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1. Bulk evidence of anisotropic s-wave pairing with no sign change in the kagome superconductor CsV3Sb5

   https://doi.org/10.1038/s41467-023-36273-x  

   Roppongi, M. ; Ishihara, K. ; Tanaka, Y. ; Ogawa, K. ; Okada, K. ; Liu, S. ; Mukasa, K. ; Mizukami, Y. ; Uwatoko, Y. ; Grasset, R. ; et al ( February 2023 , Nature Communications)

   The recently discovered kagome superconductorsAV₃Sb₅(A= K, Rb, Cs) exhibit unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries with sign-changing and chiral order parameters. Experimentally, however, superconducting phase information inAV₃Sb₅is still lacking. Here we report the impurity effects in CsV₃Sb₅using electron irradiation as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, an anisotropic fully-gapped state changes to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under pressure show that the double superconducting dome in the pressure-temperature phase diagram survives against sufficient impurities. These results support that CsV₃Sb₅is a non-chiral, anisotropics-wave superconductor with no sign change both at ambient and under pressure.

    

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2. Dirac lines and loop at the Fermi level in the time-reversal symmetry breaking superconductor LaNiGa2

   https://doi.org/10.1038/s42005-021-00771-5  

   Badger, Jackson R. ; Quan, Yundi ; Staab, Matthew C. ; Sumita, Shuntaro ; Rossi, Antonio ; Devlin, Kasey P. ; Neubauer, Kelly ; Shulman, Daniel S. ; Fettinger, James C. ; Klavins, Peter ; et al ( December 2022 , Communications Physics)

   Abstract Unconventional superconductors have Cooper pairs with lower symmetries than in conventional superconductors. In most unconventional superconductors, the additional symmetry breaking occurs in relation to typical ingredients such as strongly correlated Fermi liquid phases, magnetic fluctuations, or strong spin-orbit coupling in noncentrosymmetric structures. In this article, we show that the time-reversal symmetry breaking in the superconductor LaNiGa 2 is enabled by its previously unknown topological electronic band structure, with Dirac lines and a Dirac loop at the Fermi level. Two symmetry related Dirac points even remain degenerate under spin-orbit coupling. These unique topological features enable an unconventional superconducting gap in which time-reversal symmetry can be broken in the absence of other typical ingredients. Our findings provide a route to identify a new type of unconventional superconductors based on nonsymmorphic symmetries and will enable future discoveries of topological crystalline superconductors. 

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3. Symmetry Control of Unconventional Spin–Orbit Torques in IrO 2

   https://doi.org/10.1002/adma.202301608  

   Patton, Michael ; Gurung, Gautam ; Shao, Ding‐Fu ; Noh, Gahee ; Mittelstaedt, Joseph A. ; Mazur, Marcel ; Kim, Jong‐Woo ; Ryan, Philip J. ; Tsymbal, Evgeny Y. ; Choi, Si‐Young ; et al ( July 2023 , Advanced Materials)

   Spin–orbit torques generated by a spin current are key to magnetic switching in spintronic applications. The polarization of the spin current dictates the direction of switching required for energy‐efficient devices. Conventionally, the polarizations of these spin currents are restricted to be along a certain direction due to the symmetry of the material allowing only for efficient in‐plane magnetic switching. Unconventional spin–orbit torques arising from novel spin current polarizations, however, have the potential to switch other magnetization orientations such as perpendicular magnetic anisotropy, which is desired for higher density spintronic‐based memory devices. Here, it is demonstrated that low crystalline symmetry is not required for unconventional spin–orbit torques and can be generated in a nonmagnetic high symmetry material, iridium dioxide (IrO₂), using epitaxial design. It is shown that by reducing the relative crystalline symmetry with respect to the growth direction large unconventional spin currents can be generated and hence spin–orbit torques. Furthermore, the spin polarizations detected in (001), (110), and (111) oriented IrO₂thin films are compared to show which crystal symmetries restrict unconventional spin transport. Understanding and tuning unconventional spin transport generation in high symmetry materials can provide a new route towards energy‐efficient magnetic switching in spintronic devices.

    

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4. Optical Vortex Transmutation with Geometric Metasurfaces of Rotational Symmetry Breaking

   https://doi.org/10.1002/adom.201901152  

   Zhang, Yuchao ; Gao, Jie ; Yang, Xiaodong ( August 2019 , Advanced Optical Materials)

   The free‐space optical vortex transmutation is realized by using geometric plasmonic metasurfaces with the designed noncanonical vortex phase profiles possessing discrete rotational symmetries of finite order. Based on the introduced continuous‐to‐discrete rotational symmetry breaking in metasurfaces, the vortex transmutation phenomena are observed behind the metasurfaces from the near‐field to far‐field diffraction in free space. The near‐field optical beam profile represents the input vortex, while in the far field the input vortex is diffracted into the central output vortex with topological charge determined by the transmutation rule and the symmetrically distributed off‐axis vortices with unity topological charge bifurcating from the input vortex, with the total orbital angular momentum conserved. The demonstrated free‐space optical vortex transformation will promise many potential applications related to optical communication, particle manipulation, and quantum information processing.

    

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5. A chiral switchable photovoltaic ferroelectric 1D perovskite

   https://doi.org/10.1126/sciadv.aay4213  

   Hu, Yang ; Florio, Fred ; Chen, Zhizhong ; Phelan, W. Adam ; Siegler, Maxime A. ; Zhou, Zhe ; Guo, Yuwei ; Hawks, Ryan ; Jiang, Jie ; Feng, Jing ; et al ( February 2020 , Science Advances)

   Spin and valley degrees of freedom in materials without inversion symmetry promise previously unknown device functionalities, such as spin-valleytronics. Control of material symmetry with electric fields (ferroelectricity), while breaking additional symmetries, including mirror symmetry, could yield phenomena where chirality, spin, valley, and crystal potential are strongly coupled. Here we report the synthesis of a halide perovskite semiconductor that is simultaneously photoferroelectricity switchable and chiral. Spectroscopic and structural analysis, and first-principles calculations, determine the material to be a previously unknown low-dimensional hybrid perovskite (R)-(−)-1-cyclohexylethylammonium/(S)-(+)-1 cyclohexylethylammonium) PbI 3 . Optical and electrical measurements characterize its semiconducting, ferroelectric, switchable pyroelectricity and switchable photoferroelectric properties. Temperature dependent structural, dielectric and transport measurements reveal a ferroelectric-paraelectric phase transition. Circular dichroism spectroscopy confirms its chirality. The development of a material with such a combination of these properties will facilitate the exploration of phenomena such as electric field and chiral enantiomer–dependent Rashba-Dresselhaus splitting and circular photogalvanic effects. 

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