Determination of the phase diagram of the electron doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study maps the phase diagram of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing how electron doping suppresses magnetic and structural transitions and induces superconductivity, with evidence suggesting a quantum critical point.

Contribution

First detailed phase diagram of electron-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ showing the relationship between doping, phase transitions, and superconductivity.

Findings

Magnetic/structural transition splits and is suppressed with doping.

Superconductivity emerges above x ~ 0.025 and coexists with magnetic order.

Superconductivity peaks where magnetic/structural transitions are fully suppressed.

Abstract

Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. These data delineate an $x-T$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe$_2$As$_2$ at 134 K apparently splits into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above $x \sim 0.025$, and appears to coexist with the broken symmetry state for an appreciable range of doping, up to $x \sim 0.06$. The optimal superconducting transition temperature appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed, at least within the resolution provided by the finite step size between crystals prepared with different doping levels. Superconductivity is observed for a further range of Co concentrations, before being completely suppressed for $x \sim 0.018$ and above. The form of this $x-T$ phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions.