  Microstructura lly clean, isov alently P-doped BaFe2As2 (Ba-122) single
crystalline thin films have been prepared on MgO (001) substrates by molecular
beam epitaxy. These films show a superconducting transition temperature (Tc) of
over 30 K although P content is around 0.22, which is lower than the optimal
one for single crystals (i.e., 0.33). The enhanced Tc at this doping level is
attributed to the in-plane tensile strain. The strained film shows high
transport self-field critical current densities (Jc) of over 6 MA/cm2 at 4.2 K,
which are among the highest for Fe based superconductors (FeSCs). In-field Jc
exceeds 0.1 MA/cm2 at m0H = 35 T for H||ab and m0H = 18 T for H||c at 4.2 K,
respectively, in spite of moderate upper critical fields compared to other
FeSCs with similar Tc. Structural investigations reveal no defects or
misoriented grains pointing to strong pinning centers. We relate this
unexpected high Jc to a strong enhancement of the vortex core energy at optimal
Tc, driven by in-plane strain and doping. These unusually high Jc make P-doped
Ba-122 very favorable for high-field magnet applications.
