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1. Perspectives on field-free spin–orbit torque devices for memory and computing applications

   https://doi.org/10.1063/5.0135185  

   Lopez-Dominguez, Victor ; Shao, Yixin ; Khalili Amiri, Pedram ( January 2023 , Journal of Applied Physics)

   The emergence of embedded magnetic random-access memory (MRAM) and its integration in mainstream semiconductor manufacturing technology have created an unprecedented opportunity for engineering computing systems with improved performance, energy efficiency, lower cost, and unconventional computing capabilities. While the initial interest in the existing generation of MRAM—which is based on the spin-transfer torque (STT) effect in ferromagnetic tunnel junctions—was driven by its nonvolatile data retention and lower cost of integration compared to embedded Flash (eFlash), the focus of MRAM research and development efforts is increasingly shifting toward alternative write mechanisms (beyond STT) and new materials (beyond ferromagnets) in recent years. This has been driven by the need for better speed vs density and speed vs endurance trade-offs to make MRAM applicable to a wider range of memory markets, as well as to utilize the potential of MRAM in various unconventional computing architectures that utilize the physics of nanoscale magnets. In this Perspective, we offer an overview of spin–orbit torque (SOT) as one of these beyond-STT write mechanisms for the MRAM devices. We discuss, specifically, the progress in developing SOT-MRAM devices with perpendicular magnetization. Starting from basic symmetry considerations, we discuss the requirement for an in-plane bias magnetic field which has hindered progress in developing practical SOT-MRAM devices. We then discuss several approaches based on structural, magnetic, and chiral symmetry-breaking that have been explored to overcome this limitation and realize bias-field-free SOT-MRAM devices with perpendicular magnetization. We also review the corresponding material- and device-level challenges in each case. We then present a perspective of the potential of these devices for computing and security applications beyond their use in the conventional memory hierarchy. 

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2. Multicomponent superconducting order parameter in UTe 2

   https://doi.org/10.1126/science.abb0272  

   Hayes, I. M. ; Wei, D. S. ; Metz, T. ; Zhang, J. ; Eo, Y. S. ; Ran, S. ; Saha, S. R. ; Collini, J. ; Butch, N. P. ; Agterberg, D. F. ; et al ( July 2021 , Science)

   An unconventional superconducting state was recently discovered in uranium ditelluride (UTe₂), in which spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy-fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggests that a distinctive set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a nonzero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe₂is characterized by a two-component order parameter that breaks time-reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe₂.

    

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3. A d 8 anti-Hund’s singlet insulator in an infinite-layer nickelate

   https://doi.org/10.1088/2515-7639/ac6040  

   Jin, Hyo-Sun ; Pickett, Warren E. ; Lee, Kwan-Woo ( April 2022 , Journal of Physics: Materials)

   The status of nickelate superconductors in relation to cuprate high temperature superconductors is one of the concepts being discussed in high temperature superconductivity in correlated transition metal oxides. New additions to the class of infinite layer nickelates can provide essential input relating to connections or distinctions. A recently synthesized compound Ba₂NiO₂(AgSe)₂, which contains isolated ‘infinite layer’ NiO₂planes, may lead to new insights. Our investigations have discovered that, at density functional theory mean field level, the ground state consists of an unusual$e_{\mathrm{g}}$singlet on the Ni²⁺ion arising from large but separate Mott insulating gaps in both$e_{\mathrm{g}}$orbitals, but with different, anti-Hund’s, spin directions of their moments. This textured singlet incorporates at the least new physics, and potentially a new platform for nickelate superconductivity, which might be of an unconventional form for transition metal oxides due to the unconventional undoped state. We include in this paper a comparison of electronic structure parameters of Ba₂NiO₂(AgSe)₂with a better characterized infinite layer nickelate LaNiO₂. We provide more analysis of thed⁸anti-Hund’s singlet that emerges in this compound, and consider a minimally correlated wavefunction for this singlet in an itinerant background, and begin discussion of excitations—real or virtual—that may figure into new electronic phases.

    

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4. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R. ; Sarkar, Tarapada ; Greene, Richard L. ( March 2019 , Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H_C2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-T_Csuperconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La_2−xCe_xCuO₄(LCCO) fromx= 0.11–0.19. We observe quantum critical$\mathit{S}/\mathit{T}$versus$\mathbf{l}\mathbf{n}\left(\mathbf{1}/\mathit{T}\right)$behavior over an unexpectedly wide doping rangex= 0.15–0.17 above the QCP (x= 0.14), with a slope that scales monotonically with the superconducting transition temperature (T_Cwith H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked withT_C. Above the superconductivity dome, atx= 0.19, a conventional Fermi-liquid$\mathit{S}\propto \mathit{T}$behavior is found for$\mathit{T}\le$40 K.

    

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5. Two-fold symmetric superconductivity in few-layer NbSe2

   https://doi.org/10.1038/s41567-021-01219-x  

   Hamill, Alex ; Heischmidt, Brett ; Sohn, Egon ; Shaffer, Daniel ; Tsai, Kan-Ting ; Zhang, Xi ; Xi, Xiaoxiang ; Suslov, Alexey ; Berger, Helmuth ; Forró, László ; et al ( April 2021 , Nature Physics)

   null (Ed.)

   The strong Ising spin–orbit coupling in certain two-dimensional transition metal dichalcogenides can profoundly affect the superconducting state in few-layer samples. For example, in NbSe2, this effect combines with the reduced dimensionality to stabilize the superconducting state against magnetic fields up to ~35 T, and could lead to topological superconductivity. Here we report a two-fold rotational symmetry of the superconducting state in few-layer NbSe2 under in-plane external magnetic fields, in contrast to the three-fold symmetry of the lattice. Both the magnetoresistance and critical field exhibit this two-fold symmetry, and it also manifests deep inside the superconducting state in NbSe2/CrBr3 superconductor-magnet tunnel junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behaviour to the mixing between two closely competing pairing instabilities, namely the conventional s-wave instability typical of bulk NbSe2 and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results demonstrate the unconventional character of the pairing interaction in few-layer transition metal dichalcogenides and highlight the exotic superconductivity in this family of two-dimensional materials. 

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