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The pairing symmetry of the superconducting order parameter in iron-basedsuperconductors has been a subject of debate, with various models proposings-wave, d-wave, and mixed combinations as possible candidates. Here we probethe pairing symmetry of FeSe utilizing the new methodology of Tunneling AndreevReflection (TAR). TAR directly exploits the transparency-dependence oftunneling current to disentangle contributions of single-particle current andAndreev reflection. These measurements provided new direct evidence in favor ofthe sign-changing nature of the superconducting order in FeSe, in a distinctlycomplementary approach to nanoscale imaging of quasiparticle interference.Crucially TAR can also probe higher-order contributions to Andreev reflection.Quantitative comparison of the experimental signatures of higher-order Andreevreflections with those in concomitant tight-binding simulations revealed newevidence in support of the nodal gap structure of superconductivity in FeSe.Finally, the effect of structural topological defects can be directly probedwith TAR owing to its atomic spatial resolution. In particular, we find thatsuperconductivity is completely suppressed along the twin boundary while itselectronic structure is characterized by a V-shaped signature of a pseudogapstate. Our findings provide new insight into the pairing symmetry of anunconventional superconductor, demonstrating the potential of differentialtunneling Andreev reflection to reveal microscopic properties of emergingquantum materials.

Atomic scale imaging of sign-changing order parameter in superconducting FeSe with Tunneling Andreev Reflection