Among the iron-based superconductors, structurally-simplest FeSe exhibits tetragonal-to-orthorhombic structural transition at Ts ～ 90 K and superconducting transition at Tc ～ 9 K. FeSe is unique in that it does not exhibit antiferromagnetic transition accompanying the structural transition[1,2]. In addition to this, the previous study on powder crystals has revealed four-fold increase of Tc with applying pressure . Therefore, FeSe is considered to be a key material to understand the mechanism of iron-based high-temperature superconductor.
Recently high-quality single crystals have been synthesized which show large residual resistivity ratio, and quantum oscillations[6,7]. The study on these crystals under hydrostatic pressure has revealed a complex temperature-pressure (T-P) phase diagram, where the pressure dependence of Tc has three plateaus, and the Ts is monotonically suppressed with the sudden development of magnetic phase .
To reveal the T-P phase diagram under higher pressure, we perform the high-pressure resistivity measurements up to 15 GPa, and synchrotron X-ray scattering measurements up to 4 GPa using several pressure techniques. From resistivity measurements, we observe increase of magnetic transition temperature (Tm) with increasing pressure up to 5 GPa. Above 6 GPa, however, the drop of Tm is observed, which is accompanied by the sudden enhancement of Tc up to 38 K, indicating the competing nature of the two orders with similar energy scales. From X-ray scattering measurements, we find tetragonal-to-orthorhombic structural transitions at all pressure points. At 1 GPa, the orthorhombic distortion exhibits second-order-like temperature dependence which is similar to the one at ambient pressure. Above 2 GPa, however, it displays sudden splitting just below the Tm, indicating first-order nature of the transitions in this pressure range. Here, we report on the T-P phase diagram obtained from both measurements, and the details of the temperature dependence of resistivity and orthorhombicity at each pressure.
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