D. R. Gabbe

Phase diagram and single crystal growth of pure and Sr doped La2CuO4

Abstract Single crystals of pure and Sr doped La 2 CuO 4 have been grown from excess CuO with a top seeded solution growth technique. Crystals have been grown both in air and in oxygen. Growth in air leads to an oxygen deficiency as evidenced by a reduction in the lattice parameters. Differential Thermal Analysis measurements in oxygen on this system reveal a eutectic temperature of 1075°C for the composition 87.5 mol% CuO. Measurements in air show a reduction in the eutectic temperature to 1040°C and the formation of a 3 component system with the partial reduction of CuO to Cu 2 O.

Top seeded solution growth of La4CuO4

Abstract Single crystals of La 2 CuO 4 have been grown from a flux with a top seeded solution growth technique. X-ray and chemical analyses indicate the presence of some flux impurities that are detrimental to superconductive properties. Strontium addition was also attempted to produce superconductive crystals.

Laser action of Pr/sup 3+/ in LiYF/sub 4/ and spectroscopy of Eu/sup 2+/-sensitized Pr in BaY/sub 2/F/sub 8/

Laser action in flashlamp-pumped Pr:LiYF/sub 4/ at room temperature is observed at 640 nm with a 15-J threshold, but only approximately 0.01% slope efficiency. Increased efficiency from sensitizing the Pr with Eu/sup 2+/ is explored in the system Eu,Pr:BaY/sub 2/F/sub 8/. Co doped samples have been grown by the Czochralski growth method, and energy transfer between Eu/sup 2+/ and Pr/sup 3+/ is observed to be very weak. This is probably due to the poor overlap of the Eu/sup 2+/ emission with the Pr/sup 3+/ absorption lines, leading to the conclusion that hosts with a stronger crystal field at the Eu/sup 2+/ site need to be identified. >

Magneto‐optical properties of KTb3F10 and LiTbF4 crystals

Single crystals of KTb3F10, LiTbF4, LiTb0.5Y0.5F4 and LiTb0.25Gd0.75F4 were prepared and the wavelength, temperature, and magnetic field dependence of the Faraday rotation measured. Optical‐absorption spectra and refractive indices are also reported. The Faraday effect in these materials is described by a paramagnetic rotation with the primary oscillator strength arising from 4f8→4f75d electronic transitions of the Tb3+ ions. Whereas LiTbF4 orders ferromagnetically at low temperatures, the mixed crystals exhibited no ordering at T?1.25 K. Low‐temperature Faraday rotation measurements of KTb3F10 do not indicate ferromagnetic order above 1.66 K. KTb3F10 and LiTbF4 crystals have large Verdet constants, are transparent in the visible and near infrared and have small nonlinear refractive‐index coefficients. These materials are therefore useful for optical rotators and in high‐power laser applications.

Magneto-optical properties of KTb/sub 3/F/sub 1/0 and LiTbF/sub 4/ crystals

Single crystals of KTb3F10, LiTbF4, LiTb0.5Y0.5F4 and LiTb0.25Gd0.75F4 were prepared and the wavelength, temperature, and magnetic field dependence of the Faraday rotation measured. Optical‐absorption spectra and refractive indices are also reported. The Faraday effect in these materials is described by a paramagnetic rotation with the primary oscillator strength arising from 4f8→4f75d electronic transitions of the Tb3+ ions. Whereas LiTbF4 orders ferromagnetically at low temperatures, the mixed crystals exhibited no ordering at T?1.25 K. Low‐temperature Faraday rotation measurements of KTb3F10 do not indicate ferromagnetic order above 1.66 K. KTb3F10 and LiTbF4 crystals have large Verdet constants, are transparent in the visible and near infrared and have small nonlinear refractive‐index coefficients. These materials are therefore useful for optical rotators and in high‐power laser applications.

Growth of single crystals of pure and Ce-doped Nd2CuO4

Abstract Single crystals of Nd 2 CuO 4 have been grown from excess CuO by a top seeded solution growth technique. Ce-doped Nd 2 CuO 4 platelet-like single crystals have also been grown by slow cooling of the melt with nucleation sites provided by a platinum wire.

Frequency dependence of the conductivity and dielectric constant of La2CuO4+y near the insulator-metal transition

Abstract Measurement are reported of the conductivity and the dielectric constant (ɛ 1 ) at frequencies (ω) up to 20 MHz and magnetic fields up to 15 T for a single crystal of La 2 CuO 4+y with y varied in the range 0.001 – 0.01. The frequency dependence of the conductivity at low temperatures follows a power law, which is typical of conventional doped semiconductors. ɛ 1 at high ω is independent of ω and grows as the insulator to metal transition (IMT) is approached, but only for the electric field in the CuO 2 layers, indicating that the IMT is two-dimensional. For the highest oxygen concentration, this excess ɛ 1 was reduced by a factor of 2 at a magnetic field of 15 T perpendicular to the CuO 2 layers.

Polarized reflectivity studies of single-crystal La 2 CuO 4

Room-temperature polarized reflectivity measurements are reported for single-crystal La2CuO4 for the (E ⊥ ĉ) and (E || ĉ) polarizations, where ĉ denotes the tetragonal axis. A Kramers–Kronig analysis is performed to obtain the frequency-dependent dielectric functions. Frequencies for the seven zone-center IR-active phonons are obtained and the longitudinal-optic–transverse-optic splittings are used to evaluate the dynamic charge on the oxygen atoms. The reflectivity spectrum for the (E ⊥ ĉ) polarization is decidedly metallic and is fitted accordingly with an overdamped Drude model and a plasma frequency of 95 meV. In the high-frequency region of the spectrum, selection rules are observed for allowed and forbidden electronic transitions. The importance of this study to the interpretation of optical measurements on high-Tc, polycrystalline pellets of La2CuO4 and related materials is also discussed.

Thermal conductivity of single crystal lanthanum cuprates at very low temperature

Abstract We have undertaken a study of the thermal conductivity of single crystals of pure and strontium doped lanthanum copper oxide in the temperature range 0.1–6 K. For in plane conduction, all samples show approximately a T 2.7 dependence, whereas for conduction across the planes for x = 0 we observe κ ∼ T 2 . This suggests that dimensionality may play an important role in the lattice heat conduction properties of these materials at very low temperature.

Lattice instability in single-crystal La2−xSrxCuO4

Abstract The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La2−xSrxCuO4 has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.