  Stabilized FeSe thin films in ambient pressure with tunable superconductivity
would be a promising candidate for superconducting electronic devices yet its
superconducting transition temperature (Tc) is below 10 K in bulk materials. By
carefully controlling the depositions on twelve kinds of substrates using
pulsed laser deposition technique, high quality single crystalline FeSe samples
were fabricated with full width of half maximum 0.515? in the rocking curve and
clear four-fold symmetry in phi-scan from x-ray diractions. The films have a
maximum Tc 15 K on the CaF2 substrate and do not show obvious decay in the air
for more than half a year. Slightly tuning the stoichiometry of the FeSe
targets, the Tc becomes adjustable from 15 to < 2 K with quite narrow
transition widths less than 2 K, and shows a positive relation with the
out-of-plane (c-axis) lattice parameter of the films. However, there is no
clear relation between the Tc and the surface atomic distance of the
substrates. By reducing the thickness of the films, the Tc decreases and fades
away in samples of less than 10 nm, suggesting that the strain effect is not
responsible for the enhancement of Tc in our experiments.
