Pulsed Laser Deposition Synthesis and Photoemission Study of Superconducting Ba-Cu-O Thin Films

Ba-Cu-O films have been grown by pulsed laser deposition. Investigation of their transport properties and characterization of electronic structure by in-situ photoemission spectroscopy have been carried out. The films grown at 450 ∼ 480 deg. C from BaCuO2 target had simple infinite layer structure with lattice parameter of a and c about 0.385 and 0.405 nm, respectively. Conductivity of these films were, however, poor. Usage of BaCu0.75Ox target and addition of CO2 into the growth atmosphere resulted in an expansion of c-axis length up to 0.810 ∼ 0.835 nm and a significant rise of conductivity. This structure is represented as '2c'. For the 2c phase, conductivity above of 103 S/cm was achieved by adjusting their growth temperature in the range of 500 ∼ 530 deg. C. Most of the highly conductive 2c phase showed superconductivity. The maximum temperature of onset of the superconducting transition and zero resistance state obtained so far were 60 and 47 K, respectively. Finite spectral weights at Fermi level were successfully observed in in-situ photoemission spectra of the 2c phase. In valence band region, a prominent peak with u 3d character located at a binding energy around 1.6 eV. Though this low binding energy suggested a rather high hole concentration of mobile holes were around middle of 1021 cm-3 range. These results about electronic structure indicate a coexistence of conventional CuO2 planes and distorted ones. The latter structure might play charge-reserving blocks, though some portions of holes should localize in them. Correlations between the properties and electronic structure are to be discussed