Exfoliation of a layered high-T_c superconductor

本研究研究了层状高t_c超导体Li_x(THF)_yHfNCl的剥离，采用吡啶、二甲亚砜、N、N-二甲基甲酰胺、N-甲基甲酰胺等有机溶剂对Li_x(THF)_yHfNCl样品进行了层间距离较大（约1.87 nm）的溶解。不幸的是，我们的Li_x(THF)_yHfNCl样品（x ~ 0.5）从未在这些溶剂中溶解过。另一方面，对相关蜂窝晶格超导体Na_<0.29>HfNCl和Li_<0.19>ZrNBr的结构和物理性质也进行了研究。相关超导体Na_<0.29>HfNCl和Li_<0.19>ZrNBr的结构用中子衍射数据成功测定。我们是唯一一个成功确定这些在空气中不稳定的碱金属嵌入超导样品结构的研究小组。Na_xHfNCl的非弹性中子散射研究表明，在远高于超导能隙的几个能级上，声子态密度（PDOS）显著增加，这与La_<1.85>Sr_<0.15>CuO_4和YBa_2Cu_3O_7等高t_c铜酸盐的行为类似。最近的中子散射研究表明，与声子色散异常有关的PDOS增加可能是由于动态电荷密度波（DCDW）的存在。这表明DCDW可能是具有一定电子不稳定性的高t_c超导体（包括Ba_<1-x>K_xBiO_3）的共同特征，它们在自旋涨落方面似乎有很大的不同。

In the present research, exfoliation of a layered high-T_c superconductor Li_x (THF)_yHfNCl has been studied, where organic solvents such as pyridine, dimethylsulfoxide, N,N-dimethylformamide, and N-methylformamide were used to solve the Li_x(THF)_yHfNCl sample with large interlayer distance, i.e., about 1.87 nm. Unfortunately, our Li_x(THF)_yHfNCl sample (x〜0.5) has never been dissolved in these solvents. On the other hand, structures and physical properties on related honeycomb lattice superconductors, such as Na_<0.29>HfNCl and Li_<0.19>ZrNBr, have also been studied. Structures of related superconductors, Na_<0.29>HfNCl and Li_<0.19>ZrNBr, have been determined successfully by Rietveld analysis using neutron diffraction data. We are the only research group that has succeeded in determining the structures of these alkali metal intercalated superconducting samples, which are unstable in air. Inelastic neutron scattering study of Na_xHfNCl revealed that the phonon density of states (PDOS) at several energies much higher than the superconducting energy gap increased remarkably, which was the similar behavior to high-T_c cuprates, e.g., La_<1.85>Sr_<0.15>CuO_4 and YBa_2Cu_3O_7. Recent neutron scattering studies indicate that these increases of PDOS relating to phonon dispersion anomaly would be due to the existence of dynamical charge density wave (DCDW). It suggests that DCDW might be a common characteristic among high-T_c superconductors with some electronic instability including Ba_<1-x>K_xBiO_3, which seems to be quite different with one another in terms of spin fluctuation.