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1. Electronic stripe patterns near the fermi level of tetragonal Fe(Se,S)

   https://doi.org/10.1038/s41535-023-00592-5  

   Walker, M. ; Scott, K. ; Boyle, T. J. ; Byland, J. K. ; Bötzel, S. ; Zhao, Z. ; Day, R. P. ; Zhdanovich, S. ; Gorovikov, S. ; Pedersen, T. M. ; et al ( October 2023 , npj Quantum Materials)

   FeSe_1−xS_xremains one of the most enigmatic systems of Fe-based superconductors. While much is known about the orthorhombic parent compound, FeSe, the tetragonal samples, FeSe_1−xS_xwithx > 0.17, remain relatively unexplored. Here, we provide an in-depth investigation of the electronic states of tetragonal FeSe_0.81S_0.19, using scanning tunneling microscopy and spectroscopy (STM/S) measurements, supported by angle-resolved photoemission spectroscopy (ARPES) and theoretical modeling. We analyze modulations of the local density of states (LDOS) near and away from Fe vacancy defects separately and identify quasiparticle interference (QPI) signals originating from multiple regions of the Brillouin zone, including the bands at the zone corners. We also observe that QPI signals coexist with a much stronger LDOS modulation for states near the Fermi level whose period is independent of energy. Our measurements further reveal that this strong pattern appears in the STS measurements as short range stripe patterns that are locally two-fold symmetric. Since these stripe patterns coexist with four-fold symmetric QPI around Fe-vacancies, the origin of their local two-fold symmetry must be distinct from that of nematic states in orthorhombic samples. We explore several aspects related to the stripes, such as the role of S and Fe-vacancy defects, and whether they can be explained by QPI. We consider the possibility that the observed stripe patterns may represent incipient charge order correlations, similar to those observed in the cuprates.

    

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2. Nickelate Superconductivity without Rare‐Earth Magnetism: (La,Sr)NiO 2

   https://doi.org/10.1002/adma.202104083  

   Osada, Motoki ; Wang, Bai Yang ; Goodge, Berit H. ; Harvey, Shannon P. ; Lee, Kyuho ; Li, Danfeng ; Kourkoutis, Lena F. ; Hwang, Harold Y. ( September 2021 , Advanced Materials)

   The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite‐layer nickelate (Nd,Sr)NiO₂and (Pr,Sr)NiO₂thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare‐earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La_1−xSr_xNiO₂phase diagram can enter the regime of coherent low‐temperature transport (x = 0.14 ‐ 0.20), with subsequent superconducting transitions and a maximum onset of ≈9 K atx = 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO₂is observed, which likely reflects the self‐doped nature of the electronic structure. Combining the results of (La/Pr/Nd)_1−xSr_xNiO₂reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron‐hole populations across the lanthanides.

    

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3. Comparison of temperature and doping dependence of elastoresistivity near a putative nematic quantum critical point

   https://doi.org/10.1038/s41467-022-28583-3  

   Palmstrom, J. C. ; Walmsley, P. ; Straquadine, J. A. W. ; Sorensen, M. E. ; Hannahs, S. T. ; Burns, D. H. ; Fisher, I. R. ( February 2022 , Nature Communications)

   Strong electronic nematic fluctuations have been discovered near optimal doping for several families of Fe-based superconductors, motivating the search for a possible link between these fluctuations, nematic quantum criticality, and high temperature superconductivity. Here we probe a key prediction of quantum criticality, namely power-law dependence of the associated nematic susceptibility as a function of composition and temperature approaching the compositionally tuned putative quantum critical point. To probe the ‘bare’ quantum critical point requires suppression of the superconducting state, which we achieve by using large magnetic fields, up to 45 T, while performing elastoresistivity measurements to follow the nematic susceptibility. We performed these measurements for the prototypical electron-doped pnictide, Ba(Fe_1−xCo_x)₂As₂, over a dense comb of dopings. We find that close to the putative quantum critical point, the elastoresistivity appears to obey power-law behavior as a function of composition over almost a decade of variation in composition. Paradoxically, however, we also find that the temperature dependence for compositions close to the critical value cannot be described by a single power law.

    

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4. Orbital-flop transition of superfluid 3He in anisotropic silica aerogel

   https://doi.org/10.1038/s41467-023-44557-5  

   Nguyen, M. D. ; Simon, Joshua ; Scott, J. W. ; Zimmerman, A. M. ; Tsai, Y. C. Cincia ; Halperin, W. P. ( January 2024 , Nature Communications)

   Superfluid³He is a paradigm for odd-parity Cooper pairing, ranging from neutron stars to uranium-based superconducting compounds. Recently it has been shown that³He, imbibed in anisotropic silica aerogel with either positive or negative strain, preferentially selects either the chiral A-phase or the time-reversal-symmetric B-phase. This control over basic order parameter symmetry provides a useful model for understanding imperfect unconventional superconductors. For both phases, the orbital quantization axis is fixed by the direction of strain. Unexpectedly, at a specific temperatureT_x, the orbital axis flops by 90^∘, but in reverse order for A and B-phases. Aided by diffusion limited cluster aggregation simulations of anisotropic aerogel and small angle X-ray measurements, we are able to classify these aerogels as either “planar and “nematic concluding that the orbital-flop is caused by competition between short and long range structures in these aerogels.

    

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5. On the Remarkable Superconductivity of FeSe and Its Close Cousins

   https://doi.org/10.3390/sym12091402  

   Kreisel, Andreas ; Hirschfeld, Peter J. ; Andersen, Brian M. ( September 2020 , Symmetry)

   null (Ed.)

   Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity. 

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