| Up to now,cuprate and iron-based superconductors(FeSCs)are the only two classes of mate-rials that exhibit high-temperature superconductivity(HTSC)under ambient pressure.Both classes of materials are two dimensional in crystal structure,in which the CuO2 planes and Fe2X2(X refer-s to pnictogen or a chalcogen element)layers serve as the crucial ingredient for HTSC.In other words,the crystal structure of FeSCs(cuprates)are nothing but the stacking of Fe2X2(Cu02)sheets with other spacer layers alternately along the crystallographic c-axis.This structural feature makes it possible to explore new superconductors via a rational structural design.The Fe2X2 layers in reported FeSCs are either separated by insulating layers(for the "1111"-type compounds),or connected with monatomic layers(for the "122"-type materials),or directly contacted due to van der Waals forces(for the "11"-type system).The structural type containing double Fe2As2 layers,resembling the double Cu02 planes in cuprates exemplified by La1-xSrxCaCu2O6)was not report-ed so far.We succeeded in synthesizing the 12442-type FeSCs containing double Fe2As2 layers via rational structural design.We summarized several rules for designing the intergrowth structures.Based on these empiri-cal rules,we designed and synthesized the 12442-type FeSCs AkCa2Fe4As4F2 and AkLn2Fe4As4O2(Ak=K,Rb,Cs;Ln=Lanthanides),whose structure was actually an intergrowth of ThCr2Si2-type AkFe2As2 and ZrCuSiAs-type CaFeAsF or LnFeAsO.12442-type materials contain double Fe2As2 layers(with Ak+ ion sandwiched)separated by insulating[Ca2F2]2+or[Ln2O2]2+layers.12442-type FeSCs are the hybrid of the heavily hole-doped AkFe2As2(0.5 hole/Fe)and un-doped 111 1-type compounds,resulting in a self hole-doping level of 0.25 hole/Fe.Consequently,12442-type materials exhibit superconductivity range from 28 to 37 K without extrinsic doping.Anion height from Fe plane has and the As-Fe-As bond angle ?,the two parameters have been widely cited because the maximum Tc in FeSCs seems to fall at hAs=1.38 A,and ?=109.5°.However,the empirical rule is broken in 12442 case.We defined two new structural parameters in 12442-type FeSCs,dintra(the spacing of Fe planes within the double Fe2As2 layers)and dinter(the spacing of Fe planes separated by the insulating layers).We found that Tc tended to increase with the decreasing dintra.Given that the effective interlayer coupling increases with the decreasing spacing,our results suggest the coupling within the double Fe2As2 layers play an active role for the superconductivity of 12442-type FeSCs.The single crystals of CsCa2Fe4 As4F2 were successfully grown with CsAs flux.The anisotrop?ic transport and magnetic measurements were carried out to characterize the single crystals.We observed a large resistivity anisotropy of pc(T)/pab(T)?103 that tended to increase with decreas-ing temperature.The pc(T)data exhibit a nonmetallic behavior at high temperatures,suggesting an incoherent electronic state due to the dimension crossover.The superconducting onset tran-sition temperature in pab(T)is 0.7 K higher than that in pc(T),reflecting two-dimensional(2D)superconducting fluctuations.The lower and upper critical fields also show an exceptionally high anisotropy among iron-based superconductors.The interplane coherence length ?c(0)is estimated to be 3.6 A,which is remarkably smaller than the distance between conducting layers(8.6 A),consolidating the 2D nature in the title material.The strongly anisotropic behavior is reminiscence of those of most cuprate superconductors,implying the crucial role of two dimensionality for the superconductivity.The large anisotropy might be associated with the unique electric structure of 12442-type iron-based superconductors.In the last chapter,we designed several 12442-type materials that highly possible to be syn-thesized:CsTh2Fe4As4(N,O)2,AkCa2Fe4As4H2(Ak=K,Rb,Cs),AkAt2Fe4As4O2(Ak=K,Rb,Cs;At=Np,Pu)and CsGd2Fe4P4O2.These compounds are expected to be obtained under proper conditions. |
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