  We have investigated the step stiffness on Cu(001) surfaces as a function of
step orientation by two independent methods at several temperatures near 300 K.
Both sets of data agree well and show a substantial dependence of the stiffness
on the angle of orientation. With the exception of steps oriented along
$<110>$, the experimental stiffness is significantly larger than the stiffness
calculated within the solid-on-solid (SOS) model and the Ising-model, even if
next nearest-neighbor interactions are taken into account. Our results have
considerable consequences for the understanding and for the theoretical
modeling of equilibrium and growth phenomena, such as step meandering
instabilities.
