Search for a novel double honeycomb lattice superconductor

我们在常压下合成了Li_xZrNCl （T_c=15 K）、Li_xNBr （T_c=13 K）和Na_xHfNCl （T_c=20-23 K）等双蜂窝晶格超导体。通过合成大量样品，我们成功地利用中子粉末衍射数据逐一进行Rietveld分析，确定了这些结构参数。结果表明，哈里森近似估计的传递积分与这些化合物的超导转变温度T_c之间有密切的关系。这部分是因为尽管Na_xHfNCl原子质量很大，即晶格坚硬，但它的德拜温度却很大。它基本上表明，最近相邻过渡金属原子之间的键距越长，T_c越高。基于这一发现，我们选择了YOCl、TaON和CeSI作为双蜂窝晶格超导体的高t_c候选者。我们的简单估计预测Na_xYOCl的T_c为30k。我们在dr - lab手套箱中成功合成Li_xYOCl粉末样品后，在湿度小于1ppm的气氛下，我们发现它是一种具有奇怪的磁化温度依赖性的绝缘体。这是首次发现双蜂窝晶格绝缘体。证明了在转移积分相图中，Na_xHfNCl位于绝缘子区域附近。这一结果表明Li_xYOCl样品在压力下可能是高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.