Preparation, characterization, and upper critical field of epitaxial FeSe thin films

Epitaxial thin films of the iron chalcogenide superconductor FeSe with Tc≃7 K were prepared by Molecular Beam Epitaxy. The films grow in (001)-orientation on YAlO3(110) substrates. The influence of the Fe to Se atomic ratio on the transport properties of the samples is studied. Only within a narrow compositional range superconductivity is observed and films with a residual resistance ratio of up to RRR=R300 K/R8 K≃4 are obtained. The anisotropy of the upper critical field Hc2 is increasing with the RRR and a maximum ratio of Hc2,H∥a/Hc2,H∥c≃3 is observed. From this result, an anisotropy ratio of the coherence lengths of ξab/ξc>3 is estimated.