New(Li_1-xFe_x)OHFe_1-ySe High-T_c Superconductor:Phase Diagram,Singlecrystal Growth,and Related Physical Properties

The discovery of iron-based superconductivity by Hosono in 2008 was a real surprise and has generated tremendous interest.Investigating on the iron-based high-temperature superconductivity will promote multiband physics and application potential as well,such as ultra-strong superconducting magnet.Despite numerous progreess,there still exist important scientific problmes to be solved.For instance,do Fe As-besed and FeSe-based superconductors share a common SC mechanism? Which ordering/fluctuation plays the key role on the Fe-based high-temperature superconductivity,the spin,the electron or the orbital one? To answer these questions,we focus on the newly discovered(Li1-x Fex)OHFe1-y Se system.The main results we obtained include the complete phase diagram of the newly discovered(Li1-x Fex)OHFe1-ySe superconductor,the syntheses of large(Li0.84Fe0.16)OHFe0.98 Se superconducting single crystal and its highly 2D electronic properties,the syntheses of Mn-doped(Li1-xFex)OHFe1-ySe single crystal and related physical properties.(1)By optimizing the hydrothermal syntheses,a series of(Li1-xFex)OHFe1-ySe samples were obtained and based on them we finished the complete phase diagram of(Li1-x Fex)OHFe1-ySe for the first time.With the susceptibility measurements,we found the SDW-like transition occurs at ~130 K,which is comparable with the SDW transition temperature(130 K~150 K)in parent ROFeAs(Fe As1111,R = rare earth elements)compound.The results by the means of SEAD and HR-STM indicate that non-superconducting samples possess a basic tetragonal structure shared with FeAs1111.On the other hand,the ?2×?2 superstructure is dominant in the optimal(Li1-x Fex)OHFe1-ySe samples.This superstructure is also present in superconducting Kx Fe2-ySe2(FeSe122).Our results firstly bridge iron arsenide and selenide superconductors,indicating they share a similar electron ground state and a single superconductivity mechanism.Furthermore,unlike FeSe122,there is no insulating phase with ?5×?5 Fe vacancies in(Li1-x Fex)OHFe1-y Se samples,providing a very clean system to reveal the high temperature superconductivity in iron-based superconductors.(2)We have successfully obtained a high-quality superconducting(Li0.84Fe0.16)OHFe0.98 Se single crystal of a size over 10 mm(Tc ~ 42 K)for the first time by hydrothermal ion exchange technology developed by Xiaoli Dong.We use K0.8Fe1.6Se2(KFS245)as matrix and during this synthesized progress K+ ions are released into the solution and substituted by(Li/Fe)OH layer while FeSe blocks are remained.The(Li0.84Fe0.16)OHFe0.98 Se products almost inherit the topographic characteristics of KFS245 matrix and its chemical compoents can be controlled effectively.The electronic transport property of(Li0.84Fe0.16)OHFe0.98 Se is highly two dimensional from the linear-T dependence in electron resistivity.Furthermore,the linear temperature dependencies of the susceptibility and the resistivity indicate the two dimensional spin fluctuation might be crucial to superconducting electron pairing.(3)We have successfully doped Mn into(Li1-xFex)OHFe1-y Se system via both ion exchange(1-step)and ion release/introduction(2-step)developed by Xiaoli Dong.The micro-region x-ray diffraction and energy dispersive x-ray spectroscopy analyses indicate that the Mn has been doped into the lattice,and the content in the 1-step fabricated sample is higher than that in the 2-step.Magnetic susceptibility and electric transport properties reveal that Mn doping influences little on the superconducting transition,regardless of 1-step or 2-step routes.By contrast,the characteristic temperature T*,where the negative Hall coefficient reaches its minimum,is significantly reduced by Mn doping.This implies that the reduction of contribution from hole carriers hence the hole band might have no direct relationship with the superconductivity in(Li1-x Fex)OHFe1-ySe superconductors.The hydrothermal methods of ion exchange and ion release/introduction provide an efficient way for elements substitution/doping into(Li1-x Fex)OHFe1-ySe superconductors,which will promote the in-depth investigations on the roles of multiple electron and hole bands and their interplay with the high-temperature superconductivity in the FeSe-based superconductors.