Transport phenomena in La2-xBaxCuOy epitaxial thin films

The (001) oriented thin films of La2-xBaxCuO4+d on SrAlO4 substrates with b arium composition x = 0 to 2 were grown by DC sputtering and characterized by X-ray diffraction and resistivity measurement. The films with oxygen com position d = 0 showed superconductivity for x between 0.055 and 0.30. For x = 0.15, the superconducting transition was maximized to 4.3 K. When the fi lms had d > 0, T-c reached 47 K. These Values are higher than those for bul k samples. The depression of T-c 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 a n increase in the bond length between Cu and the apical oxygen, which is re sponsible for the enhancement of T-c. The samples with lower residual resis tivity show a higher T-c, so we suppose that the Te enhancement is caused b y reduced antiferromagnetic spin fluctuation in the CuO2 planes due to the change in the Cu-O apical bond length.