  The noncontact (van der Waals) friction is an interesting physical effect
which has been the subject of controversial scientific discussion. The "direct"
friction term due to the thermal fluctuations of the electromagnetic field
leads to a friction force proportional to 1/Z^5 where Z is the atom-wall
distance). The "backaction" friction term takes into account the feedback of
thermal fluctuations of the atomic dipole moment onto the motion of the atom
and scales as 1/Z^8. We investigate noncontact friction effects for the
interactions of hydrogen, ground-state helium and metastable helium atoms with
alpha-quartz (SiO_2), gold (Au) and calcium difluorite (CaF_2). We find that
the backaction term dominates over the direct term induced by the thermal
electromagnetic fluctuations inside the material, over wide distance ranges.
The friction coefficients obtained for gold are smaller than those for SiO_2
and CaF_2 by several orders of magnitude.
