Superconductivity Tuned by the Iron Vacancy Order in K_(x)Fe_(2−y)Se_(2)

Combining in-depth neutron diffraction and systematic bulk studies,we discover that the√5×√5 Fe vacancy order,with its associated block antiferromagnetic order,is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of K_(x)Fe_(2)-ySe_(2).The orthorhombic order,with one of the four Fe sites vacant,appears only at intermediate temperatures as a competing phase.The material experiences an insulator to metal crossover when the√5×√5 order is highly developed.Superconductivity occurs in such a metallic phase.

具有超低热导率的一维铋硒卤族化合物

本征低热导率材料的研究具有重要的科学意义,已引起广泛关注.本工作报道了一类具有简单一维晶体结构的超低热导率材料:铋硒卤族化合物(BiSeX,X=Br,I).研究发现,BiSeI的热导率在573K仅为~0.27Wm^-1K^-1,达到了最低本征热导率的理论极限值.本研究采用第一性原理计算结合粉末中子衍射和变温球差扫描透射电子显微镜表征,深入探究了其超低热导率的机制.研究表明,BiSeX的一维结构赋予了材料低热导特性:弱的成键特性、低的声子速度、声学支和光学支的强非简谐性以及Bi和卤族元素较大的偏移效应,从而有效阻碍了声子输运,使得BiSeX具有超低的热导率.本研究提出了在具有一维结构的材料中寻找低传导特性的新思路,该研究思路在热电材料和热障涂层材料等低热传导需求领域中具有广阔的应用前景.