Spin-orbit Coupling and Correlations

In materials with 4d and 5d electron systems, the intra-atomic spin-orbit coupling is comparable to the strength of the crystalline electric fields and the exchange interactions. The interplay between these parameters can generate novel phenomena such as bond-directional Kitaev interactions and excitonic magnetism in insulators, and can profoundly affect the electronic quasiparticles and superconducting pairing state in metals. Spectroscopic experiments in our department aim to unravel the underlying mechanisms.
 

In many 4d- and 5d-electron magnets, the orbital moment in the ground-state wavefunction is at least comparable to, and often larger than, the spin moment. A comprehensive theoretical study in our group demonstrated that this situation gives rise to large and unusual pseudospin-lattice interactions. Subsequent experimental work on square-lattice iridates revealed that the unusual magnetostrictive coupling strongly influences the magnetic ground state, the macroscopic magnetization, and the dispersion relations of collective excitations.  A striking consequence of the strong pseudospin-lattice coupling mediated by the orbital moment is the giant response of THz magnons in spin-orbit Mott insulators to uniaxial strain, which opens up exciting perspectives for THz magnonics.

Ultrasensitive strain modulation of terahertz magnons at a magnetic phase transition
L. Wang, S. Hameed, Y. Liu, M. Knauft, K. Higuchi, M. Krautloher, S. Francoual, G. Khaliullin, H. Liu, M. Minola, and B. Keimer
arXiv:2602.13924
Giant stress response of terahertz magnons in a spin-orbit Mott insulator
Hun-Ho Kim, Kentaro Ueda, Suguru Nakata, Peter Wochner, Andrew Mackenzie, Clifford Hicks, Giniyat Khaliullin, Huimei Liu, Bernhard Keimer, Matteo Minola
Nature Communications 13, 6674 (2022)

Influential theoretical work by Jackeli and Khaliullin in our department proposed J=1/2 pseudospin systems as solid-state realizations of the exactly soluble Kitaev model, which embodies key concepts in condensed matter physics such as topological spin liquids and spin fractionalization. The spin-orbit  Mott insulator RuCl₃ has emerged as a prime candidate for Kitaev physics. Resonant x-ray scattering experiments in our group have elucidated both the single-ion wave function and the nature of the exchange interactions in ruthenium compounds, thus establishing a generic “spectroscopic fingerprinting” methodology that can now be used to assess spin-liquid candidates.

𝐽= 1/2 pseudospins and 𝑑−𝑝 hybridization in the Kitaev spin liquid candidates Ru⁢𝑋₃ (𝑋=Cl, Br, I)
H. Gretarsson, H. Fujihara, F. Sato, H. Gotou, Y. Imai, K. Ohgushi, B. Keimer, and H. Suzuki
Physical Review B 109, L180413 (2024)
Proximate ferromagnetic state in the Kitaev model material α-RuCl₃
H. Suzuki, H. Liu, J. Bertinshaw, K. Ueda, H. Kim, S. Laha, D. Weber, Z. Yang, L. Wang, H. Takahashi, K. Fürsich, M. Minola, B. V. Lotsch, B. J. Kim, H. Yavaş, M. Daghofer, J. Chaloupka, G. Khaliullin, H. Gretarsson & B. Keimer
Nature Communications 12, 4512 (2021)
Mott Insulators in the Strong Spin-Orbit Coupling Limit: From Heisenberg to a Quantum Compass and Kitaev Models
G. Jackeli, G. Khaliullin.
Phys. Rev. Lett. 102, 017205 (2009)

The unconventional superconductor Sr₂RuO₄ has
long served as a benchmark for theories of correlated-electron materials. The determination of the superconducting pairing mechanism requires detailed experimental information on collective bosonic excitations as potential mediators of Cooper pairing. We have used Ru L-edge RIXS to obtain comprehensive maps of the electronic excitations of Sr₂RuO₄ over the entire Brillouin zone. We observe multiple branches of dispersive spin and orbital excitations associated with distinctly different energy scales. Of particular interest are incommensurate “paramagnon” excitations that are being discussed as mediators of unconventional superconductivity. Interestingly, our experiments show that such excitations are not present in the non-superconducting metal Sr₂RhO₄.

Spin-Orbit Excitons in a Correlated Metal: Raman Scattering Study of Sr₂RhO₄
L. Wang, H. Liu, V. Zimmermann, A.K. Yogi, M. Isobe, M. Minola, M. Hepting, G. Khaliullin, and B. Keimer
Physical Review Letters 132, 116502 (2024)
Distinct spin and orbital dynamics in Sr₂RuO₄
H. Suzuki, L. Wang, J. Bertinshaw, H. U. R. Strand, S. Käser, M. Krautloher, Z. Yang, N. Wentzell, O. Parcollet, F. Jerzembeck, N. Kikugawa, A. P. Mackenzie, A. Georges, P. Hansmann, H. Gretarsson, and B. Keimer
Nature Communications,14 7042 (2023)

A prime characteristic of correlated-electron materials is the near-degeneracy of quantum states with radically different macroscopic properties. In this situation, external perturbations can create non-equilibrium steady states very different from those in materials with weakly interacting electrons. Using different momentum-resolved spectroscopies, we are studying the current-induced Mott insulator-to-metal transition as a prime example of a non-equilibrium phase transition. We are aiming to understand the underlying mechanisms and to create related states in other materials and devices with a view towards novel functionalities.

Electronic response of a Mott insulator at a current-induced insulator-to-metal transition
C.T. Suen, I. Marković, M. Zonno, N. Heinsdorf, S. Zhdanovich, N.H. Jo, M. Schmid, P. Hansmann, P. Puphal, K. Fürsich, S. Smit, C. Au-Yeung, V. Zimmermann, B. Zwartsenberg, M. Krautloher, I.S. Elfimov, R. Koch, S. Gorovikov, C. Jozwiak, A. Bostwick, M. Franz, E. Rotenberg, B. Keimer, and A. Damascelli
Nature Physics 20,1757 (2024)