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2. LR. Testardi, JH. Wernick. and WA. Royer, Solid State Comm. 15, 1 (1974); J. R. Gavaler, M. H. Janoko, and C. K. Jones, J. Appi. Thys. 47, 7 (1974).
3. I. Ya. Lekhtyar, L. I. Ivanov, N. V. Karlov, G. P. Kuz’min, M. M. Nishchenko, A. M. Prokhorov, N. N. Rykalin, and V. A. Yanushkevich, Sov. J. Quant. Electron. 6, 460 (1976).
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