Segregation of antiferromagnetism and high-temperature superconductivity in ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$

We report the effect of applied pressures on magnetic and superconducting order in single crystals of the aliovalent La-doped iron pnictide material ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Using electrical transport, elastic neutron scattering, and resonant tunnel diode oscillator measurements on samples under both quasihydrostatic and hydrostatic pressure conditions, we report a series of phase diagrams spanning the range of substitution concentrations for both antiferromagnetic and superconducting ground states that include pressure-tuning through the antiferromagnetic (AFM) superconducting critical point. Our results indicate that the observed superconducting phase with a maximum transition temperature of $T_c=47$ K is intrinsic to these materials, appearing only upon suppression of magnetic order by pressure-tuning through the AFM critical point. Thus, the superconducting phase appears to exist exclusively in juxtaposition to the antiferromagnetic phase in a manner similar to the oxygen- and fluorine-based iron-pnictide superconductors with the highest transition temperatures reported to date. Unlike the lower-$T_c$ systems, in which superconductivity and magnetism usually coexist, the tendency for the highest-$T_c$ systems to show noncoexistence provides an important insight into the distinct transition temperature limits in different members of the iron-based superconductor family.

Figure 1

Lanthanum substitution phase diagram of ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$, showing the antiferromagnetic (AFM) transitions at $T_N$ (squares) and superconducting transitions (circles). Open circles indicate the “10 K” superconducting transition $T_{c1}$ (see text). Arrows indicate the La concentrations of samples S1, S2, S3, and S4 (with $x=0.10$, 0.12, 0.13 and 0.27, respectively) investigated in this study.

Figure 2

Quasihydrostatic pressure experiment on ${\mathrm{Ca}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{Fe}}_2{\mathrm{As}}_2$ (underdoped sample “S1” in Fig. 1) using a piston-cylinder clamp cell with the sample loaded in a Fluorinert liquid pressure medium. (a) Resistivity measured upon cooling temperature as a function of applied pressures. Down arrows indicate the position of the antiferromagnetic ordering temperatures $T_N$ as defined by the kink in $d\rho /dT$ (see text and Fig. 1 of the supplemental material [43]). Up arrows indicate the onset of superconducting transitions, including the high-$T_c$ phase at 44 K, that appear abruptly upon an increase of pressure from 10.5 to 12 kbar. (b) Phase diagram for sample S1, showing the evolution of $T_N$ (squares) and superconducting transition temperatures $T_c$ (closed circles) and $T_{c1}$ (open circles), which is related to the “10 K” phase observed in undoped CaFe${}_2{\mathrm{As}}_2$ at ambient pressure. An intermediate superconducting phase appears above 10.5 kbar near $\sim$20 K (dotted circles).

Figure 3

Hydrostatic pressure dependence of ordered AFM moments in ${\mathrm{Ca}}_{0.88}{\mathrm{La}}_{0.12}{\mathrm{Fe}}_2{\mathrm{As}}_2$ (underdoped sample “S2” in Fig. 1) determined by neutron scattering experiments on the BT-4 beam line at the NCNR. Integrated intensities measured at the $Q_{\text{AFM}}=\left(103\right)$ magnetic Bragg peak are shown for selected applied pressures of (a) $P=0$, (b) $P=0.24$ kbar, and (c) $P=0.44$ kbar. Intensities are proportional to the square of the magnetic order parameter $M^2$, which is the ordered moment at $(P,T)$. Values are normalized to the maximum ambient pressure value observed at 3.9 K. Solid lines are mean-field order fits to $M^2$ to obtain the Néel temperatures for each pressure. Insets show data from $\theta$-2$\theta$ scans at 3.9 K, fit to a Gaussian line shape with a sloping background.

Figure 4

(a) Magnetic intensity of the (1 0 3) Bragg peak at ambient pressure in ${\mathrm{Ca}}_{0.88}{\mathrm{La}}_{0.12}{\mathrm{Fe}}_2{\mathrm{As}}_2$ (underdoped sample “S2” in Fig. 1) determined from neutron diffraction on the BT-4 beamline at the NCNR. (b) Hydrostatic pressure dependence of $T_N$ in ${\mathrm{Ca}}_{0.88}{\mathrm{La}}_{0.12}{\mathrm{Fe}}_2{\mathrm{As}}_2$ determined by magnetic order parameter fits (see Fig. 3). (c) Variation of the $c$-axis lattice parameter vs pressure determined by measuring the (0 0 4) structural Bragg reflection at 5 K. Uncertainties are statistical in origin and represent one standard deviation.

Figure 5

Hydrostatic pressure dependence of electrical resistivity of ${\mathrm{Ca}}_{0.87}{\mathrm{La}}_{0.13}{\mathrm{Fe}}_2{\mathrm{As}}_2$ (underdoped sample “S3” in Fig. 1) using helium gas as the pressure medium. (a) Resistivity temperature dependence as a function of applied pressures. Down arrows mark the antiferromagnetic ordering transition temperature $T_N$ for each pressure, as determined by $d\rho /dT$ analysis (see text), and the up arrow indicates an example of the onset of superconductivity at $T_c$ for the sample under 4 kbar pressure. (b) Temperature vs hydrostatic pressure phase diagram of antiferromagnetic (squares) and superconducting transition temperatures $T_c$ (solid circles) and $T_{c1}$ (open circles).

Figure 6

Pressure dependence of superconducting transition of ${\mathrm{Ca}}_{0.73}{\mathrm{La}}_{0.27}{\mathrm{Fe}}_2{\mathrm{As}}_2$ (overdoped sample “S4” in Fig. 1). (a) Temperature dependence of the tunnel diode oscillator frequency of single-crystal sample S4 at various applied pressures obtained using a Moissanite anvil cell (note: the sign of the vertical axis has been inverted). Inset: example of raw data and background estimation (see text). (b) Electrical resistance of sample S4 measured using a piston-cylinder cell. Inset: magnetic-field dependence of the superconducting transition at 13.9 kbar applied pressure. (c) Phase diagram showing the pressure dependence of the superconducting transition for samples S4 (closed circles) and S4' (open circles) as shown in panels (a) and (b), respectively. The solid curve is a guide to the eye. Inset: photograph of TDO measurement setup, with a 10-turn microcoil with a diameter of 300 $\mu$m enclosing the sample and a ruby chip used for pressure determination.