A New Exponential Gravity

We propose a new exponential f（R） gravity model with f（R） = （R - λc） e^λ（c/R）n and n 〉 3, λ ≥ 1, c 〉 0 to explain late-time acceleration of the universe. At the high curvature region, the model behaves like the A CDM model. In the asymptotic future, it reaches a stable de-Sitter spaeetime. It is a cosmologically viable model and can evade the local gravity constraints easily. This model shares many features with other f（R） dark energy models like Hu-Sawicki model and ExponentiM gravity model. In it the dark energy equation of state is of an oscillating form and can cross phantom divide line ωde = -1. In particular, in the parameter range 3 〈 n ≤ 4, λ ～ 1, the model is most distinguishable from other models. For instance, when n = 4, λ = 1, the dark energy equation of state will cross -1 in the earlier future and has a stronger oscillating form than the other models, the dark energy density in asymptotical future is smaller than the one in the high curvature region. This new model can evade the local gravity tests easily when n 〉 3 and λ 〉 1.     

Cold-Fluid Analysis of the Space-Charge Effects in an Electromagnetically Pumped Free-Electron Laser with an Axial-Guide Magnetic Field

The space-charge effects are analysed and five kinds of fluid waves are obtained in the e-beam density and the axial-current density for an electromagnetically pumped free-electron laser with an axial-guide magnetic field by using the cold-fluid theory. To enhance the gain, the guide field Bo must be in this regime: -2(ω_υ + cβ_x0k_ω) < Ω < -(ω_υ + cβ_x0k_ω), where Ω₀ = eB₀/mr₀ and ω_υ(k_ω) is the frequency (wave number) of the wiggler.     

Effects of a Time-Varying Color-Luminosity Parameter β on the Cosmological Constraints of Modified Gravity Models

It has been found that, for the Supernova Legacy Survey three-year (SNLS3) data, there is strong evidence for the redshift-evolution of color-luminosity parameter β. In previous studies, only dark energy (DE) models are used to explore the effects of a time-varying β on parameter estimation. In this paper, we extend the discussions to the case of modified gravity (MG), by considering Dvali-Gabadadze-Porrati (DGP) model, power-law type f(T) model and exponential type f(T) model. In addition to the SNLS3 data, we also use the latest Planck distance priors data, the galaxy clustering (GC) data extracted from Sloan Digital Sky Survey (SDSS) data release 7 (DR7) and Baryon Oscillation Spectroscopic Survey (BOSS), as well as the direct measurement of Hubble constant H₀ from the Hubble Space Telescope (HST) observation. We find that, for both cases of using the supernova (SN) data alone and using the combination of all data, adding a parameter of β can reduce χ² by ～ 36 for all the MG models, showing that a constant β is ruled out at 6σ confidence level (CL). Moreover, we find that a time-varying β always yields a larger fractional matter density Ω_m0 and a smaller reduced Hubble constant h; in addition, it significantly changes the shapes of 1σ and 2σ confidence regions of various MG models, and thus corrects systematic bias for the parameter estimation. These conclusions are consistent with the results of DE models, showing that β's evolution is completely independent of the cosmological models in the background. Therefore, our work highlights the importance of considering the evolution of β in the cosmology-fits.     

激光在大气中驱动的强太赫兹辐射的理论和实验研究

两束双色激光脉冲能在大气中产生MV/cm的强太赫兹波.本文主要介绍了我们最近的三项理论和实验工作,澄清了双色激光方案的物理机制这个长期存在的问题,并对该方案进行了推广.为了在气体中有效地产生太赫兹波,在广泛研究的双色激光方案中两束激光的频率比ω_2/ω_1总是被取为1:2.首先从理论上预测采用其他频率比时,此方案仍能有效地工作,并通过实验进行证实.实验上观察到在新的频率比ω_2/ω_1=1:4,2:3下,也能有效地产生太赫兹波;观察到通过旋转较长波长的激光脉冲的偏振方向,能够有效地调节太赫兹波的偏振,但是旋转波长较短的激光脉冲的偏振方向,太赫兹波的偏振几乎没有变化,这违背了多波混频理论中极化率张量对称性的要求;采用不同的频率比时,太赫兹能量定标率并没有显示出明显的区别,这与多波混频理论预测的能量定标率不符.这些实验结果与等离子体电流模型及粒子模拟结果符合得很好.因此,该研究不仅对双色激光方案进行了推广,而且证实了其物理机制应该归结为等离子体电流模型.     

Dynamical Behavior of the Quant um Periodically Kicked Harmonic Oscillator

We explore the properties of the quantum kicked harmonic osciUation governed by the Hamiltonian H=1/2p²+1/2ω₀²x²-K cos x∑_n=-∞^∞δ(t/T - n): The classical version of this model is known to exhibit certain type of stochastic behavior. We reduce the quantum equation of the model td quantum mapping equations. Numerical results of the mappings show that the behavior of the average energy as a function of time depends on the ration of the frequency of the external (2π/T) and that of the unperturbed system wo. The energy increases first very rapidly and then the energy growth saturates for irrational ω₀T/2π. The break time is independent of the strength of the external force, but is roughly proportional to 1/ω₀T.For a fixed rational ω₀T/2πt,h e energy is damped oscillation for smaller T , a nd it is seemingly recurrent with time in the sense of Hogg and Huberman. In both cases, these behaviors of energy are independent of the strength of the external force and differ from the behavior of the quantum kicked rotator.     

Third-Order Optical Nonlinearity in Two-Dimensional Transition Metal Dichalcogenides

We present a detailed calculation of the linear and nonlinear optical response of four types of monolayer twodimensional (2D) transition-metal dichalcogenides (TMDCs), having the formula MX₂ with M=Mo, W and X=S, Se. The calculations are based on 6-band tight-binding model of TMDCs, and then performing a semi-classical perturbation analysis of response functions. We numerically calculate the linear χ_μν⁽¹⁾ (-ω;ω) and nonlinear surface susceptibility tensors χ_μνζη⁽³⁾(-ω_Σ; ω_r; ω_s; ω_t) with ω_Σ=ω_r+ω_s+ω_t. Both non-degenerate and degenerate cases are studied for thirdharmonic generation and nonlinear refractive index, respectively. Computational results obtained with no external fitting parameters are discussed regarding two recent reported experiments on MoS₂, and thus we can confirm the extraordinarily strong optical nonlinearity of TMDCs. As a possible application, we demonstrate generation of a π/4-rotated squeezed state by means of nonlinear response of TMDCs, in a silica micro-disk resonator covered with the 2D material. Our proposed method will enable accurate calculations of nonlinear optical response, such as four-wave mixing and highharmonic generation in 2D materials and their heterostructures, thus enabling study of novel functionalities of 2D photonic integrated circuits.     

A Prediction of a New Dibaryon System (Ω-Ω-)Jπ=o+

The structure of a new dibaryon (ΩΩ)_o+ is studied and predicted in the framework of the chiral SU(3) quark model by solving a resonating group method equation. The binding energy of this dibaryon is around 100 MeV, the mean root square of the distance between two Ωs is 0.84 fm and the preliminary estimated mean lifetime is about two times longer than that of the free Ω. All these interesting properties and also the two negative charge units it carried could make it be easily identified experimentally in the heavy-ion collision process. The production probability of this new dibaryon in 158A GeV Pb+Pb collision is estimated using the thermal model, which is of the order of 10^-6 to 10^-5 per event; And it is expected, that with the increase of temperature, the production rate will also be increased.     

Excitation of surface plasma waves over metallic surfaces by lasersand electron beams

A laser incident on a metal film (deposited on a glass substrate) from the glass side at a specific angle of incidence, excites a surface plasma wave (SPW) at the metal-free space interface. The ratio of the SPW field to the laser field increases with the laser spot size b attaining a value much greater than one at b>exp(2wα/c) where a is the film thickness and ω is the laser frequency. The SPW (ω, k_z,) can also he excited by a relativistic electron beam, propagating parallel to the interface in the free space region, via Cerenkov interaction when beam energy ϵb=(|ϵ|-1)mc² where ϵ is the effective metal permittivity, and mc² is the electron rest mass energy. When the surface has a ripple of wave number k₀, the SPW (ω, k_zz) can be excited at lower beam energy via sideband coupling, ω=(k_zz +k₀)v_b where v_bzˆ is the beam velocity. In both cases, however, the positioning of the beam in the close proximity of the interface is required. The scheme is useful for the generation of wavelengths longer than 1 μm     

Equilibrium dynamics of the sub-Ohmic spin-boson model under bias

Using the bosonic numerical renormalization group method, we studied the equilibrium dynamical correlation function C(ω) of the spin operator σ_z for the biased sub-Ohmic spin-boson model. The small-ω behavior C(ω) ∝ω~s is found to be universal and independent of the bias ε and the coupling strength α(except at the quantum critical point α = αc and ε = 0). Our NRG data also show C(ω) ∝χ~2ω~s for a wide range of parameters, including the biased strong coupling regime(ε = 0 and α α_c), supporting the general validity of the Shiba relation. Close to the quantum critical point αc,the dependence of C(ω) on α and ε is understood in terms of the competition between ε and the crossover energy scale ω_0~*of the unbiased case. C(ω) is stable with respect to ε for ε《ε~*. For ε》ε~*, it is suppressed by ε in the low frequency regime. We establish that ε~*∝(ω_0~*)~(1/θ)holds for all sub-Ohmic regime 0≤s 1, with θ = 2/(3s) for 0 s≤1/2 and θ = 2/(1 + s) for 1/2 s 1. The variation of C(ω) with α and ε is summarized into a crossover phase diagram on the α–ε plane.     

极化检测型铷原子磁力仪的研究

针对交变弱磁场的检测,研制了一种基于极化-检测双光束结构的激光抽运铷原子磁力仪.为了获得该磁力仪对磁场的响应特性,通过数值仿真分析了信号幅度随极化磁场强度、弛豫时间的变化关系,并进行了实验验证.最后通过选择合适的极化磁场使磁力仪对待测磁场的灵敏度最大.实验结果表明,优化后磁力仪灵敏度为0.2pT/(Hz)~(1/2),响应带宽3.5kHz,可用于弱磁场磁共振、高频异常物理现象等信号的检测.     

New method for polarization measurements of magnetic fields indense plasmas

Measurements of magnetic fields based on observations of the Zeeman splitting ω_B of spectral lines is a virtually impossible task in dense plasmas of powerful Z-pinches where the Stark splitting in the ion microfields ω_F is much greater than ω_B. In this situation, much better diagnostics of magnetic fields can be achieved through polarization difference contours obtained by subtracting profiles of the same spectral line observed in two orthogonal linear polarizations. In this way the obscuring role of the Stark effect is significantly diminished. In the present paper it is shown that the most sensitive and accurate measurements of the magnitude and the direction of the magnetic field in a dense plasma can be conducted employing central Stark components of hydrogen or hydrogen-like spectral lines. The polarization contour of a central Stark component turned out to be much more sensitive to the magnetic field than the polarization contour of a lateral component of the same line, namely by a factor of (ω_F/ω_B) ³≫1. This constitutes a drastic enhancement of the previously suggested method that had used the polarization contour of a lateral Stark component because in dense plasmas a typical value of the above factor is (ω_F/ω_B)³⩾10³ . The new method can also be used for laser fusion plasmas and for some astrophysical objects such as magnetic white dwarfs     

高功率激光终端KDP晶体非共线高效三倍频及远场色分离方案数值模拟分析

为满足高功率激光装置对终端光学系统的改进要求,控制3ω光路透射元件厚度以降低激光损伤风险,避免3ω非对称聚焦与色分离元件对靶场调靶产生不利影响,本文利用非共线相位匹配原理讨论了KDP晶体Ⅰ类和Ⅱ类两种和频产生351 nm(3ω)激光及其远场色分离过程.模拟结果表明,室温20?C环境中除目前常用的共线和频外,1053 nm(ω)与526.5 nm(2ω)激光可选择Ⅰ类或Ⅱ类两种非共线和频方式实现高效3ω激光输出并在激光远场实现色分离,且具有足够的高效转换失谐角容宽.计算表明,与Ⅰ类和频类似,Ⅱ类和频也存在一个非临界相位匹配过程,其匹配方向约为θ(3ω)=86.53?.可通过增加晶体厚度克服其有效非线性系数较低的缺点,实现3ω高效输出,失谐角容宽可达±20 mrad.为满足靶场需要,解决非共线角容宽苛刻带来的调节不便,并进一步使光路紧凑,将楔角为12?的熔石英楔板置于倍频晶体之后,ω与2ω激光在熔石英楔板后表面可产生约3.5 mrad分离角.经非共线和频,使用薄透镜即可实现聚焦及色分离.该方案完全满足终端光学系统的改进要求,可作为可靠的备选方案之一.     

Optimal configuration for vibration frequencies in a ring of harmonic oscillators: The nonidentical mass effect

The parameter diversity effect in coupled nonidentical elements has attracted persistent interest in nonlinear dynamics. Of fundamental importance is the so-called optimal configuration problem for how the spatial position of elements with different parameters precisely determines the dynamics of the whole system. In this work, we study the optimal configuration problem for the vibration spectra in the classical mass–spring model with a ring configuration, paying particular attention to how the configuration of different masses affects the second smallest vibration frequency (ω₂) and the largest one (ω_N). For the extreme values of ω₂ and ω_N, namely, (ω₂)min, (ω₂)max, (ω_N)min, and (ω_N)max, we find some explicit organization rules for the optimal configurations and some approximation rules when the explicit organization rules are not available. The different distributions of ω₂ and ω_Nare compared. These findings are interesting and valuable for uncovering the underlying mechanism of the parameter diversity effect in more general cases.     

Oscillating two-stream instability of a plasma wave in a plasmachannel

A long wavelength Langmuir wave (ω₀, k₀), propagating through a parabolic plasma density channel, can decay into a low-frequency mode (ω,k&oarr;) and two short wavelength Langmuir wave sidebands (ν_1,2,k&oarr;_1,2 ), via two-stream instability where ω_1,2=ω∓ω₀ and k&oarr;_1,2=k&oarr;∓k&oarr;₀. Depending on the mode number n, the growth rate maximizes in the range γ_max≃0.1ω_pi-0.4ω_pi for the range of k from 0.1(ω_pi/c_s) to 0.2(ω_pi/c_x) for ν₀~ν_th where ν₀ and ν_th are the oscillatory and thermal velocities of electrons, ω_pi is the ion plasma frequency on the axis, and c_s is the sound speed. The growth rate increases with the width a of the plasma density channel. It decreases with the mode number. The instability may be relevant to laser based charged particle accelerators     

Rashba自旋-轨道相互作用影响下量子盘中强耦合磁极化子性质的研究

本文基于Lee-Low-Pines幺正变换法,采用Tokuda改进的线性组合算符法研究了Rashba自旋-轨道相互作用效应下量子盘中强耦合磁极化子的性质.结果表明,磁极化子的相互作用能Eint的取值随量子盘横向受限强度ω0、外磁场的回旋频率ωc、电子-LO声子耦合强度α和量子盘厚度L的变化均与磁极化子的状态性质密切相关;磁极化子的平均声子数N随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在Rashba自旋-轨道相互作用效应影响下磁极化子的有效质量将劈裂为m*+,m*-两种,它们随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在研究量子盘中磁极化子问题时,电子-LO声子耦合和Rashba自旋-轨道相互作用效应的影响不可忽略,但Rashba自旋-轨道相互作用和极化子效应对磁极化子的影响只有在电子运动的速率较慢时显著.     

Cosmological Constraint Revisited from K Band Galactic Counts

The information of Ω_Λ of a flat universe is extracted from K band galaxy number counts, without using evolution (NE) models. Cowie et al.'s observations, with model input luminosity functions taken from Efstathiou et al. (1988), Loveday et al. (1992) and Mobasher et al. (1993), give (Ω_Λ) in the range: 0.59～0.81 plus a typical random error of σ_ΩΛ = 0.2 -0.1, respectively. Keck's observations give Ω_Λ = 0.9. We conclude that either the normalization of nearby luminosity function is too high or the cosmological constant cannot be too large.     

Second harmonic stimulated Raman scattering of laser radiation in aplasma

A high-power linearly polarized laser propagating through a plasma produces oscillatory electron velocity at the second harmonic due to-the ν&oarr;×B&oarr; force, This velocity couples a Langmuir wave (ω, k&oarr;) and an electromagnetic wave (ω₁, k&oarr;₁), where ω₁=ω-2ω₀ , k-2k&oarr;₀ and ω₀, k&oarr;₀ are frequency and wavenumber of the laser pump, causing second harmonic Raman scattering. The growth rate is maximum for side scattering. This process can occur above the quarter critical density, unlike the first harmonic stimulated Raman scattering which occurs below the quarter critical density     

Theoretical Study on the Low-Lying Electronic States of He2, He2+, and He2++

The equilibrium geometries, potential energy curves, spectroscopic dissociation energies of the ground and low-lying electronic states of He₂, He₂⁺ and He₂⁺⁺ are calculated using symmetry adapted cluster/symmetry adapted cluster-configuration interaction (SAC/SAC-CI) method with the basis sets CC-PV5Z. The corresponding dissociation limits for all states are derived based on atomic and molecular reaction statics. The analytical potential energy functions of these states are fitted with Murrell--Sorbie potential energy function from our calculation results. The spectroscopic constants B_e, α _e, ω _e, and ω _e χ _e of these states are calculated through the relationship between spectroscopic data and analytical energy function, which are in well agreement with the experimental data. In addition, the origin of the energy barrier in the ground state X¹Σ_g⁺ of He₂⁺⁺ energy curve are explained using the avoided crossing rules of valence bond model.