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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. Alternative modes of bonding of C ₄ F ₈ units in mononuclear and binuclear iron carbonyl complexes

   https://doi.org/10.1039/C9NJ00882A  

   Huang, Liping; Li, Jing; Li, Guoliang; Xie, Yaoming; King, R. Bruce; Schaefer, Henry F. (May 2019, New Journal of Chemistry)

   Density functional theory studies show that the lowest energy C 4 F 8 Fe(CO) 4 structure is not the very stable experimentally known ferracyclopentane isomer (CF 2 CF 2 CF 2 CF 2 )Fe(CO) 4 obtained from Fe(CO) 12 and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO) 4 structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol −1 . However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO) 4 complexes from tetrafluoroethylene are very high ( e.g. , ∼70 kcal mol −1 ). Therefore the ferracyclopentane isomer (CF 2 CF 2 CF 2 CF 2 )Fe(CO) 4 , which does not require a fluorine shift to form from Fe 3 (CO) 12 and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C 4 F 8 ) 2 Fe 2 (CO) n ( n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands. 

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4. Topology and spectral entanglement in cavity-mediated photon scattering

   https://doi.org/10.1063/5.0303703  

   Bittner, Eric R; Piryatinski, Andrei (December 2025, The Journal of Chemical Physics)

   We develop a microscopic diagrammatic theory for cavity-mediated photon scattering in a topological one-dimensional insulator described by the Su–Schrieffer–Heeger model. Within the velocity-gauge formulation, we derive the photon self-energy and vertex corrections arising from virtual electron–hole excitations coupled to a quantized cavity mode, and we evaluate the resulting polariton dispersion and two-photon correlation spectra. Our analysis shows that vacuum fluctuations of the cavity field induce a momentum-resolved self-energy that mixes conduction and valence bands through virtual photon exchange, producing interband hybridization and avoided crossings in the electronic dispersion. This “cavity dressing” is symmetry-dependent, vanishing at the Brillouin-zone edge where the dipole matrix element is zero, and its strength is controlled by the spatial coherence range ζ≈(lc/a)2 of virtual excitations. We further examine how the cavity modifies nonlinear optical observables, including the Kerr nonlinearity and biphoton spectral entanglement, and identify the regimes where these effects become sensitive to the underlying topological phase. The theoretical framework established here provides a unified description of light–matter coupling in topological and polaritonic systems, bridging solid-state cavity QED with the emerging field of cavity-modified quantum materials. Our results suggest that engineered photonic environments can coherently reshape the electronic landscape of topological insulators, offering new routes to control collective electronic and optical phenomena through vacuum-field fluctuations. 

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5. Probing fractional quantum Hall effect by photoluminescence

   https://doi.org/10.1103/gx4z-xddj  

   Makki, Aamir A; Gattu, Mytraya; Jain, J K (October 2025, Physical Review B)

   The recent discovery of fractional quantum anomalous Hall states, fractional quantum Hall states realized without an external magnetic field, in twisted transition-metal dichalcogenide (TMD) bilayers represents a significant development in condensed-matter physics. These states were first observed via photoluminescence (PL) spectroscopy. Surprisingly, a general theoretical understanding of PL is not available even for the conventional fractional quantum Hall states. Our study of a fully spin-polarized system brings out the following picture. At the Jain filling factors 𝜈=𝑛/(2⁢𝑛±1), when an electron is excited from the valence to the conduction band, the photoexcited electron and the hole lower their energies by turning into composite-fermion (CF) particles and holes. These are charge ∓𝑒/(2⁢𝑛±1) anyons and, in general, form a bound CF exciton with energy below the band gap. Away from 𝜈=𝑛/(2⁢𝑛±1), the CF exciton either binds with or annihilates an already present anyon (i.e., a CF in a partially filled level) to form a trion or a single CF quasiparticle, further lowering the energy. When the interaction between two electrons is independent of whether they occupy the conduction or the valence band, as in an ideal two-dimensional system, the resulting SU(2) symmetry implies that the energy of the emitted photon must equal the band gap, independent of correlations. In this case, the bound CF excitons and CF trions are dark, indicating an absence of photoemission at zero temperature, except for the exciton of the 2/3 (1/3) state in the valence (conduction) band, but we show that the temperature dependence of the PL peak intensity can be used to measure their binding energies. We also study, using exact diagonalization methods, the effect of an SU(2)-symmetry-breaking term; we find that as the strength of this term is increased, the dominant peaks of the SU(2)-symmetric theory evolve continuously, while additional, low-intensity peaks appear. We discuss implications for PL experiments in semiconductor quantum wells and twisted TMD bilayers. 

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