High-Tc superconductivity in FeSe at high pressure: Dominant hole   carriers and enhanced spin fluctuations
  The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (Tc). However, high-Tc superconductivity without hole carriers has been suggested in FeSe single-layer films and intercalated iron-selenides, raising a fundamental question whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-Tc phase induced by pressure in bulk FeSe from magneto-transport measurements and first-principles calculations. With increasing pressure, the low-Tc superconducting phase transforms into high-Tc phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in striking contrast to other FeSe-derived high-Tc systems. Moreover, the Hall coefficient is remarkably enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-Tc phase. These results in FeSe highlight similarities with high-Tc phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity. 
J. P. Sun, G. Z. Ye, P. Shahi, J.-Q. Yan, K. Matsuura, H. Kontani, G.   M. Zhang, Q. Zhou, B. C. Sales, T. Shibauchi, Y. Uwa...
cond-mat.supr-con