  We analyze the dynamics of a nanomechanical oscillator coupled to an
electrical tunnel junction with an arbitrary voltage applied to the junction
and arbitrary temperature of electrons in leads. We obtain the explicit
expressions for the fluctuations of oscillator position, its
damping/decoherence rate, and the current through the structure. It is shown
that quantum heating of the oscillator results in nonlinearity of the
current-voltage characteristics. The effects of mechanical vacuum fluctuations
are also discussed.
