Simultaneous suppression of superconductivity and structural phase transition under pressure in ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right)\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right){}_5$

We measured the pressure dependence of in-plane resistivity ${\rho }_{ab}$ in the recently discovered iron-based superconductor ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$, which shows a unique structural phase transition in the absence of magnetic ordering, with a superconducting transition temperature $T_{\mathrm{c}}=16$ K and a structural phase transition temperature $T_{\mathrm{s}}\simeq 100$ K at ambient pressure. $T_{\mathrm{c}}$ and $T_{\mathrm{s}}$ are suppressed on applying pressure and disappear at approximately 0.5 GPa, suggesting a relationship between superconductivity and structure. ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$ is a rather rare example in which the superconductivity appears only in a low-temperature ordered phase. The fact that the change in the crystal structure is directly linked with superconductivity suggests that the crystal structure as well as magnetism are important factors governing superconductivity in iron pnictides.

Figure 1

Schematic images of the phase diagrams in several iron pnictides; $X$ represents tuning parameters such as pressure $p$ and chemical doping $x$. T, ${\mathrm{T}}^{*}$, cT, O, and ${\mathrm{O}}^{*}$ indicate tetragonal, low-temperature phase in ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$, collapsed tetragonal, orthorhombic, and orthorhombic phases in heavily H-doped regions, respectively.

Figure 2

$T$ dependence of in-plane resistivity ${\rho }_{ab}$ under different pressures in ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$. The arrows indicate the structural phase transition temperature $T_{\mathrm{s}}$ defined by a negative peak of ${\partial }^2{\rho }_{ab}/\partial T^2$.

Figure 3

$T$ dependence of ${\rho }_{ab}$ below 40 K. $T_{\mathrm{c}}^{\mathrm{zero}}$ and $T_{\mathrm{c}}^{\mathrm{onset}}$ are defined as shown in the figure. $T_{\mathrm{c}}$ is suppressed on increasing the pressure and zero resistivity is not observed down to 2 K at above 0.5 GPa. The inset shows $log\rho$ vs $T$.

Figure 4

$T$ dependence of ${\partial }^2{\rho }_{ab}/\partial T^2$. A negative peak of ${\partial }^2{\rho }_{ab}/\partial T^2$ corresponds to a kink in the resistivity. Thus, $T_{\mathrm{s}}$ is defined as this negative peak of ${\partial }^2{\rho }_{ab}/\partial T^2$. $T_{\mathrm{s}}$ decreases with increasing pressure and disappears above 0.5 GPa.

Figure 5

Upper panel: $P$ dependence of ${\rho }_{ab}$ at 25 K. A peak at 0.5 GPa suggests the suppression of the SPT at $\sim 0.5$ GPa. Lower panel: $P-T$ phase diagram for ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$. Circles represent $T_{\mathrm{c}}$ and squares represent $T_{\mathrm{s}}$. $T_{\mathrm{c}}^{\mathrm{zero}}$ and $T_{\mathrm{c}}^{\mathrm{onset}}$ are defined as shown in Fig. 3. $T_{\mathrm{s}}$ is defined as a negative peak of ${\partial }^2{\rho }_{ab}/\partial T^2$. $T_{\mathrm{c}}$ and $T_{\mathrm{s}}$ are suppressed on applying pressure and disappear at $\sim 0.5$ GPa.