Correlation between the superconducting and structural properties in MgB2 thin films prepared by molecular-beam epitaxy

Abstract

Superconducting MgB2 films were prepared on an Al2O3(0001) substrate by codeposition of Mg and B using the molecular-beam epitaxy technique and the effects of the growth conditions on the structural and the superconducting properties were extensively investigated. The superconducting transition temperature Tc shows a peak as a function of the deposition-rate ratio of Mg to B2 and the stoichiometric composition is likely realized at the Tc peak. The optimum substrate temperature Ts was found to be 330 °C. Detailed structural characterizations indicate the strong suppression of structural coherence in the stoichiometric and Mg-rich films. This accounts for the fact that optimized Tc (26.6 K) is still lower than bulk Tc like in previous molecular-beam epitaxy studies. We argue that such structural disorder originates from epitaxial strain due to the lattice mismatch between the film and the substrate.