石墨烯力常数和弹性波波速随温度变化规律研究

在哈里森键联轨道法框架下,考虑到原子的短程相互作用和原子的非简谐振动,建立石墨烯弹性模型并求出原子振动的简谐系数和非简谐系数。在此基础上,确定了石墨烯的力常数和弹性波波速随温度和形变势参量的变化关系式,探讨了原子非简谐振动对它们的影响。结果表明：（1）石墨烯的力常数和弹性波波速均随温度升高而非线性减小,但有不同的变化特征：在C11、C12、C44这三个力常数中,以C12随温度的变化最大,以C44的变化最小;纵波波速随温度的变化要大于横波;（2）石墨烯的三个力常数随形变势参量的变化有不同的特征：C11随η和γ的增大而增大、而C12则随η的增大而减小,随γ的增大而增大;C44随η的增大而增大,而几乎不随γ而变;（3）若不考虑原子振动的非简谐项,则石墨烯的力常数和弹性波波速均为常数。考虑到非简谐项后,不仅它们均随温度的升高而减小,而且,非简谐情况的力常数Cij和弹性波波速vi与简谐近似的值Cij0、vi0的差|Cij－Cij0|、|vi－vi0|均随温度的升高而增大,即温度愈高,非简谐效应愈显著。     

石墨烯力常数和弹性波波速随温度变化规律研究

在哈里森键联轨道法框架下,考虑到原子的短程相互作用和原子的非简谐振动,建立石墨烯弹性模型并求出原子振动的简谐系数和非简谐系数。在此基础上,确定了石墨烯的力常数和弹性波波速随温度和形变势参量的变化关系式,探讨了原子非简谐振动对它们的影响。结果表明：（1）石墨烯的力常数和弹性波波速均随温度升高而非线性减小,但有不同的变化特征：在C11、C12、C44这三个力常数中,以C12随温度的变化最大,以C44的变化最小;纵波波速随温度的变化要大于横波;（2）石墨烯的三个力常数随形变势参量的变化有不同的特征：C11随η和γ的增大而增大、而C12则随η的增大而减小,随γ的增大而增大;C44随η的增大而增大,而几乎不随γ而变;（3）若不考虑原子振动的非简谐项,则石墨烯的力常数和弹性波波速均为常数。考虑到非简谐项后,不仅它们均随温度的升高而减小,而且,非简谐情况的力常数Cij和弹性波波速vi与简谐近似的值Cij0、vi0的差|Cij－Cij0|、|vi－vi0|均随温度的升高而增大,即温度愈高,非简谐效应愈显著。     

非简谐振动和形变对ZnSe类石墨烯德拜温度热容量以及极性的影响

应用哈里森(Harrison)键联轨道法和固体物理理论和方法,考虑到原子的高阶非简谐振动,计算了ZnSe类石墨烯化合物的σ键和π键的极性、简谐系数和非简谐系数,得到它的德拜温度和定容热容量随温度变化的解析式以及形变引起的极性参量的改变量,探讨了形变对ZnSe类石墨烯极性的影响,并讨论了原子非简谐振动对化合物热力学性质的影响,结果表明:在300 K到1600 K温度区间内,简谐近似下,ZnSe的德拜温度为常量,热容量随着温度的升高呈非线性增大,其中温度较低时变化较快,温度较高时变化较慢并随着温度的升高而趋于常量.考虑到非简谐项后,德拜温度随着温度升高而增大,几乎为正比关系,而热容量的值比简谐近似的值有所减小,与第一非简谐项的影响相比,第二非简谐项的影响很小.形变对ZnSe类石墨烯的极性有重要的影响,在它的几种形变中,剪切形变对σ键的极性的影响最大,而键长形变对π键的极性的影响最小.温度愈高,非简谐效应愈显著.     

石墨烯低温热膨胀和声子弛豫时间随温度的变化规律

考虑到原子非简谐振动和电子-声子相互作用,用固体物理理论和方法研究了石墨烯格林艾森参量和低温热膨胀系数以及声子弛豫时间随温度的变化规律,探讨了原子非简谐振动项对它们的影响.结果表明:1)在低于室温的温度范围内,石墨烯的热膨胀系数为负值,随着温度的升高,其热膨胀系数的绝对值单调增加,室温热膨胀系数为-3.64×10~(-6)K~(-1);2)简谐近似下的格林艾森参量为零.考虑到非简谐项后,格林艾森参量在1.40-1.42之间并随温度升高而缓慢增大,几乎成线性关系,第二非简谐项对格林艾森参量的影响小于第一非简谐项;3)石墨烯声子弛豫时间随着温度的升高而减小,其中,温度很低(T10 K)时变化很快,此后变化很慢,当温度不太低(T300 K)时,声子弛豫时间与温度几乎成反比关系.     

外延石墨烯电导率和费米速度随温度变化规律研究

考虑到原子的非简谐振动和电子-声子相互作用,建立了金属基外延石墨烯的物理模型,用固体物理理论和方法,得到金属基外延石墨烯的电导率和费米速度随温度变化的解析式.以碱金属基底为例,探讨了基底材料和非简谐振动对外延石墨烯电导率和费米速度的影响.结果表明:1)零温情况下,碱金属基外延石墨烯的电导率和费米速度均随基底元素原子序数的增大而增大;2)外延石墨烯的电导率随温度升高而减小,其中,温度较低时时变化较快,而温度较高时则变化很慢,费米速度随温度升高而增大,其变化率随基底材料原子序数的增大而增大;3)原子非简谐振动对外延石墨烯的电导率和费米速度有重要的影响,简谐近似下,费米速度为常数,电导率的温度变化率较大;考虑到原子非简谐项后,费米速度随温度升高而增大,电导率的温度变化率减小;温度愈高,原子振动的非简谐效应愈明显.     

非简谐振动对石墨烯杨氏模量与声子频率的影响

在哈里森键联轨道法框架下, 考虑到原子的短程相互作用和原子的非简谐振动, 应用固体物理理论和方法, 得到了石墨烯的力常数、杨氏模量、扭曲模量、泊松系数以及声子频率随温度的变化关系, 探讨了非简谐振动对它们的影响. 结果表明: 1)杨氏模量与声子频率等随温度变化并遵从一定的规律, 其中力常数、杨氏模量、扭曲模量随温度升高而增大, 但变化较小; 声子频率随温度升高而增大但变化较快; 泊松系数随温度升高而较快地减小; 2)石墨烯原子具有沿键长方向的纵振动和垂直键长方向的横振动, 但以纵振动为主, 纵振动的非简谐效应远大于横振动, 横振动的简谐系数ε₀' 和第二非谐系数ε₂' 均小于纵振动的相应值ε₀,ε₂; 比值为ε₀/ε₀' ≈ 8.477,ε₂/ε₂' ≈ 156; 3)若不考虑非简谐振动项, 则石墨烯的力常数、杨氏模量和扭曲模量、泊松系数、声子频率均为常量, 与实验不符合; 同时考虑到原子的第一、二非简谐振动项后, 它们均随温度升高而变化, 而且温度愈高, 原子振动的非简谐效应愈显著. 本文的结果与文献的实验结果符合较好.     

非简谐振动对SiC的热膨胀系数和介电性能的影响

利用抛物型电子能谱模型,考虑到原子的非简谐振动,求出了SiC中原子振动的简谐系数与非简谐系数,用固体物理理论和方法,得到了SiC的热膨胀系数和格林乃森参量以及介电常数随温度变化的解析式,探讨了原子非简谐振动的影响.结果表明:SiC的格林乃森参量和热膨胀系数均随温度升高而非线性增大,而介电常数随温度升高而非线性减小;原子振动的非简谐项(特别是第二非简谐项)对SiC的热膨胀等热学性质和介电性能有重要影响,温度愈高,非简谐振动项的影响愈大.     

非简谐振动对SiC的热膨胀系数数和介电性能的影响

利用抛物型电子能谱模型,考虑到原子的非简谐振动,求出了SiC中原子振动的简谐系数与非简谐系数,用固体物理理论和方法,得到了SiC的热膨胀系数和格林乃森参量以及介电常数随温度变化的解析式,探讨了原子非简谐振动对的影响。结果表明：的格林乃森参量和热膨胀系数均随温度升高而非线性增大,而介电常数随温度升高而非线性减小;原子振动的非简谐项（特别是第二非简谐项）对的热膨胀等热学性质和介电性能有重要影响,温度愈高,非简谐振动项的影响愈大。     

非简谐振动对SiC类石墨烯热输运性质的影响

考虑到原子的非简谐振动,应用固体物理理论和方法,计算了SiC类石墨烯的简谐系数和非简谐系数,得到它的德拜温度、热容量和热导率等随温度的变化规律,探讨了原子非简谐振动对它的热输运性质的影响.结果表明:SiC类石墨烯的德拜温度随温度的升高而在117-126 K之间线性增大,定容比热随温度升高而非线性增大,热导率随温度升高而非线性减小,温度较低时变化较快,而温度较高时变化较慢,并随着温度升高而趋于常量;考虑到非简谐振动后,SiC类石墨烯的德拜温度、定容比热和热导率的值分别大于、小于和大于简谐近似的相应值,温度愈高,其差值愈大,即温度愈高,非简谐效应的影响愈显著;二维平面状的SiC类石墨烯的定容比热和热导率随温度的变化规律,与三维块状SiC晶体总体趋势相同,只是具体数值不同.     

非简谐振动对SiC类石墨烯热输运性质的影响

考虑到原子的非简谐振动,应用固体物理理论和方法,计算了SiC类石墨烯的简谐系数和非简谐系数,得到它的德拜温度、热容量和热导率等随温度的变化规律,探讨了原子非简谐振动对它的热输运性质的影响.结果表明:SiC类石墨烯的德拜温度随温度的升高而在117-126 K之间线性增大,定容比热随温度升高而非线性增大,热导率随温度升高而非线性减小,温度较低时变化较快,而温度较高时变化较慢,并随着温度升高而趋于常量;考虑到非简谐振动后,SiC类石墨烯的德拜温度、定容比热和热导率的值分别大于、小于和大于简谐近似的相应值,温度愈高,其差值愈大,即温度愈高,非简谐效应的影响愈显著;二维平面状的SiC类石墨烯的定容比热和热导率随温度的变化规律,与三维块状SiC晶体总体趋势相同,只是具体数值不同.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.