Bulk and surface Dirac states accompanied by two superconducting domes in FeSe-based superconductors

TL;DR

This study reveals two distinct superconducting domes in FeSe-based superconductors, characterized by different Dirac states and normal state resistivity behaviors, suggesting separate pairing mechanisms.

Contribution

It provides the first comprehensive phase diagram showing two superconducting domes with associated Dirac states and normal state properties in FeSe$_{1-x}$S$_x$ and FeSe$_{1-x}$Te$_x$ superconductors.

Findings

Observation of two superconducting domes in FeSe-based superconductors.

Identification of bulk and surface Dirac states associated with each dome.

Distinct normal state resistivity behaviors indicating different electronic states.

Abstract

Recent investigations of FeSe-based superconductors have revealed the presence of two superconducting domes, and suggest possible distinct pairing mechanisms. Two superconducting domes are commonly found in unconventional superconductors and exhibit unique normal states and electronic structures. In this study, we conducted electromagnetic transport measurements to establish a complete phase diagram, successfully observing the two superconducting domes in FeSe$_{1-x}$S$_x$ (0 $\le x \le$ 0.25) and FeSe$_{1-x}$Te$_x$ (0 $\le x \le$ 1) superconductors. The normal state resistivity on SC1 shows the strange metal state, with a power exponent approximately equal to 1 ($\rho (T)\propto T^n$ with $n\sim 1$), whereas the exponent on SC2 is less than 1. A bulk Dirac state observed on SC1, completely synchronized with the strange metal behavior, indicating a close relationship between them. While a topological surface Dirac state is witnessed on SC2, and undergoes a sign change near the pure nematic quantum critical point. The evolution of the Dirac states indicates that the appearance of the two superconducting domes may originate from the Fermi surface reconstruction. Our findings highlight distinct Dirac states and normal state resistivity across the two superconducting domes, providing convincing evidence for the existence of the two different pairing mechanisms in FeSe-based superconductors.