E. Schönherr

Systematic Raman and infrared studies of the superconductor YBa2Cu3O7-x as a function of oxygen concentration (0≦x≦1)

Abstract We have studied the variation of Raman and infrared spectra of the new high T c superconductors with oxygen content. For the material YBa 2 Cu 3 O 7-x we varied x between 0 and 1, a range over which the material is known to lose its superconducting properties. We find that the Raman peaks associated with CuO stretching vibration (502 cm −1 , x=0) and the BaO vibrations (154 cm −1 , x=0) decrease in frequency when oxygen is removed, while the bending-stretching modes of the CuO frames (334 and 438 cm −1 , x=0) harden under the same conditions. In the infrared spectra the phonon peaks behave similarly although the vibrations associated with CuO frames appear to be stationary when oxygen is removed. The vibrations of the peak frequencies are explained with changes in the unit cell parameters associated with oxygen removal and with the decreasing electronic shielding resulting from the transition to a semiconducting material. Our results are indicative of conduction in the CuO planes of the superconductor.

The growth of laser quality NdP5O14 crystals

Neodymium pentaphosphate, NdP5O14, is the first successful result of a new approach in the synthesis of laser materials. Instead of doping the active ions as impurities into host materials, the actice ions are allowed to form specific stoichiometric compounds of their own. NdP5O14 is such a compound. It combines five desirable properties: high chemical and mechanical stability, low pump threshold, high optical gain, and small volume. We describe the growth of single crystals from polyphosphoric acid solution, and give initial results on ‘crystal engineering’ studies which yield decisive improvements of quality and laser properties of the crystals.

Czochralski growth of Li3N crystals

Abstract Large Li 3 N single crystals were grown by the Czochralski method. Starting material for crystal growth was synthesized from the elements. Decomposition of the melt was prevented by use of a stagnant N 2 atmosphere ( p = 700 Torr ). Tungsten was found to be the most suitable crucible material which was not severely attacked by the Li 3 N melt. The growth of single crystals was complicated by the easy formation of numerous misorientate grains during the seeding process.

The dielectric function of AlSb from 1.4 to 5.8 eV determined by spectroscopic ellipsometry

We have prepared AlSb substrates for optical measurements by chemomechanical polishing and etching. The quality of the surface was investigated with optical and electron microscopy and by Raman scattering and ellipsometry. We have measured the pseudodielectric function 〈e〉(ω) of AlSb in the 1.4–5.8 eV photon‐energy region with a spectroscopic ellipsometer. A peak value of 〈e2〉=24.6 at 4 eV was reached. We list the refractive index, the reflectivity, and the absorption coefficient, and obtain the critical point parameters at 300 K. Finally, we fit the index of refraction of AlSb at low photon energies with a semiempirical model.

Growth of large and untwinned single crystals of YBCO

Abstract Large (up to 5×4×1.5 mm3) single crystals of YBa2Cu3O7−σ with perfect orthorhombic morphology were successfully grown from favourable flux composition in alumina crucibles. The number of nucleation centres was reduced by using smooth inner wall crucibles and a temperature oscillation method. The crystals could be isolated from the CuO flux by tipping the crucible over while remaining in the furnace at 970°C. By applying a uniaxial stress at the annealing temperature, both macro- and micro-twin domains in the thick crystals could be removed and a single untwinned domain was obtained. The structures of bith twin- and single-domain crystals were characterized by means of high resolution electron microscopy. We also report the magnetic property of the as-grown single crystals.

Effects of oxygen deficiency on the optical spectra of YBa2Cu3O7−x

Abstract We report on the variation of ellipsometric spectra of the YBa2Cu3o7−x high Tc compounds with oxygen content 0≤x≤1. Beside the structures observed in the x=0 compound two new, rather intense peaks are observed at ∼ 1.75 eV and ∼ 4.1 eV in the semiconducting compound (x>0.5). Their intensity strongly depends on the oxygen content, whereas the energy shows a weak dependence both on oxygen content and temperature. Several minor features become significant upon cooling the samples to 7 K. We associate the 4.1 eV peak to excitations involving transitions from Ba(5d) and/or Cu(3d) states to O(2p) states. The 1.75 eV peak is possibly due to Cu(3d) and O(2p) states.

Electron-phonon coupling of apex oxygen in RBa2Cu3O7−δ

Abstract The electron-phonon coupling constants of the stretching and bending vibrations of the apex oxygen in RBa 2 Cu 3 O 7−δ are determined by Raman scattering: λ v = 0.01, λ v λ v −1 ( A g , 305 cm −1 ( B 3 g ) and 200 cm −1 ( B 2 g ), respectively. By substituting different rare earths R it was found that the frequencies of the bending vibrations depend roughly linearly on the ionic radius of the rare earth R.

Effect of isotopic substitution of oxygen on Tc and the phonon frequencies of high Tc superconductors

Abstract The Raman and infrared spectra of YBa2Cu318Oβ16O7−β and TmBa2Cu318Oβ16O7−β with various amounts β of 16O substituted by 18O have been measured. The observed phonon frequency shifts of the ir and Raman active modes are compared with predictions from a recently proposed model for the lattice dynamics of these compounds. The application of this model allows a detailed study of the frequency shifts induced by istope substitution of various inequivalent oxygen positions (OI, OII, III, OIV). The resistive transitions of these samples have also been measured. No isotope effect is ound if only one out of seven 16O is replaced by 18O. For 85% substition, a shift of ΔTc = 0.3±0.1 K is observed in agreement with previous determinations.

Growth germanium monosulfide single crystals by sublimation

Abstract Large high quality single crystals of GeS are obtained by sublimation of GeS chunks between 520 and 580°C in closed evacuated (10-6 torr) fused silica ampoules using a vertical pulling method.

Phenomenological description of crystal growth from the vapor

Abstract The results of an investigation of the growth of GeS single crystals from the vapor in the conical part of a closed ampoule are reported. The growth was forced by abrupt displacements of the ampoule through a fixed temperature gradient. The re-establishment of equilibrium was observed. The deviation from the final equilibrium length could be described by an exponential dependence on time. The total change of the crystal length was greater than the sudden displacement. The relaxation time increased linearly with the crystal length. The governing equation k(l) dl/dt+l=d 1 +R ∗ vt is deduced from the measurements, where k is the relaxation time, l the crystal length, t the time, d 1 the initial displacement for seeding and v the constant drive rate. The constant factor R ∗ ⪆1 accounts for the change of the temperature distribution by the presence of the crystal. A linear solution of the equation exists for k ( l )= a + bl , where a and b are constants.