Where is the pi particle

@article{Tchernyshyov2000WhereIT,
  title={Where is the pi particle},
  author={Oleg Tchernyshyov and M. R. Norman and Andrey V. Chubukov},
  journal={Physical Review B},
  year={2000}
}
We discuss the interplay of particle-particle and particle-hole spin-triplet channels in high-T_c superconductors using a quasiparticle dispersion motivated by angle-resolved photoemission. Within a generalized RPA, we find a well defined antibound state of two holes, the pi resonance of Demler and Zhang, as well as a bound state of a particle and a hole, the spin exciton. We show that the energy of the pi resonance always exceeds 2 Delta, twice the maximum d-wave gap, therefore the neutron… 
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