New results of infrared photoexcitation experiments on undoped La2CuO4 and Nd2CuO4 are reported. We observed two broad electronic excitations within the charge-transfer energy gap and an associated phonon bleaching, possibly originating from strong charge-lattice coupling. Our results are discussed in terms of the condensation of self-localized photocarriers in the CuO2 planes at low temperatures. © 1991 The American Physical Society.

We review the crystal chemical, electronic, magnetic and superconducting properties of La2CuO4 and related planar-cuprate materials, with emphasis on single-crystal results. Although magnetism due to divalent copper ions clearly is evident in nonsuperconducting crystals, experiments do not reveal any direct relationship between localized copper spins and high temperature superconductivity. We also indicate areas where important information is lacking for our understanding of these materials. © 1989.

A careful study of the metamagnetic transition in single-crystalline La2CuO4 is presented. From magnetic susceptibility measurements the critical exponent () of the weak ferromagnetic state, which terminates at a triple point, has been estimated to be =0.50.02, a value consistent with mean-field theory. Furthermore, the pressure dependence of the critical magnetic field for the transition has been examined from magnetoresistance measurements under hydrostatic pressure. © 1989 The American Physical Society.

A systematic study of the phase stability of La(Pr)2-x-yRxSr(Ba, Ca)yCuO4 (R = rare earth) compounds reveals a broad composition range for T*-phase formation. Several members of this series of compounds show bulk superconducting transition temperatures up to Tc ∼ 20 K. These superconducting cuprates raise further questions about the relationship between structural stability and high temperature superconductivity in oxides. © 1989 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division).

The magnetic field dependence of the three dimensional magnetic ordering transition has been examined in single crystalline La2CuO4-δ by magnetization and magnetoresistance measurements. We establish a phase diagram in the magnetic field-temperature plane and point out similarities to metamagnetic transitions observed in other magnetic systems. © 1988.

Measurements of electron paramagnetic resonance (EPR) at 9 and 35 GHz between 2 and 300 K in single crystals and powder samples of CuGeO3 are presented. Below 14 K a large decrease in the intensity of the Cu 2+ EPR signal is observed. The data can be interpreted as due to a spin-Peierls transition. However, an alternative interpretation in terms of a simple structural transition cannot be ruled out. An opening of an energy gap of ∼25 K is calculated from the analysis.

Superconducting crystals of La2CuO4+x prepared by high-pressure oxygenation have been analyzed by Raman spectroscopy. A direct comparison of the role of excess oxygen was made by examining the same crystals with and without excess oxygen. La2CuO4+x, like nonsuperconducting La2CuO4.0, is found to have a soft phonon associated with a tetragonal-to-orthorhombic phase transition. The Ag phonons of La2CuO4.0 and La2CuO4+x occur at essentially the same frequency. At room temperature, La2CuO4+x has no well-defined peak from two-magnon scattering, unlike La2CuO4.0. However, in its phase-separated form, La2CuO4+x exhibits well-defined, two-magnon scattering. This establishes that the La2CuO4.0 phase present in La2CuO4+x at low temperatures is antiferromagnetic. La2CuO4+x samples prepared by slightly different methods are found to have differing amounts of excess oxygen, as indicated by variations in the intensity of the phonon and magnetic scattering. Certain samples of La2CuO4+x had a phonon peak at 630 cm-1 that is absent in La2CuO4.0. While a definitive assignment is not possible, the frequency of this peak is consistent with a peroxidelike species in La2CuO4+x. © 1991 The American Physical Society.

The anisotropic electrical resistivity and thermoelectric power S have been determined on single crystals of La2CuO4 in which the hole doping has been varied by controlled anneals. An enormous decrease and hysteresis in both and S occur at the Néel temperature TN for small, suggesting strong coupling between magnetic order and transport. Possible origins of these transport anomalies are discussed. © 1989 The American Physical Society.

We have measured the dielectric constant, ε{lunate}, in the semiconductors La2CuO4+x and EuBa2Cu3O6+x. In both cases the crystals are grown by standard techniques and annealed in inert gases, which leads to a semiconductor with x{all equal to}0, whereas an oxygen anneal produces a superconductor. We have shown earlier that these results are intrinsic to La2CuO4 by measuring the temperature dependence of ε{lunate} at high frequencies. The intrinsic values are ε{lunate} = 23 for the c-axis of both materials, ε{lunate} = 45 in the a-b plane of La2CuO4, and ε{lunate} = 32 in the a-b plane of EuBa2Cu3O6. The implications of these large dielectric constants for high temperature superconductivity are discussed. © 1990.

A polarization Cu K-edge x-ray-absorption near-edge structure (XANES) study has been carried out on Pr2-xCexCuO4 single crystals. The spectra for x-ray polarization vector E nearly parallel to the crystal c axis suggest that electrons contributed by Ce doping are initially localized at the Cu site. The spectra for E perpendicular to the c axis exhibit an almost rigid edge shift to lower energies upon Ce doping. This suggests that the unoccupied in-plane Cu 4p states shift to lower energies. Therefore, the Ce doping donates electrons to the Cu site and also shifts the unoccupied 4p band. We propose that the upper unoccupied band consisting of predominantly Cu 3d states shifts downward and eventually joins the initially localized states near the Fermi level and thus, forms the conduction band in the n-type superconductor. © 1990 The American Physical Society.