Abstract
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.
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