The Firsrt-principles Investigation On The Electronic Structures And Magnetic Properties Of Small Metal Clusters

Low-dimension systems such as clusters, quantum dots, quantum wires and ultrathin films are different from bulk materials in properties. To understand the low-dimension effect, the investigation of clusters is a start point. In this dissertation, aluminum, manganese and aluminum-manganese mixed clusters, as well as Li₃Al₄^- clusters are chosen for study, with the latter being a typical case to reveal the influence of external magnetic field on polyatomic systems. The investigation is based on density functional theory, and has three main components.â‘ The geometric conformations of Aln (n=2～10) clusters are optimized, and their electronic structures are calculated. It is shown that smaller aluminum clusters have higher symmertries than larger oners, and that the smaller clusters favor their electronic spin polarizations. As the size increases, the spin polarization of the even-atom cluster decreases evidently. For the odd-atom cluster, such a trend is less predominant, however, detectable. The outmost molecular orbitals of the spin-polarized even-atom clusters are mostly near-degenerate, instead of degenerate, due to the Jahn-Teller effect. This orbital near-degeneracy is related to the symmetry of the cluster. Virtually, the near-degenerate outmost molecular orbitals are decisive to the electronic spin polarization. On the other hand, the nuclear spins could induce electronic spin polarization through spin dipolar and Fermi contact interactions between nuclei and electrons. In the larger cluster, these interactions become more multiple, making their effect negligible, whereas they might play a role in the smaller cluster, as supported by the study on electronic structures and nuclear spin-spin coupling constants of singlet and triplet Al2 dimers.â‘¡The Mnn (n=2～6) and AlnMnm (n=1～5, m=6-n) clusters are investigated. It is shown that the stable states of these manganese clusters are high-spin ones. For the aluminum-manganese mixed clusters, if the number of manganese atoms are dominant, they keep the high-spin states, otherwise they are in low-spin ones. The population analyses indicate that both atomic charges and spin densities are uniformally distributed in the manganese clusters, whereas uneven in the aluminum-manganese mixed clusters. Atomic spins in the manganese clusters are mainly ferromagnetically coupled, whereas both ferromagnetic and antiferromagnetic couplings are possible in the mixed clusters. The different features of electronic spin polarization between manganese and aluminum clusters are mainly attributed to the manganese atomic 3d-electrons and aluminum atomic 3p-electrons.â‘¢In order to evaluate the influence of external magnetic field on clusters, we perform nuclear magnetic resonance calculations for a interesting case, the singlet and triplet Li3Al4- clusters. Under the perturbation of an external magnetic field parallel to z-axis which is perpendicular to Al4 ring plane through the center of the ring, it is found that, in the vicinity of z-axis, current-paramagnetism is dominant above and underneath the ring in the singlet Li3Al4- cluster. In contrast, current-diamagnetism is dominant in the triplet Li3Al4- cluster within, above and underneath the ring. The field-induced currents would exist in any clusters, but more possibly significant in species which evidently have atomic rings within their geometrical frameworks.Three innovative points are included in this work.â‘ It is concluded that, as the size of an aluminum cluster (especially the even-atom cluster) increases, its electronic spin polarization decreases.â‘¡It is found that smaller aluminum clusters have higher symmertries than larger ones, their outmost orbitals are more possibly near-degenerate, resulting in the electronic spin polarizations. Due to the lower symmetries of larger aluminum clusters, their electronic spin polarizations are as low as possible.â‘¢The mechanism of field-induced ring current is firstly analyzed using first-principles method. The relation of ring currents in Li3Al4- clusters with their geometrical conformations and electronic structures is disscused.In this investigation, we have only focused on the smaller clusters. In order to obtain a universal picture of electronic structures and magnetic properties of related species, the clusters with larger sizes should be calculated, which inevitably results in computational difficulty and has to be included in future work.