The Blackett Laboratory, Imperial College, London SW7 2BZ, United Kingdom
P. Pals and A.MacKinnon
The coherent conductance and current is calculated through two quantum dots using the Hubbard model for a single level per spin. The occurrence of negative differential conductance is demonstrated. The Ohmic conductance is calculated for dots with equally spaced levels. Transport is determined by matching energy levels, even when they do not occur at the charge degeneracy points.
34.80.P, 73.20.D
The advance of lithographical techniques on a nanometre scale in recent years has made it possible to study systems that were inaccessible to experimentation before. This has opened the way to produce and investigate structures where the carriers are confined to one or even zero dimensions. This has given rise to the discovery of a number of novel effects. It has been shown by Reed et al. that discrete states can clearly be discerned in quantum dots, structures which have been confined in all three dimensions [1]. These technological advances have made it possible to study the interplay between charge quantisation effects (Coulomb blockade) and size quantisation effects (discrete energy levels).
Whereas there has been a considerable amount of experimental and theoretical study on the conductive properties of single quantum dots [2, 3] and single and double metallic dots [4, 5, 6], so far relatively little attention has been paid to the case of two quantum dots in series.
The transport properties of a single dot are insensitive to incoherent scattering, provided that the broadening of the levels is small [7, 8]. However, when two dots are connected in series, this no longer holds true. In this paper the coherent case will be considered where the phase-breaking rate is small compared to the tunnelling rate. Recently, the importance of coherence for transport through a single dots has been shown explicitly by direct measurement of the phase of the transmission coefficient [9].