and to a chiral QSL on the triangular lattice, we clarified the quantitative signatures expected in candidate materials. [1]
Interaction-induced chiral phonons and Raman circular dichroism in α-RuCl₃:
We developed a diagrammatic theory for spin-phonon coupling in the Kitaev material α-RuCl₃ to explain the recently observed Raman circular dichroism of the low-energy phonon mode in this material (Fig. 2). We demonstrated that bare phonons do not generate RCD; instead, coupling to the chiral Majorana continuum under an applied magnetic field mixes real phonon eigenvectors into complex, angular momentum-carrying modes. This mechanism produces finite RCD and characteristic Fano line shapes, both consistent with experiment. Our results establish RCD as a sensitive probe of interaction-driven chiral phonons in correlated quantum materials. [2]
Ferrimagnetic Kitaev spin liquids in mixed-spin honeycomb magnets:
We investigated a mixed-spin Kitaev model where spin-1/2 and spin-3/2 ions form a staggered pattern on the honeycomb lattice. By exploiting an exact mapping of local flux operators to gauge fields, we constructed a parton mean-field theory that reveals four distinct quantum spin-liquid phases characterized by quadrupolar order parameters. These analytical predictions were quantitatively validated using large-scale DMRG simulations. We also developed a superexchange theory identifying the microscopic conditions under which mixed-spin materials may realize dominant Kitaev interactions. [3]
Other highlights from the research of N.B. Perkins supported by the Hans Fischer Senior Fellowship:
Vacancy spectroscopy in the non-Abelian Kitaev spin liquid:
One of the major accomplishments of the PI of this Focus Group, supported by the TUM-IAS Hans Fischer Senior Fellowship, was the development of a theoretical framework for vacancy spectroscopy in the non-Abelian Kitaev spin liquid. This work, carried out in collaboration with Wen-Han Kao and Gabor Halasz and published in a coordinated PRL-PRB study (2024), demonstrated that spin vacancies act as controlled local probes of fractionalization: Each vacancy binds a quasi-zero-energy Majorana mode that leaves a clear signature in the inelastic scanning-tunneling signal (Fig. 3). Using the exactly solvable Kitaev model with a finite density of vacancies, we computed local dynamical spin correlations through a combination of analytical methods and large-scale numerical simulations. The PRL paper established that the experimentally measured second derivative of the tunneling current directly tracks the local connected spin correlator and therefore encodes the presence of vacancy-bound Majorana modes and their surrounding flux sector, most notably throug a robust quasi-zero-bias feature that scales with vacancy density. The accompanying PRB work further clarified the dynamics of these defect-induced modes, disentangling bulk and dangling-spin contributions and revealing how the in-gap response evolves with magnetic field, vacancy concentration, and flux configuration. Together, these results outline a realistic route for using STM to detect defect-bound Majorana excitations and distinguish flux sectors in candidate Kitaev materials such as α-RuCl₃.
From theory to experiment: Ultrasound signatures of Majorana excitations in Kitaev quantum spin liquids
Another major accomplishment of the PI of this Focus Group was the collaboration with the ultrasound group from Dresden, which built directly on our earlier theoretical studies of the PI and collaborators on phonon dynamics in Kitaev QSL. These studies, published in a series of Physical Seview B in the period 2023–2025, had established that acoustic phonons couple strongly to itinerant Majorana fermions, producing characteristic signatures in sound attenuation, temperature dependence, and a robust sixfold angular anisotropy governed by the ratio of the sound velocity to the Majorana Fermi velocity. The Dresden collaboration provided compelling experimental confirmation of these predictions in α-RuCl₃, revealing clear signatures of fractionalization and distinct behaviors of longitudinal and transverse modes consistent with the theoretically expected crossover between particle-hole and particle-particle scattering channels. A subsequent high-pressure ultrasound study further uncovered a suppression of magnetic order, the emergence of a dimerized phase, and a pressure-induced shift in the phonon scattering regime. Collectively, these results demonstrate that ultrasound spectroscopy serves as a sensitive, symmetry-selective, and tunable probe of fractionalization in Kitaev materials and highlight the impact of the PI’s theoretical framework on guiding and interpreting cutting-edge experiments.
Figure 2
Figure 3
source: 2024 Physical Review Letters
Selected publications
- Koller, E., Swarup, S., Knolle, J., Perkins, N. B., Spin-lattice coupling induced chiral phonons and their signature in Raman Circular Dichroism. arXiv:2511.14902 (2025a)
- Koller, E., Leeb, V., Perkins, N. B., Knolle, J., Raman Circular Dichroism and Quantum Geometry of Chiral Quantum Spin Liquids. arXiv:2503.14091 (2025b)
- Keskiner, M. A., Oktel, M. Ö., Perkins, N. B., Erten, O., Magnetic order through Kondo coupling to quantum spin liquids. Materials Today Quantum, 100038 (2025)
- Natori, W., Yang, Y., Jin, H. K., Knolle, J., Perkins, N. B., Ferrimagnetic Kitaev spin liquids in mixed spin- and spin-1/2 honeycomb magnets. Physical Review B 111 (21), 214411 (2025)
- Hauspurg, A., Singh, S., Yanagisawa, T., Tsurkan, V., Wosnitza, J., Brenig, W., Perkins, N. B., Zherlitsyn, S., Spin-strain interactions under hydrostatic pressure in α-RuCl₃. Physical Review B 112 (13), 134405, 2025