Testing the itinerancy of spin dynamics in superconducting Bi2Sr2CaCu2O8+δ

A systematic neutron-scattering study of large near-optimally doped single crystals of the cuprate superconductor, Bi 2 Sr 2 CaCu 2 O 8+ δ , indicates that its magnetic properties are governed by localized magnetic moments, and not by itinerant quasiparticles, as widely expected. Much of what we know about the electronic states of high-temperature superconductors is due to photoemission 1 , 2 , 3 and scanning tunnelling spectroscopy 4 , 5 studies of the compound Bi 2 Sr 2 CaCu 2 O 8+ δ . The demonstration of well-defined quasiparticles in the superconducting state has encouraged many theorists to apply the conventional theory of metals, Fermi-liquid theory, to the cuprates 6 , 7 , 8 , 9 . In particular, the spin excitations observed by neutron scattering at energies below twice the superconducting gap energy are commonly believed to correspond to an excitonic state involving itinerant electrons 10 , 11 , 12 , 13 , 14 . Here, we present the first measurements of the magnetic spectral weight of optimally doped Bi 2 Sr 2 CaCu 2 O 8+ δ in absolute units. The lack of temperature dependence of the local spin susceptibility across the superconducting transition temperature, T c , is incompatible with the itinerant calculations. Alternatively, the magnetic excitations could be due to local moments, as the magnetic spectrum is similar to that in La 1.875 Ba 0.125 CuO 4 (ref. 15 ), where quasiparticles 16 and local moments 17 coexist.