  The nematic instability of iron-based superconductors is an undebatable
ingredient of the physics of iron-based superconductors. Yet, its origin
remains enigmatic as it involves a fermiology with an intricate interplay of
lattice-, orbital- and spin- degrees of freedom. It is well known that
thermoelectric transport is an excellent probe for revealing even subtle
signatures of instabilities and pertinent fluctuations. In this paper, we
report a strong response of the thermoelectric transport properties of two
underdoped 1111 iron-based superconductors to a vanishingly small strain. By
introducing the strain-derivative of the Seebeck and the Nernst coefficients,
we provide a novel description of the nematic order parameter, proving the
existence of an anisotropic Peltier-tensor beside an anisotropic
conductivity-tensor. Our measurements reveal that the transport nematic
phenomenology is the result of the combined effect of both an anisotropic
scattering time and Fermi surface distortions, pointing out that in a realistic
description, abreast of the spin-fluctuations also the orbital character is a
fundamental ingredient. In addition, we show that nematic fluctuations
universally relax in a Curie-Weiss fashion above T_S in all the
elasto-transport measurements and we provide evidences that nematicity must be
band-selective.
