Common optical absorption features are observed near 0.5 eV in four undoped single-crystal copper oxides: La2CuC>4, Nd2CuC>4, Pr2Cu04, and S^CuC^Ch. These absorption bands are shown to be weakly electric dipole allowed excitations of the Cu02 layers. The features are ascribed to an exciton near 0.4 eV, probably of crystal-field origin, with strong multimagnon sidebands. Our observations provide an important basis for understanding the doped materials.

The proper basis for the calculation of transmission and reflection times for wave packets scattered off arbitrary tunneling structures in one dimension is considered. With packets narrow in wave-number space, we demonstrate that the classic phase times are indeed correct to lowest order. Explicit, general expressions for the leading correction terms for finite wave packets are given. The physics associated with these corrections is discussed. We also consider the dwell time, as it is currently defined, and derive a general relation between this dwell time and the phase times. This relation shows when the dwell time can and cannot be used. Finally, we discuss wave packets transmitted from narrow resonances, and derive an explicit, exact formula for the tunneling time with resonance transmission from a symmetric double barrier. Comparison with earlier approximate results is made.

The reflectivity and its temperature dependence in the neighborhood of the charge-transfer band is reported for carrier-free La2CuC>4. There is a sharp peak in the imaginary part of the dielectric function for light polarized parallel to the Cu02 layers. It is shown that the line shape as well as its temperature dependence is consistent with short-range electron-hole interaction in the final state of the chargetransfer excitation broadened by moderate coupling to optical phonons.

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Polarized electrore6ectance spectra are reported for single-crystal La2Cu04+y Depending on the electrode structure, the primary effect of modulating the electrode potential is addition of charge carriers or variation of the electric field in the space-charge region. By adding charge carriers we observe changes in the optical spectra similar to those caused by chemical doping. For modulation of the electric field the perturbed absorption spectrum is very different, corresponding to electric-field-induced crystal-field transitions. Bands near 1.4 and 1.6 eV respond to the electric field in ways consistent with d 2 2~d y and d 2 2~d dy transitions, respectively. Our observations are consistent with Perkins Zy Z et al. [Phys. Rev. Lett. 71, 1621] who ascribe transitions near 0.4 and 1.5 eV to the d z 2~d 2 2 and d 2 2~d"~e xcitons. z p 3z Zy 2.0

Metal-insulator-semiconductor field effect transistors have been fabricated using laser ablation to deposit YBaCuO thin films onto SrTiO3, MgO, LaAlO3, and LaGaO3 substrates. The substrates were used as gate insulators. The conductivity of two films on SrTiO3 could be modulated, while for other samples the conductivity was independent of the gate voltage. The field-effect mobility was extracted and found to be of magnitude comparable to the mobility of metallic YBa2Cu3O7.

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