Neutron Scattering Study of the One-dimensional Magnetic Anisotropy on the CuO_2 Plane of the Domain-controlled High T_c Superconductors

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Since it is difficult to grow large single crystals for highly anisotropic or domain-controlled high T_c superconductors, it is necessary to increase the effective volume by aligning multiple crystals to carry out neutron scattering. We thus have established a method to align many single crystals precisely and efficiently : Combination of a high-energy x-ray diffractometer and a CCD camera, as well as a supersensitive CCD camera with neutron scintillator have been developed. We have succeeded in obtaining the effective volume of 0.53cc by aligning six Bi2212 single crystals with a very high precision, which is about eight times as large as a typical sample volume in previous studies. We have been carrying out series of neutron scattering experiments at the Oak Ridge National Laboratory through the US-Japan cooperative program on neutron scattering to search for the magnetic excitation and the resonance peak.To elucidate the relation between superconductivity and antiferromagnetic spin dynamics, we have performed a systematic inelastic neutron scattering concerning the suppression of T_c by impurities and the change of the spin excitations, using optimally doped La_<2-x>Sr_xCuO_4 single crystals with very small amount of impurities. We have focused upon the non-magnetic impurity Zn and the magnetic impurity Ni : Since the former suppresses T_c more effectively the latter, we expected to clarify how the magnetism is related to the T_c suppression mechanism by comparing both the systems. As to the Zn doped system, a novel in-gap state appears in the middle of the previously know spin gap while the overall spin dynamics does not change much. On the other hand, the spin dynamics of the Ni doped systems is renormalized to the lower energies as T_c is suppressed. It is now clear that the Zn and Ni doping effects on the superconductivity on the CuO_2 plane is qualitatively different.