Short-lived electron excitations in $\rm FeTe_{1-x}Se_x$ as revealed by microwave absorption

The $\rm Fe_{1+y}Te_{1-x}Se_x$ single crystals with the various Se/Te ratios were studied by the microwave absorption and direct current resistivity measurements. The comparison of the microwave absorption data and the resistivity versus temperature made it possible to separate the contributions of two types of spin fluctuations. One of them is due to the anisotropic magnetic (nematic) fluctuations. It is observed over the wide temperature range from 30K to 150 or 200K. In FeSe it has the maximum close to the structural transition temperature. Another MWA anomaly is located in the narrow temperature range above the superconducting transition. It is likely due to the antiferromagnetic fluctuations. Annealing of a sample at the temperature around 300 C in the oxygen atmosphere made it possible to exclude the effect of excess iron on the observed anomalies.