Spontaneous Nernst effect in the iron-based superconductor Fe1+yTe1−xSex

We present a study of the Nernst effect in an iron-based superconductor with a nontrivial band topology ${\mathrm{Fe}}_1+y{\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{Se}}_x$. A nonzero Nernst signal is observed in a narrow temperature region around the superconducting transition temperature ${T}_c$ at a zero field. This anomalous Nernst signal shows symmetric dependence on the external magnetic field and indicates an unconventional vortex contribution in an $s$-wave superconductor with a strong spin-orbit coupling, which is originated from the local magnetic moments of the interstitial Fe atoms. Our experiments also provide the first evidence of a locally broken time-reversal symmetry in bulk ${\mathrm{Fe}}_1+y{\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{Se}}_x$ single crystals.