Coexistence of Eu antiferromagnetism and pressure-induced superconductivity in single-crystal EuFe2As2

By performing high-pressure single-crystal neutron-diffraction measurements, the evolution of structure and magnetic ordering in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ under hydrostatic pressure were investigated. Both the tetragonal-to-orthorhombic structural transition and the Fe spin-density-wave transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ display clear anomalies at 27(3) K, well consistent with the superconducting transition observed in previous high-pressure resistivity measurements. Such an anomalous thermal expansion around ${T}_{c}$ strongly suggests the appearance of bulk superconductivity and strong electron-lattice coupling in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ induced by the hydrostatic pressure. The coexistence of long-range ordered Eu antiferromagnetism and pressure-induced superconductivity is quite rare in the ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$-based iron pnictides.