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1. Valence transition theory of the pressure-induced dimensionality crossover in superconducting ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$

   https://doi.org/10.1103/PhysRevB.108.134510  

   Song, Jeong-Pil; Clay, R. Torsten; Mazumdar, Sumit (October 2023, Physical Review B)

   Stephen E. Nagler (Ed.)

   One of the strongest justifications for the continued search for superconductivity within the single-band Hubbard Hamiltonian originates from the apparent success of single-band ladder-based theories in predicting the occurrence of superconductivity in the cuprate coupled-ladder compound Sr{14−x}Ca{x}Cu{24}O{41}. Recent theoretical works have, however, shown the complete absence of quasi-long-range superconducting correlations within the hole-doped multiband ladder Hamiltonian including realistic Coulomb repulsion between holes on oxygen sites and oxygen-oxygen hole hopping. Experimentally, superconductivity in Sr{14−x}Ca{x}Cu{24}O{41} occurs only under pressure and is preceded by dramatic transition from one to two dimensions that remains not understood. We show that understanding the dimensional crossover requires adopting a valence transition model within which there occurs transition in Cu-ion ionicity from +2 to +1 , with transfer of holes from Cu to O ions [S. Mazumdar, Phys. Rev. B 98, 205153 (2018)]. The driving force behind the valence transition is the closed-shell electron configuration of Cu^{1+} , a feature shared by cations of all oxides with a negative charge-transfer gap. We make a falsifiable experimental prediction for Sr{14−x}Ca{x}Cu{24}O{41} and discuss the implications of our results for layered two-dimensional cuprates. 

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2. Data files for "Theory of electron-phonon interactions in extended correlated systems probed by resonant inelastic x-ray scattering" by J. Thomas et al.

   https://doi.org/10.5281/zenodo.15211423  

   Thomas, Jinu; Banerjee, Debshikha; Nocera, Alberto; Johnston, Steven; Thomas, Jinu; Banerjee, Debshikha; Nocera, Alberto; Johnston, Steven (January 2025, Zenodo)

   This repo contains the data files for the paper J. Thomas et al., Theory of electron-phonon interactions in extended correlated systems probed by resonant inelastic x-ray scattering. Physical Review X, in press (2025) Preprint: https://arxiv.org/abs/2412.12995 Abstract: An emerging application of resonant inelastic x-ray scattering (RIXS) is the study of lattice excitations and electron-phonon (e-ph) interactions in quantum materials. Despite the growing importance of this area of research, the community lacks a complete understanding of how the RIXS process excites the lattice and how these excitations encode information about the e-ph interactions. Here, we present a detailed study of the RIXS spectra of the Hubbard-Holstein model defined on extended one-dimensional lattices. Using the density matrix renormalization group (DMRG) method, we compute the RIXS response while treating the electron mobility, many-body interactions, and core-hole interactions on an equal footing. The predicted spectra exhibit notable differences from those obtained using the commonly adopted Lang-Firsov models, with important implications for analyzing past and future experiments. Our results provide a deeper understanding of how RIXS probes e-ph interactions and set the stage for a more realistic analysis of future experiments. 

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3. Elucidating the Role of Dimensionality on the Electronic Structure of the Van der Waals Antiferromagnet NiPS ₃

   https://doi.org/10.1002/apxr.202300096  

   DiScala, Michael_F; Staros, Daniel; de_la_Torre, Alberto; Lopez, Annette; Wong, Deniz; Schulz, Christian; Barkowiak, Maciej; Bisogni, Valentina; Pelliciari, Jonathan; Rubenstein, Brenda; et al (January 2024, Advanced Physics Research)

   Abstract The sustained interest in investigating magnetism in the 2D limit of insulating antiferromagnets is driven by the possibilities of discovering, or engineering, novel magnetic phases through layer stacking. However, due to the difficulty of directly measuring magnetic interactions in 2D antiferromagnets, it is not yet understood howintralayer magnetic interactions ininsulating, strongly correlated, materials can be modified through layer proximity. Herein, the impact of reduced dimensionality in the model van der Waals antiferromagnet NiPS₃is explored by measuring electronic excitations in exfoliated samples using Resonant Inelastic X‐ray Scattering (RIXS). The resulting spectra shows systematic broadening of NiS₆multiplet excitations with decreasing layer count from bulk down to three atomic layers (3L). It is shown that these trends originate from a decrease in transition metal‐ligand and ligand–ligand hopping integrals, and by charge‐transfer energy evolving from Δ = 0.83 eV in the bulk to 0.37 eV in 3L NiPS₃. Relevant intralayer magnetic exchange integrals computed from the electronic parameters exhibit a decrease in the average interaction strength with thickness. This study underscores the influence ofinterlayer electronic interactions onintralayer ones in insulating magnets, indicating that magnetic Hamiltonians in few‐layer insulating magnets can greatly deviate from their bulk counterparts. 

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4. 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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5. Beyond the single-site approximation modeling of electron-phonon coupling effects on resonant inelastic X-ray scattering spectra

   https://doi.org/10.21468/SciPostPhys.11.3.062  

   Bieniasz, Krzysztof; Johnston, Steven; Berciu, Mona (January 2021, SciPost Physics)

   null (Ed.)

   Resonant inelastic X-ray scattering (RIXS) is used increasingly for characterizing low-energy collective excitations inmaterials. RIXS is a powerful probe, which often requiressophisticated theoretical descriptions to interpret the data. Inparticular, the need for accurate theories describing the influence of electron-phonon (e-p) coupling on RIXS spectra is becoming timely, as instrument resolution improves and this energy regime is rapidly becoming accessible. To date, only rather exploratory theoretical work has beencarried out for such problems. We begin to bridge this gap byproposing a versatile variational approximation for calculating RIXS spectra in weakly doped materials, for a variety of models with diverse e-p couplings. Here, we illustrate some of its potential by studying the role of electron mobility, which is completely neglected in the widely used local approximation based on Lang-Firsov theory. Assuming that the e-p coupling is of the simplest, Holstein type, we discuss the regimes where the local approximation fails, and demonstrate that its improper use may grossly underestimate the e-p coupling strength. 

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