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1. 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.
2. Quest for quantum states via field-altering technology

   https://doi.org/10.1038/s41535-020-00286-2  

   Cao, Gang ; Zhao, Hengdi ; Hu, Bing ; Pellatz, Nicholas ; Reznik, Dmitry ; Schlottmann, Pedro ; Kimchi, Itamar ( November 2020 , npj Quantum Materials)

   We report quantum phenomena in spin-orbit-coupled single crystals that are synthesized using an innovative technology that “field-alters” crystal structures via application of magnetic field during crystal growth. This study addresses a major challenge facing the research community today: A great deal of theoretical work predicting exotic states for strongly spin-orbit-coupled, correlated materials has thus far met very limited experimental confirmation. These conspicuous discrepancies are due in part to the extreme sensitivity of these materials to structural distortions. The results presented here demonstrate that the field-altered materials not only are much less distorted but also exhibit phenomena absent in their non-altered counterparts. The field-altered materials include an array of4dand5dtransition metal oxides, and three representative materials presented here are Ba₄Ir₃O₁₀, Ca₂RuO₄, and Sr₂IrO₄. This study provides an approach for discovery of quantum states and materials otherwise unavailable.
3. Coexistence of metallic and nonmetallic properties in the pyrochlore Lu2Rh2O7

   https://doi.org/10.1038/s41535-019-0148-1  

   Hallas, Alannah M. ; Sharma, Arzoo Z. ; Mauws, Cole ; Chen, Qiang ; Zhou, Haidong D. ; Ding, Cui ; Gong, Zizhou ; Tachibana, Makoto ; Sarte, Paul M. ; Attfield, J. Paul ; et al ( March 2019 , npj Quantum Materials)

   Transition metal oxides of the 4dand 5dblock have recently become the targets of materials discovery, largely due to their strong spin–orbit coupling that can generate exotic magnetic and electronic states. Here, we report the high-pressure synthesis of Lu₂Rh₂O₇, a new cubic pyrochlore oxide based on 4d⁵Rh⁴⁺, and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient,γ = 21.8(1) mJ/mol-Rh K². Muon spin relaxation measurements confirm that Lu₂Rh₂O₇remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu₂Rh₂O₇is a correlated paramagnetic metal with a Wilson ratio ofR_W = 2.5. However, electric transport measurements present a striking contradiction as the resistivity of Lu₂Rh₂O₇is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu₂Rh₂O₇may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.
4. Spin–lattice and electron–phonon coupling in 3d/5d hybrid Sr3NiIrO6

   https://doi.org/10.1038/s41535-019-0184-x  

   O’Neal, Kenneth R. ; Paul, Arpita ; al-Wahish, Amal ; Hughey, Kendall D. ; Blockmon, Avery L. ; Luo, Xuan ; Cheong, Sang-Wook ; Zapf, Vivien S. ; Topping, Craig V. ; Singleton, John ; et al ( August 2019 , npj Quantum Materials)

   While 3d-containing materials display strong electron correlations, narrow band widths, and robust magnetism, 5dsystems are recognized for strong spin–orbit coupling, increased hybridization, and more diffuse orbitals. Combining these properties leads to novel behavior. Sr₃NiIrO₆, for example, displays complex magnetism and ultra-high coercive fields—up to an incredible 55 T. Here, we combine infrared and optical spectroscopies with high-field magnetization and first-principles calculations to explore the fundamental excitations of the lattice and related coupling processes including spin–lattice and electron–phonon mechanisms. Magneto-infrared spectroscopy reveals spin–lattice coupling of three phonons that modulate the Ir environment to reduce the energy required to modify the spin arrangement. While these modes primarily affect exchange within the chains, analysis also uncovers important inter-chain motion. This provides a mechanism by which inter-chain interactions can occur in the developing model for ultra-high coercivity. At the same time, analysis of the on-site Ir⁴⁺excitations reveals vibronic coupling and extremely large crystal field parameters that lead to at_2g-derived low-spin state for Ir. These findings highlight the spin–charge–lattice entanglement in Sr₃NiIrO₆and suggest that similar interactions may take place in other 3d/5dhybrids.
5. Non-magnetic ion site disorder effects on the quantum magnetism of a spin-1/2 equilateral triangular lattice antiferromagnet

   https://doi.org/10.1088/1361-648X/ac5703  

   Huang, Q ; Rawl, R ; Xie, W W ; Chou, E S ; Zapf, V S ; Ding, X X ; Mauws, C ; Wiebe, C R ; Feng, E X ; Cao, H B ; et al ( March 2022 , Journal of Physics: Condensed Matter)

   Abstract With the motivation to study how non-magnetic ion site disorder affects the quantum magnetism of Ba 3 CoSb 2 O 9 , a spin-1/2 equilateral triangular lattice antiferromagnet, we performed DC and AC susceptibility, specific heat, elastic and inelastic neutron scattering measurements on single crystalline samples of Ba 2.87 Sr 0.13 CoSb 2 O 9 with Sr doping on non-magnetic Ba 2+ ion sites. The results show that Ba 2.87 Sr 0.13 CoSb 2 O 9 exhibits (i) a two-step magnetic transition at 2.7 K and 3.3 K, respectively; (ii) a possible canted 120 degree spin structure at zero field with reduced ordered moment as 1.24 μ B /Co; (iii) a series of spin state transitions for both H ∥ ab -plane and H ∥ c -axis. For H ∥ ab -plane, the magnetization plateau feature related to the up–up–down phase is significantly suppressed; (iv) an inelastic neutron scattering spectrum with only one gapped mode at zero field, which splits to one gapless and one gapped mode at 9 T. All these features are distinctly different from those observed for the parent compound Ba 3 CoSb 2 O 9 , which demonstrates that the non-magnetic ion site disorder (the Sr doping) playsmore »a complex role on the magnetic properties beyond the conventionally expected randomization of the exchange interactions. We propose the additional effects including the enhancement of quantum spin fluctuations and introduction of a possible spatial anisotropy through the local structural distortions.« less