  Recently there has been a great deal of interest in studying the properties
of dense quark matter, with particular reference to diquark condensates and
color superconductivity. In the present work we report calculations made for
the excitations of quark matter for relatively low densities of the deconfined
phase and in the absence of meson or diquark condensation. Here, we are
interested in elucidating the role of "Pauli blocking", as such blocking
affects the calculation vector, scalar and pseudoscalar $q\bar q$ excitations.
As a byproduct of our analysis, we extend our calculations to higher densities
and explore some consequences of the use of density-dependent coupling
parameters for the Nambu--Jona-Lasinio model. (Such density-dependent
parameters have been used in some of our previous work.) For our analysis made
at large values of the matter density, we assume that at about 13 times nuclear
matter density quark matter has only minimal nonperturbative interactions. At
that high density we compare the result for hadronic current correlation
functions calculated with density-dependent and density-independent NJL
coupling constants. We find evidence for the use of density-dependent
parameters, since the results with the density-independent constants do not go
over to the perturbative description which we assume to be correct for
$\rho\simeq 6\rho_c$, where $\rho_c$ is the matter density for the
finite-density confinement-deconfinement transition. The use of
density-dependent coupling constants in the study of diquark condensates and
color superconductivity has not been explored as yet, and is a topic requiring
further investigation, particularly given the strong interest in the properties
of dense quark matter and color superconductivity.
