Paired additions of electrons into quantum dots: How and why does localization destroy Coulomb blockade to produce pairing?

Several years ago, we discovered a profound violation of Coulomb blockade in quantum dots. Not only did electron additions deviate from near perfect periodicity with electrostatic potential on the dot, the additions actually occurred in pairs and sometimes even as bunches. This behavior only occurs in quantum dots that with diameters larger than 0.4 μm. Moreover, it only arises when density of electrons within the dot is low enough such that the mean spacing between electrons is larger than the Bohr radius (r_s ≤ 1). We have completed a systematic study of dots of different sizes. For the largest dots, pairs and bunches occur randomly within the addition spectrum. In intermediate sized dots the pairing occurs periodically with each 4th and 5th electron added to the dot as a pair. In the periodic case, we have strong evidence that one of the two electrons in a pair is localized at the edge of the dot while the other four additions in the sequence are two states mainly localized to the interior. Such a bifurcation of the electron droplet into inner and outer shells may arise from negative compressibility within the dot or other interaction effects. Apparently, any type of localization is sufficient to create the pairing and bunching effect. In large dots with randomly localized electrons, the pairing appears at random, while in intermediate sized dots with a core-edge localization, the pairing occurs periodically. While we have succeeded in understanding the conditions necessary for pairing, the physics behind this extraordinary effect remains a mystery.