Hall-plot of the phase diagram for Ba(Fe1-xCox)2As2

TL;DR

This paper introduces a universal Hall-plot for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin films under strain, revealing critical fluctuations near the antiferromagnetic transition and linking magnetic and superconducting phases.

Contribution

It demonstrates that phase diagrams under different strains can be unified into a single Hall number plot, highlighting the role of critical fluctuations in multiband systems.

Findings

Universal phase diagram mapping with Hall number

Strain-induced shifts in the phase diagram

Critical fluctuations influence magnetic and superconducting properties

Abstract

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by $R_H^{-1} = -en$, where $e$ is the charge of the carrier, and $n$ is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe$_2$As$_2$ system.