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1. Fingerprints of an orbital-selective Mott phase in the block magnetic state of BaFe2Se3 ladders

   https://doi.org/10.1038/s42005-019-0155-3  

   Patel, N. D. ; Nocera, A. ; Alvarez, G. ; Moreo, A. ; Johnston, S. ; Dagotto, E. ( June 2019 , Communications Physics)

   Resonant Inelastic X-Ray Scattering (RIXS) experiments on the iron-based ladder BaFe₂Se₃unveiled an unexpected two-peak structure associated with local orbital (dd) excitations in a block-type antiferromagnetic phase. A mixed character between correlated band-like and localized excitations was also reported. Here, we use the density matrix renormalization group method to calculate the momentum-resolved charge- and orbital-dynamical response functions of a multi-orbital Hubbard chain. Remarkably, our results qualitatively resemble the BaFe₂Se₃RIXS data, while also capturing the presence of long-range magnetic order as found in neutron scattering, only when the model is in an exotic orbital-selective Mott phase (OSMP). In the calculations, the experimentally observed zero-momentum transfer RIXS peaks correspond to excitations between itinerant and Mott insulating orbitals. We provide experimentally testable predictions for the momentum-resolved charge and orbital dynamical structures, which can provide further insight into the OSMP regime of BaFe₂Se₃.

    

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2. Nematic fluctuations in an orbital selective superconductor Fe1+yTe1−xSex

   https://doi.org/10.1038/s42005-023-01154-8  

   Jiang, Qianni ; Shi, Yue ; Christensen, Morten H. ; Sanchez, Joshua J. ; Huang, Bevin ; Lin, Zhong ; Liu, Zhaoyu ; Malinowski, Paul ; Xu, Xiaodong ; Fernandes, Rafael M. ; et al ( December 2023 , Communications Physics)

   Abstract Fe 1+ y Te 1− x Se x is characterized by its complex magnetic phase diagram and highly orbital-dependent band renormalization. Despite this, the behavior of nematicity and nematic fluctuations, especially for high tellurium concentrations, remains largely unknown. Here we present a study of both B 1 g and B 2 g nematic fluctuations in Fe 1+ y Te 1− x Se x (0 ≤ x ≤ 0.53) using the technique of elastoresistivity measurement. We discovered that the nematic fluctuations in two symmetry channels are closely linked to the corresponding spin fluctuations, confirming the intertwined nature of these two degrees of freedom. We also revealed an unusual temperature dependence of the nematic susceptibility, which we attributed to a loss of coherence of the d x y orbital. Our results highlight the importance of orbital differentiation on the nematic properties of iron-based materials. 

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3. Tuning moiré excitons and correlated electronic states through layer degree of freedom

   https://doi.org/10.1038/s41467-022-32493-9  

   Chen, Dongxue ; Lian, Zhen ; Huang, Xiong ; Su, Ying ; Rashetnia, Mina ; Yan, Li ; Blei, Mark ; Taniguchi, Takashi ; Watanabe, Kenji ; Tongay, Sefaattin ; et al ( August 2022 , Nature Communications)

   Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe₂/WS₂moiré superlattice by changing WSe₂from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe₂/WS₂interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe₂/monolayer WS₂moiré superlattice. The additional WSe₂layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe₂layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction.

    

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4. Electron‐Doping Mottronics in Strongly Correlated Perovskite

   https://doi.org/10.1002/adma.201905060  

   Chen, Jikun ; Mao, Wei ; Gao, Lei ; Yan, Fengbo ; Yajima, Takeaki ; Chen, Nuofu ; Chen, Zhizhong ; Dong, Hongliang ; Ge, Binghui ; Zhang, Peng ; et al ( December 2019 , Advanced Materials)

   The discovery of hydrogen‐induced electron localization and highly insulating states in d‐band electron correlated perovskites has opened a new paradigm for exploring novel electronic phases of condensed matters and applications in emerging field‐controlled electronic devices (e.g., Mottronics). Although a significant understanding of doping‐tuned transport properties of single crystalline correlated materials exists, it has remained unclear how doping‐controlled transport properties behave in the presence of planar defects. The discovery of an unexpected high‐concentration doping effect in defective regions is reported for correlated nickelates. It enables electronic conductance by tuning the Fermi‐level in Mott–Hubbard band and shaping the lower Hubbard band state into a partially filled configuration. Interface engineering and grain boundary designs are performed for H_xSmNiO₃/SrRuO₃heterostructures, and a Mottronic device is achieved. The interfacial aggregation of hydrogen is controlled and quantified to establish its correlation with the electrical transport properties. The chemical bonding between the incorporated hydrogen with defective SmNiO₃is further analyzed by the positron annihilation spectroscopy. The present work unveils new materials physics in correlated materials and suggests novel doping strategies for developing Mottronic and iontronic devices via hydrogen‐doping‐controlled orbital occupancy in perovskite heterostructures.

    

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5. Effect of R-site element on crystalline phase and thermal stability of Fe substituted Mn mullite-type oxides: R 2 (Mn 1−x Fe x ) 4 O 10−δ (R = Y, Sm or Bi; x = 0, 0.5, 1)

   https://doi.org/10.1039/C7RA09699B  

   Thampy, Sampreetha ; Ashburn, Nickolas ; Martin, Thomas J. ; Li, Chenzhe ; Zheng, Yongping ; Chan, Julia Y. ; Cho, Kyeongjae ; Hsu, Julia W. ( January 2018 , RSC Advances)

   Combining experimental and theoretical studies, we investigate the role of R-site (R = Y, Sm, Bi) element on the phase formation and thermal stability of R 2 (Mn 1−x Fe x ) 4 O 10−δ ( x = 0, 0.5, 1) mullite-type oxides. Our results show a distinct R-site dependent phase behavior for mullite-type oxides as Fe is substituted for Mn: 100% mullite-type phase was formed in (Y, Sm, Bi) 2 Mn 4 O 10 ; 55% and 18% of (Y, Sm) 2 Mn 2 Fe 2 O 10−δ was found when R = Y and Sm, respectively, for equal Fe and Mn molar concentrations in the reactants, whereas Bi formed 54% O10- and 42% O9-mixed mullite-type phases. Furthermore, when the reactants contain 100% Fe, no mullite-type phase was formed for R = Y and Sm, but a sub-group transition to Bi 2 Fe 4 O 9 O9-phase was found for R = Bi. Thermogravimetric analysis and density functional theory (DFT) calculation results show a decreasing thermal stability in O10-type structure with increasing Fe incorporation; for example, the decomposition temperature is 1142 K for Bi 2 Mn 2 Fe 2 O 10−δ vs. 1217 K for Bi 2 Mn 4 O 10 . On the other hand, Bi 2 Fe 4 O 9 O9-type structure is found to be thermally stable up to 1227 K. These findings are explained by electronic structure calculations: (1) as Fe concentration increases, Jahn–Teller distortion results in mid band-gap empty states from unstable Fe 4+ occupied octahedra, which is responsible for the decrease in O10 structure stability; (2) the directional sp orbital hybridization unique to Bi effectively stabilizes the mullite-type structure as Fe replaces Mn. 

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