Super-slippery Carbon Nanotubes: Symmetry Breaking breaks friction

The friction between the walls of multi-wall carbon nanotubes is shown to be extremely low in general, with important details related to the specific choice of the walls. This is governed by a simple expression revealing that the phenomenon is a profound consequence of the specific symmetry breaking: super-slippery sliding of the incommensurate walls is a Goldstone mode. Three universal principles of tribology, offering a recipe for the lubricant selection are emphasized.Comment: 4 pages, 2 figures, 1 table; pdf available from: http://www.ff.bg.ac.yu/qmf/qsg_e.ht

Symmetry Based Properties of the Transition Metal Dichalcogenide Nanotubes

The full geometrical symmetry groups (the line groups) of the monolayered, 2Hb and 3R polytypes of the inorganic MoS2 and WS2 micro- and nanotubes of arbitrary chirality are found. This is used to find the coordinates of the representative atoms sufficient to determine completely geometrical structure of tubes. Then some physical properties which can be deduced from the symmetry are discussed: electron band degeneracies, selection rules, general forms of the second rank tensors and potentials, phonon spectra.Comment: 6 pages 1 figur

Irreducible Representations of Diperiodic Groups

The irreducible representations of all of the 80 diperiodic groups, being the symmetries of the systems translationally periodical in two directions, are calculated. To this end, each of these groups is factorized as the product of a generalized translational group and an axial point group. The results are presented in the form of the tables, containing the matrices of the irreducible representations of the generators of the groups. General properties and some physical applications (degeneracy and topology of the energy bands, selection rules, etc.) are discussed.Comment: 30 pages, 5 figures, 28 tables, 18 refs, LaTex2.0

Relativistic ionization-rescattering with tailored laser pulses

The interaction of relativistically strong tailored laser pulses with an atomic system is considered. Due to a special tailoring of the laser pulse, the suppression of the relativistic drift of the ionized electron and a dramatic enhancement of the rescattering probability is shown to be achievable. The high harmonic generation rate in the relativistic regime is calculated and shown to be increased by several orders of magnitude compared to the case of conventional laser pulses. The energies of the revisiting electron at the atomic core can approach the MeV domain, thus rendering hard x-ray harmonics and nuclear reactions with single atoms feasible

Modified group projectors: tight binding method

Modified group projector technique for induced representations is a powerful tool for calculation and symmetry quantum numbers assignation of a tight binding Hamiltonian energy bands of crystals. Namely, the induced type structure of such a Hamiltonian enables efficient application of the procedure: only the interior representations of the orbit stabilizers are to be considered. Then the generalized Bloch eigen functions are obtained naturally by the expansion to the whole state space. The method is applied to the electronic pi-bands of the single wall carbon nanotubes: together with dispersion relations, their complete symmetry assignation by the full symmetry (line) groups and the corresponding symmetry-adapted eigen function are found.Comment: 10 pages 1 figur

Positive emotion regulation strategies as mediators in depression and generalized anxiety disorder symptoms: A Transdiagnostic framework investigation

Positive emotion regulation (ER) strategies may contribute to the development and maintenance of generalized anxiety disorder (GAD) and depression; nonetheless, the underlying and transdiagnostic mechanisms are still unknown. To examine: 1) the mediating role of positive ER strategies in the relationship between ER deficits and experiential avoidance in GAD and depression symptoms; 2) differences in positive ER strategies among participants with elevated symptoms of GAD, depression, depression comorbid with GAD, and a nonclinical sample. 532 Tehran University students completed questionnaires assessing experiential avoidance, deficits in ER, positive ER strategies (dampening, positive rumination), and GAD and depression symptoms. Differences were assessed in four groups: GAD, depression, depression comorbid with GAD, and nonclinical. Data were analysed with path analysis and MANCOVA. Dampening and positive rumination mediated the relationship between deficits in ER and experiential avoidance in participants with depression and GAD symptoms. As for the between groups comparisons, the comorbid group obtained the highest scores on dampening and the nonclinical group the lowest. By contrast, nonclinical participants had the highest scores on positive rumination, whereas the comorbid group had the lowest. Positive ER strategies might be transdiagnostic variables. Dampening was more associated with psychopathology than positive rumination, suggesting that this strategy could be more clearly considered a transdiagnostic factor. Future studies with clinical samples must support this suggestion