Instability of a triangular Abrikosov lattice at values of the Ginzburg–Landau parameter κ close to unity

The “soft” transverse mode of gapless excitations related to the deformation of a triangular Abrikosov lattice with a single flux quantum per unit cell at an arbitrary value of the Ginzburg–Landau parameter κ is investigated. An Abrikosov lattice with the angle φ = π/3 between the unit cell vectors is shown to be unstable in a narrow range of values, 1 < κ < 1.000634. The excitation spectrum of the mode under consideration at low values of the momentum k (in the k² approximation) is isotropic at k lying in a plane perpendicular to the magnetic field.     

北京大学光阴极注入器3+1/2超导腔机械性能

 介绍了北京大学光阴极注入器的核心部件3+1/2超导腔的机械性能的设计，通过ANSYS和SUPERFISH程序优化加强筋的个数和位置，解决了超导腔在脉冲条件下由于洛仑兹力引起的超导腔失谐以及当超导腔进行低温调谐时场平滑度变化过大的问题，同时针对复杂超导腔结构进行了低温调谐结构的设计。     

A novel overmoded slow-wave high-power microwave (HPM) Generator

We present the design and experimental results of a novel overmoded slow-wave high-power microwave (HPM) generator that is featured by its compactness, low-operation magnetic field, and potentially high power and high efficiency. The device includes two slow-wave structure (SWS) sections, a resonant cavity, and a tapered waveguide. The resonant cavity was well designed and was used to achieve the axial mode selection and to decrease the length of the SWS sections. The radial mode selection is achieved using the property of "surface wave" of the device to excite the TM/sub 01/ mode while making the higher TM/sub 0n/ modes unexcited. The physical mechanisms of axial and radial mode selections ensure that the microwave is produced with a single mode and a narrow band. The feasibility of low magnetic field operation is also investigated based on the characteristics of the overmoded slow-wave devices. Experiments were carried out at the Spark-2 accelerator. At diode voltage of 474 kV, beam current of 5.2 kA, and guiding magnetic field strength of 0.6 T, a microwave was generated with power of 510 MW, mode of TM/sub 01/, and frequency of 9.54 GHz. The relative half width of the frequency spectrum is /spl Delta/f/f= 0.6%, and the beam-to-microwave efficiency is about 21% in our experiment.     

Resonant magnon—optical phonon coupling in metamagnetic FeCl2·2H2O

The resonant coupling between magnons and optical phonons of symmetryB _g was observed in the Raman spectra at different magnetic fields in the antiferro-, ferri- and ferromagnetic phases of FeCl₂·2H₂O at 2K. The line positions of the coupled modes were calculated taking a field dependence of the coupling strength into account. Varying this strength and the frequencies of the uncoupled modes as parameters of this calculation a fit of the experimental data was obtained also at low fields where former interpretations were not satisfying. However, the field dependence of the coupling strength disagrees with a corresponding term of an appropriate microscopic coupling mechanism in which the local spins are disturbed by a modulation of the ctystal field via spin-orbit coupling. In accordance with the former results of the far infrared absorption measurements zero field splitting of the magnonlike mode was found to exist. In the AF-phase two magnonlike modes can be observed, but the Raman scattering cross section of one of these modes tends to zero at zero field corresponding to its vanishing phonon contributions responsible for the scattering cross sections of the coupled modes. The relationship between cross sections of the coupled modes and their phonon contributions was confirmed by analysing the field dependence of the mode intensities in the Raman spectra.A project of the Sonderforschungsbereich 65 Festkörperspektroskopie Darmstadt-Frankfurt, supported by the Deutsche Forschungsgemeinschaft     

Simultaneous low extinction and high local field enhancement in Ag nanocubes

We theoretically investigate surface plasmon resonance properties in Au and Ag cubic nanoparticles and find a novel plasmonic mode that exhibits simultaneous low extinction and high local field enhancement properties. We analyse this mode from different aspects by looking at the distribution patterns of local field intensity, energy flux, absorption and charge density. We find that in the mode the polarized charge is highly densified in a very limited volume around the corner of the nanocube and results in very strong local field enhancement. Perturbations of the incident energy flux and light absorption are also strongly localized in this small volume of the corner region, leading to both low absorption and low scattering cross section. As a result, the extinction is low for the mode. Metal nanoparticles involving such peculiar modes may be useful for constructing nonlinear compound materials with low linear absorption and high nonlinearity.     

低频混响信号中的简正波衰减系数提取方法

提出一种从低频混响信号中提取简正波衰减系数的方法。利用简正波过滤技术对垂直阵混响信号进行分析,获得单阶简正波混响声场。假设海底反向散射矩阵可分离,从单阶简正波平均混响强度中提取出有效海底反向散射矩阵元素,最后利用不同距离上的有效海底反向散射矩阵元素计算出简正波的衰减系数。利用该方法从混响信号中提取出的简正波衰减系数预报的声传播损失和相同海域实测声传播损失一致。该简正波衰减系数提取方法有效避免了海底散射衰减和简正波传播衰减耦合的问题,同时对海底参数反演和水声环境的快速评估也具有重要的意义。     

矩形波导中超短脉冲电子束的相干同步辐射

考虑了当射频调制的超短脉冲电子束径向长度远小于辐射波长时，将其理想化为δ时间函数，提出了用波导本往模展开的方法来计算矩形波导中超短脉冲电子束的相干同步辐射及其频率特性。结果表明：（１）当谐振频率等于调制电子束微脉冲时间隔的射频的整数倍时，辐射模式表现为“纯”的波导本征模。（２）波导效应使得超短脉冲电子束在两个频率处发生相干同步辐射，而且低频支的辐射功率高于高频支的。最后指出波导自由电子激光器单横模     

IR detection by depletion of trapped charge in localized impurity states of an extrinsic semiconductor

Recently we have described a new mode of IR detection which utilizes charge storage in localized levels of an extrinsic semiconductor at low temperatures. This method is based on field-assisted impurity photoionization. IR photoionization depletes the population of stored charge in localized impurity states. The integration takes place inside the bulk of the semiconductor. A large negative going pulse causes rapid field ionization of the remaining charge after an IR exposure. Readout is accomplished by measuring this ejected charge. Using a cryogenically cooled monochromator, we have studied the spectral response and its field dependence. Extremely low dark currents (<3×10^–18 A) permit IR integration times of up to 12 hours or more. For the first time, significant photoresponse is obtained from a cryogenic Si:P detector at wavelengths up to 43 m, well beyond the nominal long wavelength cutoff of 27 m.This work was supported in part by National Science Foundation grant #ECS-8202473.     

Evolution of Crack Tip Constraint in a Mode II Elastic-Plastic Crack Problem

Numerous studies have shown that crack tip constraint has an important effect on the level of conservatism when crack extension is investigated in elastic-plastic fracture mechanics. Constraint effect has been explored extensively in the past but mainly for pure mode I problems. Very few researchers have dealt with the effects of crack tip constraint on mode II or mixed mode I/II fracture in metallic materials. In this paper, the evolution of mode II constraint parameter Q in terms of applied external load is determined numerically for a test specimen under pure mode II loading. The finite element method is utilized to model the specimen and to study the range of validity of mode II constraint parameter determined from a Q—T diagram. The parameter Q calculated from the finite element simulation (or from the full field solution) is compared with the values of Q determined from the Q—T diagram. For low levels of load, the results of full field solution are shown to be consistent well with the results obtained from the Q—T diagram. However, when the external load increases significantly, the results of Q—T diagram are no longer accurate and mode II constraint parameter Q should be calculated directly from finite element results.     

Dispersion characteristics of broadside coupled dielectric image guide at millimeter wave frequencies

Dispersion characteristics and wave impedance for the even and odd modes of the broad side coupled dielectric image guides are computed by using mode matching techniques. Dispersion curves for broadside image guide are plotted for various dielectric materials and dimensional parameters as a function of frequency. Normalized electric field for dominant electric field component E of TM even and odd modes for various half spacings between two dielectric slabs of broadside coupled image guide have been plotted. Polysterene ( r=2.56) and Stycast ( r=3.4) have been used as dielectric materials in fabricating broadside coupled image guide. This broadside coupled dielectric guide is very convenient as compared to the other dielectric guides. Since in case of broadside coupled image guide the dielectric slab can be bonded to the side metallic walls using some low loss adhesive material, while in other coupled dielectric guides the dielectric slab have to be supported by some low dielectric constant material (Eccofoam with r=1.02) to maintain the proper distance of side metallic walls from the dielectric slab. Since this structure is symmetrical, this configuration can be analysed in terms of even and odd mode phase velocities. The difference in these even and off mode phase velocities can be used to determine the coupling between coupled dielectric slab. Various couplers in this configuration can be realised.     

Two-photon resonance fluorescence

The excitation spectrum of a two-level atom interacting with a strong electromagnetic field is considered when the atomic transition frequency is nearly twice the frequency of the laser field. The spectral function consists of four Lorentzian lines describing: the central line peaked near the two-photon resonance, two sidebands peaked at the high and low frequency regions respectively and the one-photon frequency mode. In the limit of high photon densities, the low frequency photon mode is induced by the pump field which removes the singularity occurring at the two-photon resonance frequency.     

对记录脉冲声场的光学方法及对沙中埋藏的类圆柱体的散射声场的研究

本文对用于记录流体中脉冲声场的两种方法——Schlieren法和Shadowgraph法的响应进行了研究。研究表明对于Schlieren法,成像屏上的光强的响应对于声压是非线性关系;对于 Shadowgraph法来说,则是线性关系。由于 Shadowgrph法的工作不受频率限制,适用于在低超声频下研究海底模型和埋藏物的声场。本文利用Shaodowgraph法记录了埋在沙中圆柱体、类圆柱体的低超声频脉冲的散射声场,对半埋体记录到5种散射波,对全埋体则记录到3种散射波,对半埋体与全埋体都记录到两种爬波(纵波爬波、横波爬波)。     

Beat excitation of terahertz radiation in a semiconductor slab in a magnetic field

Two infrared lasers of frequencies ω₁ and ω₂ propagating in the TM/TE mode along $\stackrel{?}{z}$ direction in a rippled density semiconductor waveguide are shown to resonantly excite terahertz radiation at the beat frequency when ripple wave number is suitably chosen to satisfy the phase matching. The wave vector of the density ripple is along the direction of laser propagation while a static magnetic field is applied transverse to it. The lasers exert a ponderomotive force on the electrons at the beat frequency. This force, in the presence of density ripple and transverse magnetic field, produces a nonlinear current at the terahertz frequency. The magnetic field enhances the amplitude of the terahertz wave. However terahertz yield is significantly higher in the TM mode laser beating than in the TE mode laser beating.     

Threshold dependence of the vibrational excitation of molecules on laser radiation intensity

The collisionless vibrational excitation of a polyatomic molecule in an IR laser radiation field has been theoretically studied. It has been shown that (i) the degree of vibrational excitation (namely, number 0000 of vibrational quanta of a molecular mode near-resonant with the IR laser field that are absorbed by the molecule) is low if laser pulse intensity P (energy flux density in the laser beam) is lower than a certain critical value P _cr; (ii) the degree of excitation abruptly increases after crossing the boundary where P = P _cr; (iii) this effect is attributed to two properties inherent in polyatomic molecules, namely, the anharmonicity of the vibrational mode interacting with the laser field and the energy exchange with other modes; and (iv) at P > P _cr, number 00000 is determined only by energy density Φ = Pτ_P, where τ_P is the laser pulse duration, 00000 monotonically increases with increasing Φ. The model is in good agreement with the experimental data.     

Computer-aided analysis and design of corrugated TE11 to HE11 mode converters in highly overmoded waveguides

A theoretical and experimental study of TE11-to-HE11 mode conversion in highly oversized waveguides is given. The balanced HE11 hybrid mode propagating in a circumferentially corrugated circular waveguide is in many respects ideal for effective electron cyclotron resonance heating of fusion plasmas with high-power gyrotrons. At an aperture antenna it exhibits desirable radiation characteristics such as linear polarization with very low cross polarization and axisymmetric power distribution with low sidelobe levels. This Gaussian-like mode can be generated from a TE11 mode by means of a corrugated mode converter in which the depth of the annular slots is tapered gradually from one-half to one-quarter wavelength. Computer-aided optimization of converter length, shape of corrugations, and of non-linear slot depth variation for 28 GHz- and 70 GHz-transducers was achieved with a scattering matrix formalism employing the modal field expansion technique (Modular Analysis Concept). In both cases the measured conversion efficiency of (98.5±0.5)% and the experimental values of the maximum cross polarization (below –28 dB) are in very good agreement with the predicted values. High-power operation was successfully demonstrated using pulsed 28 GHz- and 70 GHz-VARIAN gyrotrons (200 kW).     

Morphic effects — III. Effects of an external magnetic field on the long wavelength optical phonons

The changes in the frequencies of the k ≈ 0 optical vibration modes on the application of a static, external magnetic field to a non-magnetic crystal are determined to first order in the field strength. Second order effects are equivalent to the effects of an electric field in second order and they are not considered here. It is shown that the frequency of a nondegenerate mode is not altered to first order in the magnetic field. In the case of the noncubic crystal structures it is found that the magnetic field must have a component along the axis of highest symmetry in order that the doubly degenerate modes at k ≈ 0 have their degeneracy lifted. In the case of the cubic structures a magnetic field applied in any direction can completely split the degeneracy of modes which are triply degenerate at k ≈ 0. Expressions are given for the field induced changes in the normal mode frequencies. The modes whose frequencies are shifted are found to be right or left circularly polarized. A brief discussion is given of spatial dispersion effects, that is, splitting of the mode degeneracy linear in the phonon wave-vector. Finally, a review of the symmetry aspects of acoustical activity and Faraday effects of acoustical phonons is presented.     

The near-ground structure of the nocturnal sound field

The near-ground behavior of the low-frequency (100 Hz to 500 Hz) sound field in the nocturnal sound duct is studied theoretically and experimentally. In the first few meters of the atmosphere, narrow-band sound fields are found to have a characteristic vertical structure. The sound field is the superposition of a "surface mode," whose magnitude decreases monotonically with altitude, with a sum of "higher modes," each of whose magnitudes has a pronounced minimum a few meters from the ground at approximately the same height. The surface mode attenuates to negligible levels after a few hundred meters from the source. Consequently, more than a few hundred meters from a narrow-band source, there is a "quiet height" at which the sound level is reduced by 10 to 15 dB relative to its value on the ground. The narrow-band quiet height is shown to be a robust feature of nocturnal sound propagation.     

Acoustic excitations in suspensions of soft colloids

Vibrational modes in suspensions of soft colloids in a fluid can be detected experimentally by Brillouin light scattering. Besides the usual acoustic mode, being essentially the longitudinal phonon of the liquid matrix, an "opticlike" mode is observed in giant starlike micelles at low volume fractions. We propose that this opticlike mode is due mainly to the internal vibration of each hairy particle.     

二极管激光阵列在Talbot外腔中同相模的选择

 从理论和实验上对二极管激光阵列在Talbot外腔中的锁相进行了研究。在同相模和异相模近场分布的基础上,利用1维情况下的菲涅耳衍射公式计算了其远场分布。根据同相模和异相模在Talbot腔中的分布特性,采用1/2 Talbot腔并将外腔镜倾斜一个角度a的方法既能选择同相模,又能使模式的功率损耗最小。二极管激光阵列芯片采用CD金刚石材料,“三明治”结构对其进行封装,明显地减小了阵列的“smile”效应。在实验中实现了二极管激光阵列同相模的锁相输出,远场单瓣模的半高全宽为0.11 mm。