A.R. Mackintosh

Fermi surfaces and effective masses in F.C.C. transition metals

Abstract The energy band structures of the f.c.c. transition metals Rh, Pd, Ir and Pt have been calculated by the RAPW method. The calculated Fermi surface areas are in good agreement with de Haas-van Alphen measurements. The calculated electronic heat capacity is greater than the experimental by a factor 1.4, 1.7, 1.3 and 1.6 for Rh, Pd, Ir and Pt respectively.

Energy gaps in spin-wave spectra☆

Energy gaps in the spin-wave spectra are used to calculate the temperature dependence of the saturation moment in terbium. The energy gap manifests itself at low temperatures in all properties that depend on the magnetic specific heat in a simple ferromagnet. The temperature dependence of the electrical resistivities of lutetium, terbium, dysprosium, and holmium are determined for low temperatures. (C.E.S.)

Magnetic Interactions in Tb and Tb‐10% Ho from Inelastic Neutron Scattering

The magnon dispersion relations and lifetimes have been measured in Tb and a Tb‐10% Ho alloy by inelastic neutron scattering, in regions of both ferromagnetic and spiral ordering. In the ferromagnetic phase, the magnon energy is generally finite at zero wavevector and rises quadratically at low q. The magnon energies scale approximately with the magnetization. In the spiral phase the magnon energy rises linearly from zero at low q. The Fourier‐transformed exchange parameter J(q) has pronounced peaks in the c direction, which are ascribed to transitions between states close to the Fermi surface. These peaks are less pronounced in the ferromagnetic phase. The primary mechanism limiting the magnon lifetimes appears to be interaction with the conduction electrons. In the alloy, the lifetime for magnons propagating in the c direction in the ferromagnetic phase falls abruptly at about q = 0.35 A−1, and this may be due to the exchange splitting of the conduction‐electron energy bands. The dispersion curve for ma...

Model for the Electronic Structure of Metal Tungsten Bronzes

The electronic properties of the metal tungsten bronzes of high x value are interpreted in terms of a model in which the conduction electron wavefunctions are derived principally from the metal atomic p functions. It is shown that the physical properties of the bronzes are consistent with this model, which is compared, in this respect, with others. The transition to metallic conductivity at low x values is discussed, using the theory of Mott, and a number of experiments are suggested for clarifying some features of the electronic structure of the tungsten bronzes, and also of the related vanadium bronzes.

The thermoelectric power in chromium and vanadium

Abstract The thermoelectric power in pure chromium and vanadium has been measured between 4.2 and 340°K and found to be positive in both metals over the whole temperature range. Maxima in the curves of thermopower against temperature at low temperatures are interpreted in terms of a phonon drag component. An anomaly in the results for both monocrystalline and polycrystalline chromium at the Neel temperature is interpreted in terms of the L idiard -O verhauser model of conduction electron antiferromagnetism. Hysteresis in the measurements on chromium over the whole temperature range is attributed to the effect of antiferromagnetic domains. An anomaly in the thermopower of vanadium at approximately 217°K is attributed either to a transition to an antiferromagnetic state or to a structural phase transition, possibly involving ordering of impurities. Experiments for distinguishing between these possibilities are suggested.

ELECTRONIC STRUCTURE OF LIQUID GALLIUM BY POSITRON ANNIHILATION

The observation of the angular correlation of the photons produced in electron-positron annihilation is a panticularly suitable method for studying the conduction electrons in liquid metals. It is a technique which can be applied to disordered systems, and the angular distribution of the photons reflects directly the momentum distribution of the electrons. Positron annihilation experiments on liquid and solid gallium indicate that the free electron model is not a satisfactory description of the liquid near its melting point. (R.E.U.)

MAGNETORESISTANCE IN RARE EARTH SINGLE CRYSTALS

Abstract The magnetoresistance of single crystals of holmium and dysprosium has been measured over a wide temperature range in the region of magnetic ordering. The results reflect the modification of the Fermi surface and the changes in the spin wave scattering of the conduction electrons when the magnetic ordering changes. When a large field is applied along the magnetically easy direction and the magnetic periodicity is destroyed, a large decrease in the direction is observed, primarily due to the removal of superzone energy gaps in the electron dispersion curves. Changes of resistivity in the basal plane are thought to be mainly a consequence of changes in the spin wave spectra. A short discussion is given of the motion in a magnetic field of an electron in a periodic magnetic structure.

Positron annihilation in rare-earth metals☆

Abstract The electronic structure of three rare-earth metals has been studied by measuring the angular correlations of the photons emitted when positrons annihilate with the electrons in the metal. From the results it is deduced that gadolinium and cerium at room temperature each has approximately three conduction electrons per atom, while ytterbium has approximately two as expected from the localized f -electron model. Qualitative information is obtained about the distortion of the Fermi surfaces of these three metals and it is concluded that the polarization of the conduction electrons in the magnetically ordered state of gadolinium is small. Annihilations with ion core electrons comprise a significant proportion of the total in each metal, the number relative to the conduction electron annihilations decreasing with increasing atomic number.

Novel magnetic phases in holmium

Abstract The new magnetic phases, involving spin-slip and helifan structures, which have recently been identified in Ho, are described. The genesis of these structures and the cone in the competing magnetic interactions, and their possible significance for other rare-earth systems, are discussed.

The First Electronic Computer

Until recently most Europeans interested in computing would have claimed that the first electronic computer was the Colossus, designed and constructed in Bletchley, England, by the mathematician Alan Turing and his colleagues, operational in December 1943 and used to decipher the German Enigma code, with a decisive effect on the course of World War II. Most Americans, on the ther hand, would have given the honor to the Electronic Numerical Integrator and Calculator, built by John W. Mauchly and J. Presper Eckert at the Moore School of Electrical Engineering, University of Pennsylvania, and operational in late 1945.