强迫振动方法确定黏弹性细棒的复杨氏模量

提出了一种方法--强迫纵振动法(该计算方法不同于以前的梁振动法)来确定黏弹性样品细棒的杨氏模量。在室温下,对长度分别为108 mm,100 mm,90 mm,83.5 mm,80 mm,74.5 mm,70 mm的聚碳酸酯细棒,实验测试得到了棒两端的振幅比与激励频率之间的关系曲线,并由曲线的3 dB带宽计算得到损耗因子,进一步得到棒的储能和耗能杨氏模量。依照这种方法,只要能得到足够多的数据,用最小二乘算法,可得到在感兴趣的频段内该材料相应的杨氏模量随频率变化的曲线。若测试频率步长减小到5 Hz,则测试最大误差可小于6%。与现有测试方法相比较,本方法计算简单,物理意义清晰。     

Free sets for nowhere-dense set mappings

We give some consistency results on the existence of uncountable free sets for nowhere-dense set mappings. For example, we prove that it is relatively consistent with ZFC and Martin’s Axiom that any nowhere-dense set mapping defined on the reals has an uncountable free set.     

Interpolation by splines satisfying mixed boundary conditions

We consider interpolation of Hermite data by splines of degreen withk given knots, satisfying boundary conditions which may involve derivatives at both end points (e.g., a periodicity condition). It is shown that, for a certain class of boundary conditions, a necessary and sufficient condition for the existence of a unique solution is that the data points and knots interlace properly and that there does not exist a polynomial solution of degreen?k. The method of proof is to show that any spline interpolating zero data vanishes identically, rather than the usual determinantal approach.     

Stretching of homopolymeric RNA reveals single-stranded helices and base-stacking

We have found strong supporting evidence for the helical structures of single-stranded nucleic acids by stretching individual molecules of polyadenylic acid [poly(A)] and polycytidylic acid [poly(C)]. Analyzing the force versus extension data using a two-state elastic model in which random-coil domains alternate with rigid helical domains allows one to extract the thermodynamic and structural properties. In addition, it also yields moderate to low cooperativity of the helix-coil transition for poly(A) and poly(C), respectively.     

Few-Body Approaches and Problems in Hypernuclei

It takes two nucleons to bind a Λ hyperon, and perhaps as many as three nucleons to bind two Λ hyperons. Here I review few-body calculations which consider the onset of binding in multi-strange hypernuclei, including ${\Xi}$ hyperons once the free-space strong-interaction conversion ${\Xi N \to \Lambda \Lambda}$ becomes Pauli forbidden in Λ-abundant matter. Quasibound states of ${\overline {K}}$ mesons in few-nucleon systems are also briefly discussed.     

Dimension-dependent stimulated radiative interaction of a single electron quantum wavepacket

In the foundation of quantum mechanics, the spatial dimensions of electron wavepacket are understood only in terms of an expectation value – the probability distribution of the particle location. One can still inquire how the quantum electron wavepacket size affects a physical process. Here we address the fundamental physics problem of particle–wave duality and the measurability of a free electron quantum wavepacket. Our analysis of stimulated radiative interaction of an electron wavepacket, accompanied by numerical computations, reveals two limits. In the quantum regime of long wavepacket size relative to radiation wavelength, one obtains only quantum-recoil multiphoton sidebands in the electron energy spectrum. In the opposite regime, the wavepacket interaction approaches the limit of classical point-particle acceleration. The wavepacket features can be revealed in experiments carried out in the intermediate regime of wavepacket size commensurate with the radiation wavelength.     

The equivalent nonlinearity for a nonlinear system with dither

Given a single loop nonlinear control system. The problem of constructing a dither signal in order to obtain a desired equivalent nonlinearity is considered. It is assumed that the nonlinearity may be described by a kth order polynomial. For this case it is shown that in general k different levels for the dither signal are necessary.     

Parametric interaction and backward wave oscillation in stimulated compton scattering

Stimulated Compton-Raman scattering and free electron lasers are formulated as parametric (four-photon) or three-wave interaction processes. Manley-Rowe relations and depletable pump equations are derived for both free electron lasers and optical klystron schemes. A process of backward wave oscillation is predicted when a pump radiation field is back scattered off a codirectionally propagating e beam with frequency down conversion. A practicable experiment in the optical regime is proposed to demonstrate this effect.     

Interfacial characterization of chemical solution‐deposited thin films of PbSe on GaAs(100)

The microstructure and composition of the interfacial layer between chemically deposited PbSe and GaAs substrates were studied using high‐resolution transmission electron microscopy (HRTEM), Auger electron spectroscopy (AES), x‐ray photoelectron spectroscopy (XPS) and energy‐filtered TEM. The thickness of the interfacial layer varied significantly from direct contact of the film with the substrate to 5 nm in the thickest regions. The results established the presence of a discontinuous, amorphous intermediate layer of Ga₂O₃ at the PbSe/GaAs interface. Copyright © 2008 John Wiley & Sons, Ltd.