Evidence from neutron diffraction for superconductivity in the stabilized tetragonal phase of ${\text{CaFe}}_2{\text{As}}_2$ under uniaxial pressure

${\text{CaFe}}_2{\text{As}}_2$ single crystals under uniaxial pressure applied along the $c$ axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in ${\text{CaFe}}_2{\text{As}}_2$ found below 10 K in samples subjected to nonhydrostatic pressure conditions.

Figure 1

(Color online) (a) Color map (counts per monitor are color coded in the inset) showing the temperature dependence of a portion of the diffraction pattern taken on the E4 diffractometer within the range of various structural (002) reflections of ${\text{CaFe}}_2{\text{As}}_2$. (b)–(e) $2\theta \text{−}\omega$ plots at selected temperatures showing the angular distribution of peaks tracked in (a). Panel (f) shows the temperature dependence of the integrated intensities and positions of reflections shown in panels (a)–(e).

Figure 2

(Color online) Representative examples of the magnetic diffraction peak observed at the ${\left(\frac{1}{2}\frac{1}{2}3\right)}_T$ [or ${\left(103\right)}_O$] position obtained from two ${\text{CaFe}}_2{\text{As}}_2$ samples using the D10 instrument under uniaxial pressures of (a) 0.060 GPa and (b) 0.092 GPa applied along the $c$ axis (normalized to the same monitor and crystal weight). Line through the data represents fits using a single Gaussian. (c) The temperature dependence of the integrated intensities of data shown in (a) and (b) normalized to the intensity of the ${\left(002\right)}_O$ reflections at the corresponding temperatures and pressures. (d) The normalized integrated intensities of the magnetic reflections in the vicinity of the expected superconducting transition (at $\approx 10\text{K}$) for $P=0.060$ and 0.092 GPa.

Figure 3

(Color online) Color map showing the temperature dependence of a portion of the diffraction pattern taken at E4 in that covers various structural (002) reflections of ${\text{CaFe}}_2{\text{As}}_2$ under uniaxial pressure of 0.1 GPa along the $c$ axis measured with (a) decreasing and (b) increasing temperature. While the intensity in panel (a) is color-coded in the range 60–260, the (b) panel is in the range 60–220. The simultaneously measured electrical resistance is shown in panel (c). The lower inset to (c) shows the low-temperature detail of the electrical resistance data taken upon heating. The upper inset shows the weight fraction of the pressure stabilized phase $T^{\prime }$ as a function of applied pressure. The dotted line is a guide to the eyes.