B. Heinrich

Determination of current and flux distribution in squares of thin-film high-temperature superconductors

Abstract An experimental determination of the flux distribution in square samples of thin-film superconductors has been conducted by the use of a scanning Hall probe and a magneto-optical thin film. Two 10 mm×10 mm samples, one YBa2Cu3O7 and one TlBaCaCuO, have been examined at 77 K with a field applied perpendicular to the surface of the film. For both samples, the current distribution was calculated from the measured field data. At low fields, flux penetrates at the centres of the edges giving rise to the distinctive star pattern often noted in magneto-optical studies and creating a complex current distribution. As the field penetrates fully, the current distribution pattern evolves into simple concentric squares. The evolution of the screening currents was found to be qualitatively the same in both materials, although the magnitudes of the critical currents differed by a factor of up to 10. For both samples, in full field penetration, the current is found to follow the outline of the sample. Lines of constant current are to a good approximation concentric squares. The remnant magnetization, Mrem, is found to be JcD/3, where 2D is the size of the square, in complete agreement with the critical-state model.

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 .

Magneto-optical studies of magnetic flux penetration into high- superconducting films in a `screening geometry'

Studies of magnetic field penetration into high- films ( (YBCO)) have been performed using magneto-optical techniques. The magnetic field was applied to one side of the YBCO film in a region near its centre. The field was created by using two configurations of solenoids in which the diameters of both solenoid types were much less than the lateral dimensions of the films. For a long needle-shaped solenoid (type A) of length approximately equal to the lateral film dimensions, oriented with its axis normal to the film, the magnetic flux was found to start to penetrate around the film edges. In high fields, the magnetic flux penetrated mostly through the centre. Magneto-optical images of the field distribution just above the film revealed that areas of poor superconducting properties were sites for initial flux penetration. This observation can be used for rapid identification of film defects. With a second approach using a U-shaped solenoid (type B), the magnetic field penetration was concentrated around the region of maximum magnetic field. In this case, the flux formed macroscopic domains of vortices of opposite sign.

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.

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 .

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.

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.