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1. Large spin–orbit torque in bismuthate-based heterostructures

   https://doi.org/10.1038/s41928-023-01080-1  

   Edgeton, Anthony L; Harris, Isaac A; Campbell, Neil G; Chai, Yahong; Mazur, Marcel M; Gurung, Gautam; Huang, Xiaoxi; Susarla, Sandhya; Tsymbal, Evgeny Y; Ralph, Daniel C; et al (December 2023, Nature Electronics)

   The wider application of spintronic devices requires the development of new material platforms that can efficiently manipulate spin. Bismuthate-based superconductors are centrosymmetric systems that are generally thought to offer weak spin–orbit coupling. Here, we report a large spin–orbit torque driven by spin polarization generated in heterostructures based on the bismuthate BaPb1-xBixO3 (which is in a non-superconducting state). Using spin-torque ferromagnetic resonance and d.c. non-linear Hall measurements, we measure a spin–orbit torque efficiency of around 2.7 and demonstrate current driven magnetization switching at current densities of 4×10^5 A〖cm〗^(-2). We suggest that the unexpectedly large current-induced torques could be the result of an orbital Rashba effect associated with local inversion symmetry breaking in BaPb1-xBixO3. 

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2. Superconducting diode effect via conformal-mapped nanoholes

   https://doi.org/10.1038/s41467-021-23077-0  

   Lyu, Yang-Yang; Jiang, Ji; Wang, Yong-Lei; Xiao, Zhi-Li; Dong, Sining; Chen, Qing-Hu; Milošević, Milorad V.; Wang, Huabing; Divan, Ralu; Pearson, John E.; et al (December 2021, Nature Communications)

   null (Ed.)

   Abstract A superconducting diode is an electronic device that conducts supercurrent and exhibits zero resistance primarily for one direction of applied current. Such a dissipationless diode is a desirable unit for constructing electronic circuits with ultralow power consumption. However, realizing a superconducting diode is fundamentally and technologically challenging, as it usually requires a material structure without a centre of inversion, which is scarce among superconducting materials. Here, we demonstrate a superconducting diode achieved in a conventional superconducting film patterned with a conformal array of nanoscale holes, which breaks the spatial inversion symmetry. We showcase the superconducting diode effect through switchable and reversible rectification signals, which can be three orders of magnitude larger than that from a flux-quantum diode. The introduction of conformal potential landscapes for creating a superconducting diode is thereby proven as a convenient, tunable, yet vastly advantageous tool for superconducting electronics. This could be readily applicable to any superconducting materials, including cuprates and iron-based superconductors that have higher transition temperatures and are desirable in device applications. 

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3. Angular dependence of spin-orbit torque in monolayer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$

   https://doi.org/10.1103/PhysRevB.108.144422  

   Xue, Fei; Stiles, Mark D.; Haney, Paul M. (October 2023, Physical Review B)

   In ferromagnetic systems lacking inversion symmetry, an applied electric field can control the ferromagnetic order parameters through the spin-orbit torque. The prototypical example is a bilayer heterostructure composed of a ferromagnet and a heavy metal that acts as a spin current source. In addition to such bilayers, spin-orbit coupling can mediate spin-orbit torques in ferromagnets that lack bulk inversion symmetry. A recently discovered example is the two-dimensional monolayer ferromagnet Fe3GeTe2. In this paper, we use first-principles calculations to study the spin-orbit torque and ensuing magnetic dynamics in this material. By expanding the torque versus magnetization direction as a series of vector spherical harmonics, we find that higher order terms (up to ℓ=4) are significant and play important roles in the magnetic dynamics. They give rise to deterministic, magnetic field-free electrical switching of perpendicular magnetization. 

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4. 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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5. Phase tuning of multiple Andreev reflections of Dirac fermions and the Josephson supercurrent in Al–MoTe ₂ –Al junctions

   https://doi.org/10.1073/pnas.2204468119  

   Zhu, Zheyi; Kim, Stephan; Lei, Shiming; Schoop, Leslie M.; Cava, R. J.; Ong, N. P. (July 2022, Proceedings of the National Academy of Sciences)

   When an electron is incident on a superconductor from a metal, it is reflected as a hole in a process called Andreev reflection. If the metal N is sandwiched between two superconductors S in an SNS junction, multiple Andreev reflections (MARs) occur. We have found that, in SNS junctions with high transparency ( τ   →   1 ) based on the Dirac semimetal MoTe 2 , the MAR features are observed with exceptional resolution. By tuning the phase difference φ between the bracketing Al superconductors, we establish that the MARs coexist with a Josephson supercurrent I s = I A   sin φ . As we vary the junction voltage V , the supercurrent amplitude I A varies in step with the MAR order n , revealing a direct relation between them. Two successive Andreev reflections serve to shuttle a Cooper pair across the junction. If the pair is shuttled coherently, it contributes to I s . The experiment measures the fraction of pairs shuttled coherently vs. V . Surprisingly, superconductivity in MoTe 2 does not affect the MAR features. 

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