A M Stewart

The Al-Mn decagonal phase 1. A re-evaluation of some diffraction effects 2. Relationship to crystalline phases

Abstract Abstract From a quenched, melt-spun ribbon of a specimen of Al76Mn24, bulk composition, the decagonal phase and a closely related ‘Al3Mn’ phase have been examined using analytical electron microscopic techniques. It is proposed that descriptions of the decagonal phase by analogy to an icosahedral axial system be abandoned and that a different six-axis system be adopted instead, and that the non-crystallographic space group previously proposed for the decagonal phase be changed to one containing glide planes. The ‘Al3Mn’ phase is seen to be an extremely close relative of the Al-Mn decagonal phase. However, the state of knowledge about the ‘Al3Mn’ structure is too poor to be of any use in improving the understanding of atomic arrangements in the Al-Mn decagonal phase.

Vector Potential of the Coulomb Gauge

The vector decomposition theorem of Helmholtz leads to a form of the Coulomb gauge in which the potentials are expressed in a form that is totally instantaneous. The scalar potential is expressed in terms of the instantaneous charge density, the vector potential in terms of the instantaneous magnetic field.

Angular momentum of the electromagnetic field: the plane wave paradox resolved

The angular momentum of a classical electromagnetic plane wave of arbitrary extent is predicted to be, on theoretical grounds, exactly zero. However, finite sections of circularly polarized plane waves are found experimentally to carry angular momentum and it is known that the contribution to the angular momentum arises from the edges of the beam. A mathematical model is described that gives a quantitative account of this effect and resolves the paradox.The angular momentum of a classical electromagnetic plane wave of arbitrary extent is predicted to be, on theoretical grounds, exactly zero. However, finite sections of circularly polarized plane waves are found experimentally to carry angular momentum, and it is known that the contribution to the angular momentum arises from the edges of the beam. A mathematical model is described that gives a quantitative account of this effect and resolves the paradox.

Use of magnetic forces to control distance in a surface force apparatus

A simple system that permits the surface separation to be varied smoothly and accurately in a surface force apparatus is described. It is based on the deflection of the force-measuring spring in a magnetic field gradient created by a current passing through two coils. The resulting displacement of the surface is highly linear with applied current and free of hysteresis over separations of 10 mu m or more. This allows the accurate measurement of the force between two surfaces by monitoring the difference between expected and observed spring deflection (using, for example, multiple-beam interferometry).

Specific heat study of a holmium single crystal

The magnetic transition at the Neel temperature, TN, of the heavy rare-earth metal holmium has been examined by specific heat measurements. The transition at TN=(132.245+or-0.080)K is found to be continuous and the data around TN have been analysed in terms of the equation C+or-=(A+or-/ alpha +or-) mod t mod - alpha +or-(1+E+or- mod t mod x+or-)+B+or-+D+or-t where t=(T-TN)/TN. The value of the critical exponent alpha is 0.27+or-0.02 for the data in the critical regions -3.8<log10 mod t mod <-1.2 below TN and -3.9<log10 mod t mod <-2.9 above TN. The authors find that the scaling law alpha += alpha - is valid and the condition TN+=TN- is obeyed in the critical regions. The value of alpha (=0.27) that they obtain is not close to the predictions of renormalisation group theory for magnetic systems with short-range exchange interactions. A cross-over from the critical region to a region with different behaviour is observed at log10 mod t mod =-2.9 above but not below TN. An unidentified anomaly is observed in the specific heat of holmium at 98K.

Angular momentum of light

By means of the Helmholtz theorem on the decomposition of vector fields, the angular momentum of the classical electromagnetic field is decomposed, in a general and manifestly gauge invariant manner, into a spin component and an orbital component. The method is applied to linearly and circularly polarized plane waves in their classical and quantum forms.By means of the Helmholtz theorem on the decomposition of vector fields, the angular momentum of the classical electromagnetic field is decomposed, in a general and manifestly gauge invariant manner, into a spin component and an orbital component. The method is applied to linearly and circularly polarized plane waves in their classical and quantum forms.By means of the Helmholtz theorem on the decomposition of vector fields, the angular momentum of the classical electromagnetic field is decomposed, in a general and manifestly gauge invariant manner, into a spin component and an orbital component. The method is applied to linearly and circularly polarized plane waves in their classical and quantum forms.

Force feedback surface force apparatus: Principles of operation

The principles of operation of a force feedback surface force apparatus are described. A magnetic force transducer and a bimorph displacement sensor are connected in a servo loop. The dynamic behavior of the system is analyzed and the effect of eddy currents on the frequency response of the magnetic field is discussed. The apparatus has several advantages over more traditional force measuring techniques. Cantilever instability associated with the measurement of attractive forces is reduced and the apparatus may be used for hydrodynamic measurements. A resonance technique for measuring the spring constant is also described.

The effects of oxidation and reduction on the X-ray photoelectron spectra of YBa2Cu3O7-x

Abstract Detailed X-ray photoelectron spectra (XPS) have been obtained from YBa2Cu3O7-x specimens which have a superconducting transition temperature of 93 K. The effects of reduction on heating in uacuo at 400°C and of oxidation at 400°C in air have been considered. It is found that in general the XPS signatures of the Cu 2p peaks are consistent with the presence of formally mixed valence Cu(II) and Cu(III) atoms, although a unique valence cannot be assigned. Reduction in vacuo apparently reduces all copper atoms to divalency as shown by the increasing intensity in the ‘shake-up’ satellites. Re-oxidation in air does not fully restore the sample to its original state, although the superconducting transition and the Meissner effect are recovered. It is also found that the low-binding-energy components of the O 1s and Ba 3d XPS peaks are characteristic of bulk states and cannot be ascribed to surface contamination and/or surface-specific states. Evidence is found for migration of barium to the free surface ...

Mössbauer and magnetic resonance experiments on amorphous iron-silicon films

Abstract Mossbauer measurements at room temperature and 4.2 K, and room temperature magnetic resonance measurements on a series of amorphous Fe x Si 1- x thin films (0.23⩽ x ⩽0.81) are presented. The concentration dependence of the isomer shifts and quadrupole splittings provides information on the nature of the local coordination in these amorphous materials. Analysis of the distributions of magnetic hyperfine splitting combined with the presence of multiple resonances in the magnetic resonance data indicates that magnetoanisotropy plays a dominant role in determining the magnetic behaviour of these films.

Reply to Comments on 'Vector potential of the Coulomb gauge'

The instantaneous nature of the potentials of the Coulomb gauge is clarified and a concise derivation is given of the vector potential of the Coulomb gauge expressed in terms of the instantaneous magnetic field.The instantaneous nature of the potentials of the Coulomb gauge is clarified and a concise derivation is given of the vector potential expressed in terms of the instantaneous magnetic field.