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1. Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6

   https://doi.org/10.1038/s41467-021-25567-7  

   Lin, Gaoting; Jeong, Jaehong; Kim, Chaebin; Wang, Yao; Huang, Qing; Masuda, Takatsugu; Asai, Shinichiro; Itoh, Shinichi; Günther, Gerrit; Russina, Margarita; et al (December 2021, Nature Communications)

   Abstract Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4 d /5 d -based honeycomb magnets. Recent theoretical studies predicted that 3 d -based honeycomb magnets, including Na 2 Co 2 TeO 6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T <  B (⊥ b -axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3 d -magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3 d compounds, promoting further interests on the spin-orbital effect in quantum magnets. 

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2. Momentum-independent magnetic excitation continuum in the honeycomb iridate H3LiIr2O6

   https://doi.org/10.1038/s41467-023-40769-x  

   de la Torre, A.; Zager, B.; Bahrami, F.; Upton, M. H.; Kim, J.; Fabbris, G.; Lee, G. -H.; Yang, W.; Haskel, D.; Tafti, F.; et al (August 2023, Nature Communications)

   Abstract Understanding the interplay between the inherent disorder and the correlated fluctuating-spin ground state is a key element in the search for quantum spin liquids. H₃LiIr₂O₆is considered to be a spin liquid that is proximate to the Kitaev-limit quantum spin liquid. Its ground state shows no magnetic order or spin freezing as expected for the spin liquid state. However, hydrogen zero-point motion and stacking faults are known to be present. The resulting bond disorder has been invoked to explain the existence of unexpected low-energy spin excitations, although data interpretation remains challenging. Here, we use resonant X-ray spectroscopies to map the collective excitations in H₃LiIr₂O₆and characterize its magnetic state. In the low-temperature correlated state, we reveal a broad bandwidth of magnetic excitations. The central energy and the high-energy tail of the continuum are consistent with expectations for dominant ferromagnetic Kitaev interactions between dynamically fluctuating spins. Furthermore, the absence of a momentum dependence to these excitations are consistent with disorder-induced broken translational invariance. Our low-energy data and the energy and width of the crystal field excitations support an interpretation of H₃LiIr₂O₆as a disordered topological spin liquid in close proximity to bond-disordered versions of the Kitaev quantum spin liquid. 

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3. Kitaev spin-orbital bilayers and their moiré superlattices

   https://doi.org/10.1038/s41535-023-00541-2  

   Nica, Emilian Marius; Akram, Muhammad; Vijayvargia, Aayush; Moessner, Roderich; Erten, Onur (February 2023, npj Quantum Materials)

   Abstract We determine the phase diagram of a bilayer, Yao-Lee spin-orbital model with inter-layer interactions (J), for several stackings and moiré superlattices. For AA stacking, a gapped$${{\mathbb{Z}}}_{2}$$ $Z_2$quantum spin liquid phase emerges at a finiteJ_c. We show that this phase survives in the well-controlled large-Jlimit, where an isotropic honeycomb toric code emerges. For moiré superlattices, a finite-qinter-layer hybridization is stabilized. This connects inequivalent Dirac points, effectively ‘untwisting’ the system. Our study thus provides insight into the spin-liquid phases of bilayer spin-orbital Kitaev materials. 

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4. Magnetic field effects on the quantum spin liquid behaviors of NaYbS2

   https://doi.org/10.1007/s44214-022-00011-z  

   Wu, Jiangtao; Li, Jianshu; Zhang, Zheng; Liu, Changle; Gao, Yong Hao; Feng, Erxi; Deng, Guochu; Ren, Qingyong; Wang, Zhe; Chen, Rui; et al (December 2022, Quantum Frontiers)

   Abstract Spin-orbit coupling is an important ingredient to regulate the many-body physics, especially for many spin liquid candidate materials such as rare-earth magnets and Kitaev materials. The rare-earth chalcogenides Equation missing<#comment/>(Ch = O, S, Se) is a congenital frustrating system to exhibit the intrinsic landmark of spin liquid by eliminating both the site disorders between Equation missing<#comment/>and Equation missing<#comment/>ions with the big ionic size difference and the Dzyaloshinskii-Moriya interaction with the perfect triangular lattice of the Equation missing<#comment/>ions. The temperature versus magnetic-field phase diagram is established by the magnetization, specific heat, and neutron-scattering measurements. Notably, the neutron diffraction spectra and the magnetization curve might provide microscopic evidence for a series of spin configuration for in-plane fields, which include the disordered spin liquid state, 120° antiferromagnet, and one-half magnetization state. Furthermore, the ground state is suggested to be a gapless spin liquid from inelastic neutron scattering, and the magnetic field adjusts the spin orbit coupling. Therefore, the strong spin-orbit coupling in the frustrated quantum magnet substantially enriches low-energy spin physics. This rare-earth family could offer a good platform for exploring the quantum spin liquid ground state and quantum magnetic transitions. 

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5. Sleuthing out exotic quantum spin liquidity in the pyrochlore magnet Ce2Zr2O7

   https://doi.org/10.1038/s41535-022-00458-2  

   Bhardwaj, Anish; Zhang, Shu; Yan, Han; Moessner, Roderich; Nevidomskyy, Andriy H.; Changlani, Hitesh J. (May 2022, npj Quantum Materials)

   Abstract The search for quantum spin liquids—topological magnets with fractionalized excitations—has been a central theme in condensed matter and materials physics. Despite numerous theoretical proposals, connecting experiment with detailed theory exhibiting a robust quantum spin liquid has remained a central challenge. Here, focusing on the strongly spin-orbit coupled effectiveS = 1/2 pyrochlore magnet Ce₂Zr₂O₇, we analyze recent thermodynamic and neutron-scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo, and analytical spin dynamics calculations. Its parameter values suggest the existence of an exotic phase, aπ-flux U(1) quantum spin liquid. Intriguingly, the octupolar nature of the moments makes them less prone to be affected by magnetic disorder, while also hiding some otherwise characteristic signatures from neutrons, making this spin liquid arguably more stable than its more conventional counterparts. 

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