In an interesting new experiment the electronic structure of a magnetic atom adsorbed on the surface of Cu(111), observed by STM, was projected into a remote location on the same surface. The purpose of the present paper is to interpret this experiment with a model Hamiltonian, using ellipses of the size of the experimental ones, containing about 2300 atoms. The charge distribution for the different wavefunctions is analyzed, in particular, for those with energy close to the Fermi energy of copper EF . Some of them show two symmetric maxima located on the principal axis of the ellipse but not necessarily at the foci. If a Co atom is adsorbed at the site where the wavefunction with energy EF has a maximum and the interaction is small, the main effect of the adsorbed atom will be to split this particular wavefunction in two. The total charge density will remain the same but the local density of states will present a dip at EF at any site where the charge density is large enough. We relate the presence of this dip to the observation of quantum mirages. Our interpretation suggests that other sites, apart from the foci of the ellipses, can be used for projecting atomic images and also indicates the conditions for other non magnetic adsorbates to produce mirages. The electronic structure of systems with adsorbed impurities on noble metal surfaces should depend both on the type and the geometric arrangement of the adsorbed atoms. In particular, the effect of single transition-metal impurities and also of dimers on the (111) surface of gold has been studied recently with scanning tunneling microscopy [1][2][3]. Previously, closed loops of impurities had been shown to confine the surface state wavefunctions of Cu (111), producing what are now called "quantum corrals" [4]. In an interesting new experiment the electronic structure surrounding an adsorbed magnetic atom could be projected into a remote location on the same surface [5]: a Kondo "signature" in the tunneling spectrum of a cobalt atom located on one focus of an elliptical corral was observed as a mirage on the other focus. This possibility raises many theoretical questions, the first one being the relation between this quantum system and classical image projection. Other questions regarding the experimental setup are: How does the appearance of a mirage depend on the surface type, on the adsorbed atom type, on the shape of the corral, etc? The fact that the (111) surface of noble metals presents a symmetry gap at the Γ point and a surface state at about 0.5 eV below the Fermi energy, uncoupled from the bulk states, seems of course fundamental. For the other questions one must take a closer look and this is the purpose of the present paper. It must be emphasized, however, that we are not attempting a full theoretical description of the experiment but rather a simplified approach to the previous questions.We start by assuming a model Hamiltonian for the interaction of one adsorbed atom with the surface state of the metal, using the following approximations:1. The ...

We have explored new possible phases of 3D C60-based fullerites using semiempirical potentials and ab initio density functional methods. We have found three closely related structures-two body-centered orthorhombic and one body-centered cubic-having 52, 56, and 60 tetracoordinated atoms per molecule. These 3D polymers result in semiconductors with bulk moduli near 300 GPa, and shear moduli around 240 GPa, which make them good candidates for new low density superhard materials.

The complete vibrational spectra of crystalline C6H6 and C6D6 have been calculated using semiempirical atom-atom potential functions. The calculation uses a free molecule normal coordinate basis including Eckart mass-weighted coordinates. The following factors affecting the calculated freuqencies have been investigated in some detail: (i) intermode mixing among internal vibrations and between internal and external modes, (ii) inclusion of the first derivative of the atom-atom potential with respect to interatomic distances, (iii) number of atom-atom interactions, (iv) fulfillment of equilibrium conditions, and temperature effects. The Jacobian of the crystal frequencies, equilibrium conditions, and heat of sublimation with respect to the individual atom-atom parameters have been evaluated and a refinement of the potential parameters is presented. The agreement between the calculated frequencies and available experimental data is satisfactory specially for lattice frequencies. Transition dipole-dipole calculations lead to poor agreement with observed frequencies thus showing the model to be inappropriate for crystalline benzene.

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The molecular mean-square-amplitude matrix of crystalline biphenyl, C12Hl0, is calculated with an intermolecular potential of the atom-atom type. The effect of the presence of the low-lying torsional mode, which interacts with translational modes, is discussed. The Born S-matrix method is used, and proves to be an excellent approximation, which takes about ~ of the computer time of the exact calculation. The resulting amplitudes are in fair agreement with experiment, and show that the extremely high amplitude of libration about the long molecular axis may be satisfactorily explained without assuming a double-well shape for the torsional potential.