The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy to image the electronic impact of Co atoms on the ground state of the LiFe1-xCoxAs system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strong-coupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Born-limit scattering effect in an s±-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.
Yin, J. X., Zhang, S. S., Dai, G., Zhao, Y., Kreisel, A., Macam, G., ... Hasan, M. Z. (2019). Quantum Phase Transition of Correlated Iron-Based Superconductivity in LiFe1-xCoxAs
. Physical Review Letters
(21), . https://doi.org/10.1103/physrevlett.123.217004