  We investigate the properties of crystalline phantom membranes, at the
crumpling transition and in the flat phase, using a nonperturbative
renormalization group approach. We avoid a derivative expansion of the
effective average action and instead analyse the full momentum dependence of
the elastic coupling functions. This leads to a more accurate determination of
the critical exponents and further yields the full momentum dependence of the
correlation functions of the in-plane and out-of-plane fluctuation. The flow
equations are solved numerically for D=2 dimensional membranes embedded in a
d=3 dimensional space. Within our approach we find a crumpling transition of
second order which is characterized by an anomalous exponent $\eta_c\approx
0.63(8)$ and the thermal exponent $\nu\approx 0.69$. Near the crumpling
transition the order parameter of the flat phase vanishes with a critical
exponent $\beta\approx 0.22$. The flat phase anomalous dimension is
$\eta_f\approx 0.85$ and the Poisson's ratio inside the flat phase is found to
be $\sigma_f\approx -1/3$. At the crumpling transition we find a much larger
negative value of the Poisson's ratio $\sigma_c \approx -0.71(5)$. We discuss
further in detail the different regimes of the momentum dependent fluctuations,
both in the flat phase and in the vicinity of the crumpling transition, and
extract the crossover momentum scales which separate them.
