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1. Survival of itinerant excitations and quantum spin state transitions in YbMgGaO4 with chemical disorder

   https://doi.org/10.1038/s41467-021-25247-6  

   Rao, X. ; Hussain, G. ; Huang, Q. ; Chu, W. J. ; Li, N. ; Zhao, X. ; Dun, Z. ; Choi, E. S. ; Asaba, T. ; Chen, L. ; et al ( December 2021 , Nature Communications)

   Abstract A recent focus of quantum spin liquid (QSL) studies is how disorder/randomness in a QSL candidate affects its true magnetic ground state. The ultimate question is whether the QSL survives disorder or the disorder leads to a “spin-liquid-like” state, such as the proposed random-singlet (RS) state. Since disorder is a standard feature of most QSL candidates, this question represents a major challenge for QSL candidates. YbMgGaO 4 , a triangular lattice antiferromagnet with effective spin-1/2 Yb 3+ ions, is an ideal system to address this question, since it shows no long-range magnetic ordering with Mg/Ga site disorder. Despite the intensive study, it remains unresolved as to whether YbMgGaO 4 is a QSL or in the RS state. Here, through ultralow-temperature thermal conductivity and magnetic torque measurements, plus specific heat and DC magnetization data, we observed a residual κ 0 / T term and series of quantum spin state transitions in the zero temperature limit for YbMgGaO 4 . These observations strongly suggest that a QSL state with itinerant excitations and quantum spin fluctuations survives disorder in YbMgGaO 4 . 

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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)

   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. Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet

   https://doi.org/10.1038/s41467-022-30065-5  

   Nag, A. ; Nocera, A. ; Agrestini, S. ; Garcia-Fernandez, M. ; Walters, A. C. ; Cheong, Sang-Wook ; Johnston, S. ; Zhou, Ke-Jin ( December 2022 , Nature Communications)

   Abstract The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material Y 2 BaNiO 5 using Ni L 3 -edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in Y 2 BaNiO 5 show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations. 

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4. Haldane topological spin-1 chains in a planar metal-organic framework

   https://doi.org/10.1038/s41467-023-41014-1  

   Tin, Pagnareach ; Jenkins, Michael J. ; Xing, Jie ; Caci, Nils ; Gai, Zheng ; Jin, Rongyin ; Wessel, Stefan ; Krzystek, J. ; Li, Cheng ; Daemen, Luke L. ; et al ( September 2023 , Nature Communications)

   Haldane topological materials contain unique antiferromagnetic chains with symmetry-protected energy gaps. Such materials have potential applications in spintronics and future quantum computers. Haldane topological solids typically consist of spin-1 chains embedded in extended three-dimensional (3D) crystal structures. Here, we demonstrate that [Ni(μ−4,4′-bipyridine)(μ-oxalate)]_n(NiBO) instead adopts a two-dimensional (2D) metal-organic framework (MOF) structure of Ni²⁺spin-1 chains weakly linked by 4,4′-bipyridine. NiBO exhibits Haldane topological properties with a gap between the singlet ground state and the triplet excited state. The latter is split by weak axial and rhombic anisotropies. Several experimental probes, including single-crystal X-ray diffraction, variable-temperature powder neutron diffraction (VT-PND), VT inelastic neutron scattering (VT-INS), DC susceptibility and specific heat measurements, high-field electron spin resonance, and unbiased quantum Monte Carlo simulations, provide a detailed, comprehensive characterization of NiBO. Vibrational (also known as phonon) properties of NiBO have been probed by INS and density-functional theory (DFT) calculations, indicating the absence of phonons near magnetic excitations in NiBO, suppressing spin-phonon coupling. The work here demonstrates that NiBO is indeed a rare 2D-MOF Haldane topological material.

    

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5. Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states

   https://doi.org/10.1038/s41535-020-0222-8  

   Smaha, Rebecca W. ; He, Wei ; Jiang, Jack Mingde ; Wen, Jiajia ; Jiang, Yi-Fan ; Sheckelton, John P. ; Titus, Charles J. ; Wang, Suyin Grass ; Chen, Yu-Sheng ; Teat, Simon J. ; et al ( December 2020 , npj Quantum Materials)

   null (Ed.)

   Abstract The spin- $$\frac{1}{2}$$ 1 2 kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu 4 (OH) 6 FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin- $$\frac{1}{2}$$ 1 2 kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a QSL. The presence of interlayer spins eventually leads to an interesting pinwheel q  = 0 magnetic order. Partially Zn-substituted barlowite (Cu 3.44 Zn 0.56 (OH) 6 FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu 2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states. 

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