In-situ growth and synchrotron diffraction studies of the structural properties of superconducting Ti_xO_ythin films

Understanding and enhancing high-T_c superconductivity in materials remains an active area of scientific and technological interest. Recent studies of thin films of the binary oxide Ti_xO_y have revealed higher superconducting transition temperatures than the bulk. To further understand the mechanisms for this enhancement, we have explored the effect of the growth conditions on the transport and structural properties of Ti_xO_y films that are grown on (0001)-Al₂O₃ substrates using molecular beam epitaxy. We observe that carefully tuning the ratio between Ti flux rate and oxygen partial pressure leads to a diverse range of structural phases and electronic states. Moreover, a combination of synthesis and in-situ synchrotron X-ray diffraction measurements to map out the structure-property relationship reveal that the metallicity of the films arises from the formation of the oxygen-poor cubic c-TiO phase. The superconducting γ − Ti₃O₅ Magneli phase is found to nucleate on the cubic TiO buffer. A correlation is observed between the strain state of γ − Ti₃O₅ phase and the superconducting T_c with T_c increasing with film thickness as strain is relaxed. We observe a maximum T_c onset of 7.5 K for 33 nm thick films under optimized growth conditions. These results have implications for understanding the role of strain and electronic and structural disorder in superconducting oxides.