对树液上升动力的探讨和比较

提出树液上升有三种驱动力:毛细现象、渗透压和负压.对这三种驱动力的大小进行计算和比较,指出树液上升的主要驱动力是分子距离增大处于拉伸状态后形成的负压.     

Unbiased estimate of forces from measured correlation functions,including the case of strong multiplicative noise

Starting from appropriate short-time correlation function measurements, we propose a dynamical “learning” method to derive the deterministic and stochastic forces underlying an observed process, even if this process contains strong multiplicative noise. To do this we extend the ideas of our previous paper [1] to establish mathematical relationships in this more general case between the joint distribution function of the process and its corresponding Ito-Langevin equation. A numerical example for a simulated process containing strong multiplicative noise shows good agreement with the theory.     

On a parameter-free model for treating polarisation forces in positron scattering processes

A rather simple semiclassical model is described for treating short-range correlation forces in the scattering of positrons from atoms at the energies below the thresholds of positronium formation in each of the considered systems. The present model allows one to introduce a non-empirical procedure for devising several damping functions, one for each of the perturbation terms which are included in the asymptotic expansion of the polarisation potential. It is shown by calculations, carried out for He, Ne and Ar as examples, that one can obtain very good agreement with experimental findings for surprisingly little computational effort, even for fairly complex atomic targets which would be otherwise out of reach for all but the most computationally intensive ab initio methods.     

Resonant processes causing photon multiplication in CaSO4:Tb3+

The emission spectra of single and polyterbium centers have been measured at the excitation of CaSO₄:Tb³⁺ phosphors with different charge compensators (Na⁺, calcium vacancies, etc.) by 3.8–35 eV photons or 5 and 300 keV electrons at 6–300 K. The possible mechanisms providing quantum yield above unity for green (⁵D₄ → ⁷F_J) and blue emission (⁵D₃ → ⁷F_J) of Tb³⁺ at the direct intracenter excitation, excitation of oxyanions or creation of hot (nonrelaxed) electrons and holes have been discussed. On the basis of thermally stimulated luminescence at 6–600 K, the peculiarities of the hopping diffusion of relaxed electrons and holes and their tentative low-temperature self-trapping have been considered.     

泊松统计的无偏估计

当泊松分布的平均值依赖于被估参数时, 从单次实验结果给出的关于参数的最大似然估计一般是统计学意义上有偏的. 借助矩的表达形式, 提出了一种关于被估参数的无偏估计方法.     

Quantitative determination of mineral phase effects observed in APXS analyses of geochemical reference materials

Calibration of the Curiosity Rover's alpha particle X‐ray spectrometer (APXS) was accomplished using geochemical reference materials and a fundamental parameters treatment of the X‐ray fluorescence and particle‐induced X‐ray emission (PIXE) excitation processes. For most major and minor elements the influence of different rock types was not significant. For the three light elements, Na, Mg, and Al, which are excited almost entirely by PIXE, systematic differences among felsic and mafic rocks were observed. A qualitative explanation is found in the very shallow interrogation depth (a few microns), which suggests that the X‐rays of these elements must emerge from a single mineral rather than an assumed average over the various minerals present. A quantitative explanation was sought by determining the mineralogy of several reference materials and computing their expected PIXE X‐ray yields with an adaptation of the yield prediction sub‐routine GUYLS in the Guelph PIXE software package GUPIX. The complexity of assigning the certified overall element mass fractions to specific minerals limited this exercise to cases with only a few minerals present. Good agreement was found between the X‐ray yields determined in the calibration exercise and those predicted in this new approach. It is expected that automation of the computational approach may enable examination of mineralogically more complex reference materials. This might also offer a means of coupling results from the X‐ray diffraction and APXS instruments on Mars. Copyright © 2014 John Wiley & Sons, Ltd.     

A new simulation method for the determination of forces in polymer/colloid systems

We introduce a new simulation method, which we call the contact-distribution method, for the determination of the Helmholtz potential for polymer/colloid systems from lattice Monte-Carlo simulations. This method allows one to obtain forces between finite or semi-infinite objects of any arbitrary shape and dimensions in the presence of polymer chains in solution or physisorbed or chemisorbed at interfaces. We illustrate the application of the method using two examples: (i) the interaction between the tip of an atomic force microscope (AFM) and a single, end-grafted polymer chain and (ii) the interaction between an AFM tip and a polymer brush. Numerical results for the first two cases illustrate how the method can be used to confirm and extend scaling laws for forces and Helmholtz potentials, to examine the effects of the shapes and sizes of the objects and to examine conformational transitions in the polymer chains. Received: 15 May 1998 / Revised: 11 June 1998 / Accepted: 12 June 1998     

On a proposed phenomenological determination of the strength of the dissipative forces in heavy ion reactions

We investigate a mechanism which shows that the experimental determination of the direction of the final spins of the nuclei in deep inelastic collisions yields distinct information on the strength of the radial dissipative forces since, in certain systems, the classical trajectories are accordingly scattered to positive or to negative deflection angles. The radial friction constant marking off both regions from each other amounts to c_r ≈ 2 × 10^?21 MeV fm sec for Ar + Th at E_lab^inc = 388 MeV and to c_r ≈ 1 × 10^?21 MeV fm sec for O + Pb at E_lab^inc = 315 MeV.     

Simple determination of Na2 scattering lengths using observed bound levels at the ground state asymptote

A simple model is developed to determine a scattering length from the experimental values of the energy of the last, least bound, vibrational levels of the ground state, either ¹S_g⁺^1\Sigma _g^{+} or ³S_u⁺^3\Sigma _u^{+},of an alkali dimer. It is based on an extrapolation of the positions of the few outermost nodes of the bound vibrational wave functions towards the dissociation threshold. It uses the asymptotic part of the molecular potential only. The method is applied to recently measured levels at the asymptote (3s+3s) of Na₂. We obtain precise values for the two scattering lengths respectively associated to the dissociation limits F₁=F₂=1,F=2 (55.1±\pm 1.6 a₀) and F₁=F₂=1,F=0 (50.0±\pm 1.6 a₀); the first value corresponds to the a_1,-1 scattering length usually considered in cold collisions. The extrapolation procedure is also applied to Li₂, using existing experimental data.     

Spin dependent carrier capture processes observed by ODMR on the 0.84 eV luminescence in SI-GaAs:Cr

Optically detected magnetic resonance (ODMR) observed on the 0.84eV Cr-related emission in semi-insulating GaAs:Cr implies that the excited state associated with this internal transition of the axial $\text{Cr}{}^{\mathrm{2+}}\text{-X}$ centre is formed by the spin dependent free carrier capture process:

$\text{Cr}{}^{\mathrm{3+}}\text{-X}\text{+ e}{}_{\mathrm{CB}}\text{a}\text{?}\text{Cr}{}^{\mathrm{2+}}\text{-X}{}^1$

The results imply that the luminescence is not due to the presence of $\text{Cr}{}^{\mathrm{2+}}\text{-X}$ ground state centres in the material as previously assumed. A competing charge transfer process, possibly involving $\text{Cr}{}^{\mathrm{2+}}\text{-X}$, is observed as a quenching ODMR signal on the 0.84eV luminescence.     

Unbiased sampling of globular lattice proteins in three dimensions

We present a Monte Carlo method that allows efficient and unbiased sampling of Hamiltonian walks on a cubic lattice. Such walks are self-avoiding and visit each lattice site exactly once. They are often used as simple models of globular proteins, upon adding suitable local interactions. Our algorithm can easily be equipped with such interactions, but we study here mainly the flexible homopolymer case where each conformation is generated with uniform probability. We argue that the algorithm is ergodic and has dynamical exponent z=0. We then use it to study polymers of size up to 64(3)=262 144 monomers. Results are presented for the effective interaction between end points, and the interaction with the boundaries of the system.     

The determination of fluxline gradients and volume pinning forces in type-II superconductors by means of the magnetooptical faraday effect

An experimental technique for the direct observation of flux line density gradients by means of the magnetooptical Faraday effect is proposed. For the case of cylindrical Nb-specimens this method was applied for the determination of volume pinning forces.     

Unification of the forces

Unification of the forces is achieved via an elementary application of the symmetry of parity-time-charge (PTC) inversion invariance to classical physics. The strong color force between quarks is found to be a PTC-inversion of the Coulomb force and to have a strength of about 3.75×10⁵ times the Coulomb force between electrons at a separation distance equal to the electron Coulomb radius, r₀/2 0.7045×10^–13 cm [note that what we call the electron Coulomb radius (a convenient unit) differs by a factor of 4 from the usual classical electron radius]. It displays asymptotic freedom as predicted and, consistent with the predicted spatial variation, it is found to be constant, or independent of the particle separation distance. The strong color force and other PTC-inverse EM forces are found to exist only between quarks, whereas the EM forces are found to exist only between leptons and composite particles.The weak force is the residual of long range EM and PTC-inverse EM forces (a similar situation is indicated of the short-range nucleon-nucleon force). These residual forces are of mixed parity.The net potentials between particles are polynomials in the separation variable r, which have minima and maxima at the zeros of the first derivative. The minima are the stable orbits, which provides a classical basis for quantum mechanics.Finally, the Coulomb and the gravitational forces are found to be non-reciprocal P-inversions of each other and become indistinguishable at m_x = ±(2) (q_x/e)M_p, where M_p is the Planck mass.     

Unbiased reconstruction of the dynamics underlying a noisy chaotic time series

Refined methods for the construction of a deterministic dynamical system which can consistently reproduce observed aperiodic data are discussed. The determination of the dynamics underlying a noisy chaotic time series suffers strongly from two systematic errors: One is a consequence of the so-called "error-in-variables problem." Standard least-squares fits implicitly assume that the independent variables are noise free and that the dependent variable is noisy. We show that due to the violation of this assumption one receives considerably wrong results for moderate noise levels. A straightforward modification of the cost function solves this problem. The second problem consists in a mutual inconsistency between the images of a point under the model dynamics and the corresponding observed values. For an improved fit we therefore introduce a multistep prediction error which exploits the information stored in the time series in a better way. The performance is demonstrated by several examples, including experimental data. (c) 1996 American Institute of Physics.     

A Survey of Finite Algebraic Geometrical Structures Underlying Mutually Unbiased Quantum Measurements

The basic methods of constructing the sets of mutually unbiased bases in the Hilbert space of an arbitrary finite dimension are reviewed and an emerging link between them is outlined. It is shown that these methods employ a wide range of important mathematical concepts like, e.g., Fourier transforms, Galois fields and rings, finite, and related projective geometries, and entanglement, to mention a few. Some applications of the theory to quantum information tasks are also mentioned.