  Spectral functions of the nucleon and its negative parity excited state in
nuclear matter are studied using QCD sum rules and the maximum entropy method
(MEM). It is found that in-medium modifications of the spectral functions are
attributed mainly to density dependencies of the $\langle \bar{q}q \rangle $
and $\langle q^{\dagger}q \rangle $ condensates. The MEM reproduces the
lowest-energy peaks of both the positive and negative parity nucleon states at
finite density up to $\rho \sim \rho_N$ (normal nuclear matter density). As the
density grows, the residue of the nucleon ground state decreases gradually
while the residue of the lowest negative parity excited state increases
slightly. On the other hand, the positions of the peaks, which correspond to
the total energies of these states, are almost density independent for both
parity states. The density dependencies of the effective masses and vector
self-energies are also extracted by assuming the mean-field green functions for
the peak states. We find that, as the density increases, the nucleon effective
mass decreases while the vector self-energy increases. The density dependence
of these quantities for the negative parity state on the other hand turns out
to be relatively weak.
