  Electron tunneling through a coupled quantum-dot dimer under a dc-bias is
investigated. We find that a peak in the $I$-$V$ curve appears at low
temperature when two discrete electronic states in the two quantum dots are
aligned with each other -- resonance coupling. This leads to a negative
differential resistance. The peak height and width depend on the dot-dot
coupling. At high temperature, the peak disappears due to thermal smearing
effects.
