Four metallic metamaterials with tailorable mechanical properties are designed using bimaterial star-shaped re-entrant planar lattice structures, which do not involve pins, adhesive, welding or pressure-fit joints and can be fabricated through laser-based additive manufacturing. Three length parameters, one angle parameter and three material combinations are used as adjustable design parameters to explore structure-property relations. For each of the four designed metamaterials, the effects of the design parameters on the Poisson's ratio (PR), coefficient of thermal expansion (CTE), Young's modulus and relative density are systematically investigated using unit cell-based finite element simulations that incorporate periodic boundary conditions. It is found that the bi-material lattice structures can be tailored to obtain 3-D printable metallic metamaterials with positive, near-zero or negative PR and CTE together with an uncompromised Young's modulus. In particular, it is shown that metamaterial # 1 can exhibit both a negative PR and a non-positive CTE simultaneously. These metallic metamaterials can find applications in structures or devices such as antennas and precision instruments to reduce thermomechanical stresses and extend service lives.
We present a detailed Monte Carlo simulation of electron transport incorporating both ⌫and X-valley states in GaAs-based quantum cascade lasers ͑QCLs͒. ⌫ states are calculated using the K • p method, while X states are obtained within the effective mass framework. All the relevant electron-phonon, electron-electron, and intervalley scattering mechanisms are included. We investigate the X-valley leakage in two equivalent-design GaAs/AlGaAs QCLs with 33% and 45% Al-barrier compositions. We find that the dominant X-valley leakage path in both laser structures is through interstage X → X intervalley scattering, leading to a parallel leakage current J X. The magnitude of J X depends on the temperature and occupation of the X subbands, which are populated primarily by the same-stage scattering from the ⌫-continuum ͑⌫ c ͒ states. At 77 K, J X is small up to very high fields in both QCLs. However, at room temperature the 33% QCL shows a much higher J X than the 45% QCL even at low fields. The reason is that in the 33% QCL the coupling between the ⌫-localized ͑⌫ l ͒ states and the next-stage ⌫ c states is strong, which facilitates subsequent filling of the X states through efficient intrastage ⌫ c → X scattering; with high X-valley population and high temperature, efficient interstage X → X scattering yields a large J X. In contrast, good localization of the ⌫ l states in the 45% QCL ultimately leads to low X-valley leakage current up to high fields. Very good agreement with experiment is obtained at both cryogenic and room temperatures.
We predict that within next 15 years a fundamental down-scaling limit for CMOS technology and other Field-Effect Transistors (FETs) will be reached. Specifically, we show that at room temperatures all FETs, irrespective of their channel material, will start experiencing unacceptable level of thermally induced errors around 5-nm gate lengths. These findings were confirmed by performing quantum mechanical transport simulations for a variety of 6-, 5-, and 4-nm gate length Si devices, optimized to satisfy high-performance logic specifications by ITRS. Different channel materials and wafer/channel orientations have also been studied; it is found that altering channel-source-drain materials achieves only insignificant increase in switching energy, which overall cannot sufficiently delay the approaching downscaling limit. Alternative possibilities are discussed to continue the increase of logic element densities for room temperature operation below the said limit.
The authors present a Monte Carlo simulation of GaAs/ Al 0.33 Ga 0.67 As and GaAs/ Al 0.45 Ga 0.55 As quantum cascade lasers ͑QCLs͒ that incorporates both ⌫and X-valley transport. The dominant X-valley leakage path in both lasers is interstage X → X scattering. The leakage current is much higher in the 33%-Al QCL, as strong coupling of its weakly localized ⌫-valley states to the next-stage continuum ⌫ states ͑⌫ c ͒, followed by strong same-stage ⌫ c → X scattering, ensures high X-valley population and subsequent high X → X leakage current at 300 K, even at low fields. Very good agreement with experiment is obtained at both cryogenic and room temperatures.
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