  The pairing symmetry of the superconducting order parameter in iron-based
superconductors has been a subject of debate, with various models proposing
s-wave, d-wave, and mixed combinations as possible candidates. Here we probe
the pairing symmetry of FeSe utilizing the new methodology of Tunneling Andreev
Reflection (TAR). TAR directly exploits the transparency-dependence of
tunneling current to disentangle contributions of single-particle current and
Andreev reflection. These measurements provided new direct evidence in favor of
the sign-changing nature of the superconducting order in FeSe, in a distinctly
complementary 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 Andreev
reflections with those in concomitant tight-binding simulations revealed new
evidence in support of the nodal gap structure of superconductivity in FeSe.
Finally, the effect of structural topological defects can be directly probed
with TAR owing to its atomic spatial resolution. In particular, we find that
superconductivity is completely suppressed along the twin boundary while its
electronic structure is characterized by a V-shaped signature of a pseudogap
state. Our findings provide new insight into the pairing symmetry of an
unconventional superconductor, demonstrating the potential of differential
tunneling Andreev reflection to reveal microscopic properties of emerging
quantum materials.
