Strong correlation and low carrier density in Fe_1+yTe _0.6Se_0.4 as seen from its thermoelectric response

We present a study of the Seebeck and Nernst coefficients of Fe _1+yTe_1-xSe_x extended up to 28 T. The large magnitude of the Seebeck coefficient in the optimally doped sample tracks a remarkably low normalized Fermi temperature, which like other correlated superconductors, is only one order of magnitude larger than T_c. We combine our data with other experimentally measured coefficients of the system to extract a set of self-consistent parameters, which identify Fe _1+yTe_0.6Se_0.4 as a low-density correlated superconductor barely in the clean limit. The system is subject to strong superconducting fluctuations with a sizable vortex Nernst signal in a wide temperature window.