655 research outputs found
Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles
Phosphorene, the single layer counterpart of black phosphorus, is a novel
two-dimensional semiconductor with high carrier mobility and a large
fundamental direct band gap, which has attracted tremendous interest recently.
Its potential applications in nano-electronics and thermoelectrics call for a
fundamental study of the phonon transport. Here, we calculate the intrinsic
lattice thermal conductivity of phosphorene by solving the phonon Boltzmann
transport equation (BTE) based on first-principles calculations. The thermal
conductivity of phosphorene at is
(zigzag) and
(armchair), showing an obvious anisotropy along different directions. The
calculated thermal conductivity fits perfectly to the inverse relation with
temperature when the temperature is higher than Debye temperature (). In comparison to graphene, the minor contribution around
of the ZA mode is responsible for the low thermal conductivity of
phosphorene. In addition, the representative mean free path (MFP), a critical
size for phonon transport, is also obtained.Comment: 5 pages and 6 figures, Supplemental Material available as
http://www.rsc.org/suppdata/cp/c4/c4cp04858j/c4cp04858j1.pd
The thermal and electrical properties of the promising semiconductor MXene Hf2CO2
In this work, we investigate the thermal and electrical properties of
oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes using first-principles
calculations. Hf2CO2 is found to exhibit a thermal conductivity better than
MoS2 and phosphorene. The room temperature thermal conductivity along the
armchair direction is determined to be 86.25-131.2 Wm-1K-1 with a flake length
of 5-100 um, and the corresponding value in the zigzag direction is
approximately 42% of that in the armchair direction. Other important thermal
properties of M2CO2 are also considered, including their specific heat and
thermal expansion coefficients. The theoretical room temperature thermal
expansion coefficient of Hf2CO2 is 6.094x10-6 K-1, which is lower than that of
most metals. Moreover, Hf2CO2 is determined to be a semiconductor with a band
gap of 1.657 eV and to have high and anisotropic carrier mobility. At room
temperature, the Hf2CO2 hole mobility in the armchair direction (in the zigzag
direction) is determined to be as high as 13.5x103 cm2V-1s-1 (17.6x103
cm2V-1s-1), which is comparable to that of phosphorene. Broader utilization of
Hf2CO2 as a material for nanoelectronics is likely because of its moderate band
gap, satisfactory thermal conductivity, low thermal expansion coefficient, and
excellent carrier mobility. The corresponding thermal and electrical properties
of Ti2CO2 and Zr2CO2 are also provided here for comparison. Notably, Ti2CO2
presents relatively low thermal conductivity and much higher carrier mobility
than Hf2CO2, which is an indication that Ti2CO2 may be used as an efficient
thermoelectric material.Comment: 26 pages, 5 figures, 2 table
Energy Efficiency Maximization in IRS-Aided Cell-Free Massive MIMO System
In this paper, we consider an intelligent reflecting surface (IRS)-aided
cell-free massive multiple-input multiple-output system, where the beamforming
at access points and the phase shifts at IRSs are jointly optimized to maximize
energy efficiency (EE). To solve EE maximization problem, we propose an
iterative optimization algorithm by using quadratic transform and Lagrangian
dual transform to find the optimum beamforming and phase shifts. However, the
proposed algorithm suffers from high computational complexity, which hinders
its application in some practical scenarios. Responding to this, we further
propose a deep learning based approach for joint beamforming and phase shifts
design. Specifically, a two-stage deep neural network is trained offline using
the unsupervised learning manner, which is then deployed online for the
predictions of beamforming and phase shifts. Simulation results show that
compared with the iterative optimization algorithm and the genetic algorithm,
the unsupervised learning based approach has higher EE performance and lower
running time.Comment: 6 pages, 4 figure
Diverse anisotropy of phonon transport in two-dimensional IV-VI compounds: A comparative study
New classes two-dimensional (2D) materials beyond graphene, including layered
and non-layered, and their heterostructures, are currently attracting
increasing interest due to their promising applications in nanoelectronics,
optoelectronics and clean energy, where thermal transport property is one of
the fundamental physical parameters. In this paper, we systematically
investigated the phonon transport properties of 2D orthorhombic group IV-VI
compounds of , , and by solving the Boltzmann transport
equation (BTE) based on first-principles calculations. Despite the similar
puckered (hinge-like) structure along the armchair direction as phosphorene,
the four monolayer compounds possess diverse anisotropic properties in many
aspects, such as phonon group velocity, Young's modulus and lattice thermal
conductivity (), etc. Especially, the along the zigzag and
armchair directions of monolayer shows the strongest anisotropy while
monolayer and shows an almost isotropy in phonon transport. The
origin of the diverse anisotropy is fully studied and the underlying mechanism
is discussed in detail. With limited size, the could be effectively
lowered, and the anisotropy could be effectively modulated by nanostructuring,
which would extend the applications in nanoscale thermoelectrics and thermal
management. Our study offers fundamental understanding of the anisotropic
phonon transport properties of 2D materials, and would be of significance for
further study, modulation and aplications in emerging technologies.Comment: 14 pages, 8 figures, 2 table
Polarization-based cyclic weak value metrology for angular velocity measurement
Weak value has been proved to amplify the detecting changes of the meters at
the cost of power due to post-selection. Previous power-recycling schemes
enable the failed post-selection photons to be reselected repeatedly, thus
surpassing the upper noise limit and improving the precision of interferometric
systems. Here we introduce three cyclic methods to improve the sensitivity of
polarization-based weak-value-based angular velocity measurement: power-,
signal- and dual-recycling schemes. By inserting one or two partially
transmitting mirrors inside the system, both the power and precision of
detected signals are greatly enhanced, and the dual-recycling scheme has wider
optimal region than that of power- or signal-recycling schemes. Compared to
non-polarization schemes, polarization-based schemes enjoy lower optical loss
and unique cyclic directions. These reduce the crosstalk among different paths
of light and, theoretically, eliminate the walk-off effect, thus towering in
both theoretical performance and application.Comment: 7 pages, 3 figure
Plant genetic diversity affects multiple trophic levels and trophic interactions
: Intraspecific genetic diversity is an important component of biodiversity. A substantial body of evidence has demonstrated positive effects of plant genetic diversity on plant performance. However, it has remained unclear whether plant genetic diversity generally increases plant performance by reducing the pressure of plant antagonists across trophic levels for different plant life forms, ecosystems and climatic zones. Here, we analyse 4702 effect sizes reported in 413 studies that consider effects of plant genetic diversity on trophic groups and their interactions. We found that that increasing plant genetic diversity decreased the performance of plant antagonists including invertebrate herbivores, weeds, plant-feeding nematodes and plant diseases, while increasing the performance of plants and natural enemies of herbivores. Structural equation modelling indicated that plant genetic diversity increased plant performance partly by reducing plant antagonist pressure. These results reveal that plant genetic diversity often influences multiple trophic levels in ways that enhance natural pest control in managed ecosystems and consumer control of plants in natural ecosystems for sustainable plant production
Analysis on Water Environment Management Problems in Multi-industry Towns
ABSTRACT Along with the accelerated urbanization process in China, small towns are developing rapidly and forming a multi-industry feature. While water quality of multi-industry towns is getting worse and worse at the same time. Based on the analysis of the existing problems in multi-industry towns from the perspective of water environment management, we proposed suggestions from four respects: strengthening legislation work, reconstructing management system, expanding financial resources of management and enhancing participating consciousness of resident. Our purposes were to improve water management quality in multi-industry towns and realize a coordinated and sustained development of environment and economy
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