Exploring the Dark Side of Cuprate Superconductors: s-wave Symmetry of   the Pseudogap
  We reveal the full energy-momentum structure of the pseudogap of underdoped high-Tc cuprate superconductors. Our combined theoretical and experimental analysis explains the spectral-weight suppression observed in the B2g Raman response at finite energies in terms of a pseudogap appearing in the single-electron excitation spectra above the Fermi level in the nodal direction of momentum space. This result suggests an s-wave pseudogap (which never closes in the energy-momentum space), distinct from the d-wave superconducting gap. Recent tunneling and photoemission experiments on underdoped cuprates also find a natural explanation within the s-wave-pseudogap scenario. 
S. Sakai, S. Blanc, M. Civelli, Y. Gallais, M. Cazayous, M.-A.   Measson, J. S. Wen, Z. J. Xu, G. D. Gu, G. Sangiovanni, Y. ...
cond-mat.str-el cond-mat.supr-con