| Research Abstract |
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.
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