  The resonating valence bond spin liquid model for the underdoped cuprates has
as an essential element, the emergence of a pseudogap.
  This new energy scale introduces asymmetry in the quasiparticle density of
states because it is associated with the antiferromagnetic Brillouin zone. By
contrast, superconductivity develops on the Fermi surface and this largely
restores the particle-hole symmetry for energies below the superconducting
energy gap scale. In the highly underdoped regime, these two scales can be
separately identified in the density of states and also partial density of
states for each fixed angle in the Brillouin zone. From the total density of
states, we find that the pseudogap energy scale manifests itself differently as
a function of doping for positive and negative bias. Furthermore, we find
evidence from recent scanning tunneling spectroscopy data for asymmetry in the
positive and negative bias of the extracted $\Delta(\theta)$ which is in
qualitative agreement with this model. Likewise, the slope of the linear low
energy density of states is nearly constant in the underdoped regime while it
increases significantly with overdoping in agreement with the data.
