Phase diagram of the iron arsenide superconductors Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$($0⩽x⩽0.2$)

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.

Figure 1

(a),(b) Typical flux-grown single crystals of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x_N=0.05,0.15$). The grid in each crystal is millimeter size. (c),(d) Two representative SEM images of as-grown single crystals of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ (see text for details).

Figure 2

Co composition determined by means of EDX of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals as a function of nominal concentration $x$. The dashed line is a linear fit to the data with a slope of 0.7 (see text for details).

Figure 3

Powder x-ray diffraction pattern of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x_N=0,0.20$) single crystals. Peaks marked Si are due to silicon added as an internal standard.

Figure 4

Variation of the lattice parameters as a function of nominal Co content. The lines are guides to the eye. The error bars for Δ$a$% and Δ$c$% are around ${10}^{-4}$ Å, smaller than the size of the data point.

Figure 5

X-raydiffraction pattern due to a single-crystalline CaFe${}_2$As${}_2$ in Bragg-Brentano geometry showing only [0,0,2$l$] reflections (see text for details).

Figure 6

Temperature dependence of the in-plane electrical resistivity of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals, normalized to the resistivity value at 296 K. $x_N$ and $x_{\mathrm{EDX}}$ are nominal and EDX composition, respectively.

Figure 7

Normalized resistivity ρ($T$)/ρ (296 K) of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x_N=0.00,0.03,0.05,0.075$) and its first derivate plotted as of the temperature. The arrows are indicating the magnetic ($T_N$) and the structural transition ($T_S$).

Figure 8

Low-temperature resistivity ρ($T$)/ρ (30 K) of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ showing the evolution of the SC transition with Co concentration.

Figure 9

Temperature dependence of the magnetization of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ measured under an applied field of 1 T parallel to the crystallographic basal plane in zero-field conditions.

Figure 10

Low-temperature volume magnetic susceptibility of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals under 20-Oe zero-field-cooled conditions parallel to the crystallographic basal plane.

Figure 11

Electronic phase diagram of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ obtained from magnetic and electric resistivity data, showing the suppression of the magnetic/structural phase transitions (blue circles and red squares, $T_N,T_S$) with increasing Co concentration and the appearance of the SC transition (red half-filled circles, $T_C$ determined from resistivity using the $90\%$ and the onset criteria; blue triangles, $T_C$ determined from magnetization data taken at the intersection point of the slopes extrapolated from the normal state and the SC transition region) with maximum $T_C$ of ∼20 K.

Figure 12

Evolution of SC-state parameters in Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ with Co concentration $x$. (a) SC transition temperatures determined from the susceptibility and the transport data; (b) SC volume fraction determined from the magnetic susceptibility data; (c) variation of the width in the temperature of the resistive SC transition $T_C$; (d) variation of the residual resistivity ratio with Co concentration. All lines are guides to the eye.