Sign-changing superconducting pairing revealed by Andreev transport measurement in FeSe

The symmetry of the superconducting order parameter in FeSe has been the subject of considerable debate, with possibilities including s++, s±, d, and p-wave pairings. Andreev reflection spectroscopy offers a phase-implied approach to identifying the nature of this order parameter. In our study, we investigated exfoliated FeSe single-crystal material by conducting Andreev reflection spectroscopy with current normal to the nodal direction. Here, we applied "soft" point-contact technique which avoids the hysical strain effects typically associated with the traditional point-contact method. This method revealed a zero-bias peak in the differential conductance spectra, indicating an Andreev bound state. Such observations suggest a sign-changing order parameter, as interpreted by the Blonder-Tinkham-Klapwijk (BTK) model. Furthermore, we show that the response of the Andreev bound state to an external magnetic field is consistent with the expected behavior of sign-changing superconductivity in FeSe.