Negative conductivity of graphene with direct electric current

Problem formulating. Currently, there are no compact, efficient terahertz radiation sources operating at room temperature. To create such sources and amplifiers, structures based on graphene with DC-current can be used. Goal. Finding conditions for achieving the negative real part of graphene conductivity and amplification of THz radiation in graphene with a direct electric current. Result. It is shown that for a certain value of direct electric current in graphene, the reflection coefficient of the THz wave incident on the structure based on graphene with DC-current exceeds unity, which indicates the amplification of THz radiation in the structure. The amplification of the THz radiation in graphene is achieved due to negative values of the real part of the graphene conductivity. Practical meaning. Results can be used to create sources and amplifiers of terahertz radiation.

Dynamic Response Analysis of Long-Span Continuous Bridge Considering the Effect of Train Speeds and Earthquakes

In this paper, the dynamic response analysis of long-span continuous bridge under earthquake and train load was simultaneously performed. In order to clearly reveal the mechanism of vibration coupling between vehicles and highway long-span continuous bridge, a numerical model including soil foundation, vehicle and bridge under inclined seismic wave was established utilizing finite element software. The dynamic response of the bridge with different wave incident angles and different train speeds was numerically analyzed. The results show that the wave incident angles have a significant effect on the dynamic response of the bridge, and with the increasing of the wave incident angle, the vertical displacement and velocity as well as the acceleration of mid-span constantly increase. While the dynamic response of the bridge does not increase always with the increasing of train speed, however, at a certain train speed, the dynamic response will reach the maximum. With the increasing of the train speed, the vertical displacements of mid-span points increase while the moments at mid-span reduce significantly when the soil–structure dynamic interaction was considered. The results can provide significant references to ensure the train safe running on the bridges under the earthquake.

A bi-directional dual-bandwidth microwave absorber for applications in X and Ku bands

This paper presents a microwave absorber with dual absorption bandwidth response based upon the direction of electromagnetic wave incident on the surface. The design unit cell comprises a staircase shape metallic patch on the top plane and an array of 2×2 meander square ring shape dual layer frequency selective surfaces (FSS) in the middle and bottom planes. The relative absorption bandwidth (RAB) of 39.40% (5 GHz) with more than 90% absorption of incident wave power is achieved when an electromagnetic wave impinges normally on the top plane making it suitable for wideband applications in the X and Ku bands. For the wave incident normally on the bottom plane, the same structure gives narrow band absorption with an RAB of 2.29% (260 MHz) for more than 90% absorption around 11 GHz. Thus, this bi-directional ability of the proposed design is found to be suitable for radar absorbing material, multi-bandwidth, and diverse applications. The absorption performance is also studied for different values of incident angle. The distribution of surface currents on the staircase patch and on the two FSS layers at resonant frequencies of 11 GHz and 14 GHz is analyzed to elaborate the absorption phenomenon physically. The prototype of this design is fabricated and the experimental results are found to be closely following the simulated one.