High pressure route to high-Tc superconductivity in the FeSe-based materials*

High pressure is an effective and clean tuning knob to tip the balance of competing interactions in the strongly correlated electron systems. Although FeSe is nonmagnetic within the nematic state below T_s = 90 K, the application of high pressure can induce a static magnetic order and promote high-T_c superconductivity. These facts make FeSe an ideal platform to study the interplay of superconductivity with magnetism and nematicity. In addition, high-T_c superconductivity can be achieved in various heavily electron-doped (HED) FeSe-derived materials, including A_xFe_2-ySe₂, (Li,Fe)OHFeSe, and monolayer FeSe/SrTiO₃. More intriguingly, a second high-T_c superconducting (SC-II) phase has been reported in the pressurized A_xFe_2-ySe₂ and K-doped FeSe films.To shed more light on these intriguing issues, we recently performed detailed magneto-transport measurements on bulk FeSe and the HED FeSe-based materials under high pressure. For FeSe, we constructed a comprehensive T-P phase diagram featured by a dome-shaped magnetic phase interconnecting with the nematic order and the high-T_c superconductivity [1], and further demonstrated the presence of hole-like Fermi surface and enhanced spin fluctuations near the optimal T_c = 38.5 K at 6 GPa [2]. For the HED (Li_1-xFe_x)OHFeSe, we observed the emergence of SC-II phase above P_c = 5 GPa, and uncovered a sharp transition of the normal state from a Fermi liquid for P < P_c to a non Fermi liquid for P > P_c [3]. The emergence of SC-II phase seems to be a universal phenomenon for these HED FeSe materials as also demonstrated in Li_x(NH₃)_yFe₂Se₂ with the T_c^max = 55 K achieved under high pressure [4].
References:
[1] J. P. Sun, et al., Nat. Commu. 7, 12146 (2016).
[2] J. P. Sun, et al., Phys. Rev. Lett. 118, 147004 (2017).
[3] J. P. Sun, et al., ArXiv: 17007.06735.
[4] P. Shahi, et al., ArXiv: 1709.08455.