  High-temperature superconductivity in iron-arsenic materials (pnictides) near
an antiferromagnetic phase raises the possibility of spin-fluctuation-mediated
pairing. However, the interplay between antiferromagnetic fluctuations and
superconductivity remains unclear in the underdoped regime, which is closer to
the antiferromagnetic phase. Here we report that the superconducting gap of the
underdoped pnictides scales linearly with the transition temperature, and that
a distinct pseudogap coexisting with the SC gap develops on underdoping. This
pseudogap occurs on Fermi surface sheets connected by the antiferromagnetic
wavevector, where the superconducting pairing is stronger as well, suggesting
that antiferromagnetic fluctuations drive both the pseudogap and
superconductivity. Interestingly, we found that the pseudogap and the spectral
lineshape vary with the Fermi surface quasi-nesting conditions in a fashion
that shares similarities with the nodal-antinodal dichotomous behaviour
observed in underdoped copper oxide superconductors.
