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We study the dependence of the intrinsic conductance of a nanocontact on its shape by using the recursion-transfer-matrix method. Hour-glass, torus, and spherical shapes are defined through analytic potentials, the latter two serving as rough models for ring-like and spherical molecules, respectively. The sensitivity of the conductance to geometric details is analyzed and discussed. Strong resonance effects are found for a spherical contact weakly coupled to electron reservoirs., Comment: 7 pages, 12 figures

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point contact, with G ~ 1 G_0 and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear "gold molecule" between the electrodes, and a weak coupling of this "molecule" to the tips.

We study the nuclear medium effects on the $c\bar{c}$ time evolution and charmonium production, in a relativistic proton-nucleus collision. In particular, we focus on the fragmentation region of the nucleus where the formation length of the charmonium mesons is shorter than the size of the nucleus. Little is known on the nuclear effects in this region. We use a quantum-mechanical model which includes a realistic potential for the $c\bar{c}$ system and an imaginary potential to describe the collisions of the $c\bar{c}$ with the nucleons. The imaginary potential introduces a transition amplitude among the charmonium states and produces an interference pattern on the charmonium survival probability, which is particulartly important for $\psi'$. Our results on the suppression factors are compared with data from the NA50 and E866/NuSea Collaborations. Predictions are given for the suppression of $J/\psi,~\psi',~\chi_c$ as a function of the nuclear mass and in the negative $x_F$ region, where data will be available soon., Comment: 5 pages, LaTex file, 6 eps figures. submitted to Phys.Rev.Lett