Jun 2020, Volume 15 Issue 3

Cover illustration

  • Single atom catalysts (SACs) show their unique advantages in various catalytic reactions. Graphitic carbon nitride (g-C3N4) is not only an excellent supporting material for single atom, but also an excellent photocatalyst. g-C3N4 based single-atom photocatalysts, due to their high catalysis activity, selectivity, and stability, become a hotspot in the field of photocatalysis. The preparation strategies, characterizations, and photoc [Detail] ...

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    Michael L. Goodman, Chiman Kwan, Bulent Ayhan, Eric L. Shang

    All three components of the current density are required to compute the heating rate due to free magnetic energy dissipation. Here we present a first test of a new model developed to determine if the times of increases in the resistive heating rate in active region (AR) photospheres are correlated with the subsequent occurrence of M and X flares in the corona. A data driven, 3D, non-force-free magnetohydrodynamic model restricted to the near-photospheric region is used to compute time series of the complete current density and the resistive heating rate per unit volume [Q(t)] in each pixel in neutral line regions (NLRs) of 14 ARs. The model is driven by time series of the magnetic field B measured by the Helioseismic & Magnetic Imager on the Solar Dynamics Observatory (SDO) satellite. Spurious Doppler periods due to SDO orbital motion are filtered out of the time series for B in every AR pixel. For each AR, the cumulative distribution function (CDF) of the values of the NLR area integral Qi(t) of Q(t) is found to be a scale invariant power law distribution essentially identical to the observed CDF for the total energy released in coronal flares. This suggests that coronal flares and the photospheric Qiare correlated, and powered by the same process. The model predicts spikes in Qiwith values orders of magnitude above background values. These spikes are driven by spikes in the non-force free component of the current density. The times of these spikes are plausibly correlated with times of subsequent M or X flares a few hours to a few days later. The spikes occur on granulation scales, and may be signatures of heating in horizontal current sheets. It is also found that the times of relatively large values of the rate of change of the NLR unsigned magnetic flux are also plausibly correlated with the times of subsequent M and X flares, and spikes in Qi.

    X.-J. Hao, R.-Y. Yuan, J.-J. Jin, Y. Guo

    Using the transfer matrix method, spin- and valley-dependent electron transport properties modulated by the velocity barrier were studied in the normal/ferromagnetic/normal monolayer MoS2 quantum structure. Based on Snell’s Law in optics, we define the velocity barrier as ξ=v2/v1 by changing the Fermi velocity of the intermediate ferromagnetic region to obtain a deflection condition during the electron transport process in the structure. The results show that both the magnitude and the direction of spin- and valley-dependent electron polarization can be regulated by the velocity barrier. –100% polarization of spin- and valley-dependent electron can be achieved for ξ>1, while 100% polarization can be obtained for ξ<1. Furthermore, it is determined that perfect spin and valley transport always occur at a large incident angle. In addition, the spin- and valley-dependent electron transport considerably depends on the length kFL and the gate voltage U(x) of the intermediate ferromagnetic region. These findings provide an effective method for designing novel spin and valley electronic devices.

    Yuan Gan, Jiyuan Liang, Chang-woo Cho, Si Li, Yanping Guo, Xiaoming Ma, Xuefeng Wu, Jinsheng Wen, Xu Du, Mingquan He, Chang Liu, Shengyuan A. Yang, Kedong Wang, Liyuan Zhang

    Recently, the layered transition metal dichalcogenide 1T′-MoTe2 has generated considerable interest due to their superconducting and non-trivial topological properties. Here, we present a systematic study on 1T′-MoTe2 single-crystal and exfoliated thin-flakes by means of electrical transport, scanning tunnelling microscope (STM) measurements and band structure calculations. For a bulk sample, it exhibits large magneto-resistance (MR) and Shubnikov–de Hass oscillations in ρxx and a series of Hall plateaus in ρxy at low temperatures. Meanwhile, the MoTe2 thin films were intensively investigated with thickness dependence. For samples, without encapsulation, an apparent transition from the intrinsic metallic to insulating state is observed by reducing thickness. In such thin films, we also observed a suppression of the MR and weak anti-localization (WAL) effects. We attributed these effects to disorders originated from the extrinsic surface chemical reaction, which is consistent with the density functional theory (DFT) calculations and in-situ STM results. In contrast to samples without encapsulated protection, we discovered an interesting superconducting transition for those samples with hexagonal Boron Nitride (h-BN) film protection. Our results indicate that the metallic or superconducting behavior is its intrinsic state, and the insulating behavior is likely caused by surface oxidation in few layer 1T′-MoTe2 flakes.

    Qian Shen, Yun You, Jie Xu, Yun Shen, Xiaohua Deng, Zhuoyuan Wang, Weidong Min, Linfang Shen, Sanshui Xiao

    We propose a uniform backfire-to-endfire leaky-wave antenna (LWA) based on a topological one-way waveguide under external bias magnetic field. We systematically analyze the dispersion, showing that the proposed structure supports leaky mode arisen from total internal reflection. By means of tuning frequency or magnetic field, we obtain fixed-bias frequency and fixed-frequency bias LWA with continuous beam scanning from backward, broadside to forward direction. More importantly, we, for the first time, demonstrate that this proposed LWA shows mechanical tunability, allowing us to manipulate the radiation direction from backward, broadside to forward direction by mechanically tuning the air layer thickness. The simulated results show that our system exhibits super low 3dB beam width, high radiation efficiency as well as high antenna gain. Being provided such multiple controlled (especially mechanically) beam scanning manners, the present LWA paves an advanced approach for continuous beam scanning, holding a great potential for applications in modern communication and radar system.

    Xiao-Ming Huang, Li-Zhao Liu, Si Zhou, Ji-Jun Zhao

    Graphene oxide (GO), the functionalized graphene with oxygenated groups (mainly epoxy and hydroxyl), has attracted resurgent interests in the past decade owing to its large surface area, superior physical and chemical properties, and easy composition with other materials via surface functional groups. Usually, GO is used as an important raw material for mass production of graphene via reduction. However, under different conditions, the coverage, types, and arrangements of oxygen-containing groups in GO can be varied, which give rise to excellent and controllable physical properties, such as tunable electronic and mechanical properties depending closely on oxidation degree, suppressed thermal conductivity, optical transparency and fluorescence, and nonlinear optical properties. Based on these outstanding properties, many electronic, optical, optoelectronic, and thermoelectric devices with high performance can be achieved on the basis of GO. Here we present a comprehensive review on recent progress of GO, focusing on the atomic structures, fundamental physical properties, and related device applications, including transparent and flexible conductors, field-effect transistors, electrical and optical sensors, fluorescence quenchers, optical limiters and absorbers, surface enhanced Raman scattering detectors, solar cells, light-emitting diodes, and thermal rectifiers.

    Junwei Fu (傅俊伟), Shuandi Wang (王栓娣), Zihua Wang (王自华), Kang Liu (刘康), Huangjingwei Li (李黄经纬), Hui Liu (刘恢), Junhua Hu (胡俊华), Xiaowen Xu (徐效文), Hongmei Li (李红梅), Min Liu (刘敏)

    Single-atom photocatalysts, due to their high catalysis activity, selectivity and stability, become a hotspot in the field of photocatalysis. Graphitic carbon nitride (g-C3N4) is known as both a good support for single atoms and a star photocatalyst. Developing g-C3N4-based single-atom photocatalysts exhibits great potential in improving the photocatalytic performance. In this review, we summarize the recent progress in g-C3N4-based single-atom photocatalysts, mainly including preparation strategies, characterizations, and their photocatalytic applications. The significant roles of single atoms and catalysis mechanism in g-C3N4-based single-atom photocatalysts are analyzed. At last, the challenges and perspectives for exploring high-efficient g-C3N4-based single-atom photocatalysts are presented.

    Xiao-Dong Wu, Yi-Jun Wang, Duan Huang, Ying Guo

    We propose a novel scheme for measurement-device-independent (MDI) continuous-variable quantum key distribution (CVQKD) by simultaneously conducting classical communication and QKD, which is called “simultaneous MDI-CVQKD” protocol. In such protocol, each sender (Alice, Bob) can superimpose random numbers for QKD on classical information by taking advantage of the same weak coherent pulse and an untrusted third party (Charlie) decodes it by using the same coherent detectors, which could be appealing in practice due to that multiple purposes can be realized by employing only single communication system. What is more, the proposed protocol is MDI, which is immune to all possible side-channel attacks on practical detectors. Security results illustrate that the simultaneous MDI-CVQKD protocol can secure against arbitrary collective attacks. In addition, we employ phasesensitive optical amplifiers to compensate the imperfection existing in practical detectors. With this technology, even common practical detectors can be used for detection through choosing a suitable optical amplifier gain. Furthermore, we also take the finite-size effect into consideration and show that the whole raw keys can be taken advantage of to generate the final secret key instead of sacrificing part of them for parameter estimation. Therefore, an enhanced performance of the simultaneous MDI-CVQKD protocol can be obtained in finite-size regime.