Effects of Co substitution on thermodynamic and transport properties and anisotropicHc2inBa(Fe1−xCox)2As2single crystals

Abstract

Single crystalline samples of $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ with $x<0.12$ have been grown and characterized via microscopic, thermodynamic, and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high-temperature tetragonal to low-temperature orthorhombic) and magnetic (high-temperature nonmagnetic to low-temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K/% Co. In addition, for $x\ensuremath{\ge}0.038$ superconductivity is stabilized, rising to a maximum ${T}_{c}$ of approximately 23 K for $x\ensuremath{\approx}0.07$ and decreasing for higher $x$ values. The $T\text{\ensuremath{-}}x$ phase diagram for $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ indicates that either superconductivity can exist in both low-temperature crystallographic phases or that there is a structural phase separation. Anisotropic superconducting upper critical-field data $[{H}_{c2}(T)]$ show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.