STRUCTURES AND INCOMMENSURATE SPIN EXCITATIONS IN EXCESS OXYGEN-DOPEDLA2CUO4+Y

Over the past decade, we have studied in detail the low-energy spin fl uctuations in La2-xSrxCuO4 for x between 0 and 0.18. Our experiments, as well as those by others, have revealed a fascinating interplay betw een the hole doping, the static and dynamic spin fluctuations and supe rconductivity. Recently, using electrochemical techniques, we have lea rned how to produce large single crystals of La2CuO4+y which are relat ively homogenous. In this latter system, the dopants are characterized by annealed rather than quenched disorder. Furthermore, we have demon strated staging behavior of the excess oxygen analogous to staging in intercalated graphite. We have now succeeded in carrying out measureme nts of the low-energy spin fluctuations in stage-6 La2CuO4+y. Specific ally, we find that in this system which has T-c = 31 K, the spin fluct uations are incommensurate with energy and temperature dependencies ve ry much like those in La1.85Sr0.15CuO4 with T-c = 33 K. In addition, w e find that T-c/delta is a constant for high-quality materials in both La2-xSrxCuO4 and La2CuO4+y, here delta Is the incommensurability.