Bistable tunneling current through a molecular quantum dot

An exact solution is presented for tunneling through a negative-U degenerate molecular quantum dot weakly coupled to electrical leads. The tunnel current exhibits hysteresis if the level degeneracy of the negative-U dot is larger than 2. Switching occurs in the voltage range $V_1<V<\mathrm{}{V}_2$ as a result of attractive electron correlations in the molecule, which open up a new conducting channel when the voltage is above the threshold bias voltage $V_2.$ Once this current has been established, the extra channel remains open as the voltage is reduced down to the lower threshold voltage $V_1.$ The current hysteresis vanishes above some critical temperature. Possible realizations of bistable molecular quantum dots are fullerenes, especially ${\mathrm{C}}_{60},$ and mixed-valence compounds.