Comment to "Imaging the atomic orbitals of carbon atomic chains with field-emission electron microscopy"

The observation of a stable doublet pattern in the field-emission electron microscopy of a linear atomic chain requires a stable mechanism breaking the axial symmetry, which is not identified correctly by Mikhailovskij et al. [Phys. Rev. B 80, 165404 (2009)]. Using microscopic calculations, we attribute the observed pattern to the symmetry breaking produced by the ligand where the chain is attached, plus carbon pi-bonding alternation.Comment: 2 pages 1 fig, PRB commen

Jahn-Teller distortions and excitation energies in C60(n+)

Based on previously computed parameters for the electron-phonon couplings and the Coulomb exchange, we compute and classify the static Jahn-Teller distortions, i.e. the minima of the lowest adiabatic potential energy surface, of C60(n+), for all values of charge 1 <= n <= 9 and spin. We compute the intra-band electronic excitation energies in the different optimal geometries in the sudden approximation, and find a spread of the electronic states of roughly 1 eV. We also obtain the leading vibronic quantum corrections to the ground-state energy, equal to zero-point energy lowering due to the softening of the phonons at the adiabatic Jahn-Teller minima: these non-adiabatic corrections are so large that for 4 <= n <= 6 states of different spin symmetry turn lower than the high-spin adiabatic ground state.Comment: 19 pages, proceedings of the XVI Jahn-Teller Conference - Leuven (Be), 26/8 - 1/9 200

An optimized algebraic basis for molecular potentials

The computation of vibrational spectra of diatomic molecules through the exact diagonalization of algebraically determined matrixes based on powers of Morse coordinates is made substantially more efficient by choosing a properly adapted quantum-mechanical basis, specifically tuned to the molecular potential. A substantial improvement is achieved while still retaining the full advantage of the simplicity and numerical light-weightedness of an algebraic approach. In the scheme we propose, the basis is parameterized by two quantities which can be adjusted to best suit the molecular potential through a simple minimization procedure.Comment: 29 pages, 4 tables and 4 figures, latex. Sumbitted to J. Phys. Chem

Influence of substrate potential shape on the dynamics of a sliding lubricant chain

We investigate the frictional sliding of an incommensurate chain of interacting particles confined in between two nonlinear on-site substrate potential profiles in relative motion. We focus here on the class of Remoissenet-Peyrard parametrized potentials $V_{\rm RP}(x,s)$, whose shape can be varied continuously as a function of $s$, recovering the sine-Gordon potential as particular case. The observed frictional dynamics of the system, crucially dependent on the mutual ratios of the three periodicities in the sandwich geometry, turns out to be significantly influenced also by the shape of the substrate potential. Specifically, variations of the shape parameter $s$ affects significantly and not trivially the existence and robustness of the recently reported velocity quantization phenomena [Vanossi {\it et al.}, Phys. Rev. Lett. 97, 056101 (2006)], where the chain center-of-mass velocity to the externally imposed relative velocity of the sliders stays pinned to exact "plateau" values for wide ranges of the dynamical parameters.Comment: 7 pages, 6 figure

Crystalline misfit-angle implications for solid sliding

For the contact of two finite portions of interacting rigid crystalline surfaces, we compute the dependence of the pinning energy barrier on the misfit angle and contact area. The resulting data are used to investigate the distribution of static frictional thresholds for a contact of polycrystal surfaces, as occurs at the touching points of dry or even lubricated friction. The simplicity of the model allows us to investigate a broad contact-size and angular range, thus obtaining the statistical properties of the energy barriers opposing sliding for a single asperity. These statistical properties are used as the input of a master-equation model to predict the sliding properties of two macroscopic surfaces in contact. The model is consistent with the well-established result that low temperature should generally favor stick-slip motion, while at high temperature sliding should be smooth.Comment: 11 pages, including 7 figure

Hund's rule Magnetism in C60 ions?

We investigate the occurrence of Hund's rule magnetism in C60(n+-) molecular ions, by computing the ground-state spin for all charge states n from -3 to +5. The two competing interactions, electron-vibration (e-v, including Jahn Teller, favoring low spin) and electron-electron (e-e, including Hund-rule exchange, favoring high spin), are accounted for based on previously computed ab-initio coupling parameters. Treating the ion coordinates as classical, we first calculate and classify the static Jahn-Teller distorted states for all n, inclusive of both e-v and e-e effects. We then correct the adiabatic result by including the zero-point energy lowering associated with softening of vibrations at the adiabatic Jahn-Teller minima. Our overall result is that while, like in previous investigations, low-spin states prevail in negative ions, Hund's rule high spin dominates all positive C60(n+) ions. This suggests also that Hund-rule magnetism could arise in fullerene cation-based solid state compounds, particularly those involving C60(2+).Comment: 12 pages, 2 figures, epj styl