The latest process and challenges of microwave dielectric ceramics based on pseudo phase diagrams

The explosive process of 5G communication evokes the urgent demand of miniaturized and integrated dielectric ceramics filter. It is a pressing need to advance the development of dielectric ceramics utilization of emerging technology to design new materials and understand the polarization mechanism. This review provides the summary of the study of microwave dielectric ceramics (MWDCs) sintered higher than 1000 from 2010 up to now, °C with the purpose of taking a broad and historical view of these ceramics and illustrating research directions. To date, researchers endeavor to explain the structure-property relationship of ceramics with multitude of approaches and design a new formula or strategy to obtain excellent microwave dielectric properties. There are variety of factors that impact the permittivity, dielectric loss, and temperature stability of dielectric materials, covering intrinsic and extrinsic factors. Many of these factors are often intertwined, which can complicate new dielectric material discovery and the mechanism investigation. Because of the various ceramics systems, pseudo phase diagram was used to classify the dielectric materials based on the composition. In this review, the ceramics were firstly divided into ternary systems, and then brief description of the experimental probes and complementary theoretical methods that have been used to discern the intrinsic polarization mechanisms and the origin of intrinsic loss was mentioned. Finally, some perspectives on the future outlook for high-temperature MWDCs were offered based on the synthesis method, characterization techniques, and significant theory developments.

Compact artificial neuron based on anti-ferroelectric transistor

Neuromorphic machines based on spiking neural networks (SNNs) provide a more fascinating platform over traditional computers on building energy-efficient intelligent systems, in which spiking neuron are pivotal components. Recently, memristive neurons, with promising bio-plausibility and density, have been developed, but with limited reliability or bulky capacitors for integration or additional circuits for reset. Here, we propose a novel anti-ferroelectric field-effect transistor (AFeFET) neuron based on the inherent polarization and depolarization of Hf0.2Zr0.8O2 anti-ferroelectric film to meet these challenges. In this neuron, the intrinsic polarization accumulation effect in the Hf0.2Zr0.8O2 film increases the channel current of AFeFET gradually, which implements the integration feature of the neuronal membrane and avoids using external capacitors. Also, the spontaneous depolarization effect in AFeFET emulates the leaky behavior of neurons, saving the hardware overhead of neuron circuits by getting rid of external reset circuits. Moreover, the AFeFET neuron exhibits other comprehensive merits, such as low energy consumption (37 fJ/spike), excellent endurance (>1012), high uniformity and high stability. Using such an AFeFET neuron, we further construct a two-layer fully ferroelectric (784×400×10) SNN combining established FeFET synapse, achieving 96.8% recognition accuracy on MNIST datasets. This work opens the way to emulate spiking neurons with anti-ferroelectric materials and provides a more competitive approach to build high efficient neuromorphic hardware systems.

High-Frequency Polarization Variability from Active Galactic Nuclei

The linear polarization of non-thermal emission encodes information about the structure of the magnetic fields, either from the region where the emission is produced (i.e., the intrinsic polarization angle) and/or from the screens of magnetized plasma that may be located on its way towards Earth (i.e., the effect of Faraday rotation). In addition, the variability timescale of the polarized emission, or its Faraday rotation, can be used to estimate the size of the region where the emission (or the Faraday rotation) originates. The observation of polarized emission from active galactic nuclei (AGN) and, in particular, its time evolution, also provides information about the critical role that magnetic fields may play in the process of jet launching and propagation. In this paper, we review some recent results about polarization variability from the cores of AGN jets, including observations at high spatial resolutions and/or at high radio frequencies.

Constraints on Lorentz Invariance Violation with Multiwavelength Polarized Astrophysical Sources

Possible violations of Lorentz invariance (LIV) can produce vacuum birefringence, which results in a frequency-dependent rotation of the polarization plane of linearly polarized light from distant sources. In this paper, we try to search for a frequency-dependent change of the linear polarization angle arising from vacuum birefringence in the spectropolarimetric data of astrophysical sources. We collect five blazars with multiwavelength polarization measurements in different optical bands (UBVRI). Taking into account the observed polarization angle contributions from both the intrinsic polarization angle and the rotation angle induced by LIV, and assuming that the intrinsic polarization angle is an unknown constant, we obtain new constraints on LIV by directly fitting the multiwavelength polarimetric data of the five blazars. Here, we show that the birefringence parameter η quantifying the broken degree of Lorentz invariance is limited to be in the range of −9.63×10−8<η<6.55×10−6 at the 2σ confidence level, which is as good as or represents one order of magnitude improvement over the results previously obtained from ultraviolet/optical polarization observations. Much stronger limits can be obtained by future multiwavelength observations in the gamma-ray energy band.

Coherence scale of magnetic fields generated in early time forward shocks of Gamma Ray Bursts

We report the earliest-ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured 129.5 − 204.3 s after the burst using the multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from 86 s to ∼2200 s post-burst is typical of classical forward shocks with α = 1.091 ± 0.008. The low optical polarization $P_{BV} = 2.8 _{- 1.6} ^{+ 2.0} \, \%$ (2σ) at mean time ∼168 s post-burst is compatible with being induced by the host galaxy dust (AV, HG = 0.71 ± 0.15 mag), leaving low polarization intrinsic to the GRB emission itself —as theoretically predicted for forward shocks and consistent with previous detections of low degrees of optical polarization in GRB afterglows observed hours to days after the burst. The current sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P \sim 1\%-3\%$), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. $P \le 2\%$), which depends on the magnetic field coherence length scale and the size of the observable emitting region (burst energetics, circumburst density).