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1. 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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2. Signature of a randomness-driven spin-liquid state in a frustrated magnet

   https://doi.org/10.1038/s42005-022-00879-2  

   Khatua, J. ; Gomilšek, M. ; Orain, J. C. ; Strydom, A. M. ; Jagličić, Z. ; Colin, C. V. ; Petit, S. ; Ozarowski, A. ; Mangin-Thro, L. ; Sethupathi, K. ; et al ( April 2022 , Communications Physics)

   Collective behaviour of electrons, frustration induced quantum fluctuations and entanglement in quantum materials underlie some of the emergent quantum phenomena with exotic quasi-particle excitations that are highly relevant for technological applications. Herein, we present our thermodynamic and muon spin relaxation measurements, complemented by ab initio density functional theory and exact diagonalization results, on the recently synthesized frustrated antiferromagnet Li₄CuTeO₆, in which Cu²⁺ions (S= 1/2) constitute disordered spin chains and ladders along the crystallographic [101] direction with weak random inter-chain couplings. Our thermodynamic experiments detect neither long-range magnetic ordering nor spin freezing down to 45 mK despite the presence of strong antiferromagnetic interaction between Cu²⁺moments leading to a large effective Curie-Weiss temperature of − 154 K. Muon spin relaxation results are consistent with thermodynamic results. The temperature and magnetic field scaling of magnetization and specific heat reveal a data collapse pointing towards the presence of random-singlets within a disorder-driven correlated and dynamic ground-state in this frustrated antiferromagnet.

    

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3. Pressure-tuned quantum criticality in the large-D antiferromagnet DTN

   https://doi.org/10.1038/s41467-024-46527-x  

   Povarov, Kirill Yu. ; Graf, David E. ; Hauspurg, Andreas ; Zherlitsyn, Sergei ; Wosnitza, Joachim ; Sakurai, Takahiro ; Ohta, Hitoshi ; Kimura, Shojiro ; Nojiri, Hiroyuki ; Garlea, V. Ovidiu ; et al ( March 2024 , Nature Communications)

   Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized byz = 1 orz = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl₂ ⋅ 4SC(NH₂)₂(aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigatez = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

    

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4. On the use of radio‐frequency offsets for improving double‐quantum homonuclear dipolar recoupling of half‐integer‐spin quadrupolar nuclei

   https://doi.org/10.1002/mrc.5142  

   Duong, Nghia Tuan ; Lee, Daniel ; Mentink‐Vigier, Frédéric ; Lafon, Olivier ; De Paëpe, Gaël ( May 2021 , Magnetic Resonance in Chemistry)

   Detecting proximities between nuclei is crucial for atomic‐scale structure determination with nuclear magnetic resonance (NMR) spectroscopy. Different from spin‐1/2 nuclei, the methodology for quadrupolar nuclei is limited for solids due to the complex spin dynamics under simultaneous magic‐angle spinning (MAS) and radio‐frequency irradiation. Herein, the performances of several homonuclear rotary recoupling (HORROR)‐based homonuclear dipolar recoupling sequences are evaluated for²⁷Al (spin‐5/2). It is shown numerically and experimentally on mesoporous alumina thatoutperforms the supercycled S₃sequence and its pure double‐quantum (DQ) (bracketed) version, [S₃], both in terms of DQ transfer efficiency and bandwidth. This result is surprising since the S₃sequence is among the best low‐power recoupling schemes for spin‐1/2. The superiority ofis thoroughly explained, and the crucial role of radio‐frequency offsets during its spin dynamics is highlighted. The analytical approximation of, derived in an offset‐toggling frame, clarifies the interplay between offset and DQ efficiency, namely, the benefits of off‐resonance irradiation and the trough in DQ efficiency forwhen the irradiation is central between two resonances, both for spin‐1/2 and half‐integer‐spin quadrupolar nuclei. Additionally, density matrix propagations show that thesequence, applied to quadrupolar nuclei subject to quadrupolar interaction much larger than radio‐frequency frequency field, can create single‐ and multiple‐quantum coherences for near on‐resonance irradiation. This significantly perturbs the creation of DQ coherences between central transitions of neighboring quadrupolar nuclei. This effect explains the DQ efficiency trough for near on‐resonance irradiation, in the case of both cross‐correlation and autocorrelation peaks. Overall, this work aids experimental acquisition of homonuclear dipolar correlation spectra of half‐integer‐spin quadrupolar nuclei and provides theoretical insights towards improving recoupling schemes at high magnetic field and fast MAS.

    

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5. 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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