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1. Kinetic Simulations of Instabilities and Particle Acceleration in Cylindrical Magnetized Relativistic Jets

   https://doi.org/10.3847/1538-4357/ac6acd  

   Ortuño-Macías, José; Nalewajko, Krzysztof; Uzdensky, Dmitri A.; Begelman, Mitchell C.; Werner, Gregory R.; Chen, Alexander Y.; Mishra, Bhupendra (June 2022, The Astrophysical Journal)

   Abstract Relativistic magnetized jets, such as those from AGN, GRBs, and XRBs, are susceptible to current- and pressure-driven MHD instabilities that can lead to particle acceleration and nonthermal radiation. Here, we investigate the development of these instabilities through 3D kinetic simulations of cylindrically symmetric equilibria involving toroidal magnetic fields with electron–positron pair plasma. Generalizing recent treatments by Alves et al. and Davelaar et al., we consider a range of initial structures in which the force due to toroidal magnetic field is balanced by a combination of forces due to axial magnetic field and gas pressure. We argue that the particle energy limit identified by Alves et al. is due to the finite duration of the fast magnetic dissipation phase. We find a rather minor role of electric fields parallel to the local magnetic fields in particle acceleration. In all investigated cases, a kink mode arises in the central core region with a growth timescale consistent with the predictions of linearized MHD models. In the case of a gas-pressure-balanced (Z-pinch) profile, we identify a weak local pinch mode well outside the jet core. We argue that pressure-driven modes are important for relativistic jets, in regions where sufficient gas pressure is produced by other dissipation mechanisms. 

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2. Spectroscopic Observation and Modeling of Photonic Modes in CeO2 Nanostructures

   https://doi.org/10.1093/micmic/ozad059  

   Wang, Yifan; Yang, Shize; Crozier, Peter A (July 2023, Microscopy and Microanalysis)

   Abstract Photonic modes in dielectric nanostructures, e.g., wide gap semiconductor like CeO2 (ceria), have the potential for various applications such as information transmission and sensing technology. To fully understand the properties of such phenomenon at the nanoscale, electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope was employed to detect and explore photonic modes in well-defined ceria nanocubes. To facilitate the interpretation of the observations, EELS simulations were performed with finite-element methods. The simulations allow the electric and magnetic field distributions associated with different modes to be determined. A simple analytical eigenfunction model was also used to estimate the energy of the photonic modes. In addition, by comparing various spectra taken at different location relative to the cube, the effect of the surrounding environment on the modes could be sensed. This work gives a high-resolution description of the photonic modes' properties in nanostructures, while demonstrating the advantage of EELS in characterizing optical phenomena locally. 

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3. High-resolution particle-in-cell simulations of two-dimensional Bernstein–Greene–Kruskal modes

   https://doi.org/10.1063/5.0187853  

   McClung, J.; Franciscovich, M_T; Germaschewski, K.; Ng, C_S (April 2024, Physics of Plasmas)

   We present two-dimensional (2D) particle-in-cell (PIC) simulations of 2D Bernstein–Greene–Kruskal modes, which are exact nonlinear steady-state solutions of the Vlasov–Poisson equations, on a 2D plane perpendicular to a background magnetic field, with a cylindrically symmetric electric potential localized on the plane. PIC simulations are initialized using analytic electron distributions and electric potentials from the theory. We confirm the validity of such solutions using high-resolutions up to a 20482 grid. We show that the solutions are dynamically stable for a stronger background magnetic field, while keeping other parameters of the model fixed, but become unstable when the field strength is weaker than a certain value. When a mode becomes unstable, we observe that the instability begins with the excitation of azimuthal electrostatic waves that ends with a spiral pattern. 

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4. Diocotron modes in pure electron plasmas in the APEX levitating dipole trap

   https://doi.org/10.1088/1361-6587/ad9e70  

   Deller, A.; Bayer, V_C; Steinbrunner, P.; Card, A.; Danielson, J_R; Stoneking, M_R; Stenson, E_V (December 2024, Plasma Physics and Controlled Fusion)

   Abstract Observations of persistent toroidal modes in dipole-trapped pure electron plasmas are presented. Non-neutral electron plasmas were confined by the poloidal magnetic field of the APEX (A Positron Electron eXperiment) levitating dipole trap. The negative bias of the electron emitter was gated to control the filling time. Large amplitude, narrow-band oscillations were consistently observed in wall probe measurements after the fill. The oscillatory image-charge currents induced by the trapped electron plasma typically persisted for many seconds with fundamental frequencies,f₀ = 50 to 500 kHz, that scale with the $\overrightarrow{E}\times \overrightarrow{B}$drift velocity. We attribute this feature to the dipole-equivalent of the diocotron mode that is commonly observed in non-neutral plasmas in linear traps. 

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5. Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment, and valley spin

   https://doi.org/10.1126/sciadv.aax7407  

   Hsu, Wei-Ting; Lin, Bo-Han; Lu, Li-Syuan; Lee, Ming-Hao; Chu, Ming-Wen; Li, Lain-Jong; Yao, Wang; Chang, Wen-Hao; Shih, Chih-Kang (December 2019, Science Advances)

   Excitons in monolayer semiconductors have a large optical transition dipole for strong coupling with light. Interlayer excitons in heterobilayers feature a large electric dipole that enables strong coupling with an electric field and exciton-exciton interaction at the cost of a small optical dipole. We demonstrate the ability to create a new class of excitons in hetero- and homobilayers that combines advantages of monolayer and interlayer excitons, i.e., featuring both large optical and electric dipoles. These excitons consist of an electron confined in an individual layer, and a hole extended in both layers, where the carrier-species–dependent layer hybridization can be controlled through rotational, translational, band offset, and valley-spin degrees of freedom. We observe different species of layer-hybridized valley excitons, which can be used for realizing strongly interacting polaritonic gases and optical quantum controls of bidirectional interlayer carrier transfer. 

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