Parity of the Pairing Bosons in a High-Temperature Superconductor
  We report the observation of a novel effect in the bilayer Pb-Bi2212 high-TC superconductor by means of angle-resolved photoemission with circularly polarized excitation. Different scattering rates, determined as a function of energy separately for the bonding and antibonding copper-oxygen bands, strongly imply that the dominating scattering channel is odd with respect to layer exchange within a bilayer. This is inconsistent with a phonon-mediated scattering and favours the participation of the odd collective spin excitations in the scattering mechanism in near-nodal regions of the k-space, suggesting a magnetic nature of the pairing mediator. 
S. V. Borisenko, A. A. Kordyuk, A. Koitzsch, J. Fink, J. Geck, V.   Zabolotnyy, M. Knupfer, B. Buechner, H. Berger, M. Falub...