Combined effects of Sr substitution and pressure on the ground states in ${\mathrm{CaFe}}_2{\text{As}}_2$

We present a detailed study of the combined effects of Sr substitution and hydrostatic pressure on the ground-state properties of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. Measurements of the electrical resistance and magnetic susceptibility, both at ambient and finite pressure $P\le 2$ GPa, were performed on ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals grown out of Sn flux. We find that by Sr substitution the transition temperature to the magnetic/structural phase is enhanced and therefore a higher pressure is needed to suppress the transition to lowest temperature. In addition, the transition to the collapsed tetragonal phase is found at a pressure, which is distinctly higher than in the pure compound. This implies that the stability ranges of both phases shift on the pressure-axis upon doping, but the latter one with a higher rate. These observations suggest the possibility of separating the two phase lines, which intersect already at elevated temperatures for $x=0$ and low Sr concentration levels. For $x=0.177$, we find strong evidence that both phases remain separated down to the lowest temperature and that a zero-resistance state emerges in this intermediate pressure window. This observation indicates that Sr substitution combined with hydrostatic pressure provides another route for stabilizing superconductivity in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. Our results are consistent with the notion that (i) preserving the fluctuations associated with the structural-magnetic transition to low temperatures is vital for superconductivity to form in this material and that (ii) the nonmagnetic collapsed tetragonal phase is detrimental for superconductivity.

Figure 1

Results of the chemical analysis and lattice parameters of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ as a function of $x$. (a) Measured Sr concentrations $x_{\mathrm{WDS}}$ vs nominal concentration. (b) Lattice parameters $a$ (left axis, full black triangles) and $c$ (right axis, full red squares) as a function of $x_{\mathrm{WDS}}$. Data for $x=1$ (open symbols) are taken from Yan et al. [23].

Figure 2

Temperature dependence of (a) the electrical resistance $R\left(T\right)$, normalized to its value at 300 K, and (b) the magnetic susceptibility $\chi \left(T\right)$ for ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals with $x=0$, 0.105 and 0.177. (c) Transition temperature of the structural-magnetic phase transition at $T_{s,N}$ vs $x_{\mathrm{WDS}}$ for all concentrations prepared. Triangles correspond to magnetic susceptibility data $\left(\chi \right)$, squares correspond to electrical resistance $\left(R\right)$ data. The solid line is a guide to the eyes.

Figure 3

Single crystalline ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ for $x=0.105$ studied by (a) electrical resistance under gas-pm conditions for selected pressures $P\le 600$ MPa on crystal F1 (data are taken on decreasing the temperature) and by (b) magnetic susceptibility under gas-pm conditions for $P\le 500$ MPa on crystal F2. Closed symbols represent data taken with increasing temperature, open symbols represent data taken with decreasing temperature. The anomaly for $P=500\mathrm{MPa}$ at $T=45\mathrm{K}$ is assigned to solidification of helium. (c) Electrical resistance data for $x=0.105$ (crystal A1) under liquid-pm conditions at higher pressure $610\text{MPa}\le P\le 1660$ MPa taken on decreasing temperature. The arrows indicate positions of anomalies assigned to the transition into the cT phase.

Figure 4

(a) Anomalies in $\mathrm{d}R/\mathrm{d}T$ vs $T$ for the $x=0.105$ data shown in Fig. 3 corresponding to the structural-magnetic transition (left axis) and Fig. 3 corresponding to the cT transition (right axis). The inset shows the anomaly around the transition at $P=610\mathrm{MPa}$ on enlarged scales. (b) Low-temperature part of the resistance data taken under liquid-pm conditions. Note that the pressure value of 560 MPa corresponds to the same measurement as $P=610\mathrm{MPa}$ in Fig. 3.

Figure 5

Temperature-pressure phase diagram of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ for $x=0.105$. Closed (open) red squares correspond to the magnetic-structural transition temperatures, $T_{s,N}$, as seen in resistance (susceptibility) data taken under gas-pm conditions, orange circles correspond to the transition to the cT phase seen in resistance measurements performed under liquid-pm conditions. Green triangles indicate zero-resistance in measurements performed using liquid-pm. The o/afm (cT) phase is marked by the red (orange) area.

Figure 6

Single crystalline ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ for $x=0.177$ studied by (a) resistance measurements under gas-pm conditions for selected pressures $P\le 800$ MPa on two different crystals upon decreasing temperature and by (b) magnetic susceptibility under gas-pm conditions at $P\le 500$ MPa. Closed symbols represent data taken with increasing temperature, open symbols represent data taken with decreasing temperature. (c) Resistance data taken under liquid-pm conditions at higher pressure $1070\text{MPa}\le P\le 1880$ MPa, on crystal A2 upon decreasing temperature. The arrows indicate the position of anomalies assigned to the transition into the cT phase.

Figure 7

(a) Anomalies in $\mathrm{d}R/\mathrm{d}T$ vs $T$ for the $x=0.177$ data shown in Fig. 6 corresponding to the structural-magnetic transition (left axis) and 6 the cT transition (right axis). (b) Low-temperature part of the resistance, normalized to its value at 22 K, under liquid-pm conditions as well as gas-pm conditions, measured on different crystals. Note that the pressure values of 950 and 1370 MPa correspond to the same measurements as $P=1070$ and 1540 MPa in Fig. 6. The inset shows the $\left(R=0\right)$ criterion used to determine the superconducting transition temperature.

Figure 8

(a) Resistance data of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ for $x=0.177$ taken at constant temperature $T=75$ and 100 K with increasing pressure under gas-pm conditions. The arrows indicate the onset pressure of the transition from the o/afm to the tet/pm phase. (b) $T$-dependent measurements at $P=1000$ and 1100 MPa under gas-pm conditions. The change of slope at $\approx 60$ K (65 K) is due to the solidification of helium.

Figure 9

Temperature-pressure phase diagram of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ for $x=0.177$. Red symbols mark the phase transition line from the tet/pm to the o/afm phase as determined under gas-pm conditions: light red symbols correspond to $T_{s,N}$, determined from temperature-dependent measurements, whereas the dark red hexagons correspond to $P_{s,N}$ obtained from pressure-dependent measurements at constant temperature. Full orange spheres mark the transition to the cT phase as obtained under liquid-pm conditions. Green symbols indicate the transition to a state of zero resistance. Solid black lines at $P=1000$ and 1100 MPa correspond to temperature-pressure traces underlying the experiments shown in Fig. 8, where no indications for a phase transition were found. The solid gray line shows the solidification line of helium.

Figure 10

$T\text{−}{P}^{*}$ phase diagram for ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ for various $x$. Closed (open) symbols represent the transition to the o/afm (cT) phase. Black triangles correspond to data obtained on crystals with $x=0$, red squares to $x=0.105$, green circles to $x=0.177$, blue stars to $x=0.31$ and purple triangles to $x=1$. The inset shows the shift $\mathrm{\Delta }P$ as function of concentration $x$ as described in the text. The dashed, respectively dotted lines are guide to the eyes. Data for $x=0$ and 1 are taken from Yu et al. [7] and Colombier et al. [30].