In many unconventional superconductors the pairing of electrons is driven by the repulsive interaction, which leads to the sign reversal of superconducting gaps along the Fermi surfaces or between them. However, to measure this sign change is not easy and straightforward. It is known that, in superconductors with sign reversal gaps, nonmagnetic impurities can break Cooper pairs leading to the quasiparticle density of states in the superconducting state. The standing waves of these quasiparticles will interfere with each other leading to the quasiparticle interference (QPI) pattern which carries the phase message reflecting also the superconducting gap structure. Based on the recently proposed defect-bound-state QPI technique, we explore the applicability of this technique to a typical iron-based superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ with roughly equivalent gap values on the electron and hole pockets connected by the wave vector ${\mathbf{q}}_2=(0,\pi )$. It is found that, on the negative energy side, with the energy slightly below the gap value, the phase reference quantity $\left|g(\mathbf{q},-E)\right|cos({\theta }_{\mathbf{q},+E}-{\theta }_{\mathbf{q},-E})$ becomes negative and the amplitude is strongly enhanced with the scattering vector ${\mathbf{q}}_2$, but that corresponding to the scattering between the electron-electron pockets, namely ${\mathbf{q}}_3=(\pi ,\pi )$, keeps all positive. This is well consistent with the theoretical expectation of the $s^{\pm }$ pairing gap and thus serves as a direct visualization of the sign reversal gaps. This experimental observation is also supported by the theoretical calculations with the Fermi surface structure and $s^{\pm }$ pairing gap.

• Received 15 October 2018

DOI:https://doi.org/10.1103/PhysRevB.99.014507