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1. Artificial Pinning Centers in (Y,RE)-Ba-Cu-O Superconductors: Recent Progress and Future Perspective

   https://doi.org/1088/1361-6668/ab4ccd  

   Timothy J. Haugan, Teresa Puig ( October 2019 , Superconductor science technology)

   A microscopic understanding of vortex pinning in type II superconductors began with the theoretical discovery of magnetic vortices by Abrikosov, which received the 2003 Nobel Prize in Physics [1, 2]. When type II superconductors are exposed to magnetic fields (H), the magnetic field enters as quantized vortices, each with a fundamental flux j0 = 2.07 × 10−11 T cm−2 , or 2.07 × 10−15 Wb. The vortex core size on the order of the superconducting coherence length can be very small, e.g. ∼1–2 nm for the cuprate family of high-temperature superconductors (HTSs). The vortices electrically interact with each other by repelling, and act collectively together as a flux lattice that is affected by the intrinsic crystal lattice properties and microstructure defects. For superconducting power applications where applied magnetic fields are in the range of 0.1 T to >30 T, the areal number density of the vortices can reach incredibly high values. For example, for an applied magnetic field of 5 T, the vortex areal density is around 2.5 × 1011 cm−2 , which translates to inter-vortex spacing of about 20 nm (assuming a square lattice for vortices). Somewhat surprisingly, if the crystal lattice for type II superconductors, such as HTS cuprates [3] is nearly perfect without any defects to pin vortices, the vortices can move collectively and almost freely in an applied magnetic field due to Lorentz forces, which results in electrical resistance at a fairly low critical current density Jc(H, T) at an applied magnetic field (H) and temperature (T). In order to realize useful critical current densities in type II superconductors, imperfections and defects must be added to the crystal lattice to effectively pin vortices. The simplest example of this was achieved in the (Y, RE)Ba2Cu3O7 (where RE is rare earth elements) family by depositing thin films, in which high densities of dislocations and other growth defects are added into the film microstructure and dramatically increase the critical current density Jc(77 K, H//c-axis) > 106 A cm−2 compared to Jc (77 K) < 103 A cm−2 for single crystals [4–6] 

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2. Nonaxisymmetric Precession of Magnetars and Fast Radio Bursts

   https://doi.org/10.3847/1538-4357/ac38a6  

   Wasserman, I. ; Cordes, J. M. ; Chatterjee, S. ; Batra, G. ( March 2022 , The Astrophysical Journal)

   The repeating fast radio bursts (FRBs) 180916.J0158 and 121102 are visible during periodically occurring windows in time. We consider the constraints on internal magnetic fields and geometries if the cyclical behavior observed for FRB 180916.J0158 and FRB 121102 is due to the precession of magnetars. In order to frustrate vortex line pinning we argue that internal magnetic fields must be stronger than about 10¹⁶G, which is large enough to prevent superconductivity in the core and destroy the crustal lattice structure. We conjecture that the magnetic field inside precessing magnetars has three components: (1) a dipole component with characteristic strength ∼ 10¹⁴G; (2) a toroidal component with characteristic strength ∼ 10¹⁵–10¹⁶G that only occupies a modest fraction of the stellar volume; and (3) a disordered field with characteristic strength ∼ 10¹⁶G. The disordered field is primarily responsible for permitting precession, which stops once this field component decays away, which we conjecture happens after ∼1000 yr. Conceivably, as the disordered component damps bursting activity diminishes and eventually ceases. We model the quadrupolar magnetic distortion of the star, which is due to its ordered components primarily, as triaxial and very likely prolate. We address the question of whether the spin frequency ought to be detectable for precessing, bursting magnetars by constructing a specific model in which bursts happen randomly in time with random directions distributed in or between cones relative to a single symmetry axis. Within the context of these specific models, we find that there are precession geometries for which detecting the spin frequency is very unlikely.

    

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3. Laser‐Induced Cooperative Transition in Molecular Electronic Crystal

   https://doi.org/10.1002/adma.202103000  

   Hu, Yong ; Adhikari, Dasharath ; Tan, Andrew ; Dong, Xi ; Zhu, Taishan ; Wang, Xiaoyu ; Huang, Yulong ; Mitchell, Travis ; Yao, Ziheng ; Dasenbrock‐Gammon, Nathan ; et al ( August 2021 , Advanced Materials)

   The competing and non‐equilibrium phase transitions, involving dynamic tunability of cooperative electronic and magnetic states in strongly correlated materials, show great promise in quantum sensing and information technology. To date, the stabilization of transient states is still in the preliminary stage, particularly with respect to molecular electronic solids. Here, a dynamic and cooperative phase in potassium‐7,7,8,8‐tetracyanoquinodimethane (K‐TCNQ) with the control of pulsed electromagnetic excitation is demonstrated. Simultaneous dynamic and coherent lattice perturbation with 8 ns pulsed laser (532 nm, 15 MW cm⁻², 10 Hz) in such a molecular electronic crystal initiates a stable long‐lived (over 400 days) conducting paramagnetic state (≈42 Ωcm), showing the charge–spin bistability over a broad temperature range from 2 to 360 K. Comprehensive noise spectroscopy, in situ high‐pressure measurements, electron spin resonance (ESR), theoretical model, and scanning tunneling microscopy/spectroscopy (STM/STS) studies provide further evidence that such a transition is cooperative, requiring a dedicated charge–spin–lattice decoupling to activate and subsequently stabilize nonequilibrium phase. The cooperativity triggered by ultrahigh‐strain‐rate (above 10⁶s⁻¹) pulsed excitation offers a collective control toward the generation and stabilization of strongly correlated electronic and magnetic orders in molecular electronic solids and offers unique electro‐magnetic phases with technological promises.

    

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4. Enhancing magnetic pinning by BaZrO 3 nanorods forming coherent interface by strain-directed Ca-doping in YBa 2 Cu 3 O 7−x nanocomposite films

   https://doi.org/10.1088/1361-6668/ac1fd3  

   Ogunjimi, Victor ; Sebastian, Mary Ann ; Zhang, Di ; Gautam, Bibek ; Jian, Jie ; Huang, Jijie ; Zhang, Yifan ; Haugan, Timothy ; Wang, Haiyan ; Wu, Judy ( September 2021 , Superconductor Science and Technology)

   BaZrO₃(BZO) one-dimensional artificial pinning centers (1D-APCs) aligned along thec-axis of the YBa₂Cu₃O₇(YBCO) have been adopted to enhance the magnetic vortex pinning in BZO/YBCO nanocomposite films. However, the pinning force densityF_pof the BZO 1D-APCs remains moderate at temperatures near 77 K. A hypothesis of the major limiting factor is the defective BZO 1D-APCs/YBCO interface as a direct consequence of the large interfacial strain originated from the BZO/YBCO lattice mismatch of ∼7.7%. Herein, we explore enlarging thec-axis of the YBCO dynamically to reduce the lattice mismatch and hence to prevent formation of the defective BZO 1D-APCs/YBCO interface. Specifically, thec-axis enlargement was achieved by partial replacement of Cu with Ca on the YBCO lattice using strain-directed Ca diffusion into YBCO from two Ca_0.3Y_0.7Ba₂Cu₃O_7−x(CaY-123) spacers of only 10 nm in thickness inserted into the 2 vol% BZO 1D-APC/YBCO nanocomposite thin films of ∼150 nm in total thickness. The achieved elongatedc-axis is attributed to the formation of stacking faults induced by Ca-replacement of Cu on YBCO lattice. The reduced BZO/YBCO lattice mismatch allows formation of a coherent BZO 1D-APC/YBCO interface with negligible defects. This leads to an enhancedF_pvalue up to 98 GN m⁻³at 65 K, which is 70% higher than that of the reference 2 vol% BZO 1D-APC/YBCO sample. Furthermore, the benefit of the enhanced pinning of the BZO 1D-APCs with a coherent interface with YBCO can be extended to a large angular range of the magnetic field orientation. This study reveals the significant effect of the BZO/YBCO interface on the pinning efficiency of BZO 1D-APCs and provides a promising approach to achieve a coherent interface in BZO/YBCO nanocomposite films.

    

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5. Weyl Fermions and broken symmetry phases of laterally confined 3 He films

   https://doi.org/10.1088/1361-648X/acf42b  

   Wu, Hao ; Sauls, J. A. ( September 2023 , Journal of Physics: Condensed Matter)

   Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D)³He-A consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum,$L_z=(N/2)\hslash$, for$N/2$Cooper pairs. The power law suppression of the angular momentum,$L_z(T)\simeq (N/2)\hslash [1-\frac{2}{3}(\pi T/\mathrm{\Delta }{)}^2]$for$0⩽T\ll T_c$, in the fully gapped 2D chiral A-phase reflects the thermal excitation of the chiral edge Fermions. We discuss the effects of wave function overlap, and hybridization between edge states confined near opposing edge boundaries on the edge currents, ground-state angular momentum and ground-state order parameter of superfluid³He thin films. Under strong lateral confinement, the chiral A phase undergoes a sequence of phase transitions, first to a pair density wave (PDW) phase with broken translational symmetry at$D_{c2}\sim 16{\xi }_0$. The PDW phase is described by a periodic array of chiral domains with alternating chirality, separated by domain walls. The period of PDW phase diverges as the confinement length$D\to D_{c_2}$. The PDW phase breaks time-reversal symmetry, translation invariance, but is invariant under the combination of time-reversal and translation by a one-half period of the PDW. The mass current distribution of the PDW phase reflects this combined symmetry, and originates from the spectra of edge Fermions and the chiral branches bound to the domain walls. Under sufficiently strong confinement a second-order transition occurs to the non-chiral ‘polar phase’ at$D_{c1}\sim 9{\xi }_0$, in which a single p-wave orbital state of Cooper pairs is aligned along the channel.

    

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