Thermoelectric response of Fe${}_{1+y}$Te${}_{0.6}$Se${}_{0.4}$: evidence for strong correlation and low carrier density

We present a study of the Seebeck and Nernst coefficients of Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$ 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${}_{1+y}$Te${}_{0.6}$Se${}_{0.4}$ as a low-density correlated superconductor barely in the clean limit. The system is subject to strong superconducting fluctuations with a sizeable vortex Nernst signal in a wide temperature window.