High-Tc Superconductivity up to 55 K under High Pressure in a Heavily Electron-Doped Li0.36(NH3)yFe2Se2 Single Crystal
To find out the approaches to raise the critical temperature Tc of unconventional superconductors is one of the most enduring problems in contemporary condensed matter physics. The principal route to raise the Tc of FeSe is to dope electron, which has been successfully achieved via the interlayer intercalations [AxFe2−ySe2 (A = K, Rb), Ax(NH3)yFe2Se2, and (Li,Fe)OHFeSe], interface charge transfer, surface K dosing, and gate-voltage regulation. Further enhancement of Tc via adding more electrons seems to be plagued by the observed insulating state in the over doped regime. Given the limitations of electron doping, it is imperative to explore other routes to enhance Tc of these heavily electron doped (HED) FeSe materials further. In contrast to electron-doping approaches, the application of high pressure can provide an alternative means.
In this work , we had performed high-pressure measurement on Li0.36(NH3)yFe2Se2 single crystal, which can reach an optimal Tconset ≈ 44.3 K at ambient pressure. From high-pressure resistivity and AC- susceptibility measurement, we can conclude that superconducting transition temperature Tc ≈ 44 K at ambient pressure is first suppressed to below 20 K upon increasing pressure to Pc ≈ 2 GPa above which the pressure dependence of Tc(P) reverses and Tc increases steadily to 55 K at 11 GPa. These results thus evidence a pressure-induced second high-Tc superconducting (SC-II) phase in Li0.36(NH3)yFe2Se2 with the highest Tcmax ≈ 55 K among the FeSe-based bulk materials. Also it is evident that the SC-II phase is bulk in nature for 6 GPa, whereas the sample contains two superconducting phases with different Tc above 6 GPa: The high-Tc (50K) phase has a small but nearly constant volume fraction~30% to 11 GPa, whereas the low-(33K) phase shrinks and vanishes completely Tc above 11 GPa.
Figure 1 summarizes the pressure dependences of Tconset Tczero, and Tcχ for Li0.36(NH3)yFe2Se2 together with the Tczero, of FeSe for comparison. The temperature-pressure phase diagram depicts explicitly the evolution of the superconducting phase of Li0.36(NH3)yFe2Se2 intitially achived at ambient pressure via doping electron through inserting lithium Li+ and ammonia in between the FeSe layers, is quikly suppressed by pressure, and the supercondiucting phase emerges above Pc ≈ 2 GPa and exists in a broad pressure range.
To further characterize the SC-II phase, we tentatively probe the information about Fermi surface under pressure by measuring the Hall effect in the normal state just above Tc. Hall data confirms that in the emergent SC-II phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as Tc(P). Interestingly, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-II phases of Li0.36(NH3)yFe2Se2.
Our present study thus demonstrates a way for high pressure to raise Tc of these HED FeSe-based materials by increasing the effective charge-carrier concentration via a possible Fermi-surface reconstruction at Pc.
-  P. Shahi, J.P. Sun, S.H. Wang, Y.Y. Jiao, K.Y. Chen, S.S. Sun, H.C. Lei, Y. Uwatoko, B.S. Wang, and J-G Cheng, Phys. Rev. B, 97, 020508 (2018).