Probing the nature of excitations in spin-orbit-coupled materials from first principles
Final Report Abstract
In this project, theoretical models describing Kitaev candidate material α-RuCl3 and related materials were studied, which feature strongly anisotropic magnetic exchange interactions. A main focus here was on the controversy in α-RuCl3 regarding the presence or absence of a field-induced intermediate spin liquid state for in-plane magnetic fields of circa 7 to 10 Tesla. In collaborations with experiment, this question was investigated from both a spectroscopic (Raman scattering and terahertz absorption) and from a thermodynamic (magnetic Grüneisen parameter and specific heat) view. In both cases the experimental response showed rather unconventional features in the field range of question. However, at the same time, this response did not resemble closely than expected for an intermediate Kitaev spin liquid. Instead, we could reconcile the unconventional features in realistic extended Kitaev models, originating from strong magnetic frustration, a proximate ferromagnetic state, and magnetic anisotropy, not inherently related to potential spin liquid states. Indeed, the overall experimental observations could be well described by extended Kitaev models featuring a direct transition between low-field zigzag antiferromagnetic and a high-field partially-polarized phase with strong quantum fluctuations near the critical field, and no intermediate spin liquid. Additionally, in this project, magnetoelastic coupling in α-RuCl3 was studied. This covered two aspects, one on hand the possibility to tune the system by externally applied uniaxial and hydrostatic strains and pressures, and on the other hand to model emergent "magnetoelastic properties" such as magnetostriction and a variant of the structural Grüneisen parameter as a function of magnetic field, which directly couple lattice and pseudospin degrees of freedom. In all of these studies, we compared our theoretical calculations closely to measurements (partly already available at the time, partly in collaborations with experiment), finding very satisfactory agreement. Overall, an interesting aspect that should generalize to similarly strongly geometry-sensitive anisotropic spin exchanges in other materials, is that the form of the magnetoelastic couplings strongly deviated from the form of the magnetic couplings. Additionally, uniaxial compressive strain in α-RuCl3 is proposed as a future interesting avenue to increase the Kitaev coupling while weakening the other anisotropic coupling. Lastly, "variations" of α-RuCl3 and other materials were considered in the realm of this project. The simplest "variation" was studied in a collaboration with experiment in the form of few-layer and single-layer α-RuCl3. Here, we aided in the interpretation of our collaborators’ experiments by classical and quantum calculations, and as the main particularity of single-layer α-RuCl3, a reversal of the magnetic anisotropy is observed. Due to the latter, the out-of-plane critical field reduces from above 30 Tesla to only circa 7 Tesla, putting out-of-plane field physics much closer to usual experimental feasibility. In another study, we considered bulk RuBr3 and RuI3, which are structurally akin to α-RuCl3, but feature the heavier ligand atoms Br or I. While our analysis finds RuBr3 to have qualitatively similar magnetic properties to α-RuCl3, RuI3 is rather distinct and could be interesting for future studies. Finally, in another study we also considered the triangular-lattice material NaRuO2, which realizes a Mott-insulating j eff =1/2 state as well. Here, strongly anisotropic exchange, concomitant with a ferromagnetic Heisenberg coupling, lead to a ferromagnetic ground state. Additionally, we used this material as a test case system to study strongly anisotropic, plaquette-directional dependent ring exchange.
Publications
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Kitaev honeycomb models in magnetic fields: Dynamical response and dual models. Physical Review B, 100(14).
Kaib, David A. S.; Winter, Stephen M. & Valentí, Roser
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High-field quantum disordered state in α−RuCl3: Spin flips, bound states, and multiparticle continuum. Physical Review B, 101(14).
Sahasrabudhe, A.; Kaib, D. A. S.; Reschke, S.; German, R.; Koethe, T. C.; Buhot, J.; Kamenskyi, D.; Hickey, C.; Becker, P.; Tsurkan, V.; Loidl, A.; Do, S. H.; Choi, K. Y.; Grüninger, M.; Winter, S. M.; Wang, Zhe; Valentí, R. & van Loosdrecht, P. H. M.
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Thermodynamic Perspective on Field-Induced Behavior of α. Physical Review Letters, 125(9).
Bachus, S.; Kaib, D. A. S.; Tokiwa, Y.; Jesche, A.; Tsurkan, V.; Loidl, A.; Winter, S. M.; Tsirlin, A. A.; Valentí, R. & Gegenwart, P.
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Magnetoelastic coupling and effects of uniaxial strain in α−RuCl3 from first principles. Physical Review B, 103(14).
Kaib, David A. S.; Biswas, Sananda; Riedl, Kira; Winter, Stephen M. & Valentí, Roser
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Modified Curie-Weiss law for j eff magnets. Physical Review B, 103(22).
Li, Ying; Winter, Stephen M.; Kaib, David A. S.; Riedl, Kira & Valentí, Roser
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Combined experimental and theoretical study of hydrostatic He-gas pressure effects in α−RuCl3. Physical Review B, 106(13).
Wolf, B.; Kaib, D. A. S.; Razpopov, A.; Biswas, S.; Riedl, K.; Winter, S. M.; Valentí, R.; Saito, Y.; Hartmann, S.; Vinokurova, E.; Doert, T.; Isaeva, A.; Bastien, G.; Wolter, A. U. B.; Büchner, B. & Lang, M.
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Electronic and magnetic properties of the RuX3 (X = Cl, Br, I) family: two siblings—and a cousin?. npj Quantum Materials, 7(1).
Kaib, David A. S.; Riedl, Kira; Razpopov, Aleksandar; Li, Ying; Backes, Steffen; Mazin, Igor I. & Valentí, Roser
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Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3. Nature Materials, 22(1), 50-57.
Yang, Bowen; Goh, Yin Min; Sung, Suk Hyun; Ye, Gaihua; Biswas, Sananda; Kaib, David A. S.; Dhakal, Ramesh; Yan, Shaohua; Li, Chenghe; Jiang, Shengwei; Chen, Fangchu; Lei, Hechang; He, Rui; Valentí, Roser; Winter, Stephen M.; Hovden, Robert & Tsen, Adam W.
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Magnetoelastic coupling anisotropy in the Kitaev material α−RuCl3. Physical Review B, 105(9).
Kocsis, Vilmos; Kaib, David A. S.; Riedl, Kira; Gass, Sebastian; Lampen-Kelley, Paula; Mandrus, David G.; Nagler, Stephen E.; Pérez, Nicolás; Nielsch, Kornelius; Büchner, Bernd; Wolter, Anja U. B. & Valentí, Roser
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A jeff = 1/2 Kitaev material on the triangular lattice: the case of NaRuO2. npj Quantum Materials, 8(1).
Razpopov, Aleksandar; Kaib, David A. S.; Backes, Steffen; Balents, Leon; Wilson, Stephen D.; Ferrari, Francesco; Riedl, Kira & Valentí, Roser
