一种从辐射噪声中提取调制谱的方法

在水下航行器的辐射噪声中,螺旋桨节拍对航行器的宽带辐射噪声存在着明显的振幅调制,调制频率与航行器螺旋桨的转速有关。该特征可用于估计航行器的速度及目标分类与识别。本文利用子波变换原理,从实测的航行器辐射噪声中提取了它的调制谱,并据此对螺旋桨转速进行了估计,计算机仿真结果与实测的螺旋桨的转速具有良好的一致性。     

嵌入锂原子的zigzag型单壁碳纳米管的电子传导特性

运用基于第一性原理的密度泛函理论（DFT）的非平衡格林函数（NEGF）方法对Li原子嵌入后的zigzag型单壁碳纳米管(SWCNT)的电子输运性质进行了研究.在构建和优化了Li原子嵌入的zigzag型单壁碳纳米管的电子输运模型后,研究了该系统的电子传输概率、能态密度、电子透射谱,还研究了电子能量和偏置电压设置与变化对其电子输运特性的影响.结果显示zigzag型单壁碳纳米管嵌入Li原子后,电子输运特性发生了较大变化,具有电子输运拓宽效应和量子台阶复苏效应. 关键词： Li原子 碳纳米管 电子输运 拓宽效应     

Experimental line broadening and line shift coefficients of the acetylene ν1 + ν3 band pressurized by hydrogen and deuterium and comparison with calculations

Theoretical and experimental values have been determined for the pressure broadening of the ν₁ + ν₃ band of acetylene by hydrogen and deuterium at 195 K, and experimental values of the pressure shifts have been determined. Theoretical values have been calculated on the basis of a recent potential energy surface using the close coupling scheme. We discuss the detailed contribution of the various rotational angular momenta of the perturbing gas and the ortho and para contribution to the total pressure broadening cross-sections. We give routes to circumvent the computational cost of such calculations. Experimental values have been measured using a tunable diode laser spectrometer assuming a Voigt line shape. These pressure broadening parameters are compared with measurements performed recently at room temperature and with present measurements performed at 195 K in the ν₁ + ν₃ band of acetylene. A satisfactory agreement is obtained with the present results and available ones at 295 K.     

A simple line shape technique for electron number density diagnostics of helium and helium-seeded plasmas

The results of an experimental study of the He I 447.1 nm line and its forbidden component at high electron number density are presented and compared with profiles calculated using computer simulation method. Michelson interferometer at 632.8 nm was used to measure plasma electron number density in the range (1–7) × 10²³ m^− 3 while electron temperatures for the same experimental conditions in the range of 25 000 K to 35 000 K were determined using several spectroscopic techniques. The agreement of experimental overall line shape with computer simulation results is within 10% of what is well within theoretical and experimental uncertainty. This favorable comparison enabled the development of a simple approximate formula for the evaluation of electron number density from the measurement of wavelength separation between peaks of allowed and forbidden lines. This technique of plasma diagnostics is not sensitive to the presence of self-absorption of strong He I allowed line. The derivation of approximate formula with estimated accuracy of 15% was followed by detailed comparison with other experimental and theoretical data.     

Transient Nutation EPR Spectroscopy of Condensed Media (Review)

The application of transient nutations in EPR spectroscopy of condensed media is considered. The main methods of formation and observation of transient nutations are presented. The laws governing this phenomenon in twolevel and multilevel spin systems and also in inhomogeneous broadening of EPR lines are described. Recent advances in the use of transient nutations to separate overlapping spectra, identify quantum numbers and quantum transitions, investigate the kinetics of photoinduced paramagnetic centers, and determine relaxation times for a wide range of crystalline and disordered media are presented.     

Beatings in the Spin Echo Signal in Media Having a Doublet Structure of the Spectrum

We have obtained an analytical expression for the twopulse echo signal arising in nonresonant excitation of a condensed medium with a nonuniformly broadened spectrum of doublet structure by two pulses of an electromagnetic field of different durations and of the same amplitude. It is shown that in the case where the differences between the carrying frequency of exciting pulses and the central frequencies of the spectral lines differ considerably, beatings are formed in the signal of the twopulse echo. The rate of decay of the amplitude of these beatings is found to depend only on the time of reversible and irreversible relaxation, and their circular frequency is determined by the shift of the spectral lines relative to each other. A technique for separate determination of the inhomogeneous width of the lines and time of the irreversible transverse relaxation is suggested which is based on the analysis of the twopulse echo signal beatings.     

Theoretical analysis of collisional shift and broadening of antiprotonic helium spectral lines

Effect of medium density on shift and broadening of ( ) spectral lines is considered in the framework of binary-collision approximation with an effective ( )–He potential including the long-range attraction and short-range repulsion. Quantum calculations of the scattering phase shifts at reasonable values of the potential parameters allow to explain the main features of the observed density effect on the shape of ( ) spectral lines in low-temperature helium. This revised version was published online in August 2006 with corrections to the Cover Date.     

Derivation of a new class of distributions for the microcanonical ensemble

A new class of distributions for the microcanonical ensemble, which are shown to be stable laws, are derived by applying the central limit theorem to the canonical ensemble. This opens up a whole new host of phenomena that can be treated from a unified thermodynamic point of view. Pressure broadening of line shapes is used as an illustration.1. Work supported, in part, by contributions from the Consiglio Nazionale di Ricerche and the Ministero dell'Università e della Ricerca Scientifica e Tecnologica.2. The interaction parameterC is proportional to the mean square dipole moment. As an order of magnitude of the dipole moment, we can take it as the product of the electric charge and a typical atomic radius for a moderately excited state, which is several times the Bohr radius. Multiplication by the fine structure constant converts the Bohr radius into the Compton wavelength, thereby reducing the magnitude of the interaction by 1/137.     

Unusual resistivity broadening in the mixed state of NdBa2Cu3O7−δ crystals grown by a crystal-pulling technique

In order to clarify the origin of a strong peak effect in the magnetization curve of NdBa₂Cu₃O_7−δ crystals, we investigated the superconductivity transition behavior of the in-plane resistivity in static magnetic fields up to 8 T. Comparing the results for the samples exhibiting and not exhibiting the peak effect, we found that the former (peak effect sample) shows lower resistivity above the vortex melting temperature. This implies that the pinning force is effective in the vortex liquid state. We also found that the normal state resistivity behaviors of some samples were unusual, indicating inhomogeneous current flow. These results suggest an existence of sheet-like pinning centers perpendicular to the conduction planes.