Effect of pressure on the structural phase transition and superconductivity in $\left({\text{Ba}}_{1-x}{\text{K}}_x\right){\text{Fe}}_2{\text{As}}_2$ ($x=0$ and 0.45) and ${\text{SrFe}}_2{\text{As}}_2$ single crystals

The effects of pressure up to $\sim 20\text{kbar}$ on the structural phase transition of ${\text{SrFe}}_2{\text{As}}_2$ and lightly Sn-doped ${\text{BaFe}}_2{\text{As}}_2$ as well as on the superconducting transition temperature and upper critical field of $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$ single crystals have been studied. All the transition temperatures decrease with pressure in an almost linear fashion. Under pressure, the upper critical-field curve $H_{c2}\left(T\right)$ for $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$ shifts down in temperature to follow the zero-field $T_c$ with very little change in slope. Composite $P\text{−}T$ phase diagrams for three parent compounds $A{\text{Fe}}_2{\text{As}}_2$ ($A=\text{Ba}$,Sr,Ca) are constructed and appear to be remarkably similar: (i) having a structural (antiferromagnetic) phase-transition line with a negative slope and (ii) showing signs of the emerging superconducting state at intermediate pressures.

Figure 1

(Color online) (a) Pressure dependence of in-plane resistance of ${\text{BaFe}}_2{\text{As}}_2$. [For ${\text{BaFe}}_2{\text{As}}_2$ sample ${\rho }_{300\text{K}}\left(P=0\right)\approx 1\text{m}\Omega \text{cm}$.] Inset: pressure dependent transition temperatures, determined as shown in panel (b), the lines are from linear fits. (b) Derivatives $dR/dT$ at different pressures in the transition region. Arrows show two definitions of the transition temperature (Ref. 5).

Figure 2

(Color online) (a) Temperature-dependent in-plane resistance of $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$ under pressure (pressure values in the legend are in the order of runs). [For $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$ sample ${\rho }_{300\text{K}}\left(P=0\right)\approx 0.9\text{m}\Omega \text{cm}$.] Dashed line–20.4 kbar data scaled in a way that brings the last, 12.9 kbar, data between 8.9 and 15.6 kbar runs. Inset: normalized temperature-dependent resistivity as a function of pressure. (b) Resistive superconducting transition in $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$ (pressure values in the legend are in the order of runs). Inset: superconducting transition temperatures (defined as onset and offset of resistive transition) as a function of pressure. The lines are from linear fits (Ref. 5).

Figure 3

(Color online) Upper critical fields $H_{c2}\left(T\right)$ measured up to 90 kOe at zero and maximum pressures as defined from onset (filled symbols) and offset (open symbols) of resistive transitions. The lines are guides to the eye.

Figure 4

(Color online) Temperature-dependent in-plane resistance of ${\text{SrFe}}_2{\text{As}}_2$ measured at $P=0$, 3.1, 6.4, 12.1, 15.7, and 18.9 kbar (pressure values at low temperatures). Arrow indicates the direction of the pressure increase. Upper left inset: enlarged region of the $R\left(T\right)$ curves near the structural (antiferromagnetic) phase transition. Arrow indicates the direction of the pressure increase. Lower right inset: enlarged low-temperature part of the $R\left(T\right)$ curves in the region of the $T^{\ast }$ (see text). Lower left inset: low-temperature part of the $R\left(T\right)$ curves taken at $P=18.9\text{kbar}$ for $H=0$ and 90 kOe. Vertical arrows mark $T^{\ast }$.

Figure 5

(Color online) Pressure dependence of the structural (antiferromagnetic) phase transition (top panel) and $T^{\ast }$ low-temperature anomaly (bottom panel). Pressure values interpolation consistent with Ref. 14 were used for the top panel and low-temperature values of pressure for $T^{\ast }\left(P\right)$. Two criteria, onset and maximum of the $dR/dT$ derivatives, were used for the structural (antiferromagnetic) transition.

Figure 6

(Color online) Schematic $P\text{−}T$ phase diagrams for (a) ${\text{CaFe}}_2{\text{As}}_2$, (b) ${\text{BaFe}}_2{\text{As}}_2$, and (c) ${\text{CaFe}}_2{\text{As}}_2$. Data in panel (a) are from Ref. 10. Panel (b) combines data from this work (●), Ref. 11 $(\ast )$, and shifted by 40 kbar data for $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$ $(★)$ from this work. Panel (c) presents data from this work $\left(●,★\right)$, Ref. 11 $(\ast )$, and Ref. 17 $\left(+,\times \right)$.