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1. 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) plays 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. 

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2. Emergence of a metallic surface state for narrow bandgap Mott insulator Sr ₃ Ir ₂ O ₇ (001) thin films

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

   Komesu, Takashi; Zhang, Yuanyuan; Kumar, Shiv; Kumar, Amit; Miyai, Yudai; Shimada, Kenya; Adegbite, Peace_Ikeoluwa; Hong, Xia; Dowben, P_A (March 2025, Journal of Physics: Condensed Matter)

   Abstract We report evidence of a finite density of states at the Fermi level at the surface of epitaxial thin films of the narrow bandgap Mott insulator Sr₃Ir₂O₇(001). The Brillouin zone critical points for Sr₃Ir₂O₇(001) thin films have been determined by a comparison of the band mapping from angle-resolved photoemission spectroscopy and low energy electron diffraction. Angle-resolved x-ray photoemission studies reveal the surface termination of Sr₃Ir₂O₇(001) is Sr–O. The absence of dispersion with photon energy, or changing wave vector along the surface normal, indicates the two-dimensional character of the bands contributing to the density of states close to the Fermi level for Sr₃Ir₂O₇(001) thin films. Thus, the finite density of states at the Fermi level is attributed to surface states or surface resonances. The appearance of a finite density of states at the Fermi level is consistent with the increased conductivity with decreasing film thickness for ultrathin Sr₃Ir₂O₇(001) films. 

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3. Enhanced CO ₂ electrolysis with a SrTiO ₃ cathode through a dual doping strategy

   https://doi.org/10.1039/c8ta10188d  

   Ye, Lingting; Hu, Xiuli; Wang, Xin; Chen, Fanglin; Tang, Dian; Dong, Dehua; Xie, Kui (February 2019, Journal of Materials Chemistry A)

   The significant role of perovskite defect chemistry through A-site doping of strontium titanate with lanthanum for CO 2 electrolysis properties is demonstrated. Here we present a dual strategy of A-site deficiency and promoting adsorption/activation by making use of redox active dopants such as Mn/Cr linked to oxygen vacancies to facilitate CO 2 reduction at perovskite titanate cathode surfaces. Solid oxide electrolysers based on oxygen-excess La 0.2 Sr 0.8 Ti 0.9 Mn(Cr) 0.1 O 3+δ , A-site deficient (La 0.2 Sr 0.8 ) 0.9 Ti 0.9 Mn(Cr) 0.1 O 3−δ and undoped La 0.2 Sr 0.8 Ti 1.0 O 3+δ cathodes are evaluated. In situ infrared spectroscopy reveals that the adsorbed and activated CO 2 adopts an intermediate chemical state between a carbon dioxide molecule and a carbonate ion. The double strategy leads to optimal performance being observed after 100 h of high-temperature operation and 3 redox cycles, suggesting a promising cathode material for CO 2 electrolysis. 

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4. 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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5. Antiferromagnetic excitonic insulator state in Sr3Ir2O7

   https://doi.org/10.1038/s41467-022-28207-w  

   Mazzone, D. G.; Shen, Y.; Suwa, H.; Fabbris, G.; Yang, J.; Zhang, S. -S.; Miao, H.; Sears, J.; Jia, Ke; Shi, Y. G.; et al (February 2022, Nature Communications)

   Abstract Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr₃Ir₂O₇. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr₃Ir₂O₇indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase. 

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