Testing the itinerancy of spin dynamics in superconducting Bi2212

TL;DR

This study measures magnetic spectral weight in Bi2212 and finds results inconsistent with itinerant electron models, suggesting local moments may play a significant role in its spin dynamics.

Contribution

First absolute measurement of magnetic spectral weight in Bi2212 showing evidence against itinerant electron models in its spin excitations.

Findings

Magnetic spectral weight measured in absolute units.

Temperature independence of local spin susceptibility across T_c.

Results incompatible with itinerant electron calculations.

Abstract

Much of what we know about the electronic states of high-temperature superconductors is due to photoemission and scanning tunneling spectroscopy studies of the compound Bi2212. 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. 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. Here, we present the first measurements of the magnetic spectral weight of optimally-doped Bi2212 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 LBCO where quasiparticles and local moments coexist.