Showing papers in "Physical Review Letters in 1959"

Spontaneously Growing Transverse Waves in a Plasma Due to an Anisotropic Velocity Distribution

Abstract: Self-excited transverse electromagnetic waves which involve only the electrons were found to exist in plasmas provided the velocity distribution is sufficiently anisotropic. Their existence and rate of growth are derived from the Boltzmam transport equation, neglecting the collision term and retaining only linear terms of the perturbation. (W.D.M.)

Precession of the Polarization of Particles Moving in a Homogeneous Electromagnetic Field

Abstract: The problem of the precession of the “spin” of a particle moving in a homogeneous electromagnetic field — a problem which has recently acquired considerable experimental interest — has already been investigated for spin ½ particles in some particular cases.1 In the literature the results were derived by explicit use of the Dirac equation, with the occasional inclusion of a Pauli term to account for an anomalous magnetic moment. On the other hand, following a remark of Bloch2 in connection with the nonrelativistic case, the expectation value of the vector operator representing the “spin” will necessarily follow the same time dependence as one would obtain from a classical equation of motion. To solve the problem for arbitrary spin in the relativistic case, it will thus suffice to produce a consistent set of covariant classical equations of motion. Such equations have been indicated a long time ago by Frenkel3 and are discussed by Kramers.4 These authors use an antisymmetric tensor M as the relativistic generalization of the intrinsic angular momentum observed in the rest-frame of the particle. A formulation in terms of the (axial) four-vector s which describes the polarization in a covariant fashion5 — though basically equivalent — is however much more convenient for our problem. We shall therefore derive first the equations of motion directly in terms of this four-vector s.

Field theory commutators

Abstract: It is customary to assert that the electric charge density of a Dirac field commutes with the current density at equal times, since the current vector is a gauge-invariant bilinear combination of the Dirac fields. It follows from the conservation of charge that the charge density and its time derivative, referring to any pair of spatial points at a common time, are commutative. (W.D.M.)

Crystal Stability and the Theory of Ferroelectricity

Abstract: The phenomenon of ferroelectricity in pseudo-cubic crystals is discussed in terms of the normal modes of vibration. It is shown that the parameters which determine the lattice vibrations of a diatomic ionic crystal may be chosen in such a way that the crystal will exhibit ferroelectric properties, and that ferroelectric or anti-ferroelectric transitions may be regarded as the result of an instability of the crystal for a certain normal mode of vibration. The theory is extended to apply to other cubic crystals, including barium titanate, and the concepts of ‘ionic polarizability’ and ‘a polarizability catastrophe’ are discussed in terms of lattice dynamics. Certain of the parameters which appear in Devonshire's phenomenological theory of ferroelectricity are found to be expressible in terms of atomic parameters. Values for the latter which are physically reasonable are found to account quite well for the dielectric properties of barium titanate and for the relative movements of the atoms which occ...

Dipole State in Nuclei

Abstract: It is shown that the energies of the positions of singleparticle excitations lying between zero bombarding energy and the binding energy of the last neutron cannot be compared directly due to dipole absorption. Because many particle-hole states can be formed, with the states almost degenerate in energy, the particle-hole interaction can have a profound effect in redistributing dipole transition strength. These effects are demonstrated by a schematic model which shows how coherent effects are able to push the dipole transitions to high energies. Protons in a potential well are considered, and the discussion is extended to protons and neutrons in a nucleus. The discussion is limited to nuclei with double closed shells, neglecting the influence of the few valence nucleons, spin, and considering only transitions from 1 to 1 + 1. (C.J.G.)

Gravitational Red-Shift in Nuclear Resonance

Abstract: The several theories of relativity predict the frequency of electromagnetic spectral lines to be proportional to the gravitational potential. Experiments are proposed to observe the timekeeping of a "clcck" based on an atomic or molecular transition, when held aloft in a rocket-launched satellite, relative to a similar one kept on the ground. Methods for testing this on the ground are discussed. (W.D.M.)

Stimulated Emission of Radiation by Relativistic Electrons in a Magnetic Field

Abstract: The Schroedinger equation of an electrons moving perpendicularly to a magnetic field H, can be reduced to the equation of the harmonic oscillator. Using the relativistic Schroedinger equation and neglecting the spin, equations are found for the kinetic energy levels of the electron. Formulas were derived by means of the Boltzman transfer equation, taking into account the dependence of the electron mass on kinetic energy. (A.C.)

Unstable plasma oscillations in a magnetic field

Abstract: >The small amplitude oscillations of a fully ionized quasi-neutral plasma in a uniform time-independent externally produced magnetic field are considered. Motions of the ions and perturbations of the magnetic field are neglected. The distribution function for the electrons is assumed to depart only slightly from the zeroth order distribution. (W.D.M.)

A formal optical model

Abstract: A construction is given of an equivalent one-body pocomplex target. The case of incident and target particles being identical nonrelativistic fermions, and allowing fully for antisymmetry, is considered explicitly. (A.C.)

Possible Resonant State in Pion-Hyperon Scattering

Abstract: It is pointed out that there is a probability for a resonant state in pion-hyperon scattering at an energy of about 20 Mev below the K--p (C.M.) threshold energy. This possibility is discussed in some detail. Charge- dependent refinements due to the Coulcomb interaction and the K--K/sup 0/ mass difference are neglected. (W.D.M.)

Effect of a Pion-Pion Scattering Resonance on Nucleon Structure

Abstract: The electromagnetic properties of the nucleon have recently been studied by the dispersion-relation method. Although qualitatively successful in accounting for the isotopic vector properties of the nucleon, these treatments proved incapable of explaining simultaneously the value of the magnetic moment and the radii of the charge and moment distributions. It is shown that the inclusion of a strong pion-pion interaction could explain these aspects of nucleon structure. (W.D.M.)

Pion-Pion Correlations in Antiproton Annihilation Events

Abstract: Author(s): Goldhaber, Gerson; Fowler, William B.; Goldhaber, Sulamith; Hoang, T.F.; Kalogeropoulos, Theodore E.; Powell, Wilson M.

Novel Method of Spectroscopy With Applications to Precision Fine Structure Measurements

Abstract: A new spectroscopic method which promises to yield very precise measurements of some atomic fine structure intervals is described. This technique exploits interference phenomena that can occur in the resonance fluorescence of an atom in which two of the excited Zeeman substates cross. The method will be described in reference to the apparatus utilized for the determination of the helium 2^P^-2 ^P^ separation. See Fig. 1(a). A beam of helium light is projected through a helium discharge tube and detected by a PbS cell sensitive to the helium 1-micron line. The discharge tube contains ~ 0.2 mm of pure He and is situated between the poles of a Varian 12-in. magnet. The discharge is maintained by a high-

Observation of Nuclear Resonance in a Ferromagnet

Abstract: A strong resonance was detected at room temperature in a sample of finely divided face-centered-cubic cobalt (Co/sup 59/) metal. The resonance frequency was measured to be 213.1 Mc/sec in zero applied field. The observed frequency indicates a magnetic field of 213,400 oersteds at the site of the cobalt nuclei. This very sizeable magnetic field arises from both core and conduction electron contributions. (auth)