Theory of Asymmetric Tunneling in the cuprate superconductors
  We explain quantitatively, within the Gutzwiller-Resonating Valence Bond theory, the puzzling observation of tunneling conductivity between a metallic point and a cuprate high-$T_c$ superconductor which is markedly asymmetric between positive and negative voltage biases. The asymmetric part does not have a "coherence peak" but does show structure due to the gap. The fit to data is satisfactory within the over-simplifications of the theory; in particular, it explains the marked "peak-dip-hump" structure observed on the hole side and a number of other qualitative observations. This asymmetry is strong evidence for the projective nature of the ground state and hence for "t-J" physics. 
P. W. Anderson and N. P. Ong