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1. Two superconducting states with broken time-reversal symmetry in FeSe _1−x S _x

   https://doi.org/10.1073/pnas.2208276120  

   Matsuura, Kohei; Roppongi, Masaki; Qiu, Mingwei; Sheng, Qi; Cai, Yipeng; Yamakawa, Kohtaro; Guguchia, Zurab; Day, Ryan P.; Kojima, Kenji M.; Damascelli, Andrea; et al (May 2023, Proceedings of the National Academy of Sciences)

   Iron-chalcogenide superconductors FeSe_1−xS_xpossess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe_1−xS_xsuperconductors for0≤x≤0.22covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperatureT_cfor all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-fieldμSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x>0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe_1−xS_x, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity. 

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2. Visualizing symmetry-breaking electronic orders in epitaxial Kagome magnet FeSn films

   https://doi.org/10.1038/s41467-023-41831-4  

   Zhang, Huimin; Oli, Basu Dev; Zou, Qiang; Guo, Xu; Wang, Zhengfei; Li, Lian (December 2023, Nature Communications)

   Abstract Kagome lattice hosts a plethora of quantum states arising from the interplay of topology, spin-orbit coupling, and electron correlations. Here, we report symmetry-breaking electronic orders tunable by an applied magnetic field in a model Kagome magnet FeSn consisting of alternating stacks of two-dimensional Fe₃Sn Kagome and Sn₂honeycomb layers. On the Fe₃Sn layer terminated FeSn thin films epitaxially grown on SrTiO₃(111) substrates, we observe trimerization of the Kagome lattice using scanning tunneling microscopy/spectroscopy, breaking its six-fold rotational symmetry while preserving the translational symmetry. Such a trimerized Kagome lattice shows an energy-dependent contrast reversal in dI/dV maps, which is significantly enhanced by bound states induced by Sn vacancy defects. This trimerized Kagome lattice also exhibits stripe modulations that are energy-dependent and tunable by an applied in-plane magnetic field, indicating symmetry-breaking nematicity from the entangled magnetic and charge degrees of freedom in antiferromagnet FeSn. 

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3. Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd ₃ S ₄ under high pressure

   https://doi.org/10.1073/pnas.2310779120  

   Huyan, Shuyuan; Ryan, Dominic H.; Slade, Tyler J.; Lavina, Barbara; Jose, Greeshma; Wang, Haozhe; Wilde, John M.; Ribeiro, Raquel A.; Zhao, Jiyong; Xie, Weiwei; et al (December 2023, Proceedings of the National Academy of Sciences)

   We present a comprehensive study of the inhomogeneous mixed-valence compound, EuPd₃S₄, by electrical transport, X-ray diffraction, time-domain¹⁵¹Eu synchrotron Mössbauer spectroscopy, and X-ray absorption spectroscopy measurements under high pressure. Electrical transport measurements show that the antiferromagnetic ordering temperature,T_N, increases rapidly from 2.8 K at ambient pressure to 23.5 K at ~19 GPa and plateaus between ~19 and ~29 GPa after which no anomaly associated withT_Nis detected. A pressure-induced first-order structural transition from cubic to tetragonal is observed, with a rather broad coexistence region (~20 GPa to ~30 GPa) that corresponds to theT_Nplateau. Mössbauer spectroscopy measurements show a clear valence transition from approximately 50:50 Eu²⁺:Eu³⁺to fully Eu³⁺at ~28 GPa, consistent with the vanishing of the magnetic order at the same pressure. X-ray absorption data show a transition to a fully trivalent state at a similar pressure. Our results show that pressure first greatly enhancesT_N, most likely via enhanced hybridization between the Eu 4fstates and the conduction band, and then, second, causes a structural phase transition that coincides with the conversion of the europium to a fully trivalent state. 

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4. Electron doping of Sr ₂ FeMoO _6−δ as high performance anode materials for solid oxide fuel cells

   https://doi.org/10.1039/c8ta10061f  

   Yang, Xin; Chen, Jing; Panthi, Dhruba; Niu, Bingbing; Lei, Libin; Yuan, Zhihao; Du, Yanhai; Li, Yongfeng; Chen, Fanglin; He, Tianmin (January 2019, Journal of Materials Chemistry A)

   Electron doping in perovskites is an effective approach to design and tailor the structure and property of materials. In A 2 BB′O 6−δ -type double perovskites, B-site cation order can be tunable by A-site modification, potentially leading to significant effect on the oxygen nonstoichiometry of the compounds. La 3+ -doped Sr 2 FeMoO 6−δ (Sr 2−x La x FeMoO 6−δ , SLFM with 0 ≤ x ≤ 1) double perovskites have been designed and characterized systematically in this study as anode materials for solid oxide fuel cells. Rietveld refinement of powder X-ray diffraction reveals a crystalline symmetry transition of SLFM from tetragonal to orthorhombic with the increase of La content, driven by the extra electron onto the antibonding orbitals of e g and t 2g of Fe/Mo cations. An increase in Fe/Mo anti-site defect accompanies this phase transition. Solid oxide fuel cells incorporating the Sr 1.8 La 0.2 FeMoO 6−δ (SLFM2) anode demonstrate impressive power outputs and stable performance under direct CH 4 operation because of its altered electronic structure, desired oxygen vacancy concentration and enhanced reducibility. Density functional theory plus U correction calculations provide an insight into how La doping affects the Fe/Mo anti-site defects and consequently the oxygen transport dynamics. 

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5. 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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