Canadian Journal of Physics
0008-4204
1208-6045
Canada
Cơ quản chủ quản: Canadian Science Publishing , National Research Council of Canada
Lĩnh vực:
Physics and Astronomy (miscellaneous)
Các bài báo tiêu biểu
Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis We assess various approximate forms for the correlation energy per particle of the spin-polarized homogeneous electron gas that have frequently been used in applications of the local spin density approximation to the exchange-correlation energy functional. By accurately recalculating the RPA correlation energy as a function of electron density and spin polarization we demonstrate the inadequacies of the usual approximation for interpolating between the para- and ferro-magnetic states and present an accurate new interpolation formula. A Padé approximant technique is used to accurately interpolate the recent Monte Carlo results (para and ferro) of Ceperley and Alder into the important range of densities for atoms, molecules, and metals. These results can be combined with the RPA spin-dependence so as to produce a correlation energy for a spin-polarized homogeneous electron gas with an estimated maximum error of 1 mRy and thus should reliably determine the magnitude of non-local corrections to the local spin density approximation in real systems.
Tập 58 Số 8 - Trang 1200-1211 - 1980
INTERACTION OF SLOW ELECTRONS WITH CHEMISORBED OXYGEN 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.
Tập 42 Số 5 - Trang 886-905 - 1964
THE TRANSPORT OF HEAT BETWEEN DISSIMILAR SOLIDS AT LOW TEMPERATURES 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.
Tập 37 Số 3 - Trang 334-349 - 1959
Drying of solids wetted by thin liquid films (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 < eS (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.
Tập 68 Số 9 - Trang 1084-1088 - 1990
ON VOLTERRA'S DISLOCATIONS IN A SEMI-INFINITE ELASTIC MEDIUM 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.
Tập 36 Số 2 - Trang 192-205 - 1958
GAMMA-RAY ANGULAR CORRELATIONS FROM ALIGNED NUCLEI PRODUCED BY NUCLEAR REACTIONS 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(h1 h2 ) 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.
Tập 39 Số 6 - Trang 788-824 - 1961
SHELL EFFECTS ON THE SPACING OF NUCLEAR LEVELS 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.
Tập 34 Số 8 - Trang 804-829 - 1956
DISLOCATION DYNAMICS IN DEFORMATION AND RECOVERY 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.
Tập 45 Số 2 - Trang 493-509 - 1967
A METHOD OF DETERMINING THE ELECTRONIC TRANSITION MOMENT FOR DIATOMIC MOLECULES 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.
Tập 32 Số 8 - Trang 515-521 - 1954
NUCLEAR LEVEL SPACINGS T. D. Newton's shell-dependent level spacing formula [Formula: see text] depends upon the densities of single-particle orbits near the Fermi level in the nucleon gas. It has been found that these densities can be computed by taking appropriate averages of the second differences between adjacent atomic masses, using the writer's semiempirical mass formula from which electrostatic and pairing energy terms have been omitted. With this procedure, observed nuclear level spacings have been fitted with a root mean square error factor of 1.74. This fit shows that the level spacings are proportional to (2J + 1)−1 to a good approximation. Since the average density of single-particle orbits depends on the nuclear excitation energy, and since its computation takes a long time even with an electronic computer, an approximation formula with five coefficients has been fitted to the computed orbit densities for each Z and N in the ranges [Formula: see text] and [Formula: see text]. These coefficients are tabulated.
Tập 36 Số 8 - Trang 1040-1057 - 1958