Evidence for a cos(4\varphi) Modulation of the Superconducting Energy Gap of Optimally Doped FeTe_{0.6}Se_{0.4} Single Crystals Using Laser Angle-Resolved Photoemission Spectroscopy

TL;DR

This study reveals a fourfold symmetric modulation of the superconducting gap in FeTe_{0.6}Se_{0.4} using laser ARPES, indicating non-conventional pairing interactions likely involving electronic correlations.

Contribution

First direct observation of cos(4φ) superconducting gap modulation in FeTe_{0.6}Se_{0.4} using laser ARPES, supporting s_±-wave pairing mediated by electronic interactions.

Findings

Superconducting gap exhibits cos(4φ) modulation.

Gap anisotropy suggests non-phononic pairing mechanism.

Electronic interactions between iron sites are important.

Abstract

We study the superconducting(SC)-gap anisotropy of the \Gamma-centered hole Fermi surface in optimally doped FeTe_{0.6}Se_{0.4} (T_c = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (ARPES). We observe sharp superconducting coherence peaks at T = 2.5 K. In contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a \cos(4\varphi) modulation of the SC-gap anisotropy. The observed SC-gap anisotropy strongly indicates that the pairing interaction is not a conventional phonon-mediated isotropic one. Instead, the results suggest the importance of second-nearest-neighbor electronic interactions between the iron sites in the framework of s_\pm-wave superconductivity.