Pressure-enhanced superconducting transition in the inter-block-layer electron-transfer superconductor BaTh2Fe4As4(N0.7O0.3)2

The 12442-type hybrid structure compound $\mathrm{Ba}{\mathrm{Th}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{({\mathrm{N}}_{0.7}{\mathrm{O}}_{0.3})}_{2}$ is a recently synthesized inter-block-layer electron-transfer superconductor with separated double ${\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ layer and shows the superconducting transition at ${T}_{\mathrm{c}}^{\mathrm{onset}}\ensuremath{\sim}30$ K and ${T}_{\mathrm{c}}^{\mathrm{zero}}\ensuremath{\sim}20$ K. Here, we perform the resistivity measurements under various hydrostatic pressures up to $\ensuremath{\sim}12$ GPa to track the evolution of its superconducting ${T}_{\mathrm{c}}$ and normal-state properties as a function of pressure. We found that ${T}_{\mathrm{c}}$($P$) exhibits a nonmonotonic variation with pressure forming a dome-shaped superconducting phase, i.e., ${T}_{\mathrm{c}}^{\mathrm{onset}}$ first increases quickly from $\ensuremath{\sim}30$ K at ambient pressure to the optimal values of $\ensuremath{\sim}46$ K at 2.6 GPa and then decreases gradually to $\ensuremath{\sim}23$ K upon further increasing pressure to $\ensuremath{\sim}12$ GPa. However, ${T}_{\mathrm{c}}^{\mathrm{zero}}$ merely increases from $\ensuremath{\sim}21$ K at ambient pressure to $\ensuremath{\sim}24$ K at 2.6 GPa and then to $\ensuremath{\sim}12$ K at $\ensuremath{\sim}12$ GPa, leading to an obvious broadening of the superconducting transition at $2.6$ GPa. Concomitant with the domed-shaped superconducting phase, the linear-temperature resistivity coefficient ${A}_{1}$ described by the empirical formula $\ensuremath{\rho}={\ensuremath{\rho}}_{0}+{A}_{1}T+{A}_{2}\phantom{\rule{0.16em}{0ex}}{T}^{2}$ also exhibits a similar dome-shaped structure, which may establish the positive correlation between ${T}_{\mathrm{c}}$ and ${A}_{1}$ as found in the unconventional high-${T}_{\mathrm{c}}$ superconductors. Our high-pressure x-ray diffraction measurements rule out the occurrence of structural transition or a half-collapsed phase transition in the measured pressure range. Thus, our results indicate that the application of pressure may optimize the superconducting ${T}_{\mathrm{c}}$ by enhancing the inter-block-layer electron-transfer and/or the spin fluctuations in $\mathrm{Ba}{\mathrm{Th}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{({\mathrm{N}}_{0.7}{\mathrm{O}}_{0.3})}_{2}$.