Tunneling and Photoemission in an SO(6) Superconductor
  Combining the results of tunneling, photoemission and thermodynamic studies, the pseudogap is unambiguously demonstrated to be caused by Van Hove nesting: a splitting of the density of states peak at $(\pi, 0)$. The fact that the splitting remains symmetric about the Fermi level over an extended doping range indicates that the Van Hove singularity is pinned to the Fermi level. Despite these positive results, an ambiguity remains as to what instability causes the pseudogap. Charge or spin density waves, superconducting fluctuations, and flux phases all remain viable possibilities. This ambiguity arises because the instabilities of the two-dimensional Van Hove singularity are associated with an approximate SO(6) symmetry group, which contains Zhang's SO(5) as a subgroup. It has two 6-component superspins, one of which mixes Zhang's (spin-density wave plus d-wave superconductivity) superspin with a flux phase operator. This is the smallest group which can explain striped phases in the cuprates. Evidence for a prefered hole density in the charged stripes is discussed. 
R.S. Markiewicz, C. Kusko, and M.T. Vaughn (Northeastern U., Boston)