Canadian Journal of Physics

Chúng tôi đánh giá các hình thức gần đúng khác nhau cho năng lượng tương quan trên mỗi phần tử của khí điện tử đồng nhất có phân cực spin, những hình thức này đã được sử dụng thường xuyên trong các ứng dụng của xấp xỉ mật độ spin địa phương vào chức năng năng lượng trao đổi-tương quan. Bằng cách tính toán lại chính xác năng lượng tương quan RPA như là một hàm của mật độ điện tử và phân cực spin, chúng tôi chứng minh sự không đầy đủ của các xấp xỉ thông thường trong việc nội suy giữa các trạng thái para-magnet và ferro-magnet, đồng thời giới thiệu một công thức nội suy mới chính xác. Một kỹ thuật xấp xỉ Padé được sử dụng để nội suy chính xác các kết quả Monte Carlo gần đây (para và ferro) của Ceperley và Alder vào phạm vi mật độ quan trọng cho các nguyên tử, phân tử và kim loại. Các kết quả này có thể được kết hợp với sự phụ thuộc vào spin của RPA để tạo ra một năng lượng tương quan cho một khí điện tử đồng nhất có phân cực spin với sai số tối đa được ước tính là 1 mRy và do đó có thể xác định đáng tin cậy mức độ của các hiệu chỉnh không địa phương đối với xấp xỉ mật độ spin địa phương trong các hệ thống thực.

Irregularities and irregular motions in the upper atmosphere have been detected and studied by a variety of techniques during recent years, but their proper interpretation has yet to be established. It is shown here that many or most of the observational data may be interpreted on the basis of a single physical mechanism, namely, internal atmospheric gravity waves.A comprehensive picture is envisaged for the motions normally encountered, in which a spectrum of waves is generated at low levels of the atmosphere and propagated upwards. The propagational effects of amplification, reflection, inter-modulation, and dissipation act to change the spectrum continuously with increasing height, and so produce different types of dominant modes at different heights. These changes, coupled with an observational selection in some cases, lead to the various characteristics revealed by the different observing techniques. The generation of abnormal waves locally in the ionosphere appears to be possible, and it seems able to account for unusual motions sometimes observed.

Measurements have been made of the effects of low-energy electron bombardment (0–300 ev) on a chemisorbed layer of oxygen on polycrystalline molybdenum. Oxygen is adsorbed in two states at room temperature with approximate heats of adsorption of 60 and 110 kcal/mole, respectively. O⁺ ions are released from the weakly bound state with a maximum probability of 10⁻⁵ ion/electron at an electron energy of 90 ev. The ion-energy distribution, measured by a retarding-field method, has a maximum at about 6 ev and a half-width of 5 ev. About 50 neutrals are released for every ion. The measured threshold energy, 17.6 ± 0.2 ev, for ion formation is in good agreement with the calculated value. The probability of ion and neutral release from the strongly bonded state is about 2 × 10³ times smaller than for the weakly bonded state. A qualitative explanation of the formation of neutrals is proposed in terms of Auger neutralization of the ions near the surface. Reflection of the ion-energy distribution in the calculated curve of the potential energy of the O⁺ ion yields, after correction for Auger neutralization, the probability density distribution of the adsorbed oxygen atom. The equilibrium spacing of the chemisorbed atom is then estimated to be 1.67 Å.

The resistance offered to the flow of heat by the mismatch of the elastic constants at the interface between two materials has been calculated. It is shown that for a perfectly joined interface the heat flow is proportional to the difference of the fourth powers of the temperature on each side of the interface. Deviations from this temperature dependence are to be expected for rough surfaces and for surfaces pressed into contact with one another. The calculated contact resistance between some common solids is given, and graphs are presented from which the heat flow between any two materials may be computed. It is shown too that the spin–phonon, phonon–electron, and phonon–phonon relaxation processes give rise to additional resistive processes in some solids, some of which restrict the heat flow at the surface. The theoretical results compare well with the available experimental data. However, the problem of the contact resistance between helium and metals is still unresolved. A tentative explanation of this is presented which predicts that there should be an appreciable difference between the contact resistance of a metal in the normal and in the superconducting state.

(i) A film of a nonwetting liquid is not stable. If it is thick (thickness e ≥ 1000 Å) it is metastable and evolves via nucleation and growth of a dry spot. If it is thin, it is unstable against spinodal decomposition (amplification of thermal undulations) and breaks into microscopic droplets of size ~e²/a (a = a molecular size), (ii) A film made with a wetting liquid is metastable and tends to shrink whenever e < e_S (the equilibrium thickness of the wetting layer). Contrary to case (i) where the growth of a dry patch is controlled by capillary forces, the central role for drying in case (ii) is played by long-range forces, and the only process is nucleation and growth.

In this paper a Green's function method is developed to deal with the problem of a Volterra dislocation in a semi-infinite elastic medium in such a way that the boundary surface of the medium remains free from stresses. (A Volterra dislocation is here defined as a surface across which the displacement components show a discontinuity of the type Δu = U + Ω ×r, where U and Ω are constant vectors.) It is found that the general problem requires the construction of six sets of Green's functions. The method for the construction is outlined and applied to one of the six sets, which is of the type of two double forces with moments in a plane parallel with the boundary. The displacement field thus generated is computed. Several of the results obtained are believed to be of geophysical interest, but a more detailed discussion of these applications is postponed to a further communication which is being prepared.

Two general procedures for the measurement and analysis of angular correlations of gamma radiations from nuclear reactions are described which have wide applications in nuclear spectroscopy for the determination of spins and gamma-ray multipolarities. Cases can be studied by these methods when the reaction proceeds through a compound state too complex to allow the usual analysis to be made, for example where several levels overlap or where direct interaction is dominant. The basis of these procedures is to exploit the simplifications brought about by making the reacting system axially symmetric. A sharp gamma-ray-emitting state formed in such a system can be regarded as aligned and described in terms of a relatively small number of population parameters for the magnetic substates. In the first procedure, a state Y* is prepared by a nuclear reaction X(h₁h₂) Y* in which h₂ is unobserved. The state Y* has axial symmetry about the beam axis. From coincidence angular correlation measurements of two cascade gamma rays from Y*, the unknown population parameters for Y* together with the nuclear spins and gamma-ray multipolarities can be determined. In the second procedure, h₂ is measured in a small counter at 0° or 180° relative to the incident beam. It is then shown that the quantum numbers of the magnetic substates of Y* which can be populated do not exceed the sum of the spins of X, h₁, and h₂. In cases where the sum of the spins does not exceed [Formula: see text], the angular correlation of the gamma rays from the aligned state depends only upon the properties of the states in the residual nucleus. Theoretical expressions for angular correlations from aligned states are given, together with a method whereby existing extensive tables of coefficients can be used to calculate them. The results of two recent experiments are discussed as examples.

Recent measurements of resonances in slow neutron total cross sections yield good estimates of the average level spacing, D, in medium and heavy nuclei. These spacings show large variations, by factors of 10³ to 10⁵, in the region of magic numbers of nucleons. There are also variations by smaller factors between nuclei with even and odd numbers of protons or neutrons. The even–odd effect is a co-operative phenomenon; it can be approximately treated by redefining the ground state to be used for a Fermi gas model. After this correction the gas model should predict D with reasonable accuracy since it is required only to define the density of a complete set of states. The magic number variations are shown to be fitted by an improved approximation to the single-nucleon level density. This is obtained from the observed sequence of single-particle spins and the assumption that the energy interval between spin subshells is constant. Fifty-two observed spacings are fitted by a two-parameter formula with an average uncertainty factor 3. Many of the larger remaining differences between observation and the predictions of the model are qualitatively explicable as expected departures from this uniform spacing hypothesis.

Phenomenological equations between dislocation velocity, effective shear stress, temperature, and pressure are presented. It is suggested that the term "activation volume" be properly assigned to the pressure dependence of dislocation velocity. Indirect methods of determining velocity–stress relations are reviewed, and a new method is introduced using the stress–time data in stress relaxation. Some results are reported for high-purity iron, LiF, and NaCl crystals. Phenomenological relations near absolute zero are examined in the light of the third law of thermodynamics. These relations are illustrated with the deformation behavior of potassium at liquid helium temperatures.

A method is described that will give the variation, as a function of internuclear distance, of the electronic transition moment governing intensities in diatomic molecular band systems. Reliable theoretical results and good experimental intensities are used conjointly to find this variation. Once smoothed, the trend may be replaced into the overlap integrals to give relative vibrational transition probabilities better than those given by the approximation ‘overlap integrals squared’, and presumably better than those deduced directly from experiment, since the latter are not smoothed. Limits on the application of the method are suggested; however many band systems fall within these limits.