Search for High Critical Temperature Superconductivity in Li- or Mg-doped Boron Icosahedral Cluster Solids

硼二十面体簇状固体β-菱面体硼（β-B^<105>）中掺杂金属元素后，有可能产生高tc超导性。我们尝试将镁（Mg）掺杂到β-B^<105>中，并讨论了金属和超导转变的可能性。我们获得了较高浓度的Mg掺杂到β-B^<105>，达到MgB_<11.5> (8.6 Mg/cell)，即电子掺杂足以使费米能（E_F）达到超过本禀受体水平（IAL）。然而，既没有观察到金属转变，也没有观察到超导转变。通过x射线粉末衍射（XRD）、Rietveld法、电导率和磁化率测量，分别讨论了结构和电子性能的变化。我们估计了E_F的态密度（DOS），并讨论了β-B^<105>的电子态。结果表明，在IAL上方存在局域态，可能来源于存在结构缺陷的B28星团。我们测量了具有α-四方晶结构的单晶硼纳米带的电导率。采用气相扩散法对Mg进行掺杂实验。纯硼纳米带是一种p型半导体，其电导率在室温下估计为10^<-3> （Ωcm）^<-1>。在室温下，纯硼纳米带的载流子迁移率为10^<-3> (cm^2/Vs)，活化能为~ 0.19 eV。掺镁硼纳米带具有与原始纳米带相同的α-四方晶结构。Mg蒸气扩散后，纳米带仍是半导体性质，但电导率提高了100 ~ 500倍。没有观察到掺杂向金属或超导体的转变。

There is a possibility of high-Tc superconductivity upon metallic-element doping into β-rhombohedral boron (β-B^<105>), which is one of the boron icosahedral cluster solids. We attempted magnesium (Mg) doping into β-B^<105> and discussed the possibility of metal and superconductive transitions. We achieved a higher concentration of Mg doping into β-B^<105>, up to MgB_<11.5> (8.6 Mg/cell), i.e., electron doping sufficient for the Fermi energy (E_F) to reach over the intrinsic accepter level (IAL). However, neither metal nor superconductive transition was observed. The change in the structure and electronic properties are discussed on the basis of the X-ray powder diffraction (XRD), using the Rietveld method, and electrical conductivity and magnetic susceptibility measurements, respectively. We estimated the density of states (DOS) at the E_F and discussed the electronic states of β-B^<105>. As a result, it is suggested that a localized state exists above the IAL probably originating from B28 cluster with structural defects.We measured electrical conductance of single crystalline boron nanobelts having α-tetragonal crystalline structure. The doping experiment of Mg was carried out by vapor diffusion method. The pure boron nanobelt is a p-type semiconductor and its electrical conductivity was estimated to be on the order of 10^<-3> (Ωcm)^<-1> at room temperature. The carrier mobility of pure boron nanobelt was measured to be on the order of 10^<-3> (cm^2/Vs) at room temperature and has an activation energy of 〜0.19 eV. The Mg-doped boron nanobelts have the same α-tetragonal crystalline structure as the pristine nanobelts. After Mg vapor diffusion, the nanobelts were still semiconductor, while the electrical conductance increased by a factor of 100〜500. Transition to metal or superconductor by doping was not observed.