Novel in-gap spin state in Zn-doped La1.85Sr0.15CuO4.

Low-energy spin excitations of La 1 . 8 5 Sr 0 . 1 5 Cu 1 - y Zn y O 4 were studied by neutron scattering. In y = 0.004, the incommensurate magnetic peaks show a well-defined "spin gap" below T c . The magnetic signals at w = 3 meV decrease below T c = 27 K for y = 0.008, also suggesting the gap opening. At lower temperatures, however, the signal increases again, implying a novel in-gap spin state. In y = 0.017, the spin gap vanishes and elastic magnetic peaks appear. These results clarify that doped Zn impurities induce the novel in-gap state, which becomes larger and more static with increasing Zn.