Mechanism of high-Tc superconductivity studied by neutron scattering

La2-xSrxCuO4 is one of the typical high-Tc cuprates where Sr-doping creates many phases from the Mott insulator at x=0 nonsuperconducting metal for x>0.26; the high-Tc superconductivity appears for 0.06≤x≤0.26. We have grown large single crystals of La2-xSrxCuO4 over a wide the doping rate up to x=0.3 and performed systematic neutron scattering experiments for the first time. We obtained several results indicating an intimate relation between the dynamical spin correlations and the superconductivity. Incommensurate spatial modulation appears in the antiferromagnetic spin correlations beyond x=0.05 close to the lower boundary of the superconducting phase. We found that the degree of the spatial modulation or the incommensurability δ increases with doping and Tc is linearly scaled with δ for x≤0.15. A well-defined spin excitation gap was observed only for x=0.15 where the Tc reaches the maximum value. And the dynamical spin coherence degrades upon doping with x>>0.15. There results strongly suggest the essential role of the magnetically correlated region and the spatial spin modulation in the CuO2 planes to sustain or create the superconductivity. (author)