J. Dorantes-Dávila

Energetics of giant fullerenes and Matrjoschka structures

Abstract The electronic structure of the giant fullerene family C n , n = 60( ν + 1) 2 , was calculated for ν = 0,1,2,3,4. The calculation was carried out within the tigh-binding Hamiltonian in the Hartree-Fock approximation. We present results for the energy per atom as a function of size. An analytic expression is derived for the energy of giant fullerenes of arbitrary size in terms of the energy per pentagon, and plain and folded hexagons occuring in the truncated icosahedra. We computed also the energy of formation of multi-shelled matrjoschka structures and found that those are the most stable forms of carbon clusters.

The s and p character of the electronic structure of C20, C60 and C70

Abstract The electronic structure of the carbon small clusters, C20, C60 and C70, is calculated. The calculation is carried out within a Hubbard like Hamiltonian in which s- and p-electrons are taken into account. Results are obtained for the selfconsistent densities of states by means of the recursion method and are discussed and compared with those of diamond and the hexagonal lattice.

A possible route to macroscopic formation of endohedral fullerenes

Abstract A possible mechanism to encapsulate atoms in the internal cavities of C 60 and higher fullerenes is proposed. It involves the production of C 60 molecules with two carbon isotopes A C and B C ( A , B = 12, 13, 14). The molecules A C 59 B C 1 and A C 58 B C 2 are separated from the total production and collected in a chamber under partial pressure of the element to be inserted. The proposed mechanism is to excite selectively the minority isotopes by laser irradiation in such a way that the bonds formed by the excited atoms are stretched to open a gate, allowing the foreign atoms to get into the distorted cages. The calculation of the electronic structure and the energy needed to deform C 60 supports this insertion mechanism.

Electronic structure of some semiconductor fullerenes

Abstract The electronic structure of small germanium, silicon, and carbon clusters is calculated. The calculation is carried out within a Hubbard-like Hamiltonian in which s- and p-electrons are taken into account. Charge transfer is allowed between the various atomic sites in order to achieve global charge neutrality. The local electronic density of states is calculated by means of the recursion method. The results for atomic aggregates with 20, 60 and 70 atoms are presented and compared with those obtained with other methods and with experimental data.

Magnetic configurations of rhodium

Abstract Two configurations are, a priori, favorable for a possible magnetic ground state of rhodium: small clusters and very thin films. In both cases, the reduction of the average coordination number could allow the onset of magnetism. For clusters, experimental works indicate that Rh N clusters with N 60 have a non-zero average magnetic moment. By using a d-band Hubbard Hamiltonian, the size and the geometric dependence of the magnetic properties of Rh clusters is studied. Different structures and atomic relaxations are considered. Significant magnetic moments are found up to N 50. For thin films of rhodium on silver, the Ag/Rh interdiffusion inhibits a long range magnetic order. The only favorable case is to consider a magnetic substrate. For thin Rh layers on Fe(001) only mono- and bi-layers are polarized. For larger thicknesses, only an interfacial polarization remains. Finally, in agreement with other studies, the Rh semi-infinite crystal does not display a local magnetic moment.

Magnetism of small vanadium clusters

Abstract The onset of magnetism of small V N clusters is studied by using a d-band Hubbard like Hamiltonian in the unrestricted Hartree-Fock approximation. We determine the local moments, magnetic order and average magnetization as a function of the intraatomic exchange integral J. Different structures are considered and charge transfer effects are discussed. We found that the magnetic behavior of small clusters is very different from the bulk behavior. For the considered clusters, two different magnetic configurations are obtained. The role of sp-electrons and spd-hybridization on the magnetic properties is also discussed.

Calculation of the magnetic properties of FeN clusters embedded in 3d transition-metal matrices

Abstract The magnetic properties of Fe N clusters ( N = 1,9,15) embedded in non-magnetic and anti-ferromagnetic matrices (e.g., V, Cr) are calculated self-consistently by solving a realistic spd-band model Hamiltonian including intra-atomic Coulomb interactions in the unrestricted Hartree-Fock approximation. Our results are compared with those obtained for free Fe N clusters and for the Fe, V and Cr solids. The role of the structural and chemical local environment on cluster magnetic properties is analyzed. Some of the implications of the present study for magnetic alloys and ferromagnetic clusters on surfaces are also briefly discussed.

Magnetic properties of small 3-d transition metal clusters: Role of the sp-electrons and spd-hybridization

Abstract The magnetic properties of small FeN, and CrN clusters are calculated by using a Hubbard-like Hamiltonian for spd-electrons in the unrestricted Hartree-Fock approximation. Results are given for the average magnetic moment, local moments and charge distribution within the cluster. The role of sp-electrons and sp-d hybridization are discussed by comparison with previous d-band calculations. In most cases a reduction of the average magnetic moment is obtained for FeN clusters by including sp-electrons, whereas the oppositee happens to Cr clusters.

Calculation of Spin-Fluctuation Energies in Fe N Clusters

A functional-integral theory of itinerant d-electron magnetism is applied to small transition-metal clusters. The spin-fluctuation energies ?Fi(ξ) at different atoms i in FeN (N ≤ 15) are calculated using a real-space recursive expansion of the local Green’s function. The size, structural, and local-environment dependences of ?Fi(ξ) are determined. The interplay between fluctuations of the module and orientation of the local exchange fields ξi is investigated, and the applicability of phenomenological spin models is discussed.

Tight Binding Theory of Overlap Interactions: Applications to Magnetic Materials

The magnetic properties of low dimensional systems are studied using a recently developed local approach to overlap interactions in the tight-binding (TB) method. The effects of overlap and local atomic environment on the orthogonalized interaction parameters are determined from a set of coupled linear equations. The accuracy of the method is demonstrated by calculations on transition metals having periodic and complex structures. This theory combines efficiently the simplicity of the orthogonal TB method with the transferability of the non-orthogonal approach, and is thus particularly useful for the study of complex systems.