  The vibration of the dimer excited by STM current on Si(001) surface is
investigated. We describe this system by the Hamiltonian which has the
electron-vibration coupling term as the key ingredient. In order to
characterize the transition rates induced by STM current between vibrational
states we have introduced the effective temperature of the vibration which
differs from the temperature of the substrate. The behavior of the effective
temperature depends on the substrate temperature and STM current in highly
nonlinear manner and qualitatively changes around 50K of the substrate
temperature. At lower temperatures, the effective temperature strongly deviates
from the substrate temperature and reaches a few hundred Kelvin for the typical
values of STM current. At higher substrate temperatures, the effective
temperature reduces to the substrate temperature. On the basis of these
behaviors of the effective temperature, we solve the puzzle of the
symmetric-asymmetric crossover in dimer images of STM observation in the
ordered state of c(4 x 2).
