  It has recently been shown that submonolayer Pd on W(110) forms
highly-ordered linear mesoscopic stripes at high temperatures. The stripes
display an internal Pd superstructure with a nano-scale periodicity along the
direction perpendicular to the periodicity of the stripes. The same type of
superstructure is also observed in a wide range of temperatures below the
stripe formation temperature. We present a combined experimental and
theoretical study of this superstructure of Pd on W(110) and investigate its
influence on the appearance of the linear mesoscopic stripes. By means of
low-energy electron diffraction and low-energy-electron microscopy we show that
it has a far more peculiar dependence on temperature and coverage than expected
from a regular surface reconstruction. Using density-functional theory, we
model the Pd superstructures as periodic vacancy-line type of configurations
and investigate their energetics and elastic properties. From our calculated
surface stresses and anisotropies for the vacancy-line type of configurations,
and based on the continuum elasticity theory, we demonstrate quantitatively
that the vacancy-line type of structure is a prerequisite for the formation of
the linear mesoscopic stripes. Moreover, we show that the physics driving the
formation of the internal superstructure is very similar to the one at play in
forming the mesoscopic stripes themselves.
