A two-dimensional artificial crystal, which is made up of artificial atoms-identical Al clusters with nanometer size and spacing, was fabricated by taking advantage of surface-mediated clustering on a growth template. In situ scanning tunneling microscopy analysis and first-principles total energy calculations were used to determine the atomic structure of the Al nanoclusters. The Al clusters exhibit more remarkable thermal stability than the In clusters we reported previously. Based on our systematic observations and calculations, the formation mechanism and the high stability of these magic clusters are discussed.

The structural stability of type-I K8Si46 clathrate has been investigated at high pressure by synchrotron x-ray diffraction. In contrast to that observed in the Na-doped structure-II analogue [A. San-Miguel, Phys. Rev. Lett. 83, 5290 (1999)]], no phase separation into the beta-Sn Si structure was identified at 11 GPa. Instead, K8Si46 is found to undergo a transition to an isostructural positional disordered phase at around 15 GPa. Ab initio phonon band structure calculations reveal a novel phenomenon of phonon instabilities of K atoms in the large cavities is responsible for this transition. Above 32 GPa, the new structure transforms into an amorphous phase.

The exact spectra of two electrons confined by two-dimensional and three-dimensional quantum dots ͑2D and 3D QD's͒ with parabolic potentials are obtained. Using the present results, the size and shape effects of QD's on the spectra are revealed. It is found that the spectra are dramatically changed with the variation of the dot size, and then the crossover of two levels can appear. The variation of spectra with size and the positions of crossover points are quite different between 2D and 3D QD's. It is well explained based on the study of electron-electron interaction energies in 2D and 3D QD's. The size and shape effects predict a possibility to observe phenomena related to electron-electron interactions in QD's. ͓S0163-1829͑97͒06120-1͔

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