Research Of New Layered Superconductors

As a macroscopic quantum phenomenon, superconductivity was first discovered in 1911, which plays an important role in condensed matter physics. In theory, researchers are commit-ted to unravel the superconducting mechanism. The famous BCS theory explain the phenomena of conventional superconductors while those of unconventional superconductors are still not fully understood. On the other hand, much efforts are devoted to search for new superconductors with high transition temperature, which are viewed as carrier to reveal mechanism of unconvention-al superconductors. Superconducting materials include elementary substance, alloys, oxides and compounds with complex layered structure. Since superconductivity was discovered in cuprates and oxypnictide, superconductors with layered structure have triggered enormous research activi-ties. From the point view of crystal structure, these layered materials consist of superconducting "active layer" and "carrier base layer", which stack alternatively along the c-axis. Based on the physical properties, the parent compounds of these layered superconductors are anti-ferromagnetic semimetal or semiconductor, in which superconductivity can be introduced by chemical doping or physical pressure. Therefore, it is rational to explore new superconductors by doping upon specific parent compounds or designing layered structure with superconducting "active layer". In this paper, we report our research on exploring new layered superconductors.Quaternary oxypnictide LaMnAsO, isostructural to the parent compound of iron-based su-perconductor LaFeAsO, is a Mott insulator with antiferromagnetism transition temperature 317 K. Metallic conduction is expected to be introduced and antiferromagnetic order is also expected to be suppressed by doping hole carrier in LaMnAsO. Similar to cuprates and iron-based compounds, su-perconductivity can also be introduced after suppression of antiferromagnetism. La1-xSrxMnAsO (x=0,0.02,0.06,0.08,0.1) polycrystalline samples were synthesized and their physical proper-ties were studied. We demonstrated an insulator-to-metal transition by hole doping. Further, we found that the antiferromagnetism in this material was relatively robust against the hole carrier doping. However, no superconductivity was observed in this system.The iron-based superconductors mainly consist of four families, and the number of members remains increasing. These iron-based superconductors contain the same [Fe2As2] layer, which is the superconducting "active layer". Designing new structure with this layer seems to be a short-cut to seek for new layered superconductors. The [Fe2As2] layer is relatively "flexible", which means that it can stack with various two-dimensional layers and form stable compounds. Among them, [Ti2As20] layer has the similar cell parameter with that of [Fe2As2] layer, and both of them can bind with Ba2+ ion stably. Thus we designed a new structure with these layers, and finally we synthesized a new compound Ba2Ti2Fe2 As4O. Revealed by electrical and magnetic measurements, Ba2Ti2Fe2As4O is a self-doping superconductor with Tc up to 21 K, in which superconductivity is introduced by interlayer charge transfer. Further, we successfully synthesized Ba2Ti2Fe2As4O single crystals by self-flux method and studied its superconducting properties.Bismuth-oxysulfide is another class of two-dimensional layered superconductors containing [Bi2S4] layers. From the point view of crystal structure, [Bi2S4] layer is just a case of [MnXn+2] (M=Pb, Bi; X=S, Se, Te) for n=2, which implies that it is possible to design layered struc-ture with [MnXn+2]layers to search for new superconductors. After considering lattice match and charge balance, LaPbBiS3O containing [La2O2] and [Pb2Bi2S6] layers was then designed and final-ly synthesized. We studied its physical properties and concluded that LaPbBiS3O is a narrow gap semiconductor with high thermoelectric performance. Similar to the parent compound of bismuth-oxysulfide superconductors like LaBiS2O, LaPbBiS3O may be a new layered superconductor after specific chemical doping.