FREQUENCY AND MAGNETIC-FIELD DEPENDENCE OF THE DIELECTRIC-CONSTANT AND CONDUCTIVITY OF LA2CUO4+Y

Detailed measurements are reported of the conductivity and dielectric constant frequencies up to 20 MHz for single crystals of La2CuO4+y, with y varied in the range 0.001-0.01. The hole concentration is determined for each y by Hall measurements. The conductivity shows a power-law dependence on frequency (w(s)), which is typical of thermally assisted tunneling between localized states. The exponent s < 1 decreases with increasing temperature and increasing oxygen content. The dielectric constant decreases with increasing frequency at low frequency with the power law w(s)-1, showing that it is dominated by the same hopping mechanism. At high frequency the dielectric constant saturates and, for the electric field parallel to the CuO2 layers, the saturation value increases with oxygen content. From the polarizability at low acceptor concentration the radius of the bound hole is found to be approximately 8 angstrom, a value that is consistent with a simple hydrogenic model of the impurity state. The small binding energy of the hole to the impurity, 35 meV as determined from Hall measurements, together with this radius of 8 angstrom, requires that the mass of the hole be fairly small, approximately 2 m(e). The growth of the localization length with hole density is almost purely two dimensional, implying that there is not true insolator-to-metal transition, but rather a crossover from strong to weak localization. From studies of the magnetic-field dependence of the frequency-dependent conductivity and dielectric constant one finds new evidence for the strong coupling of the excess holes to the antiferromagnetically ordered Cu spins.