Possible Kondo scattering and its signature in Seebeck coefficient in topological superconductor Fe_1+yTe_0.55Se_0.45

Fe_1+yTe_1-xSe_x is an iconic topological superconductor with Majorana zero mode in the Abrikosov vortex core, holding potential for topological quantum computing. However, the presence of interstitial iron impurities in this system induces spontaneous magnetization, which often hinders access to the intrinsic behavior. Here, we show that in high-quality Fe_1+yTe_0.55Se_0.45 single crystals, no trace of spontaneous magnetization can be found using the highly sensitive Nernst measurement. Instead, a negative magnetoresistance with intriguing temperature and field dependences is observed, with corresponding signatures also found in the magneto Seebeck effect. These observations point to possible Kondo scattering in the normal state of the superconductor originating from the fluctuating magnetic moment of the interstitial irons, which constitute a non-negligible factor in interpreting the low-temperature transport behaviors.