  The optimally doped and underdoped region of the $t-J$ model at large N (N is
the number of spin components) is governed by the competition of d-wave
superconductivity (SC) and a d Charge-Density Wave (d-CDW).The partial
destruction of the Fermi surface by the d-CDW and the resulting density of
states are discussed. Furthermore, c-axis conductances for incoherent and
coherent tunneling are calculated, considering both an isotropic and an
anisotropic in-plane momentum dependence of the hopping matrix element between
the planes. The influence of self-energy effects on the conductances is also
considered using a model where the electrons interact with a dispersionless,
low-lying branch of bosons. We show that available tunneling spectra from
break-junctions are best explained by assuming that they result from incoherent
tunneling with a strongly anisotropic hopping matrix element of the form
suggested by band structure calculations. The conductance spectra are then
characterized by one single peak which evolves continuously from the
superconducting to the d-CDW state with decreasing doping. The intrinsic c-axis
tunneling spectra are, on the other hand, best explained by coherent tunneling.
Calculated spectra show at low temperatures two peaks due to SC and d-CDW. With
increasing temperature the BCS-like peak moves to zero voltage and vanishes at
T$_c$,exactly as in experiment.Our results thus can explain why break junction
and intrinsic tunneling spectra are different from each other. Moreover, they
support a scenario of two competing order parameters in the underdoped region
of high-T$_c$ superconductors.
