摘要:

A self‐consistent effective medium method of computing the effective conductivity tensor of polycrystalline metals in a magnetic field is presented. The method is applied subsequently to the study of the influence of three different factors 1) fluctuations of the electron density 2) open orbits 3) extended orbits on the galvanomagnetic properties of polycrystalline samples. Formulas obtained by the author are compared with those obtained earlier by Herring and later by Dreizin and Dykhne.It is concluded that a prolonged linear growth of the magnetoresistance cannot follow from a single relaxation time application of the LAK theory. Some possibilities of obtaining such a linear growth are pointed out, like small angle scattering and broken orbits.

ISSN:0094-243X    

DOI:10.1063/1.31133

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The anomalous magnetoresistance of the simple metals is considered to be one of the outstanding unsolved porblems in metal physics. These metals, of which potassium is the archetype, have a transverse electrical magnetoresistance which increases linearly with the applied magnetic field. This is in strong disagreement with the semiclassical theory of transport in metals, which, for these metals, predicts a saturating (field independent) electrical magnetoresistivity. Many different approaches have been investigated in an attempt to explain the linear magnetoresistivity, including both intrinsic and extrinsic theories. It has recently been shown that a linear electrical magnetoresistivity of the correct order of magnitude can be obtained by considering the conductivity to be inhomogeneous. These ideas are examined, insofar as they apply to potassium, in the light of the transverse and longitudinal electrical magnetoresistivity data and in the light of recent measurements of the other transport coefficients of potassium (the Hall and Righi‐Leduc coefficients and the transverse and longitudinal thermal magnetoresistivity). We conclude that inhomogeneous conduction is not a likely cause of the magnetoresistance anomalies.

ISSN:0094-243X    

DOI:10.1063/1.31134

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Current jetting and current channelling are discussed as representative models of magnetoresistance due to inhomogeneities. Induced torque experiments on the linear magnetoresistance of potassium show that its orientational dependance has uniaxial symmetry; and that the axis of symmetry is determined by the direction of growth of the single crystal. These, and a variety of other contactless measurements are consistent with a boundary‐conditions model for the current flow around non‐spherical inhomogeneities.

ISSN:0094-243X    

DOI:10.1063/1.31135

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Small metal particle composite systems have been produced from a number of metallic elements both by co‐evaporation of a metal and dielectric and by the evaporation of a metal in an inert gas atmosphere. The physical properties of these systems will be discussed. The optical response of several of these composite systems has been measured in the visible and near‐infrared spectral regions. Comparison between the data and predictions of effective medium theories will be made, and possible causes for the cases of disagreement discussed.

ISSN:0094-243X    

DOI:10.1063/1.31136

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

This paper reports on optical transmittance through samples comprised of ultrafine gold particles. These were prepared by two techniques: gas evaporation and island growth in discontinuous films. The measured data could be brought into detailed quantitative agreement with computed data based on the Maxwell‐Garnett theory provided an accurate model for the particle morphology was employed and the local‐field effects were accounted for properly.

ISSN:0094-243X    

DOI:10.1063/1.31137

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Optical properties of composite materials, such as aggregated metal milms, rough surfaces, cermets (metal‐insulator mixtures), semiconductor‐insulator mixtures and other multiphase systems have been of continuing interest since the beginning of the century. A review is given of their optical properties in the infrared and the visible. When the wavelength of light is large compared to the grain size, the composite material can be described in terms of a dielectric constant which is a function of the dielectric constants of the component materials and their relative concentrations. The applicability of theoretical models to the various composite materials is discussed. The importance of composite optical materials in solar energy applications is pointed out.

ISSN:0094-243X    

DOI:10.1063/1.31138

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Current interest in optimum surfaces for solar collectors has focussed on rough surfaces. We consider surface texture on a scale less than the wavelength of incident light. We utilize effective medium theories to relate a spatial variation in mass density to a spatial variation in dielectric constant. An iterative solution to Maxwell’s equations for the above configuration is stated in terms of reflectivity, transmission and absorption for the surface grade as a function of angle of incidence, polarization and wavelength. Comparison is made to an exact analytic solution. Experimental data are presented for some examples of microscopic texturing. Finally we consider within the framework of the effective medium theory the contribution of polarization phenomena.

ISSN:0094-243X    

DOI:10.1063/1.31139

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The problem of calculating the optical properties of an assembly of small metallic spheres (radius a) dispersed in a dielectric host is approached from the standpoint of multiple scattering theory as formulated for scalar fields and periodic systems by Morse. If k is the largest wave vector of the radiation in either the bulk constituent or in the composite, the leading term for the dielectric function in the limit ka→0 emerges as the Maxwell‐Garnett result. The result is subject to significant modification, however, if structure in the composite large on the scale of a wavelength (e.g. clustering or periodicity in the dilute limit) is present. For both Maxwell‐Garnett and effective medium theories the effects of introducing a third component in the form of an insulator coating (spatially correlated, therefore, to the metallic component) are examined, particularly in the frequency dependent reflectivity. Finally, an argument is given for the two component system which leads to the Maxwell‐Garnett result from consideration of an effective medium approach for insulator‐coated metallic inclusions, these properly reflecting the topology of such composites. For m(≳2) component systems an expression for the effective dielectric function is derived by applying a similar argument.

ISSN:0094-243X    

DOI:10.1063/1.31140

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The far‐infrared absorption coefficients of &agr; of 100 A˚ of a number of metals and also of amorphous and crystalline Al203have been measured at 4.2K and are orders of magnitude larger than effective medium calculations of &agr; which rely on bulk dielectric constants. These results complement earlier measurements on Al particles1. We find that: 1. &agr; varies linearly with filling fraction f for f?15% 2. The magnitude of &agr; does not depend to any great extent on whether the particles are free standing or are embedded in a KBr matrix, and 3. Particles of Au and Pd which do not form oxide coatings have larger absorption than do particles of Al or Cu with oxide coatings. A comparison of the resonance absorption in the visible part of the spectrum for Au and oxide‐coated Cu particles dispersed in KBr reveals that the key role of the oxide coating is to transfer oscillator strength from the far‐infrared to the small particle resonance in the visible.

ISSN:0094-243X    

DOI:10.1063/1.31141

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The reflectance in the infrared and visible (0.08 eV to 3.2 eV) has been measured for aluminum small particles compacted into potassium chloride. The aluminum particles were prepared by evaporation in inert gas atmosphere and had mean diameters of 200A˚ and 600A˚. The volume fraction, f, of metal in the samples studied was between f=0.03 and f&angupr;1. For f&angupr;0.15 the specimens exhibited finite dc conductivity. The main feature of the data is a decreasing reflectance with increasing frequency. The magnitude of the reflectance increases with metal concentration. Comparison of experiment with simple theories shows that the self consistent theory predicts the general form of the reflectance if the low concentration for first achieving conduction is taken into consideration. Kramers‐Kronig inversion of the reflectances yield frequency dependent conductivities which are not like those of a simple metal.

ISSN:0094-243X    

DOI:10.1063/1.31142

出版商:AIP    

年代:1978

数据来源: AIP