Nonmonotonic Evolution of the Superconducting Transition Temperature and Robust Multigap Extended s-wave + s-wave Pairing in Zn-Substituted FeSe Single Crystals

TL;DR

This study reveals the nonmonotonic evolution of Tc in Zn-doped FeSe, demonstrating robust multigap superconductivity with an extended s-wave pairing symmetry that persists despite impurity effects.

Contribution

It provides the first detailed analysis of how Zn doping affects multigap superconductivity and pairing symmetry in FeSe, emphasizing the robustness of the extended s-wave gap.

Findings

Tc shows nonmonotonic dependence on Zn doping.

Superconductivity is described by a two-gap s-wave and extended s-wave model.

Zn doping induces enhanced scattering but preserves multiband superconductivity.

Abstract

We report a systematic study of superconductivity on Fe1-xZnxSe single crystals synthesized over a broad Zn doping range (x = 0-0.023). High-quality single crystals across all compositions range exhibit superconducting transitions, while the transition temperature Tc shows a pronounced nonmonotonic dependence on Zn doping concentration, indicating that the underlying mechanism govering Tc its evolution cannot be explained solely by simple impurity pair breaking alone. Magnetization and transport measurements confirm the bulk behavior of superconductivity and reveal enhanced scattering effects with Zn doping. Low-temperature specific heat is consistently described by a two-gap scenario composed of an isotropic s-wave gap and an anisotropic extended s-wave gap, whereas single-gap and alternative pairing symmetries fail to describe the data. The nearly unchanged relative weights of the two gap components suggest the weak interband scattering induced by Zn substitution, thereby preserving multiband superconductivity. These results demonstrate the robustness of multigap superconductivity in FeSe and impose stringent constraints on candidate pairing mechanisms, highlighting the role of multiband electronic structure and anisotropic gap formation.