J. Chrzanowski

Raman scattering from single crystals of cupric oxide

Abstract Raman scattering experiments have been carried out on single crystals of cupric oxide (CuO) in the temperature range from 15 K to 300 K. The symmetry of the three first order Raman active phonons has been determined. A broad peak occurs at approximately 1100 cm −1 and is assigned to multiphonon scattering on the basis of the observed temperature dependence. Another peak appears in the Raman spectrum as the temperature of the sample is lowered below the Neel temperature. The frequency, intensity and linewidth of this peak have been studied as a function of temperature. The frequency of this peak cannot be reconciled with the known spin wave spectrum of CuO and thus, on the basis of the measured temperature dependence, it has been assigned to scattering from a magnetic exciton. The Raman spectrum also contains a background continuum which is weakly peaked at about 2000 cm −1 and the possible origin of this scattering is discussed. The observed optical and magnetic properties of CuO are compared, where possible, to results that have been obtained for the high- T c cuprates.

ELECTRON-PHONON INTERACTIONS OF RAMAN ACTIVE PHONONS IN YBA2CU3O7-Y

Abstract Polarized Raman measurements have been carried out on twinned single crystals of YBa 2 Cu 3 O 7− y . The temperature dependence of the frequencies and linewidths of the 340, 435 and 500 cm −1 phonons have been investigated for temperatures from 15 to 300 K. Both the frequency and linewidth of the 340 cm −1 phonon have been found to decrease as the sample temperature is reduced from about 90 K ( T c ) to 15 K whereas the frequencies and linewidths of the 435 and 500 cm −1 modes both increase as the temperature is lowered through this same interval. For T > T c the 500 cm −1 phonon exhibits an anisotropic coupling to the electronic excitations in the crystal but all evidence of this anisotropy vanishes when the sample temperature is reduced below T c . The superconductivity induced changes in the frequency and linewidth of all three modes have been compared to the predictions of the strong-coupling model of Zeyher and Zwicknagl for high temperature superconductors. Good agreement between theory and measurements is obtained if it is assumed that the superconducting gap parameter is 2 Δ =370±20 cm −1 which corresponds to a ratio 2 Δ / k B T c =5.9±0.3. This result is compared to previously published values for 2 Δ.

The oxygen isotope effect in superconducting YBa2Cu3O7−δ

Abstract We report on measurements of the effect of 18O-16O substitution on the superconducting properties of Y1Ba2Cu3O7−δ. Substitutions were made by gas-exchange alone, and by gas exchange in conjunction with substituted BaCO3 and CuO. From Raman scattering we find that gas-exchange alone leads to lower substitution levels, particularly in the bridging oxygen O(4) position. Gas-exchanged samples give an isotope effect of 0.3 to 0.4 K. Use of substituted materials leads to samples with an isotope effect of up to 0.9 K in the resistance transition and 1.7 K in in the magnetically determinded transition.

Characterization of films prepared by laser ablation of precursor targets and ex situ thalliation

Thin films of (Tl-2212) were grown on (001) single crystals by excimer laser ablation using Tl-free targets and subsequent ex situ thalliation. The thalliation process of the film precursor () took place in an crucible containing the precursor and Tl-2212 powder. The procedure was carried out at a temperature and under a reduced oxygen pressure = 20 - 400 Torr. The resulting Tl-2212 films were characterized by XRD, AES, EDAX, ECP, Raman spectroscopy and a.c. susceptibility measurements. The films exhibited epitaxial, c-axis-oriented growth with critical temperatures = 98 - 106.5 K, transition widths = 0.5 - 1 K, and critical current densities at 77 K of . EDAX measurements showed that the films were as a rule somewhat Tl and Ba deficient but Ca rich . AES depth profiling revealed a noticeable depth dependence of the Ca and Ba distribution, though the combined concentration of these elements (Ba + Ca) was fairly constant throughout the whole film thickness. It is postulated that cation disorder (Ba Ca substitution) is responsible, at least in part, for the high critical current density observed in the Tl films. These point defects would contribute to the strong pinning of the flux lines in the films by creating a dense network of pinning centres.

The oxygen isotope effect in Pr, Ca, and Zn substituted YBa2Cu3O7−δ and EuBa2Cu3O7−δ

Abstract The oxygen isotope effect α was studied for Pr and Ca substituted YBCO, and for Zn substituted EuBCO and YBCO, α increases to values near 0.5 for the highest Pr concentrations, and lowest transition temperatures. In the Zn substituted system α remains small to the lowest transition temperatures. It is shown that the isotope exponent is a function of mobile hole concentration, it decreases with increasing hole concentration and remains low for over-doping.

Oxygen isotope effect in the superconductor YBa2Cu3O7-x

Abstract Shifts in the superconducting transition in 18 O substituted YBa 2 Cu 3 O 7-x are reported. Shifts as large as 1.31 K (resistively) and 1.70 K (magnetically) are found.

Characterization of sputtered films of Tl2Ba2CaCu2O8 using Raman scattering and x-ray diffraction

Abstract Thin superconducting films of Tl2Ba2CaCu2O8 deposited on MgO (001) single crystal substrates using a single RF magnetron sputtering source, were investigated using Raman spectroscopy and X-ray diffractometry. A correlation between the apical oxygen vibration and critical temperature Tc in the Tl-2212 superconducting films was observed for the films grown at fixed calcination conditions and various oxygenation times. Another correlation between small shifts in spectral positions of the lowest-frequency Raman active modes (108 and 132 cm-1) and Tc has been found and discussed in terms of partial cation substitution. The Raman spectra can also be used to obtain an estimate of the degree to which the film is crystallized during the annealing process.

A study of the optimization of sputtering and sintering parameters for the growth of Tl2Ba2Ca2Cu3O10+χ superconducting films

Abstract Thin superconducting films of Tl-2223 were grown on YSZ, MgO, LaAlO 3 and SrTiO 3 single crystal (001) faces using a single target RF magnetron sputtering source and different geometrical configurations. The sputtered films were post annealed in various crucibles (Al 2 O 3 , Pt) and in gas atmospheres composed of Ar/O 2 mixtures. XRD, EDAX, SEM, AES and Raman spectroscopy were used to characterize the quality of the resulting films. These examinations have yielded a simplified phase diagram correlating the sintering temperature ( T s ), and a parameter β (β= f (O 2 )/[ f (O 2 ) + f (Ar)]), that characterizes the oxidizing-reducing properties of the gaseous atmosphere used during sintering, with the resulting film phase-content, structure and morphology. A specific “time-window” for optimal film sintering with respect to the highest T c has been found for the experimental arrangement used. The highest critical temperature observed in the Tl-2223 films was ~ 114 K ( R =0) and the critical current density at 77 K reached ~5×10 4 A/cm 2 .

Temperature dependence of the linewidth of the 500 cm−1 Raman mode of YBa2Cu3O7−x

Abstract The temperature dependence of the 500 cm−1 Raman active phonon has been carefully investigated in Raman spectra obtained from pressed and sintered pellets of YBa2Cu3O7−x with x ≈ 0. The 500 cm−1 phonon arises from vibrations of the O (4) bridging oxygen atoms which are believed to be situated in a strongly anharmonic potential well. The linewidth of the 500 cm−1 mode has thus been studied as a function of temperature. In several samples the linewidth was observed to decrease by about 3 cm−1 near Tc and to increase again at lower temperature. A linear chain model is used to estimate the effect of charge transfer fluctuations on the 500 cm−1 mode and the results suggest that coupling of such fluctuations to the 500 cm−1 mode may influence the observed behaviour near Tc.

Temperature dependence of the Raman spectra of Tl2Ba2CaCu2O8

Abstract The A 19 phonon mode at 494 cm −1 in Tl 2 Ba 2 CaCu 2 O 9 has been observed to soften as the sample temperature is reduced below T c . This mode involves vibrations of the bridging or apical oxygen atoms and the observed temperature dependence is compared to that of the analogous modes in YBa 2 Cu 3 O 7 and Bi 2 CaSr 2 Cu 2 O 8 .