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We investigate the dissociation of few-electron circular vertical semiconductor double quantum dot artificial molecules at 0 T as a function of interdot distance. A slight mismatch introduced in the fabrication of the artificial molecules from nominally identical constituent quantum wells induces localization by offsetting the energy levels in the quantum dots by up to 2 meV, and this plays a crucial role in the appearance of the addition energy spectra as a function of coupling strength particularly in the weak coupling limit. DOI: 10.1103/PhysRevLett.87.066801 PACS numbers: 73.21. -b, 36.40.Ei, 71.15.Mb, 85.35.Be Semiconductor quantum dots (QD's) are widely considered as artificial atoms, and are uniquely suited to study fundamental electron-electron interactions and quantum effects [1]. There are many analogies with "natural" atoms. One of the most appealing is the capability of forming molecules. Indeed, systems composed of two QD's, artificial quantum molecules (QM's), coupled either laterally or vertically, have recently been investigated experimentally [2,3] and theoretically [4 -7]. Nevertheless, the direct observation of a systematic change in the addition energy spectra for few-electron (number of electrons, N , 13) QM's as a function of interdot coupling has not been reported, and calculations of QM properties widely assume a priori that the constituent QD's are identical [4][5][6]. Special transistors incorporating QM's [8] made by vertically coupling two well defined and highly symmetrical QD's [9] are ideally suited to observe the former and test the latter.In this Letter we present experimental and theoretical addition energy spectra characterizing the dissociation of slightly asymmetric vertical diatomic QM's on going from the strong to the weak coupling limits that correspond to small and large interdot distances, b, respectively. We also show that spectra calculated for symmetric diatomic QM's resemble only those actually observed when the coupling is strong. The interpretation of our experimental results is based on the application of local-spin density-functional theory (LSDFT) [10][11][12]. It follows the development of the method thoroughly described in Ref. [12], which includes finite thickness effects of the dots, and uses a relaxation method to solve the partial differential equations arising from a high order discretization of the Kohn-Sham equations on a spatial mesh in cylindrical coordinates [13]. Axial symmetry is imposed, and the exchange-correlation energy has been taken from Perdew and Zunger [10].The molecules we study are formed by coupling, quantum mechanically and electrostatically, two QD's which individually can show clear atomiclike features [8,9]. For the materials we typically use, the energy splitting between the bonding and antibonding sets of single particle (sp) molecular states, D SAS , can be varied from about 3.5 meV for b 2.5 nm (strong coupling) to about 0.1 meV for b 7.5 nm (weak coupling) [8]. This is expected to have a dramatic effect on the electronic pr...

We have investigated the structure of double quantum dots vertically coupled at zero magnetic field within local-spin-density functional theory. The dots are identical and have a finite width, and the whole system is axially symmetric. We first discuss the effect of thickness on the addition spectrum of one single dot. Next we describe the structure of coupled dots as a function of the interdot distance for different electron numbers. Addition spectra, Hund's rule, and molecular-type configurations are discussed. It is shown that self-interaction corrections to the density-functional results do not play a very important role in the calculated addition spectra.