B1: New faces of Kitaev materials: chemical, pressure, and strain tuning
Philipp Gegenwart, Alexander Tsirlin, Anton Jesche
Kitaev model of bond-anisotropic exchange interactions offers a promising route toward topological quantum computing, but its existing realizations in solid-state materials have shown significant deviations from the idealized interaction regime. In this project, we seek to tailor both well-known and recently discovered material candidates by chemical substitutions, hydrostatic pressure, and strain with the ultimate goal of approaching the limit of dominant Kitaev interactions and securing experimental access to the full parameter range of the model. Our work will establish main structural and electronic parameters that control manifestations of the Kitaev physics in solid-state materials.
Publications
2024 |
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Shen, B.; Ginga, V. A.; Arévalo-López, A. M.; Garbarino, G.; Uykur, E.; Goncalves-Faria, M.; Mukharjee, P. K.; Gegenwart, P.; Tsirlin, A. A. Magnetic vs. nonmagnetic polymorphs of RuBr3 under pressure Journal Article Phys. Rev. B 109, 224402, 2024. @article{shen_magnetic_2024,Pressure evolution of the crystal structure and magnetism of the honeycomb 𝛼-RuBr_3 is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic 𝛼-RuBr_3 (R-3) into paramagnetic 𝛼'-RuBr_3 (P-1) where short Ru-Ru bonds cause magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this critical pressure, the Ntextbackslash'eel temperature of 𝛼-RuBr_3 increases with the slope of 1.8 K/GPa. Pressure tunes 𝛼-RuBr_3 away from the Kitaev limit, whereas increased third-neighbor in-plane coupling and interlayer coupling lead to a further stabilization of the collinear zigzag state. Both 𝛼- and 𝛼'-RuBr_3 are metastable at ambient pressure, but their transformation into the thermodynamically stable 𝛽-polymorph is kinetically hindered at room temperature. |  |
Reinhoffer, C.; Esser, S.; Esser, S.; Mashkovich, E.; Germanskiy, S.; Gegenwart, P.; Anders, F.; Loosdrecht, P. H. M.; Wang, Z. Strong Terahertz Third-Harmonic Generation by Kinetic Heavy Quasiparticles in CaRuO3 Journal Article Phys. Rev. Lett. 132, 196501, 2024. @article{reinhoffer_strong_2024,We report on time-resolved nonlinear terahertz spectroscopy of a strongly correlated ruthenate, CaRuO3, as a function of temperature, frequency, and terahertz field strength. Third-harmonic radiation for frequencies up to 2.1 THz is observed evidently at low temperatures below 80 K, where the low-frequency linear dynamical response deviates from the Drude model and a coherent heavy quasiparticle band emerges by strong correlations associated with the Hund’s coupling. Phenomenologically, by taking an experimentally observed frequency-dependent scattering rate, the deviation of the field driven kinetics from the Drude behavior is reconciled in a time-dependent Boltzmann description, which allows an attribution of the observed third-harmonic generation to the terahertz field driven nonlinear kinetics of the heavy quasiparticles. | |
Mukharjee, P. K.; Shen, Bin; Erdmann, S.; Jesche, A.; Kaiser,; Baral, P. R.; Zaharko, O.; Gegenwart, P.; Tsirlin, A. A. Intermediate field-induced phase of the honeycomb magnet BaCo2(AsO4)2 Unpublished 2024, arXiv.2403.04466. @unpublished{mukharjee_intermediate_2024,We use magnetometry, calorimetry, and high-resolution capacitive dilatometry, as well as single-crystal neutron diffraction to explore temperature-field phase diagram of the anisotropic honeycomb magnet BaCo$_2$(AsO$_4)_2$. Our data reveal four distinct ordered states observed for in-plane magnetic fields. Of particular interest is the narrow region between 0.51 and 0.55 T that separates the up-up-down order from the fully polarized state and coincides with the field range where signatures of the spin-liquid behavior have been reported. We show that magnetic Bragg peaks persist in this intermediate phase, thus ruling out its spin-liquid nature. However, the simultaneous nonmonotonic evolution of nuclear Bragg peaks suggests the involvement of the lattice, witnessed also in other regions of the phase diagram where large changes in the sample length are observed upon entering the magnetically ordered states. Our data highlight the importance of lattice effects in BaCo$_2$(AsO$_4)_2$. | |
Abdeldaim, A. H.; Gretarsson, H.; Day, S. J.; Le, M. D.; Stenning, G. B. G.; Manuel, P.; Perry, R. S.; Tsirlin, A. A.; Nilsen, G. J.; Clark, L. 2024, arXiv.2403.19406. @unpublished{abdeldaim_kitaev_2024,Magnetic materials are composed of the simple building blocks of magnetic moments on a crystal lattice that interact via short-range magnetic exchange interactions. Yet from these simple building blocks emerges a remarkable diversity of magnetic states. Some of these, such as ferromagnetism, are familiar in our everyday lives, while others reveal the deep quantum mechanical origins of magnetism. A prime example of the latter are quantum spin liquid (QSL) states in which – unlike in a ferromagnet where magnetic moments are driven by their exchange interactions to adopt long-range order – magnetic moments remain disordered at low temperatures but are simultaneously correlated over long length scales through quantum entanglement. A particularly promising theoretical model of a QSL is the Kitaev model, composed of unusual bond-dependent exchange interactions between magnetic moments on a honeycomb lattice. However, the Kitaev QSL is extremely challenging to realise experimentally as it is unstable to competing exchange interactions and crystal lattice perturbations that inevitably arise in real materials. This makes it essential to understand the relationship between the structure and interactions that may give rise to Kitaev interactions in new candidate materials. Here we show that the material requirements for the Kitaev QSL survive for an extended pseudo-edge-sharing superexchange pathway of Ru3+ 4d5 octahedra within the honeycomb layers of the inorganic framework solid, RuP3SiO11. Through materials synthesis and structural characterisation, resonant inelastic X-ray and neutron scattering experiments, we confirm the requisite jeff = 1/2 state of Ru3+ in RuP3SiO11 and resolve the hierarchy of exchange interactions that provide experimental access to an otherwise unexplored region of the extended Kitaev phase diagram. |  |
2023 |
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Solana-Madruga, E.; Mentre, O.; Tsirlin, A. A.; Huve, M.; Khalyavin, D.; Ritter, C.; Arevalo-Lopez, A. M. CoVO3 High-Pressure Polymorphs: To Order or Not to Order? Journal Article Adv. Sci. 11, 2307766, 2023. @article{solana-madruga_covo_3_2023,Materials properties are determined by their compositions and structures. In ABO(3) oxides different cation orderings lead to mainly perovskite- or corundum like derivatives with exciting physical properties. Sometimes, a material can be stabilized in more than one structural modification, providing a unique opportunity to explore structure-properties relationship. Here, CoVO3 obtained in both ilmenite-(CoVO3-I) and LiNbO3-type (CoVO3-II) polymorphs at moderate (8-12 GPa) and high pressures (22 GPa), respectively are presented. Their distinctive cation distributions affect drastically the magnetic properties as CoVO3-II shows a cluster-glass behavior while CoVO3-I hosts a honeycomb zigzag magnetic structure in the cobalt network. First principles calculations show that the influence of vanadium is crucial for CoVO3-I, although it is previously considered as non-magnetic in a dimerized spin-singlet state. Contrarily, CoVO3-II shows two independent interpenetrating antiferromagnetic Co- and ferromagnetic V-hcp sublattices, which intrinsically frustrate any possible magnetic order. CoVO3-II is also remarkable as the first oxide crystallizing with the LiNbO3-type structure where both metals contain free d electrons. CoVO3 polymorphs pinpoint therefore as well to a much broader phase field of high-pressure A-site Cobaltites. | |
Panther, A.; Tsirlin, A. A.; Rousochatzakis, I. Frustration relief and reorientation transition in the kagomelike dolerophanite Cu2OSO4 Journal Article Phys. Rev. B 108, 224410, 2023. @article{panther_frustration_2023,We present a theoretical study of dolerophanite Cu2OSO4, a layered kagomelike spin-21 magnetic insulator that can be described either as a system of chains coupled through dimers or as a kagome lattice where every third spin is replaced by a ferromagnetic spin dimer. Building on insights from ab initio calculations, classical numerical minimizations, and semiclassical expansions, we arrive at a minimal microscopic description that accounts for the experimental data reported so far, including the nature of the magnetic order, the reported spin length, and the observed anisotropy. The latter arises by a peculiar competition between the antisymmetric (DzyaloshinskiiMoriya) and the symmetric part of the exchange anisotropy, which gives rise to a two-step reorientation process involving two successive continuous phase transitions. In this paper, we uncover mechanisms stabilizing canted ferrimagnetic order in kagome systems and highlight strong magnetic anisotropy in the presence of dissimilar magnetic orbitals on crystallographically nonequivalent Cu sites. We also show how these anisotropy terms affect the spin-wave spectrum and how they can be tracked experimentally. | |
Breitner, F. A.; Kaiser, J.; Jesche, A.; Gegenwart, P. Metallic conductivity in Na-deficient structural domain walls in the spin-orbit Mott insulator Na2IrO3 Journal Article Phys. Rev. B 108, 235130, 2023. @article{breitner_metallic_2023,Honeycomb Na2IrO3 is a prototype spin-orbit Mott insulator and Kitaev magnet. We report a combined structural and electrical resistivity study of Na2IrO3 single crystals. Laue back-scattering diffraction indicates twinning with +/- 120 degrees rotation around the c* axis while scanning electron microscopy displays nanothin lines parallel to all three b-axis orientations of twin domains. Energy dispersive x-ray analysis line scans across such domain walls indicate no change of the Ir signal intensity, i.e., intact honeycomb layers, while the Na intensity is reduced down to similar to 2/3 of its original value at the domain walls, implying significant hole doping. Utilizing focused-ion-beam microsectioning, the temperature dependence of the electrical resistance of individual domain walls is studied. It demonstrates the tuning through the metal-insulator transition into a correlated-metal ground state by increasing hole doping. | |
Hua, N.; Breitner, F.; Jesche, A.; Huang, S. -W.; Rüegg, C.; Gegenwart, P. Structural and magnetic properties of β-Li2IrO3 after grazing-angle focused ion beam thinning Unpublished 2023, arXiv:2310.18644. @unpublished{hua_structural_2023, | |