Transport Phenomena in La2-xBaxCuOy epitaxial thin films

The (001) oriented thin films of La2-xBaxCuO4+d on SrAlO4 substrates with barium composition x=0 to 2 were grown by DC sputtering and characterized them by X-ray diffraction and resistivity measurement. The films with oxygen composition d=0 showed superconductivity for x between 0.055 and 0.30. For x=0.15, the superconducting transition was maximized to 43 K. When the films had d>0, Tc reached 47 K. These values are higher than those for bulk samples. The depression of T, around x=0.125 is smaller than that for the bulk samples. The compressive strain expands the c-axis and suppresses formation of the low-temperature tetragonal phase. Both effects result in an increase in the bond length between Cu and the apical oxygen, which is responsible for the enhancement of Tc. The samples with lower residual resistivity show a higher Tc, so we suppose that the Tc enhancement is caused by reduced antiferromagnetic spin fluctuation in the CuO2 planes due to the change in the Cu-O apical bond length. (authors)