Search for a novel double honeycomb lattice superconductor

在常压下合成了Li_xZrNCl(T_c=15K)、Li_xNbR(T_c=13K)和Na_xHfNCl(T_c=20-23K)双蜂窝晶格超导体。在合成大量样品的基础上，利用中子粉末衍射数据，通过Rietveld分析，成功地确定了这些结构参数。结果发现，在这些化合物中，哈里森近似估计的转移积分与超导转变温度T_c之间存在着密切的关系。这部分是因为尽管Hf原子的质量很大，即刚性晶格，Na_xHfNCl仍具有较高的德拜温度。结果表明，近邻过渡金属原子间的键距越大，T_c越高。基于这一发现，我们选择YOCl、Taon和CeSi作为双蜂窝晶格超导体的高T_c候选者。我们预测Na_xYOCl的T_c为30K。我们在DRI-LAB手套箱中，在湿度小于1ppm的气氛中成功地合成了Li_xYOCl粉末样品，发现它是一种具有奇怪的磁化强度温度依赖性的绝缘体。这是首次发现双蜂窝格子绝缘子。证明了在我们的传递积分相图中，Na_xHfNCl位于绝缘区附近。这一结果表明，Li_xYOCl样品在压力下可能是一种高T_c超导体。

We have synthesized double honeycomb lattice superconductors, such as Li_xZrNCl (T_c=15 K), Li_xNBr (T_c=13 K), and Na_xHfNCl (T_c=20-23 K) under ambient pressure. By synthesizing large amount of sample, we have succeeded to determine these structural parameters by Rietveld analysis using neutron powder diffraction data one by one. As a result, we found the close relationship between the transfer integral estimated by Harrison approximation and the superconducting transition temperature T_c in these compounds. This is partially because Na_xHfNCl has large Debye temperature in spite of the large mass of a Hf atom, I.e., stiff lattice. It basically suggests that the longer bond distance between the nearest neighbor transition metal atoms are, the higher T_c is. Based on this finding, we have selected YOCl, TaON, and CeSI as high-T_c candidates of double honeycomb lattice superconductors. Our simple estimation predicts that Na_xYOCl would have a T_c of 30 K. After we successfully sysnthesized Li_xYOCl powder sample in DRI-LAB glove box under an atmosphere of less than 1 ppm moisture, we found it was an insulator with curious temperature dependence of magnetization. This is the first discovery of a double honeycomb lattice insulator. It proves that Na_xHfNCl locates close to an insulator region in our transfer integral phase diagram. This result suggests that Li_xYOCl sample might be a high-T_c superconductor under pressure.