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1. 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 inmore »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.« less
2. 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 photoferroelectricmore »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.« less
3. Chaotic cold accretion in giant elliptical galaxies heated by AGN cosmic rays

   https://doi.org/10.1093/mnras/staa550  

   Wang, Chaoran ; Ruszkowski, Mateusz ; Yang, H-Y Karen ( April 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Black hole feedback plays a central role in shaping the circumgalactic medium (CGM) of elliptical galaxies. We systematically study the impact of plasma physics on the evolution of ellipticals by performing three-dimensional non-ideal magnetohydrodynamic simulations of the interactions of active galactic nucleus (AGN) jets with the CGM including magnetic fields, and cosmic rays (CRs) and their transport processes. We find that the physics of feedback operating on large galactic scales depends very sensitively on plasma physics operating on small scales. Specifically, we demonstrate that (i) in the purely hydrodynamical case, the AGN jets initially maintain the atmospheres in globalmore »thermal balance. However, local thermal instability generically leads to the formation of massive cold discs in the vicinity of the central black hole in disagreement with observations; (ii) including weak magnetic fields prevents the formation of the discs because local B-field amplification in the precipitating cold gas leads to strong magnetic breaking, which quickly extracts angular momentum from the accreting clouds. The magnetic fields transform the cold clouds into narrow filaments that do not fall ballistically; (iii) when plasma composition in the AGN jets is dominated by CRs, and CR transport is neglected, the atmospheres exhibit cooling catastrophes due to inefficient heat transfer from the AGN to CGM despite Coulomb/hadronic CR losses being present; (iv) including CR streaming and heating restores agreement with the observations, i.e. cooling catastrophes are prevented and massive cold central discs do not form. The AGN power is reduced as its energy is utilized efficiently.« less
4. Quantum sensing and imaging of spin‐orbit‐torque‐driven spin dynamics in noncollinear antiferromagnet Mn ₃ Sn

   https://doi.org/10.1002/adma.202200327  

   Yan, Gerald Q. ; Li, Senlei ; Lu, Hanyi ; Huang, Mengqi ; Xiao, Yuxuan ; Wernert, Luke ; Brock, Jeffrey A. ; Fullerton, Eric E. ; Chen, Hua ; Wang, Hailong ; et al ( March 2022 , Advanced Materials)

   Novel noncollinear antiferromagnets with spontaneous time-reversal symmetry breaking, nontrivial band topology, and unconventional transport properties have received immense research interest over the past decade due to their rich physics and enormous promise in technological applications. One of the central focuses in this emerging field is exploring the relationship between the microscopic magnetic structure and exotic material properties. Here, the nanoscale imaging of both spin-orbit-torque-induced deterministic magnetic switching and chiral spin rotation in noncollinear antiferromagnet Mn3Sn films using nitrogen-vacancy (NV) centers is reported. Direct evidence of the off-resonance dipole-dipole coupling between the spin dynamics in Mn3Sn and proximate NV centers ismore »also demonstrated with NV relaxometry measurements. These results demonstrate the unique capabilities of NV centers in accessing the local information of the magnetic order and dynamics in these emergent quantum materials and suggest new opportunities for investigating the interplay between topology and magnetism in a broad range of topological magnets.« less
5. Multi-Agent Path Finding with Mutex Propagation

   Zhang, H. ; Li, J. ; Surynek, P ; Koenig, S. ; Kumar, S. ( June 2020 , Proceedings of the International Conference on Automated Planning and Scheduling)

   Mutex propagation is a form of efficient constraint propagation popularly used in AI planning to tightly approximate the reachable states from a given state. We utilize this idea in the context of Multi-Agent Path Finding (MAPF). When adapted to MAPF, mutex propagation provides stronger constraints for conflict resolution in Conflict-Based Search (CBS), a popular optimal MAPF algorithm, and provides it with the ability to identify and reason with symmetries in MAPF. While existing work identifies a limited form of symmetries using rectangle reasoning and requires the manual design of symmetry-breaking constraints, mutex propagation is more general and allows for themore »automated design of symmetry-breaking constraints. Our experimental results show that CBS with mutex propagation is capable of outperforming CBSH with rectangle reasoning, a state-of-the-art variant of CBS, with respect to runtime and success rate.« less