External Transverse Field Research Articles

Diamagnetic properties of the electron-exciton bound states in charge-transfer organic solids

An effective Hamiltonian is established which describes the excitation spectrum of independent quasiparticles arising from the coherent electron-exciton pairing in molecular solids of the charge-transfer type. This Hamiltonian is then used to study the interaction of the quasiparticles with an external transverse electromagnetic field. An expression for the dielectric function is derived which describes the dielectric properties of the system. In the presence of electron-exciton pairing and in the long-wavelength limit, the diamagnetic current is finite and is of the London type, indicating the existence of the Meissner effect. The corrections to the London value for the diamagnetic current depend on the magnitudes of the binding energy of the exciton, the gap function, and the energy difference between the excited and ground states of the system. In the absence of electron-exciton pairing, the diamagnetic current vanishes and the dielectric function corresponds to that for the exciton band.

Magnetic Domain Structures in Thin Uniaxial Plates with Perpendicular Easy Axis

We present a convenient formulation of the single-domain energy in the wall-energy model and minimize the total energy of a lattice of cylindrical domains and of a parallel-stripe array in an infinite plate. The latter structure is shown to have the lower energy for external transverse fields Hex<H1 while the lattice is favored for H1≤Hex<H2. The critical domain sizes and fields bounding these structures are calculated as functions of the plate thickness and the material parameters. The factors which affect the evolution of the domain structures in finite and nearly defect-free plates are discussed and the following conclusions drawn: Domain nucleation is favored by the presence of nonuniformities such as cracks or mounds. Stripes nucleated at cracks or boundaries expand and contract with one end tied to the nucleation point and there may be only as many stripes as nucleation centers. An energy barrier inhibits the separation of a domain from (or the joining to) a free edge boundary or another domain. Closed domains are repelled by boundaries and alternate between stripe- and bubble-domain configurations as the local field varies below and above the critical stripe-bubble instability value. The qualitative conclusions can be quantified to some extent in the wall-energy model and provide a satisfactory explanation of the domain structures observed in a high-mobility garnet film.

On the Influence of Magnetic and Electrical Fields in Gas Lubrication

The paper examines some of the possibilities of MGD lubrication by considering some typical examples of external electromagnetic conditions and some simple types of bearings. Thus, for self-acting films, the case when an external tangential magnetic field combined with a transversal electrical field is studied. Pressures and load capacity are thus improved and, in addition, the friction forces can be diminished and even eliminated. For externally pressurized films, insulated surfaces subjected to an external transversal magnetic field can reduce the flow, provided that the Hartmann number has a sufficiently high value. Finally, it is pointed out that it is possible to produce pressures by electromagnetic means only. The electromagnetic pumping effect is produced, for instance, when a transversal electrical field is applied, combined with a tangential magnetic field, even when both lubricated surfaces are at rest. Some considerations concerning the practical possibilities of obtaining an electrically conductive gas at relatively low temperatures are also given.

CRITICAL CURRENT DENSITIES IN SUPERCONDUCTING NIOBIUM FILMS SPUTTERED REACTIVELY IN THE PRESENCE OF NITROGEN GAS

A critical current density of about 2 × 105 A/cm2 in a transverse external field of 73 kOe, parallel to the film plane, was obtained at liquid-helium temperature for niobium films deposited on glass substrates by sputtering technique in an argon atmosphere with a dilute addition of nitrogen. This critical current density is larger than any other reported values for refractory metal carbides and nitrides with the NaCl structure.

Comments on Intensity-Dependent Frequency Shift in Compton Scattering and Its Possible Detection

We show that the Hamiltonian of an electron coupled to an external radiation field is not Hermitian within the space of Volkov solutions with scattering boundary conditions. This is demonstrated by explicitly showing that the inner product of two states which are asymptotically orthogonal to each other fails to vanish at a finite time. It is further demonstrated that if the momentum operator is Hermitian within the (asymptotically) free-electron states, it fails to be Hermitian within the fully developed Volkov states. Consequently the parameters $p$ and ${p}^{\ensuremath{'}}$ entering into the Volkov solution with scattering boundary conditions cannot be interpreted as the momenta of the incident and emerging electron, respectively. The only consistent use of the Volkov solutions is for nonscattering situations, viz., the emission or absorption of a photon by an electron which is never decoupled from the external field. In the latter case the parameters $p$ and ${p}^{\ensuremath{'}}$ which enter into the energy-momentum conservation formula refer to average initial and final electron momenta. A consistent interpretation requires us also to compute the dispersion in the electron momenta. On this basis we also obtain a dispersion in the scattered frequency, a physically detectable quantity, which is much larger than the intensity-dependent frequency shift. On the basis of general quantum principles we also demonstrate: (a) that the Feynman-Dyson $S$-matrix technique is the correct procedure to use inasmuch as it gives agreement with energy-momentum conservation and unitarity, (b) that Kibble's explanation of energy-momentum conservation is incompatible with quantized perturbation theory, and (c) that in the case of external transverse fields the perturbation expansion yields a polynomial in the field strength. An experiment involving frequency mixing is suggested as a means of obtaining a maximum effect in the beat frequency due to the nonlinearity parameter ${\ensuremath{\nu}}^{2}$.

Topology of steady-state plasma configurations in transverse self-consistent fields: Spatial-periodic plasma structures: I

Neglecting collisions, we consider a kinetic approximation for the Boltzmann function of particle distribution with a longitudinal macroscopic velocity that does not depend on the coordinates. With it we solve the two-dimensional problem (∂/∂x≠0, ∂/∂y≠0, ∂/∂z=0) of the equilibrium configurations of an l-component plasma in self-consistent internal and external transverse magnetic fields (and also in electric fields caused by a non-compensation of plasma charge as a whole).The solution obtained with the given assumptions is a general one and permits one to make a broad analysis of equilibrium plasma configurations with longitudinal currents in self-consistent external and internal transverse fields. It is shown that existence of steady plasma states with a spatial-periodic structure is possible. This corresponds to the data of experiments in which it has been reliably established that a plasma arising in pinch discharges has a fibrous structure characterized by a definite space periodicity.

Magnetization Curves of Superconductive Tin Alloys

We have supplemented a previous investigation of flux trapping by obtaining the magnetization curves of very fine impure tin wires in a transverse external field. For annealed specimens (containing In, Bi, or Cd) in which the impurity is completely dissolved and homogeneously distributed, we find that some flux is frozen-in in decreasing magnetic field when the external field equals half the critical value. However, this flux leaks out monotonically as the field is further reduced, and none remains in zero field. In pure specimens virtually no flux trapping occurs. However, in a specimen containing more Cd than was completely soluble, considerable flux remains frozen-in even in zero field. These results confirm our previous inference that suitably prepared and annealed homogeneous alloy specimens are incapable of freezing-in appreciable amounts of magnetic flux. We further conclude that this is due to the fact that the flux which is initially trapped when the specimens begin to become superconducting readily leaks or migrates out, probably because of the tendency of lines of force to contract as suggested by Faber and Pippard.

Measurements of the magnetic anisotropies and the thermodynamic temperatures of potassium chromium alum below 1°K

Synopsis The magnetic susceptibility of CrK(SO 4 ) 2 .12H 2 O has been studied in zero external field for different amplitudes of the measuring field. In section 3 the dependence of the susceptibility on the direction of an external transverse field is described. The anisotropy pattern measured in this way at low entropies, proves to depend on the strength of the field and the entropy. The susceptibility in longitudinal fields has been measured and both magnetization curves and temperature variations along isentropics have been derived from these measurements. In section 5 new determinations of the thermodynamic temperatures are discussed, giving marked differences with our previous results. The discussion in section 6 gives a comparison between CrK(SO 4 ) 2 .12H 2 O and CrCH 3 NH 3 (SO 4 ) 2 .12H 2 O based on the occurrence of ferromagnetic walls between antiferromagnetic domains. The variation of the susceptibility with entropy and in external fields has been interpreted by means of a recent theoretical study of Miss Mary C. M. O'Brien on the antiferromagnetic state in chrome alums. In addition the new temperature scale for this salt and some consequences of this scale are discussed.