  We investigate the morphologies of the Ge(001) surface that are produced by
bombardment with a normally incident, broad argon ion beam at sample
temperatures above the recrystallization temperature. Two previously-observed
kinds of topographies are seen, i.e., patterns consisting of upright and
inverted rectangular pyramids, as well as patterns composed of shallow,
isotropic basins. In addition, we observe the formation of an unexpected third
type of pattern for intermediate values of the temperature, ion energy and ion
flux. In this type of intermediate morphology, isolated peaks with rectangular
cross sections stand above a landscape of shallow, rounded basins. We also
extend past theoretical work to include a second order correction term that
comes from the curvature dependence of the sputter yield. For a range of
parameter values, the resulting continuum model of the surface dynamics
produces patterns that are remarkably similar to the intermediate morphologies
we observe in our experiments. The formation of the isolated peaks is the
result of a term that is not ordinarily included in the equation of motion, a
second order correction to the curvature dependence of the sputter yield.
