We report measurements of resistance and ac magnetic susceptibility on FeSe single crystals under high pressure up to 27.2 kbar. The structural phase transition is quickly suppressed with pressure, and the associated anomaly is not seen above ∼18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at ∼12 kbar. We find another anomaly at 21.2 K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in μSR measurements [M. Bendele \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to ∼25 kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists.
@article{arxiv.1502.03548,
title = {Pressure-induced antiferromagnetic transition and phase diagram in FeSe},
author = {Taichi Terashima and Naoki Kikugawa and Shigeru Kasahara and Tatsuya Watashige and Takasada Shibauchi and Yuji Matsuda and Thomas Wolf and Anna E. Böhmer and Frédéric Hardy and Christoph Meingast and Hilbert v. Löhneysen and Shinya Uji},
journal= {arXiv preprint arXiv:1502.03548},
year = {2015}
}