[Paper Review] Topological Semimetal Transport Modulated by Interstitial Fe in Ba(Fe$_{1-x}$Co$_x$)$_{2+δ}As$_2$ Superconductors

Ze-Xian Deng, Qiang-Jun Cheng+15 more|arXiv (Cornell University)|Jan 22, 2026

Iron-based superconductors research0 citations

TL;DR

The study combines magneto-transport measurements and first-principles calculations to show tunable topological semimetal states in Ba(Fe1−x Cox)2+δAs2 films, modulated by interstitial Fe, coexisting with superconductivity. It reports ultrahigh mobility, electron-hole compensation, and an unsaturated magnetoresistance up to 1206% at high fields.

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$_x$)$_{2+δ}As$_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.

Motivation & Objective

• Motivate understanding of how interstitial Fe influences topology and transport in iron-based superconductors.
• Demonstrate the emergence and tunability of topological semimetal states in Ba(Fe1−x Cox)2+δAs2 epitaxial films.
• Explore the coexistence of superconductivity with topological transport in these ferropnictide films.
• Investigate how carrier mobility and magnetoresistance respond to magnetic field orientation and temperature.

Proposed method

• Combine angle-resolved magneto-transport measurements with first-principles calculations to identify topological semimetal states.
• Characterize residual resistivity, carrier mobility, and electron-hole compensation in Ba(Fe1−x Cox)2+δAs2 epitaxial films.
• Measure magnetoresistance under varying field orientations, including in-plane fields up to 56 T.
• Use first-principles theory to link interstitial Fe content to electronic topology.
• Assess coexistence of superconductivity with topological transport.
• analyze linear positive magnetoresistance behavior below 110 K.

Experimental results

Research questions

• RQ1 Do interstitial Fe atoms modulate the emergence of topological semimetal states in Ba(Fe1−x Cox)2+δAs2?
• RQ2 What are the transport signatures (mobility, compensation, magnetoresistance) associated with these topological states?
• RQ3 How does field orientation affect magnetoresistance and its saturation behavior in these films?
• RQ4 Is superconductivity preserved while topological semimetal transport is present?
• RQ5 How do first-principles calculations corroborate the experimental observations?

Key findings

• Topological semimetal transport emerges and is tunable by interstitial Fe in Ba(Fe1−x Cox)2+δAs2 films.
• The films exhibit ultralow residual resistivity with coexisting high-mobility electron and hole carriers.
• Linear positive magnetoresistance appears below 110 K.
• Magnetoresistance becomes more pronounced with in-plane magnetic field, reaching 1206% at 56 T (unsaturated).
• Superconductivity persists in these ferropnictide films, enabling study of interplay among electron correlation, topology, and superconductivity.