Quantum charge fluctuations in quantum dots

Influence of the quantum charge fluctuations in a quantum dot on its chargi ng effect has been investigated by using a quantum-dot device having four q uantum point contacts (QPC's) which connect the dot to the reservoirs. The charging effect was detectable at combined tunnel conductance of four QPC's up to near 8e(2)/h beyond the intuitive criterion e(2)/h. We observed anom alously nonlinear dependences of the conductances at the tops (G(top)) and bottoms (G(bot)) of the Coulomb oscillation on the tunnel conductances of Q PC's, G(QPC)'s The result on G(top) indicates that a transition from Coulom bically regulated tunnelings to non-correlated tunnelings occurs by increas ing G(QPC)'s. The result on G(bot) shows the existence of leakage conductan ce beyond the 2nd-order cotunneling contribution. The offset voltage in the current-voltage characteristics of the device showed significant decrease with the increase of G(QPC)'s, indicating the shrinkage of the effective ch arging energy. The results on G(top) and G(bot) can be explained in the lig ht of the shrinkage of the effective charging energy.