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1. First-Principles Prediction of Structural Distortions in the Cuprates and Their Impact on the Electronic Structure

   https://doi.org/10.1103/PhysRevX.14.041053  

   Jin, Zheting; Ismail-Beigi, Sohrab (December 2024, Physical Review X)

   Materials-realistic microscopic theoretical descriptions of copper-based superconductors are challenging due to their complex crystal structures combined with strong electron interactions. Here, we demonstrate how density functional theory can accurately describe key structural, electronic, and magnetic properties of the normal state of the prototypical cuprate ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$(Bi-2212). We emphasize the importance of accounting for energy-lowering structural distortions, which then allows us to (a) accurately describe the insulating antiferromagnetic (AFM) ground state of the undoped parent compound (in contrast to the metallic state predicted by previous studies); (b) identify numerous low-energy competing spin and charge stripe orders in the hole-overdoped material nearly degenerate in energy with the AFM ordered state, indicating strong spin fluctuations; (c) predict the lowest-energy hole-doped crystal structure including its long-range structural distortions and oxygen dopant positions that match high-resolution scanning transmission electron microscopy measurements; and (d) describe electronic bands near the Fermi energy with flat antinodal dispersions and Fermi surfaces that are in agreement with angle-resolved photoemission spectroscopy (ARPES) measurements and provide a clear explanation for the structural origins of the so-called “shadow bands.” We also show how one must go beyond band theory and include fully dynamic spin fluctuations via a many-body approach when aiming to make quantitative predictions to measure the ARPES spectra in the overdoped material. Finally, regarding spatial inhomogeneity, we show that the local structure at the ${\mathrm{CuO}}_2$layer, rather than dopant electrostatic effects, modulates the local charge-transfer gaps, local correlation strengths, and by extension the local superconducting gaps. Published by the American Physical Society2024 

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2. Realisation of symmetry-enforced Dirac fermions in nonsymmorphic α-bismuthene

   https://doi.org/doi.org/10.1021/acsnano.9b08136  

   Pawel J. Kowalczyk, Simon A. (January 2020, ACS nano)

   Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and nontrivial Berry phase play a pivotal role in the electronic, optical, mechanical, and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of spin–orbit coupling (SOC). Here, we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC. Through μ-ARPES measurements, we observe Dirac-like band dispersions in α-bismuthene. The nonsymmorphic lattice symmetry is confirmed by μ-low-energy electron diffraction and scanning tunneling microscopy. Our first-principles simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results enlighten the search of symmetry-enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials. 

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3. Electronic band structure of iridates

   https://doi.org/10.1039/d1mh00063b  

   Dhingra, Archit; Komesu, Takashi; Kumar, Shiv; Shimada, Kenya; Zhang, Le; Hong, Xia; Dowben, Peter A. (January 2021, Materials Horizons)

   null (Ed.)

   In this review, an attempt has been made to compare the electronic structures of various 5d iridates (iridium oxides), with an effort to note the common features and differences. Both experimental studies, especially angle-resolved photoemission spectroscopy (ARPES) results, and first-principles band structure calculations have been discussed. This brings to focus the fact that the electronic structures and magnetic properties of the high- Z 5d transition iridates depend on the intricate interplay of strong electron correlation, strong (relativistic) spin–orbit coupling, lattice distortion, and the dimensionality of the system. For example, in the thin film limit, SrIrO 3 exhibits a metal–insulator transition that corresponds to the dimensionality crossover, with the band structure resembling that of bulk Sr 2 IrO 4 . 

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4. Evolution of the electronic structure in open-shell donor-acceptor organic semiconductors

   https://doi.org/10.1038/s41467-021-26173-3  

   Chen, Zhongxin; Li, Wenqiang; Sabuj, Md Abdus; Li, Yuan; Zhu, Weiya; Zeng, Miao; Sarap, Chandra S.; Huda, Md Masrul; Qiao, Xianfeng; Peng, Xiaobin; et al (December 2021, Nature Communications)

   Abstract Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship. 

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5. High Symmetry Metal‐Dielectric Photonic Crystal Organic Light Emitting Diodes With Single‐Cavity Unit Cells

   https://doi.org/10.1002/adom.202201631  

   Allemeier, David; Sobolew, Nicholas; Magnifico, Sean; Henry, Katherine; Abua, Edward; White, Matthew_S (November 2022, Advanced Optical Materials)

   Abstract Vertically‐stacked organic light emitting diode (OLED) microcavities form 1D metal‐dielectric photonic crystals (MDPC) with many degrees of freedom for engineering complex emission profiles. The photonic band structure of the MDPC OLED is determined by the underlying unit cell and is particularly sensitive to the properties of the metallic electrodes. The electronic requirements of microcavity OLED fabrication often necessitate dissimilar metallic electrodes to achieve good performance. This can profoundly impact the photonic properties of a MDPC by doubling the unit cell length. This work presents a MDPC OLED formed with single‐cavity unit cells by employing optically similar Ag alloys as the semi‐transparent electrode materials. The crystal is found to display a single photonic band without a band gap up to eight stacked cavities. The states within the band are evenly‐spaced and clearly resolved, which is critical for applications seeking to utilize specific photonic states. Design considerations are presented for optimizing the photonic behavior of MDPC OLEDs through selective control of the optical properties of metallic alloys. 

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