Distinct Modulation Behavior of Superconducting Coherence Peaks Associated with Sign-Reversal Gaps in FeTe$_{0.55}$Se$_{0.45}$

Abstract

Using high-resolution scanning tunneling microscopy, we reveal two distinct types of superconducting (SC) gap modulations in bulk superconductor FeTe$_{0.55}$Se$_{0.45}$. By analyzing the phase relation between modulations at positive and negative bias, we identify in-phase (particle-hole asymmetric) and anti-phase (particle-hole symmetric) oscillations, corresponding to sign-reversing and sign-preserving scattering processes, respectively. The observed features are consistent with predictions from pair-breaking scattering interference (PBSI) theory and are distinguishable from other alternative mechanisms such as pair density waves. Our results provide compelling evidence that PBSI is the dominant mechanism behind the SC gap modulations in FeTe$_{0.55}$Se$_{0.45}$, offering new insights into the role of impurity scattering in iron-based superconductors.

Figures (4)

• Figure 1: (a) Atomically resolved topography measured on FeTe$_{0.55}$Se$_{0.45}$. (b) Average tunneling spectrum of 112×112 spectra measured in the area of (a). (c),(d) QPI images $g(\textbf{r}, E)$ in the real space measured at +1.75 and -1.75 mV, respectively. (e),(f) FT-QPI patterns $|g(\textbf{q}, E)|$ obtained from the Fourier transform results of (c) and (d), respectively. (g) Schematic image of the Fermi surfaces in the unfolded Brillouin zone. The hole pocket is centered at $\Gamma$ point, while the electron pockets are around M points. (h) PR-QPI pattern $g_r(\textbf{q}, E)$ obtained by the DBS-QPI technique. The pattern is fourfold-symmetrized.
• Figure 2: (a),(b) Spatial distributions of dI/dV extracted along the yellow and red lines in Fig.\ref{['fig1']}(c) or \ref{['fig1']}(d), respectively. (c),(d) Schematic results based on the PBSI theory of the dI/dV along $\boldsymbol{q_3}$ and $\boldsymbol{q_2}$ directions, respectively. The SC gaps determined from the coherence peaks can be easily identified, as marked by $\Delta_+$ and $\Delta_-$. (e),(f) Distributions of dI/dV at $\pm 0.8 \Delta_0$ extracted from (c),(d), as marked by red and blue dashed lines in (c),(d). The curves are offset for clarity.
• Figure 3: (a),(b) Spatial distributions of the positive ($\Delta_+$) and negative ($\Delta_-$) superconducting gap positions and (c),(d) the corresponding FT results. (e),(f) Spatial distributions of $(\Delta_+ - \Delta_-)$ (usually referred as the SC gap size) and $(\Delta_+ + \Delta_-)$ and (g),(h) the corresponding FT results.
• Figure 4: (a),(b) Color plots of tunneling spectra measured along the red (a) and the yellow (b) lines in Fig. \ref{['fig3']}(a) or \ref{['fig3']}(b), respectively. The positive and negative sides of the tunneling spectra are normalized by setting the integrated part from 0 to +3.5 mV and from –3.5 to 0 mV to the same value to remove the asymmetric feature of the tunneling spectra. The coherence peak locations are marked by the circles, and the translucent lines with oscillations connecting the circles are guides for the eyes. (c),(d) Typical tunneling spectra with symmetric (c) and asymmetric (d) modulation extracted from (a) and (b) at the positions marked by dashed lines.