Infrared Absorption by Granular Metals
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INFRARED ABSORPTION BY GRANULAR METALS. D.B. Tanner, Y.H. Kim,* and C.L. Carr Department of Physics, University of Florida, Gainesville, FL 32611. ABSTRACT The infrared properties of granular metals and superconductors are qualitatively in accord with effective medium ideas, with insulating behavior below a percolation transition and metallic response above. An exception is the far-infrared absorption at low metallic concentrations, which is much stronger than theoretical predictions. Measurements of superconductors and of normal metals in different hosts suggest that this absorption is predominately electric dipole rather than the magnetic dipole (eddy current) absorption which is expected to be the dominant low-frequency loss in highly conducting particles. Measurements of clustered and non-clustered samples suggest that the strong far-infrared absorption does not arise from the clustering together of the individual metallic particles, although clustering does lead to about a tenfold increase in absorption. INTRODUCTION The far-infrared properties of small metallic particles have been studied for many years 6 8 now.' 2,'A'4 Early measurements, ',7, found that the absorption is substantially larger than the predictions of classical theory. More recently it has been suggested that clustering of the 9 10 particles could explain the absorption. ' For relatively large (-. 400-1200 A), well isolated particles, it has been shown that a combination of eddy current (magnetic dipole) absorption in the metal particle and electric dipole absorption in an oxide coating around the metal is 1 consistent with the measured absorption.' However, measurements for unclustered particles 2 this picture.' with consistent not in the 100 A range are When superconducting small particles are measured, another "anomalous" result is 3 found:' the absorption below T, is larger than above T,. This result is inconsistent with eddy current loss in the small-particle system. It does show that the absorption is due to the metal and not some oxide coating. EXPERIMENTAL Small-particle/insulator composites 8 3 The samples were prepared as previously described7, ,12," Al and Sn metal particles were made by gas evaporation. These particles were mixed with either KCI or paraffin powders at low volume fraction (0.1%-5%) and compressed into a thin wafer. The wafers were reground (at 77 K) and recompressed up to five times to better disperse the metal in the sample.
The metal grain size was determined from electron microscopy. The small particle samples had log-normal size distributions with a geometric standard deviation of 1.4-1.6. Dark field/bright field comparisons show that there is an oxide coating about 15 A thick surrounding the Al particles. In addition, STEM studies for the Al/KC1 samples showed that they contained isolated particles and not clusters.12 * Present address: Department of Physics, University of Cincinnati, Cincinnati, OH 45221 Mat. Res. Soc. Symp. Proc. Vol. 195. @1990Materials Research Society
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