  During the course of atomic transport in a host material, impurity atoms need
to surmount an energy barrier driven by thermodynamic bias or at ultra-low
temperatures by quantum tunneling. In the present article we demonstrate using
atomistic simulations that at ultra-low temperature transient inter-layer
atomic transport is also possible without tunneling when the Pt/Al(111)
impurity/host system self-organizes itself spontaneously into an intermixed
configuration. No such extremely fast athermal concerted process has been
reported before at ultra low temperatures. The outlined novel transient atomic
exchange mechanism could be of general validity. We find that the source of
ultra-low temperature heavy particle barrier crossing is intrinsic and no
external bias is necessary for atomic intermixing and surface alloying in Pt/Al
although the dynamic barrier height is few eV. The mechanism is driven by the
local thermalization of the Al(111) surface in a self-organized manner arranged
spontaneously by the system without any external stimulus. The core of the
short lived thermalized region reaches the local temperature of $\sim 1000$ K
(including few tens of Al atoms) while the average temperature of the
simulation cell is $\sim 3$ K. The transient facilitated intermixing process
also takes place with repulsive impurity-host interaction potential leading to
negative atomic mobility hence the atomic injection is largely independent of
the strength of the impurity-surface interaction. We predict that similar
exotic behaviour is possible in other materials as well.

Materials Science;Other Condensed Matter