  We use transport measurements to determine the in-plane anisotropy of the
upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co,
Ni) single crystals. In previous measurements on twinned single crystals, the
charge carrier doping dependence (x) of the upper critical field anisotropy for
fields along the inter-planar (c-axis) and in-plane field directions was found
to increase in the overdoped regime. For underdoped samples, which exhibit a
spin nematic phase below the tetragonal to orthorhombic structural transition
temperature Ts , we find that Hc2 along the a-axis is considerably lower than
that along the b-axis. The upper critical field anisotropy disappears in the
over-doped regime when the system becomes tetragonal. By combining these
results with inelastic neutron scattering studies of spin excitations, and
angle-resolved photoemission spectroscopy, we conclude that superconductivity
in under-doped iron pnictides is orbital selective - with a dominant
contribution from electrons with the dyz orbital character and being intimately
associated with spin excitations.
