C1: Kinetic arrest and quantum annealing in model systems
Christian Franz, Christian Pfleiderer
The effects of constrained kinetic energy versus quantum tunneling will be explored systematically in selected model materials using pump-probe ac susceptibility measurements and ultra-high-resolution neutron spectroscopy. The objectives of this project are (i) to establish empirical differences of kinetic arrest and quantum annealing, (ii) to explore relaxation pathways as mediated by different hybrid excitations formed, e.g., by lattice, spin, charge, and nuclear degrees of freedom, (iii) to study spin freezing in classical spin glasses versus quantum materials, (iv) to study spin freezing at quantum phase transitions, and (v) to examine freezing transitions of spin textures subject to non-trivial topological winding.
Publications
2026 |
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Gaggl, T.; Misawa, R.; Kriener, M.; Tanaka, Y.; Yamada, R.; Hirschberger, M. Bulk superconductivity in the kagome metal YRu3B2 Journal Article Phys. Scr. 101, 055912 (2026), datadoi: (10.5281/zenodo.18276372). @article{gagglBulkSuperconductivityKagome2026,Materials with a kagome lattice have been heavily studied for their exotic electronic band structure, geometrical frustration, high-temperature charge-order transitions, and unconventional electron-phonon coupling. In LaRu3Si2, it was proposed that electronic flat bands conspire with the characteristic phonon spectrum of the kagome lattice to drive enhanced superconductivity at Tc = 6.8 K. Here, we report bulk superconductivity in the structural analogue YRu3B2, which hosts a pristine kagome lattice. We observe a superconducting transition at Tc = 0.7 K through magnetization, resistivity, and heat-capacity measurements in this novel kagome metal. |  |
2025 |
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Misawa, R.; Kitou, S.; Yamada, R.; Gaggl, T.; Nakano, R.; Shibata, Y.; Okamura, Y.; Kriener, M.; Baral, P. R.; Nakamura, Y.; Ōnuki, Y.; Takahashi, Y.; Arima, T.; Jovanovic, M.; Schoop, L. M.; Hirschberger, M. Successive orthorhombic distortions in kagome metals by molecular orbital formation Journal Article Adv. Mater. e13015 (2025). @article{misawaSuccessiveOrthorhombicDistortions2025,Abstract The kagome lattice, with its inherent frustration, hosts a plethora of exotic phenomena, including the emergence of 3q charge-density-wave order. The high rotational symmetry required to realize such an unconventional charge order is broken in many kagome materials by orthorhombic distortions at high temperature, the origin of which remains much less examined despite their ubiquity. In this study, synchrotron X-ray diffraction reveals a structural phase transition from a parent hexagonal structure to an orthorhombic groundstate, mediated by a critical regime with diffuse scattering in the prototypical kagome metals RRu3Si2 (R = Nd, Pr). Structural analysis uncovers partially ordered bonds between kagome layers in the orthorhombic phases. Accordingly, a short-range correlated dimer model on the kagome layers reproduces the diffuse scattering, with the short-range order arising from competing structures induced by the geometrical frustration of the kagome lattice. The observations point to molecular orbital formation between Ru $4d_zˆ2$ orbitals as the driving force behind the transition, consistent with ab initio calculations. A framework based on electronegativity and a tolerance factor is proposed to evaluate the stability of the hexagonal phase in various kagome metals, guiding the design of highly symmetric materials. | |
2024 |
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Säubert, S.; Franz, C.; Jochum, J. K.; Benka, G.; Bauer, A.; Shapiro, S. M.; Böni, P.; Pfleiderer, C. Evolution of spin dynamics during freezing in the spin-glass FexCr1-x Journal Article Phys. Rev. B 110, 094422 (2024). @article{saubert_evolution_2024,In the iron–chromium system, Fe𝑥Cr1−𝑥, a wide dome of spin-glass behavior emerges when the ferromagnetism of iron is suppressed and the antiferromagnetism of chromium emerges as a function of increasing iron content 𝑥. As both, the high-temperature state and the characteristic cluster size vary as a function of 𝑥, different regimes of spin-glass behavior may be compared in a single, isostructural material system. Here, we report a study of the spin dynamics across the freezing process into the spin-glass state for different iron concentrations (𝑥=0.145, 0.175, 0.21) using modulation of intensity with zero effort (MIEZE) spectroscopy. In the parameter range studied, the relaxation process observed experimentally may be described well in terms of a stretched exponential. In the reentrant cluster-glass regime, 𝑥=0.145, this behavior persists up to high temperatures. In comparison, in the superparamagnetic regime, 𝑥=0.175 and 𝑥=0.21, a single relaxation time at elevated temperatures is observed. For all samples studied, the spin relaxation exhibits a momentum dependence consistent with a power law, providing evidence of a dispersive character of the spin relaxation. |  |
Kalin, J.; Sievers, S.; Schumacher, H. W.; Füser, H.; Bieler, M.; Bauer, A.; Pfleiderer, C. Influence of the magnetovolume effect on the transient reflectivity of MnSi Journal Article Phys. Rev. B 110, 014415 (2024). @article{kalin_influence_2024,The magnetovolume effect is a well established yet frequently overlooked phenomenon in magnetic materials that may affect a wide range of physical properties. Our study explores the influence of the magnetovolume effect on the transient reflectivity of MnSi, a well-known chiral magnet with strong magnetoelastic coupling. We observe a unipolar reflectivity transient in the paramagnetic phase, contrasting with a bipolar response in phases with magnetic long-range order. Comparing our findings with thermal expansion from literature, we establish that the bipolar response originates in the magnetovolume effect which dominates the thermal expansion and influences the optical reflectivity. Our results highlight not only that the magnetovolume effect must be considered when discussing transient reflectivity measurements of magnetic materials but also that such measurements permit to study the characteristic time scales of the magnetovolume effect itself, contributing to a deeper understanding of this often-neglected phenomenon. | |