|Budget Amount *help
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2002: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2001: ¥2,700,000 (Direct Cost: ¥2,700,000)
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.