A.E. Curzon

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.

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 .

Raman investigation of Pb-substituted Bi2Sr2Cu6+δ single crystals

Abstract Raman scattering experiments have been carried out to investigate the vibrational spectrum of Pb-doped Bi 2 Sr 2 CuO 6+δ single crystals with molar Pb concentrations x in the range 0 ⩽ x ⩽ 0.4. The intensities and frequencies of several of the vibrational modes, with frequencies ω in the range 15 ⩽ ω ⩽ 650 cm − , are found to depend sensitively on the degree of Pb substitution. This information has been used to clarify, and in some cases confirm, the vibrational origin of the modes. The introduction of Pb into the single crystals also reduces the structural modulation that is present in the undoped crystals. The degree of modulation was monitored using electron diffraction techniques and it was found that the modulation vanished at a Pb doping level of x ≈ 0.35. The influence of the modulation on the Raman spectra was investigated and it was found that two-modes at 30 and 52 cm −1 , which were previously associated with the structural modulation, soften with Pb doping but are still present in spectra obtained from unmodulated samples. It is suggested that these modes arises from rigid layer vibrations, similar to those observed in layered dichalcagonides.

Epitaxial bilayer and trilayer heterostructures grown on LaAlO3 and SrTiO3(001) single crystals by laser ablation

High-quality S/I and S/I/S epitaxial heterostructures (S=YBa2Cu3Ox, l=SrTiO3) have been grown in situ on LaAlO3 and SrTiO3(001) single crystals, by the single-chamber pulsed laser deposition technique. YBa2Cu3Ox and SrTiO3 layers were deposited sequentially, at a substrate temperature Th=760 degrees C, and an oxygen pressure rho (O2)=300 mTorr. Structural data showed that the SrTiO3 layer, grown either on a YBCO film or sandwiched by two YBaCuO films, was strained in the (001) plane and exhibited an elongation of the lattice parameter in the (001) direction. This behaviour was consistent with the epitaxial growth of the S/I and S/I/S structures as revealed by the RHEED and ECP patterns. Ion milling in conjunction with AES analysis showed that the thicknesses of the interfacial regions were <or=35 AA, and resulted from residual roughness of the substrates. The critical temperature (Tc) and critical current density (jc) of the multilayers at 77 K, as determined by an inductive AC method, were found to be Tc=89.5 and 90.5 K, and jc=3 and 5*106 A cm-2, for trilayers and bilayers, respectively.

Characterization of sputtered films of Tℓ-Ba-Ca-Cu-O

Abstract Thin, non-superconducting films of Tl-Ba-Ca-Cu-O have been deposited on Yttrium-stabilized-zirconia single crystal substrates. After annealing, the films became superconducting with critical temperatures between 90 and 96 K. The annealed films have been investigated using X-ray diffraction, energy dispersive X-ray analysis and Raman scattering and the unannealed films were characterized using energy dispersive X-ray analysis. The data acquired by the three techniques yields information on the elemental composition and phase purity of the films. This information has been correlated with the parameters used during growth and annealing. The results of this correlation have in turn been used to optimize the parameters used during the growth process.

Current and potential distributions in highly anisotropic superconductors with inclusions

Abstract Thin whiskers of orthorhombic Bi 2 Sr 2 Ca 1 Cu 2 O x have been grown from powders and their resistive transitions have been studied at constant current. The potential difference between the voltage electrodes on some of the whiskers showed an abrupt increase when they were cooled below ∼ 105 K. It is shown that this increase may be explained by one or more intercalated layers of superconducting Bi 2 Sr 2 Ca 2 Cu 3 O x inside the bulk Bi 2 Sr 2 Ca 1 Cu 2 O x material. The whisker is represented by an orthorhombic solid which extends from − ∞ to +∞ in the b -direction and has a thickness in the c -direction which is much less than the length in the a -direction. The ratio of the electrical conductivities σ aa σ cc is taken to be of the order of 10 4 as is the case for Bi 2 Sr 2 Ca 1 Cu 2 O x . The two-dimensional potential distribution in the whisker satisfies Laplaces equation in a co-ordinate system in which the displacements along the c -direction are scaled by a factor of √( σ aa σ cc ). This equation is solved by a standard relaxation method. Two cases have been considered. In the first, the entire crystal is in its normal state and in the second there is an intercalated superconducting layer of Bi 2 Sr 2 Ca 2 Cu 3 O x perpendicular to the c -direction and inside normally conducting Bi 2 Sr 2 Ca 1 Cu 2 O x which constitutes the bulk material. It is this layer which gives rise to the observed anomalous resistive transitions at 105 K and to changes in potential distributions. Figures are presented to illustrate the changes.

Structural and electrical analysis of RF magnetron sputtered large-area YBa2Cu3Oz films on LaAlO3(001) single crystals

The uniformity of the structural and superconducting properties of large-area thin films of YBa2Cu3Oz (YBCO) deposited by RF magnetron sputtering on (001) faces of LaAlO3 single crystals have been investigated using XRD, AES, SEM, EDAX, Raman spectroscopy and an inductive a.c. method. The YBCO films were deposited over areas up to approximately=23*23 mm2, and were approximately=1600-2100 AA thick. The films were highly c-axis oriented, exhibited Tc0=89+or-1 K, and jc=1.7-2+or-0.5*106 A cm-2 at 77 K across the entire film area. Raman spectra showed that oxygenation of the films was z>or=6.83, and the degree of epitaxy was eta approximately=88%. EDAX measurements showed a very uniform distribution of Y, Ba and Cu across the film area. Local changes in mosaic spread and the c-axis length, observed near the edges of the films, corresponded with the measured variations in the critical current density (jc). Reproducible sputtering conditions have been determined for the growth of high-quality large-area YBCO superconducting films.

Pressure contacts to Bi2212 whiskers

Eight electrical silver contacts were attached to very small and thin orthorhombic Bi2212 single-crystal whiskers by the application of pressure in a small cell developed especially for this purpose. No heating treatment of the sample and the contacts was necessary; thus, diffusion of the contact material into the sample was avoided. Four contacts were made to the top ab-face of the whisker, and four to the bottom ab-face, the crystallographic c-axis being along the thickness of the whisker. This arrangement allowed electrical measurements to be made in a variety of ways. These measurements revealed that the current distribution is not uniform through the highly anisotropic Bi2212 superconductor. A contact resistance of a few ohms was seen and it changed by less than 30% between the superconducting transition of the whiskers and 4.2 K. This contact resistance is due to the normal surface layers of the whiskers and shows a temperature dependence similar to that of metallic glasses. However, the contact resistance is only of the order of 10% of the resistance of the whiskers in the normal state and stays constant at any given temperature below the superconducting transition of the whiskers which makes this pressure contact technique valuable.

C-axis IV-curves in Bi2212 whiskers with applied magnetic fields parallel to the a-axis

Abstract We have studied c -axis IV -curves of nominal Bi2212 whiskers in a magnetic field applied in the crystalline a -direction. In zero applied field we observe branches in the IV -plane which for low fields are retained. For high currents, they all merge into a linear IV dependence which extrapolates to the origin. The critical current as a function of field shows a linear decrease to a level that stays constant in higher fields.