Two-electronic component behavior in the multiband FeSe$_{0.42}$Te$_{0.58}$ superconductor

TL;DR

This study investigates the coexistence of localized and itinerant electronic states in FeSe$_{0.42}$Te$_{0.58}$ superconductor using EPR and NMR, revealing unconventional superconductivity influenced by strong electronic correlations.

Contribution

It provides new insights into the multiband electronic structure and electronic correlations affecting superconductivity in FeSe$_{0.42}$Te$_{0.58}$, highlighting the coexistence of localized and itinerant states.

Findings

Localized moments couple to itinerant electrons in the normal state.

Spin fluctuations are rapidly suppressed below $T_c$.

Unconventional superconducting state with faster $1/T_1$ reduction.

Abstract

We report X-band EPR and $^{125}$Te and $^{77}$Se NMR measurements on single-crystalline superconducting FeSe$_{0.42}$Te$_{0.58}$ ($T_c$ = 11.5(1) K). The data provide evidence for the coexistence of intrinsic localized and itinerant electronic states. In the normal state, localized moments couple to itinerant electrons in the Fe(Se,Te) layers and affect the local spin susceptibility and spin fluctuations. Below $T_c$, spin fluctuations become rapidly suppressed and an unconventional superconducting state emerges in which $1/T_1$ is reduced at a much faster rate than expected for conventional $s$- or $s_\pm$-wave symmetry. We suggest that the localized states arise from the strong electronic correlations within one of the Fe-derived bands. The multiband electronic structure together with the electronic correlations thus determine the normal and superconducting states of the FeSe$_{1-x}$Te$_x$ family, which appears much closer to other high-$T_c$ superconductors than previously anticipated.