Synergy and competition between superconductivity and antiferromagnetism in FeSe under pressure

Temperature dependence of resistivity under high pressures with magnetic fields up to 16 T is measured in FeSe single crystals with field directions parallel and perpendicular to the FeSe planes. It is found that the tetragonal to orthorhombic structural transition (nematic) temperature is suppressed by pressure and ends at around P = 1.18 GPa. Below around 0.85 GPa, the superconducting transition shows a narrow width with no indication of antiferromagnetic order. Above this pressure, the superconducting transition temperature drops slightly, forming a small dome of superconducting region with the maximum T-c at around 0.825 GPa. Furthermore, just above this pressure, the superconducting transition exhibits an unusual large transition width which reaches about 6-8 K. This large transition width is an intrinsic feature and does not change with magnetic field. In the high-pressure region above 1.18 GPa, just accompanying the onset of superconducting transition, an upturn of resistivity immediately occurs, which is attributed to the formation of an antiferromagnetic order. This closely attached behavior of superconductivity and antiferromagnetic order indicates that these two orders have a synergy feature. The wide transition width in high-pressure region is interpreted as the concurrence but spatially phase-separated regions of the superconductivity and antiferromagnetic order. Near the critical pressure 0.825 GPa and below, our data illustrate that an antiferromagnetic order emerges when superconductivity is suppressed. From the weak influence of magnetic field to the antiferromagnetic order, we conclude that it may exist already below the superconducting transition temperature in the low-pressure region. This shows a competing feature between superconductivity and antiferromagnetic order. Our results show duality features, namely, synergy and competition between superconductivity and antiferromagnetic order under pressure in FeSe.