Two different self-consistency conditions are used in the literature to derive the effective-mediumapproximation equations. They are (a) that the net polarization of the medium should be zero, and (b) that the effective electric field should be the same as the average of the electric field in every grain.These two approaches lead to different predictions, as we show, for conducting spheroidal particles in an insulating background. It is found that the self-consistency condition of the zero net polarization field should be used to explain the transport properties of randomly inhomogeneous media.

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Our attempts to observe the antiferromagnetic resonance (AFMR) modes in La2Cu04+~in both single-crystal and small-particle forms have been unsuccessful. An argument based on a magnetic-susceptibility sum rule shows that the resonance previously identified with the AFMR in polycrystalline La2Cu04+~[R. T. Collins et al. , Phys. Rev. B 37, 5817 (1988)l is too strong to be compatible with a magnetic spin-wave mode. No evidence of an AFMR mode is found in our single-crystal La2Cu04+~transmission measurements versus magnetic field. Additional measurements on small particles show that the broad resonant feature observed around 8 cm ' is produced by an electric-dipole-active superconducting sphere resonance of the La2CuOq+~particles. The recent discovery of superconductivity' at temperatures above 30 K in La2 -"Ba,Cu04+~has brought much attention to the parent compound La2Cu04+y. Early magnetic susceptibility measurements on La2Cu04+s howed anomalies around 250 K which suggested antiferromagnetism in this compound. The antiferromagnetic ordering on the Cu sites was confirmed by elastic neutron scattering. Moreover, inelastic neutron scattering and two-magnori Raman-scattering measurements showed that the intraplanar antiferromagnetic exchange interaction in La2Cu04+~, as well as in YBaqCu306+y, is quite strong. Collins et a/. have reported a very broad antiferromagnetic resonance (AFMR) feature at 9 cm '. Using neutron scattering together with the results of Ref. 6, Peters eta/.reported in-plane and out-of-plane spinwave excitations at energies of 8+ 2 and 20+ 4 cm ', respectively. The identification of the modes would appear complete were it not for the fact that we find the measured strength of the reported AFMR mode to be more than an order of magnitude larger than the value required by the measured dc susceptibility. In this paper, we first show that the far-infrared line strength of the reported AFMR in powder samples is not consistent with the measured dc magnetic susceptibility. Next we demonstrate with single-crystal samples that an absorption line of the same strength is not observed. To identify the origin of the observed resonance we have made additional far-infrared measurements on LaqCu-04+~, in small particle form. Our study shows that the resonant feature observed by Collins et al. is most likely a superconducting sphere resonance similar to that previously observed in La2 -"Sr"Cu04+y.To show that the resonance feature reported by Collins et a/. is not consistent with AFMR, we use a susceptibility sum rule which relates the magnetic absorption strength of the AFMR to the dc magnetic susceptibility for linearly polarized light, namely j~h , a;ay~; (0) =, ", ' dto . n "p co Here g~;(0) is the contribution to the dc magnetic susceptibility in emu/cm produced by the ac response of magnetic mode i Both A. tt; and to are given in cm, and n is the low-frequency index of refraction produced by the electric dipole active TO modes of the lattice vibration spectrum; n is assumed to be a constant over the FIR frequen...

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