Complete pressure-dependent phase diagrams for ${\text{SrFe}}_2{\text{As}}_2$ and ${\text{BaFe}}_2{\text{As}}_2$

The temperature-dependent electrical resistivity of single-crystalline ${\text{SrFe}}_2{\text{As}}_2$ and ${\text{BaFe}}_2{\text{As}}_2$ has been measured in a liquid-medium modified Bridgman anvil cell for pressures in excess of 75 kbar. These data allow for the determination of the pressure dependence of the higher-temperature structural/antiferromagnetic phase transitions as well as the lower-temperature superconducting phase transition. For both compounds the ambient-pressure higher-temperature structural/antiferromagnetic phase transition can be fully suppressed with a domelike region of zero resistivity found to be centered about its critical pressure. Indeed, qualitatively, the temperature dependence of the resistivity curves closest to the critical pressures is the closest to linear, suggesting the possibility of quantum criticality. For pressures significantly higher than the critical pressure the zero-resistivity state is suppressed and the low-temperature resistivity curves asymptotically approach a universal low-temperature manifold. These results are consistent with the hypothesis that correlations/fluctuations associated with the ambient-pressure, high-temperature, tetragonal phase have to be brought to low enough temperature to allow superconductivity, but if too fully suppressed it can lead to the loss of the superconducting state.

Figure 1

(Color online) (a) Top view (photograph) of the pressure chamber with a spot-welded lead manometer (top) and a ${\text{SrFe}}_2{\text{As}}_2$ sample (bottom) to which wires were fixed with silver epoxy. A second Teflon ring is then placed on top of the first one, just before filling with liquid. (b) Sketch of the side view.

Figure 2

(Color online) Resistivity at ambient pressure for a ${\text{SrFe}}_2{\text{As}}_2$ sample before and after a pressure cycle up to around 50 kbar. Two different resistivity scales were used for a better comparison. A low-temperature zoom is shown in the inset.

Figure 3

(Color online) Lead resistance at low temperature from one pressure cell. The superconducting transition is shown at six different pressures. From right to left: 0, 16.6, 27.9, 38.8, 46.5, and 56.8 kbar.

Figure 4

(Color online) (a) and (b) summarize the resistivity measurements under pressure for two different ${\text{SrFe}}_2{\text{As}}_2$ samples cell 1 and cell 2, respectively. (c) presents an enlargement of the low-temperature behavior with resistivity on logarithmic scale (from $5\times {10}^{-5}$ to $0.4\text{m}\Omega \text{cm}$ and $0.2\text{m}\Omega \text{cm}$, respectively, for cell 1 and cell 2). Arrows show the structural/antiferromagnetic transition temperature deduced from a maximum of the resistivity derivative criterion.

Figure 5

(Color online) Relative resistivity decrease at 300 K ${\rho }_{300\text{K}}\left(P\right)/{\rho }_{300\text{K}}\left(0\right)$ versus pressure from cell 1 (black squares) and cell 2 (red circles). Green crosses up to 20 kbar are data from Ref. 12. The dashed line is a guide for the eyes.

Figure 6

(Color online) Phase diagram $T\left(P\right)$ of ${\text{SrFe}}_2{\text{As}}_2$ deduced from resistivity measurements in a modified Bridgman pressure cell. Blue and green data correspond to cell 1 and cell 2, respectively. Low-pressure data up to 20 kbar from Ref. 12 were added in pink. Circles and crosses correspond, respectively, to the structural/antiferromagnetic transition and the onset of superconductivity. Vertical error bars indicate the shift between cooling and warming due to a not perfect thermalization. Triangles represent the offset temperature of the full superconducting transition. The hatched area shows the zero-resistance superconducting region. The very large horizontal error bars are due to pressure uncertainties caused by a small remanent field. These uncertainties were estimated from one pressure (60 kbar), where the pressure cell was measured in two different PPMS, one without remanent field. Smaller error bars were estimated from the superconducting width, indicating at the same time pressure gradients and a not perfect thermalization. Dotted lines link the onset temperature of the superconducting transition.

Figure 7

(Color online) Resistivity of ${\text{SrFe}}_2{\text{As}}_2$ under different magnetic fields measured up to 14 T. The criterions used to determine the onset, half-width, and offset temperatures are shown for the superconducting transition at 14 T. The inset summarizes the field dependence of the onset (triangles), the offset (circles), and the half-width (crosses) temperatures.

Figure 8

(Color online) (a) and (b) summarize the resistivity measurements under pressure for two different ${\text{BaFe}}_2{\text{As}}_2$ samples. Two sets of measurements are, respectively, called cell 1 and cell 2. For cell 1, pressure uncertainty is around 3 kbar for the two last pressures due to a small remanent field. The arrows show the structural/antiferromagnetic transition temperature deduced from our criterion (maximum of the resistivity derivative). (c) presents an enlargement of the low-temperature behavior with resistivity in logarithmic scale from $5\times {10}^{-5}$ to $0.3\text{m}\Omega \text{cm}$. The higher and lower panels correspond to cell 1 and cell 2, respectively.

Figure 9

(Color online) Relative resistivity decrease at 300 K ${\rho }_{300\text{K}}\left(P\right)/{\rho }_{300\text{K}}\left(0\right)$ versus pressure from cell 1 (black squares) and cell 2 (red circles). The dashed line is a guide for the eyes.

Figure 10

(Color online) Phase diagram $T\left(P\right)$ of ${\text{BaFe}}_2{\text{As}}_2$ deduced from resistivity measurements in a modified Bridgman pressure cell. Green and blue colors refer to two different cells, respectively, called cell 1 and cell 2. Circles correspond to the structural transition deduced from the local maximum of the resistivity derivative. Vertical error bars indicates the shift between cooling and warming due to a not perfect thermalization. Crosses correspond to the onset of the superconducting transition. Triangles represent offset temperature of the full superconducting transition. The hatched area estimates the true zero-resistance superconducting region.

Figure 11

(Color online) Resistivity of ${\text{BaFe}}_2{\text{As}}_2$ under different magnetic fields measured up to 14 T. The criterions used to determine the onset, half-width, and offset temperatures are shown for the superconducting transition at 14 T. The inset summarizes the field dependence of the onset (triangles), the offset (circles), and the half-width (crosses) temperatures.

Figure 12

(Color online) Superconducting transition measured in resistivity shown for different currents 0.01, 0.1, and 1 mA for three pressures. ${\text{SrFe}}_2{\text{As}}_2$ at 33 and 52.9 kbar and ${\text{BaFe}}_2{\text{As}}_2$ at 38.9 kbar.

Figure 13

(Color online) Pressure dependence of the resistivity at 40 K (right axis) (a) for ${\text{SrFe}}_2{\text{As}}_2$ and (b) for ${\text{BaFe}}_2{\text{As}}_2$. For reference, the zero-resistivity dome (hatched area) from the $T\left(P\right)$ phase diagram is also shown (left axis). The ${\rho }_{40\text{K}}\left(P\right)$ data have been fit (solid red line) and the pressure derivative of this fit (scaled by a negative factor) is also shown (dashed red line).