Physica最新文献

A free-energy functional is proposed for the two-dimensional Ising model below T_c which incorporates the correct critical exponents and the anomalous behaviour of the order-parameter correlation function.

We consider the dissipative effects of electron-phonon interactions on electron states which are not eigenstates of the pure electron hamiltonian. A master-equation formalism is developed to calculate the time evolution of the occupation probabilities of these states as well as other elements of the reduced single-electron density matrix. Conditions are found which ensure that the master equations are markoffian. It is suggested that markofficity rather than approximate stationarity is the important criterion for the usefulness of a basis of states for the description of the evolution of a physical system.

Experimental results for six binary mixtures of Ne and Kr obtained in a two-bulb apparatus operated at 77.6 K and 308 K are presented. The composition dependence of α_T is compared with the results of the Chapman-Enskog theory. The empirical potential of Kestin, Ro and Wakeham is able to explain the experimental dependence of α_T on the composition at our working temperature. The same comparison with the results of other authors, at around room temperature, indicates an equally good agreement.

Two recent versions of the perturbation theory of liquids, the optimized chain approximation of Andersen, Chandler and Weeks (ACW) and the renormalized one-chain approximation (ROC) by one of us (AF), are discussed. The ACW optimized version leads to better results than the ROC one. The main difference between these versions lies in the definition of the renormalized perturbation bond. The ACW form enables the optimization of the chain sum by the adjustment of the perturbation potential inside the hard core. This is impossible in the ROC approximation, which, however, may be ‘optimized’ by the replacement of the complete hard-sphere radial distribution function by its zeroth-order approximation. Both of these optimalization procedures are equivalent to the effective weakening of the repulsive interactions of the reference system. We infer that the hard-sphere fluid is too strongly repulsive to be the best reference system for perturbation theories of liquids.

The viscosities of the binary gas mixtures He-NH₃, Ne-NH₃, Kr-NH₃ and Xe-NH₃ have been measured by the oscillating-disc method in the temperature range from -35°C to 35°C. The experimental data are systematically higher than the calculated values obtained by using the conventional combination rules for the parameters of the interaction potential for the polar-nonpolar interaction which includes the dipole-induced dipole term only. The discrepancy increases from He to Xe. These results are consistent with those obtained previously for mixtures of CH₃Cl gas with the inert gases. It has been suggested that the major cause of the discrepancy is the neglect of the r⁻⁷ term in the interaction between a polar and a nonpolar molecule.

The molecular-field model of an Ising ferromagnet is modified by cutting interaction bonds so as to leave a system of M chains with long-range interactions between nearest-neighbor chains only. Equilibrium properties of the model, such as the critical temperature.

The theory of the long-time photocount statistics of a linear gaussian light wave is extended into the unstable region characterized by a gain constant Λ. Exact recurrence relations for the photocount distribution and the normalized factorial moments are given and the distribution function P(E) of the integrated light intensity E is discussed. Due to the exponential increase of mean intensity during the counting interval T only a limited amount of amplitude stabilization can occur in E provided ΛT ≲1. For ΛT ⪡1 and ΛT ⪢1 the distribution P(E) is thermal. A new parameter r appears in the theory measuring the initial wave intensity in relation to the quantum fluctuations. For r ⪢ 1, P(E) is practically thermal for all values of ΛT. For r≲1 characteristic deviations from a thermal distribution appear which provide a means to measure the linear gain Λ of the light wave by photon counting.

It is shown that the momentum transfer theorem implies a rough correlation of the diffusion cross section with the slope of the intermolecular potential energy. This is a particular case of the collision Ehrenfest theorem which can be applied in like manner to all of the collision cross sections arising in kinetic theory, thus allowing a qualitative discussion of their dependence on the potential. As an example, relative to the diffusion cross section the viscosity has an increased sensitivity to the curvature of the potential.

An expression is derived for the Van der Waals force between two semi-infinite bodies with small surface irregularities. Calculations are given both for the plane-plane and the plane-sphere configuration. The value of the correction from the surface irregularities upon the Van der Waals force is shown to amount easily to 10–50%.

The Yonezawa diagrammatic method has been generalized in order to describe the electric conductivity both in compositionally disordered systems (substitution alloys) and translationally disordered systems with short-range correlations (liquid metals). Results equivalent to those by Velicky and Leath have been obtained in the case of substitutional alloys. A generalized resistivity formula has been derived for liquid metals with the approximation of a long mean free path of conducting electrons. In the case of weak scattering potential this formula can be reduced to the Ziman expression. This formula includes also, as an approximiation, the result of the Ashcroft-Schaich theory. An important role of the special expression of high-order atomic correlations in liquid metals by pair correlations is pointed out. New integral equations determining the pair correlation function in fluids are suggested on the basis of this expression.