Topological Semimetal Transport Modulated by Interstitial Fe in Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}As$_2$ Superconductors

TL;DR

This study uncovers tunable topological semimetal states in Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}$As$_2$ superconductors, modulated by interstitial Fe, exhibiting high mobility, large magnetoresistance, and coexistence with superconductivity.

Contribution

It demonstrates the emergence and tunability of topological semimetals in iron-based superconductors through interstitial Fe manipulation, combining experimental and theoretical insights.

Findings

Ultralow residual resistivity and high mobility carriers.

Linear positive magnetoresistance below 110 K.

Unsaturated magnetoresistance reaching 1206% at 56 T.

Abstract

Topological semimetals are renowned for exhibiting large, unsaturated magnetoresistance arising from ultrahigh carrier mobility and electron-hole compensation. However, such behaviors remain poorly understood in iron-based superconductors that have been recently recognized to harbor rich nontrivial topology. Here, we combine angle-resolved magneto-transport measurements with first principles calculations to reveal the emergence and tunability of topological semimetals in ferropnictide Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}As$_2$ epitaxial films, modulated by interstitial Fe. These states exhibit ultralow residual resistivity, coexisting high-mobility electron and hole carriers, and linear positive magnetoresistance below 110 K. Remarkably, the magnetoresistance becomes more pronounced when the magnetic field is applied parallel to the film plane, reaching an unsaturated 1206% at 56 T. Furthermore, superconductivity persists in these ferropnictide films, establishing them as a tunable platform for investigating the interplay among electron correlation, topology, and superconductivity.