Connection between Near Millimeter Range Dielectric Loss and High Frequency Acoustic Attenuation
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Connection between Near Millimeter Range Dielectric Loss and High Frequency Acoustic Attenuation Boris M. Garin, Sergey N. Ivanov*, Efim N. Khazanov, Ivan P. Nikitin Institute of Radio Engineering and Electronics of Russian Academy of Sciences 11, Mokhovaya str., Moscow 103907, Russia Tel.: +7(095)203-4774, Fax: +7(095)203-8414, E-mail: [email protected] ABSTRACT A series of crystalline isomorphic solid solutions of Yttrium-Lutetium Aluminum Garnets (Y1-cLuc)3Al5O12 (c = 0 − 1) has been studied. The measurements of dielectric loss (DL) tangent tgδ has been performed at electromagnetic wavelengths 1 − 0.6 mm and temperature T = 300 K. The results obtained has been compared with longitudinal acoustic waves (AW) attenuation data for frequencies f = 1 − 9.4 GHz, T = 4.2 − 300 K and with theory. The correlation between the DL and the AW attenuation is observed. The dependencies of DL (at T = 300 K) and AW attenuation (at T ≥ 77 K) on concentration c are qualitatively identical in the whole interval c = 0 − 1. From comparison with theory it follows that the observed DL is due to the two-phonon intrinsic lattice loss. It is caused by the lattice anharmonicity as well as the AW attenuation at not too low temperatures. Also the prediction for concentration dependence of DL at very low temperatures is discussed that follows from comparison with the results of “heat pulse” propagation experiments for the same samples at T = 2 K and theory. INTRODUCTION The connection between dielectric loss (DL) at near millimeter (NM) and neighbouring electromagnetic wavelengths range and the high frequency lattice vibrations properties, including acoustic wave (AW) attenuation in crystals, follows from theory of lattice loss [1,2]. Particularly it follows that low attenuation of high frequency AW and high thermal conductivity are important criteria of low DL materials at the given range [3]. On the other hand, the problems of revealing and creation of both extremely low DL materials and materials with low AW attenuation are of key significance for various applications (high quality electromagnetic and acoustic resonators, waveguide systems, most powerful gyrotron windows, delay lines, etc.) in the microwave technology [1,3-5]. So the experimental research of connection between mentioned dielectric and acoustic phenomena is of considerable current interest and the lattice nature of DL in the given range needs direct confirmations. As mentioned above, in insulators and semi-insulators the DL is determined by the lattice loss. In ideal crystals only the multiphonon (two-phonon [1,2], three-phonon [1], etc.) absorption processes are allowed at the given range (except of the immediate vicinities of the frequencies of the infrared active fundamental optical lattice modes). They are determined by the lattice anharmonicity. The latter is connected with higher order terms in a series expansion of the potential energy and the dipole moment of a crystal in terms of displacements of atoms. This kind of loss may be called «intrinsic lattice loss» (IL
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