Persistent Fe moments in the normal-state collapsed-tetragonal phase of the pressure-induced superconductor ${\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$

Using nonresonant Fe $K\beta$ x-ray emission spectroscopy, we reveal that Sr substitution into ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ decouples the Fe moment from the volume collapse transition, yielding a collapsed-tetragonal, paramagnetic normal state out of which superconductivity develops. X-ray diffraction measurements implicate the $c$-axis lattice parameter as the controlling criterion for the Fe moment, promoting a generic description for the appearance of pressure-induced superconductivity in the alkaline-earth-based 122 ferropnictides (A${\mathrm{Fe}}_2{\mathrm{As}}_2)$. The evolution of $T_c$ with pressure lends support to theories for superconductivity involving unconventional pairing mediated by magnetic fluctuations.

Figure 1

(a) Example, ambient-pressure XES spectra at 298 and 75 K as well as the difference between the two spectra. (b) The Fe moment of ${\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$ at ambient pressure as a function of temperature. Error bars are estimated from noise in the spectra to be 0.075 ${\mu }_B$. The solid line is guide to the eye.

Figure 2

Room-temperature, unit-normalized XES spectra of $({\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2{\mathrm{As}}_2$ under pressure (dashed lines, closed symbols) at (a) 1.5, (b) 3.6, and (c) 6.6 GPa as compared with the spectrum at 0.1 GPa (black line, no symbols). The differences between the XES data at each pressure and those at 0.1 GPa are shown as the gray solid lines below the data. Measurement errors due to counting statistics are less than 1%.

Figure 3

(a) The pressure-dependent Fe moment of $({\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2{\mathrm{As}}_2$ at 295, 125, and 27 K; solid lines through the data are linear fits. Horizontal error bars are the larger of 0.1 GPa or the difference between the pressures before and after collection of the spectra. Vertical error bars are 0.1 ${\mu }_B$ estimated from the noise in the spectra. The inset displays the crystal structure of $({\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2{\mathrm{As}}_2$, and labels the two As-Fe-As bond angles. (b) The electronic and structural phase diagram of $({\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2{\mathrm{As}}_2$, including a contour plot of the Fe moment in ${\mu }_B$. Open symbols represent data acquired with a pentane:isopentane pressure-transmitting medium, while closed symbols are those with steatite as a pressure-transmitting medium. Error bars are from Ref. [18] and represent transition widths determined from the derivative of the electrical resistivity. The arrows indicate the approximate paths of the XES measurements.

Figure 4

Example refinement of the XRD data at 125 K and $P=0.9$ GPa. Red tick marks correspond to the orthorhombic $\left(Fmmm\right)$ phase of (Ca,Sr)${\mathrm{Fe}}_2{\mathrm{As}}_2$, blue tick marks are from the Cu pressure marker, the background is the green line, the refinement is the red line through the black data points, and the residual of the refinement is the light blue line beneath the pattern.

Figure 5

Crystal structure parameters of $({\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2{\mathrm{As}}_2$ as a function of Fe moment for different temperatures: (a) $a_T,a_O$, and $b_O$, lattice parameters; (b) $c$ lattice parameter; and (c) unit cell volume per formula unit $V/\mathrm{f}.\mathrm{u}.$ The internal degrees of freedom of the crystal structure: (d) As-Fe-As bond angles (see inset of Fig. 3), and (e) the Fe-As and As-As bond distances. The $a$- and $c$-axis lattice parameters (from temperature-dependent measurements) for $({\mathrm{Ca}}_{0.92}{\mathrm{Nd}}_{0.08}){\mathrm{Fe}}_2{\mathrm{As}}_2$ are included in (a) and (b), respectively [25]. Error bars for the structure are smaller than the data markers.

Figure 6

Representative x-ray diffraction acquired at 125 K of the region around the ambient-pressure (202)/(022) orthorhombic Bragg peaks showing their collapse to a single (112) tetragonal Bragg reflection. Configuration 1 is shown in (a) and configuration 2 in (b); both use neon as a pressure-transmitting medium. Subscripts associated with the labeled peaks indicate the structural phase: orthorhombic (O), tetragonal (T), and collapsed tetragonal (CT).

Figure 7

(a) The Fe moment, determined from the IAD analysis of the XES data, as a function of pressure using neon and silicone oil as pressure-transmitting media. (b) Normalized electrical resistivity $\rho /\rho$ (300 K) versus temperature $T$ showing a clear superconducting transition near $T_c=24$ K in a pentane:isopentane pressure-transmitting medium.