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1. Quadrupolar charge dynamics in the nonmagnetic FeSe 1−x S x superconductors

   https://doi.org/10.1073/pnas.2020585118  

   Zhang, Weilu ; Wu, Shangfei ; Kasahara, Shigeru ; Shibauchi, Takasada ; Matsuda, Yuji ; Blumberg, Girsh ( May 2021 , Proceedings of the National Academy of Sciences)

   null (Ed.)

   We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic F e S e 1 − x S x superconductor. We observe two types of long-wavelength X Y symmetry excitations: 1) a low-energy quasi-elastic scattering peak (QEP) and 2) a broad electronic continuum with a maximum at 55 meV. Below the tetragonal-to-orthorhombic structural transition at T S ( x ) , a pseudogap suppression with temperature dependence reminiscent of the nematic order parameter develops in the X Y symmetry spectra of the electronic excitation continuum. The QEP exhibits critical enhancement upon cooling toward T S ( x ) . The intensity of the QEP grows with increasing sulfur concentration x and maximizes near critical concentration x c r ≈ 0.16 , while the pseudogap size decreases with the suppression of T S ( x ) . We interpret the development of the pseudogap in the quadrupole scattering channel as a manifestation of transition from the non-Fermi liquid regime, dominated by strong Pomeranchuk-like fluctuations giving rise to intense electronic continuum of excitations in the fourfold symmetric high-temperature phase, to the Fermi liquid regime in the broken-symmetry nematic phase where the quadrupole fluctuations are suppressed. 

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2. Magnons and magnetic fluctuations in atomically thin MnBi2Te4

   https://doi.org/10.1038/s41467-022-29996-w  

   Lujan, David ; Choe, Jeongheon ; Rodriguez-Vega, Martin ; Ye, Zhipeng ; Leonardo, Aritz ; Nunley, T. Nathan ; Chang, Liang-Juan ; Lee, Shang-Fan ; Yan, Jiaqiang ; Fiete, Gregory A. ; et al ( December 2022 , Nature Communications)

   Abstract Electron band topology is combined with intrinsic magnetic orders in MnBi 2 Te 4 , leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin fluctuations in atomically thin MnBi 2 Te 4 flakes using Raman spectroscopy. In a two-septuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers. 

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3. Role of electron-phonon coupling in excitonic insulator candidate Ta2NiSe5

   https://doi.org/10.1103/PhysRevResearch.5.043089  

   Chen, Cheng ; Chen, Xiang ; Tang, Weichen ; Li, Zhenglu ; Wang, Siqi ; Ding, Shuhan ; Kang, Zhibo ; Jozwiak, Chris ; Bostwick, Aaron ; Rotenberg, Eli ; et al ( October 2023 , Physical Review Research)

   Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and condense into a new insulating ground state—the so-called excitonic insulator—when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs. However, a similar structural transition can also be induced by electron-phonon coupling itself, therefore hindering the clean identification of bulk excitonic insulators (e.g., which instability is the driving force of the phase transition). Using high-resolution synchrotron x-ray diffraction and angle-resolved photoemission spectroscopy, we identify key electron-phonon coupling effects in a leading excitonic insulator candidate Ta 2 NiSe 5 . These include an extensive unidirectional lattice fluctuation and an electronic pseudogap in the normal state, as well as a negative electronic compressibility in the charge-doped broken-symmetry state. In combination with first principles and model calculations, we use the normal state electronic spectra to quantitatively determine the electron-phonon interaction vertex g and interband Coulomb interaction V in the minimal lattice model, the solution to which captures the experimental observations. Moreover, we show how the Coulomb and electron-phonon coupling effects can be unambiguously separated based on the solution to quantified microscopic models. Finally, we discuss how the strong lattice fluctuations enabled by low dimensionality relate to the unique electron-phonon interaction effects beyond the textbook Born-Oppenheimer approximation. 

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4. Trimerization and cyclization of reactive P-functionalities confined within OCO pincers

   https://doi.org/10.1039/d1ra05926b  

   Chinen, Beatrice L. ; Hyvl, Jakub ; Brayton, Daniel F. ; Riek, Matthew M. ; Yoshida, Wesley Y. ; Chapp, Timothy W. ; Rheingold, Arnold L. ; Cain, Matthew F. ( August 2021 , RSC Advances)

   null (Ed.)

   In order to stabilize a 10–P–3 species with C 2v symmetry and two lone pairs on the central phosphorus atom, a specialized ligand is required. Using an NCN pincer, previous efforts to enforce this planarized geometry at P resulted in the formation of a C s -symmetric, 10π-electron benzazaphosphole that existed as a dynamic “bell-clapper” in solution. Here, OCO pincers 1 and 2 were synthesized, operating under the hypothesis that the more electron-withdrawing oxygen donors would better stabilize the 3-center, 4-electron O–P–O bond of the 10–P–3 target and the sp 3 -hybridized benzylic carbon atoms would prevent the formation of aromatic P-heterocycles. However, subjecting 1 to a metalation/phosphination/reduction sequence afforded cyclotriphosphane 3, resulting from trimerization of the P( i ) center unbound by its oxygen donors. Pincer 2 featuring four benzylic CF 3 groups was expected to strengthen the O–P–O bond of the target, but after metal–halogen exchange and quenching with PCl 3 , unexpected cyclization with loss of CH 3 Cl was observed to give monochlorinated 5. Treatment of 5 with ( p -CH 3 )C 6 H 4 MgBr generated crystalline P-( p -Tol) derivative 6, which was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The complex 19 F NMR spectra of 5 and 6 observed experimentally, were reproduced by simulations with MestreNova. 

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5. Mayfly emergence production and body length response to hydrology in a tropical lowland stream

   https://doi.org/10.7717/peerj.9883  

   Gutiérrez-Fonseca, Pablo E. ; Ramírez, Alonso ( January 2020 , PeerJ)

   null (Ed.)

   Background Hydrological impacts on aquatic biota have been assessed in numerous empirical studies. Aquatic insects are severely affected by population declines and consequent diversity loss. However, many uncertainties remain regarding the effects of hydrology on insect production and the consequences of energy transfer to the terrestrial ecosystem. Likewise, sublethal effects on insect morphology remain poorly quantified in highly variable environments. Here, we characterized monthly fluctuation in benthic and emerged biomass of Ephemeroptera in a tropical lowland stream. We quantified the proportion of mayfly production that emerges into the riparian forest. We also examined the potential morphological changes in Farrodes caribbianus (the most abundant mayfly in our samples) due to environmental stress. Methods We collected mayflies (nymphs and adults) in a first-order stream in Costa Rica. We compared benthic and adult biomass from two years’ worth of samples, collected with a core sampler (0.006 m 2 ) and a 2 m 2 -emergence trap. The relationship between emergence and annual secondary production (E/P) was used to estimate the Ephemeroptera production that emerged as adults. A model selection approach was used to determine the relationship between environmental variables that were collected monthly and the emergent biomass. To determine potential departures from perfect bilateral symmetry, we evaluated the symmetry of two morphological traits (forceps and forewing) of F. caribbianus adults. We used Spearman’s rank correlation coefficients (ρ) to examine potential changes in adult body length as a possible response to environmental stress. Results Benthic biomass was variable, with peaks throughout the study period. However, peaks in benthic biomass did not lead to increases in mayfly emergence, which remained stable over time. Relatively constant mayfly emergence suggests that they were aseasonal in tropical lowland streams. Our E/P estimate indicated that approximately 39% and 20% (for 2002 and 2003, respectively) of the nymph production emerged as adults. Our estimated proportion of mayfly production transferred to terrestrial ecosystems was high relative to reports from temperate regions. We observed a strong negative response of F . caribbianus body length to increased hydrology (Spearman: ρ = −0.51, p < 0.001), while slight departures from perfect symmetry were observed in all traits. Conclusion Our two years study demonstrates that there was large temporal variability in mayfly biomass that was unrelated to hydrological fluctuations, but potentially related to trophic interactions (e.g., fish predation). Body length was a good indicator of environmental stress, which could have severe associated costs for mayfly fitness in ecosystems with high temporal variation. Our results highlight the complex ecological and evolutionary dynamics of tropical aquatic insects, and the intricate connection between aquatic and terrestrial ecosystems. 

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