Enhanced electron-phonon coupling at the Mo and W(110) surfaces induced by adsorbed hydrogen

The possible occurrence of either a charge-density-wave or a Kohn anomaly is governed by the presence of Fermi-surface nesting and the subtle interaction of electrons and phonons. Recent experimental and theoretical investigations suggest such an effect for the hydrogen covered Mo and W(110) surfaces. Using density-functional theory we examine the electronic structure and the electron-phonon coupling of these systems. Besides good agreement with the experimental phonon frequencies our study provides a characterization and quantitative analysis of an unusual scenario determining the electronic, vibrational, and structural properties of these surfaces.Comment: 4 Pages, RevTe

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

Nanomechanical dissipation at a tip-induced Kondo onset

The onset or demise of Kondo effect in a magnetic impurity on a metal surface can be triggered, as sometimes observed, by the simple mechanical nudging of a tip. Such a mechanically driven quantum phase transition must reflect in a corresponding mechanical dissipation peak; yet, this kind of signature has not been focused upon so far. Aiming at the simplest theoretical modeling, we treat the impurity as an Anderson impurity model, the tip action as a hybridization switching, and solve the problem by numerical renormalization group. Studying this model as function of temperature and magnetic field we are able to isolate the Kondo contribution to dissipation. While that is, reasonably, of the order of the Kondo energy, its temperature evolution shows a surprisingly large tail even above the Kondo temperature. The detectability of Kondo mechanical dissipation in atomic force microscopy is also discussed

Jahn-Teller Spectral Fingerprint in Molecular Photoemission: C60

The h_u hole spectral intensity for C60 -> C60+ molecular photoemission is calculated at finite temperature by a parameter-free Lanczos diagonalization of the electron-vibration Hamiltonian, including the full 8 H_g, 6 G_g, and 2 A_g mode couplings. The computed spectrum at 800 K is in striking agreement with gas-phase data. The energy separation of the first main shoulder from the main photoemission peak, 230 meV in C60, is shown to measure directly and rather generally the strength of the final-state Jahn-Teller coupling.Comment: 5 pages, 3 figure

robotic am system for plastic materials tuning and on line adjustment of process parameters

Abstract Additive Manufacturing (AM) techniques based on thermoplastic polymer extrusion allow the manufacture of complex parts, but their slow printing speed limits their use for mass production. To overcome this drawback, an industrial screw-based extruder has been mounted on an anthropomorphic robot, realizing a flexible AM platform for big objects. The most important process parameters have been set by a suitable experimental campaign, ensuring a regular deposited layer geometry. A closed-loop control has been implemented to further improve the process parameter setting based on data measured during the deposition, in this way compensating the material withdrawal or other unexpected defects