  The superconducting phase in iron based high Tc superconductors (FeSC) as in
other unconventional superconductors such as the cuprates neighbours a
magnetically ordered one in the phase diagram. This proximity hints at the
importance of electron correlation effects in these materials, and Hund
exchange interaction has been suggested to be the dominant correlation effect
in FeSCs because of their multiband nature. By this reasoning, correlation
should be strongest for materials closest to a half filled 3d shell (Mn
compounds, hole doped FeSCs) and decrease for systems with both higher
(electron doped FeSCs) and lower (Cr pnictides) 3d counts. Here we address the
strength of correlation effects in BaCr2As2 by means of angle resolved
photoelectron spectroscopy (ARPES) and first principles calculations. This
combination provides us with two handles on the strength of correlation, First,
a comparison of the experimental and calculated effective masses yields the
correlation induced mass renormalisation. In addition, the lifetime broadening
of the experimentally observed dispersions provides another measure of the
correlation strength. Both approaches reveal a reduction of electron
correlation in BaCr2As2 with respect to systems with a 3d count closer to five.
Our results thereby support the theoretical predictions that Hund's exchange
interaction is important in these materials.
