Phase diagram of the iron arsenide superconductors Ca(Fe 1− x Co x ) 2 As 2 ( 0 ⩽ x ⩽ 0.2)

Abstract

Plateletlike single crystals of the Ca(Fe 1 − x Co x ) 2 As 2 series having lateral dimensions up to 15 mm and thicknesses up to 0.5 mm were obtained from the high-temperature solution-growth technique using Sn flux. Upon Co doping, the c axis of the tetragonal unit cell decreases, while the a axis shows a less significant variation. Pristine CaFe 2 As 2 shows a combined spin-density wave and structural transition near T = 166 K, which gradually shifts to lower temperatures and splits with increasing Co doping. Both transitions terminate abruptly at a critical Co concentration of x c = 0.075. For x ⩾ 0.05, superconductivity (SC) appears at low temperatures with a maximum transition temperature T C of around 20 K. The SC volume fraction increases with Co concentration up to x = 0.09 followed by a gradual decrease with a further increase of the doping level. The electronic phase diagram of the Ca(Fe 1 − x Co x ) 2 As 2 (0 ⩽ x ⩽ 0.2) series is constructed from the magnetization and electric resistivity data. We show that the low-temperature SC properties of Co-doped CaFe 2 As 2 differ considerably from those of BaFe 2 As 2 reported previously. These differences seem to be related to the extreme pressure sensitivity of CaFe 2 As 2 relative to its Ba counterpart.