  We have systematically studied the effects of in-plane uniaxial pressure $p$
on the superconducting transition temperature $T_c$ in many iron-based
superconductors. The change of $T_c$ with $p$ is composed of linear and
nonlinear components. The latter can be described as a quadratic term plus a
much smaller fourth-order term. In contrast to the linear component, the
nonlinear $p$ dependence of $T_c$ displays a pronounced in-plane anisotropy,
which is similar to the anisotropic response of the resistivity to $p$. As a
result, it can be attributed to the coupling between the superconducting and
nematic orders, in accordance with the expectations of a phenomenological
Landau theory. Our results provide direct evidences for the interplay between
nematic fluctuations and superconductivity, which may be a common behavior in
iron-based superconductors.
