Pressure-induced superconducting state of antiferromagnetic ${\text{CaFe}}_2{\text{As}}_2$

The antiferromagnet ${\text{CaFe}}_2{\text{As}}_2$ does not become superconducting when subject to ideal hydrostatic pressure conditions, where crystallographic and magnetic states also are well defined. By measuring electrical resistivity and magnetic susceptibility under quasihydrostatic pressure, however, we find that a substantial volume fraction of the sample is superconducting in a narrow pressure range where collapsed tetragonal and orthorhombic structures coexist. At higher pressures, the collapsed tetragonal structure is stabilized with the boundary between this structure and the phase of coexisting structures strongly dependent on pressure history. Fluctuations in magnetic degrees of freedom in the phase of coexisting structures appear to be important for superconductivity.

Figure 1

(Color online) Electrical resistivity of ${\text{CaFe}}_2{\text{As}}_2$ under pressure. Curves were obtained on cooling, except for $P=1\text{bar}$ where measurements were made on both cooling and warming. The legends apply to both (a) and (b) and give the pressure in GPa. (a) In-plane resistivity ${\rho }_{ab}$ from 1.2 to 300 K for pressures of 1 bar, 0.01, 0.14, 0.41, 0.48, 0.56, 0.7, 0.73, 0.82, 0.98, 1.23, and 1.52 GPa. Arrows denote temperatures for a phase change, as discussed in the text. For clarity, only one out of 50 data points is shown. (b) An expanded view of the low-temperature, in-plane resistivity ${\rho }_{ab}$ shows the evolution of the superconducting transition temperature $T_c$. Sharp transitions appear only in a phase of coexisting O and cT structures. Curves corresponding to pressure regimes in Fig. 2b are circled. For clarity, one out of ten data points is shown.

Figure 2

(Color online) (a) Temperature-dependent resistivity upon cooling and warming in representative pressure ranges, corresponding to those in phases I, II, and III in (b). Note the emergence of strong thermal hysteresis at $T_S^h$ in phase III. (b) Temperature-pressure phase diagram. Solid squares (increasing pressure) denote the structural transition temperature $T_S$ from a tetragonal to a orthorhombic for $P<0.3\text{GPa}$ (phase I), a phase of coexisting O and cT structures for $0.3<P<0.8\text{GPa}$ (phase II), and the cT structure (phase III) for $P>0.8\text{GPa}$. Open squares represent structural transitions determined with decreasing pressure. The transition temperatures were defined from a break or maximum in the slope of $d\rho /dT$. Hashed vertical lines separate these structural phases. Circles denote the midpoint of the resistive SC transition and diamonds the onset of superconductivity in ac magnetic susceptibility ${\chi }_{\text{ac}}$. Triangles describe the magnetic transition from a paramagnetic to a SDW phase deduced from $\mu \text{SR}$ measurements (Ref. 22).

Figure 3

(Color online) (a) Pressure evolution of normalized ac magnetic susceptibility $\Delta {\chi }_{\text{ac}}/\Delta {\chi }_{\text{ac}}\left(=0.56\text{GPa}\right)$ on a single crystal sample, where $\Delta {\chi }_{\text{ac}}={\chi }_{\text{ac}}-{\chi }_{\text{ac}}\left(T_c\right)$. In these units, 1.0 corresponds to at least $0.5\left(1/4\pi \right)$ as explained in the text. The amplitude of the ac magnetic field was approximately 1 Oe. (b) Diamagnetic response $-\Delta {\chi }_{\text{ac}}\left(P\right)/\Delta {\chi }_{\text{ac}}\left(0.56\text{GPa}\right)$ (circles) at 1.2 K and volume fraction of a paramagnetic phase estimated by $\mu \text{SR}$ measurements (squares), where the dotted line is an extrapolation (Ref. 22). Hashed vertical lines are the same as those shown in Fig. 2b.

Figure 4

(Color online) (a) Pressure dependence of the residual resistivity ${\rho }_0$ (squares) of ${\text{CaFe}}_2{\text{As}}_2$ plotted on the left ordinate and superconducting transition temperature $T_c$ (circles) plotted on the right ordinate. Solid and open symbols are data obtained with increasing pressure, for the same crystal (1) reported in previous figures and for a second crystal (2), respectively. The residual resistivity of (2) was normalized to that of (1) at 1.2 GPa. Hashed vertical lines are the same as those shown in previous figures. (b) Residual resistivity and resistive midpoint $T_c$ of crystal (1) (solid symbols) and for (2) (open symbols) obtained on decreasing pressure from 1.52 GPa. Open triangles are the normalized diamagnetic response, defined in Fig. 3, for decreasing pressure. Hashed lines from (a) are shown for comparison.

Figure 5

(Color online) Temperature-dependent resistance of a constantan strain gauge glued along the tetragonal $a$ axis of ${\text{CaFe}}_2{\text{As}}_2$. One of nine data points is shown for clarity. The change in resistance is proportional to strain, as discussed in the text. For comparison, the solid line is plot of the temperature-dependent resistance of the free-standing strain gauge at $P=0$. Measurements are plotted for various increasing pressures, except for the $P=0$ data that were obtained on decreasing pressure from 0.6 GPa. Note the sharp jump in resistance at the structural transition for $P=0$ and 0.25 GPa but only a broad peak for pressures corresponding to range II in Fig. 2b. The inset is a plot of the structural transition temperature determined by these measurements, which are in good agreement with transition temperatures from resistivity measurements.