Room temperature Peierls distortion in small radius nanotubes

By means of {\it ab initio} simulations, we investigate the phonon band structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We show that both the C(5,0) and C(3,3) tubes undergo above room temperature a Peierls transition mediated by an acoustical long-wavelength and an optical $q=2k_F$ phonons respectively. In the armchair geometry, we verify that the electron-phonon coupling parameter $\lambda$ originates mainly from phonons at $q=2k_F$ and is strongly enhanced when the diameter decreases. These results question the origin of superconductivity in small diameter nanotubes.Comment: submitted 21oct2004 accepted 6jan2005 (Phys.Rev.Lett.

The role of the dopant in the superconductivity of diamond

We present an {\it ab initio} study of the recently discovered superconductivity of boron doped diamond within the framework of a phonon-mediated pairing mechanism. The role of the dopant, in substitutional position, is unconventional in that half of the coupling parameter $\lambda$ originates in strongly localized defect-related vibrational modes, yielding a very peaked Eliashberg $\alpha^2F(\omega)$ function. The electron-phonon coupling potential is found to be extremely large and T$_C$ is limited by the low value of the density of states at the Fermi level

Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes

Using first-principles total-energy pseudopotential calculations, we have studied the properties of chains of potassium and aluminum in nanotubes. For BN tubes, there is little interaction between the metal chains and the tubes, and the conductivity of these tubes is through carriers located at the inner part of the tube. In contrast, for small radius carbon nanotubes, there are two types of interactions: charge-transfer (dominant for alkali atoms) leading to strong ionic cohesion, and hybridization (for multivalent metal atoms) resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show that both effects contribute. New electronic properties related to these confined atomic chains of metal are analyzed.Comment: 12 pages + 3 figure

Optical excitations in hexagonal nanonetwork materials

Optical excitations in hexagonal nanonetwork materials, for example, Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. The bonding of BN systems is positively polarized at the B site, and is negatively polarized at the N site. There is a permanent electric dipole moment along the BN bond, whose direction is from the B site to the N site. When the exciton hopping integral is restricted to the nearest neighbors, the flat band of the exciton appears at the lowest energy. The higher optical excitations have excitation bands similar to the electronic bands of graphene planes and carbon nanotubes. The symmetry of the flat exciton band is optically forbidden, indicating that the excitons related to this band will show quite long lifetime which will cause strong luminescence properties.Comment: 4 pages; 3 figures; proceedings of "XVIth International Winterschool on Electronic Properties of Novel Materials (IWEPNM2002)

BN domains included into carbon nanotubes: role of interface

We present a density functional theory study on the shape and arrangement of small BN domains embedded into single-walled carbon nanotubes. We show a strong tendency for the BN hexagons formation at the simultaneous inclusion of B and N atoms within the walls of carbon nanotubes. The work emphasizes the importance of a correct description of the BN-C frontier. We suggest that BN-C interface will be formed preferentially with the participation of N-C bonds. Thus, we propose a new way of stabilizing the small BN inclusions through the formation of nitrogen terminated borders. The comparison between the obtained results and the available experimental data on formation of BN plackets within the single walled carbon nanotubes is presented. The mirror situation of inclusion of carbon plackets within single walled BN nanotubes is considered within the proposed formalism. Finally, we show that the inclusion of small BN plackets inside the CNTs strongly affects the electronic character of the initial systems, opening a band gap. The nitrogen excess in the BN plackets introduces donor states in the band gap and it might thus result in a promising way for n-doping single walled carbon nanotubes

Stochastic Heterostructures in B/N-Doped Carbon Nanotubes

Carbon nanotubes are one-dimensional and very narrow. These obvious facts imply that under doping with boron and nitrogen, microscopic doping inhomogeneity is much more important than for bulk semiconductors. We consider the possibility of exploiting such fluctuations to create interesting devices. Using self-consistent tight-binding (SCTB), we study heavily doped highly compensated nanotubes, revealing the spontaneous formation of structures resembling chains of random quantum dots, or nano-scale diode-like elements in series. We also consider truly isolated impurities, revealing simple scaling properties of bound state sizes and energies.Comment: 4 pages RevTeX, 4 PostScript figure

Neurophysiological approach by self-control of your stress-related autonomic nervous system with depression, stress and anxiety patients

Background: Heart Rate Variability Biofeedback (HRVB) is a treatment in which patients learn self-regulation of a physiological dysregulated vagal nerve function. While the therapeutic approach of HRVB is promising for a variety of disorders, it has not yet been regularly offered in a mental health treatment setting. Aim: To provide a systematic review about the efficacy of HRV-Biofeedback in treatment of anxiety, depression, and stress related disorders. Method: Systematic review in PubMed and Web of Science in 2020 with terms HRV, biofeedback, Post-Traumatic Stress Disorder (PTSD), depression, panic disorder, and anxiety disorder. Selection, critical appraisal, and description of the Random Controlled Trials (RCT) studies. Combined with recent meta-analyses. Results: The search resulted in a total of 881 studies. After critical appraisal, nine RCTs have been selected as well as two other relevant studies. The RCTs with control groups treatment as usual, muscle relaxation training and a "placebo"-biofeedback instrument revealed significant clinical efficacy and better results compared with control conditions, mostly significant. In the depression studies average reduction at the Beck Depression Inventory (BDI) scale was 64% (HRVB plus Treatment as Usual (TAU) versus 25% (control group with TAU) and 30% reduction (HRVB) at the PSQ scale versus 7% (control group with TAU). In the PTSD studies average reduction at the BDI-scale was 53% (HRV plus TAU) versus 24% (control group with TAU) and 22% (HRVB) versus 10% (TAU) with the PTSD Checklist (PCL). In other systematic reviews significant effects have been shown for HRV-Biofeedback in treatment of asthma, coronary artery disease, sleeping disorders, postpartum depression and stress and anxiety. Conclusion: This systematic review shows significant improvement of the non-invasive HRVB training in stress related disorders like PTSD, depression, and panic disorder, in particular when combined with cognitive behavioral therapy or different TAU. Effects were visible after four weeks of training, but clinical practice in a longer daily self-treatment of eight weeks is more promising. More research to integrate HRVB in treatment of stress related disorders in psychiatry is warranted, as well as research focused on the neurophysiological mechanisms.Stress-related psychiatric disorders across the life spa

Oscillator strengths with pseudopotentials

The time-dependent local-density approximation (TDLDA) is shown to remain accurate in describing the atomic response of IB elements under the additional approximation of using pseudopotentials to treat the effects of core electrons. This extends the work of Zangwill and Soven who showed the utility of the all-electron TDLDA in the atomic response problem.Comment: 13 pages including 3 Postscript figure

First-Principles Studies of Hydrogenated Si(111)--7×\times7

The relaxed geometries and electronic properties of the hydrogenated phases of the Si(111)-7$\times$7 surface are studied using first-principles molecular dynamics. A monohydride phase, with one H per dangling bond adsorbed on the bare surface is found to be energetically favorable. Another phase where 43 hydrogens saturate the dangling bonds created by the removal of the adatoms from the clean surface is found to be nearly equivalent energetically. Experimental STM and differential reflectance characteristics of the hydrogenated surfaces agree well with the calculated features.Comment: REVTEX manuscript with 3 postscript figures, all included in uu file. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.htm

The Post Anachronism: The Temporal Dimension of Facebook Privacy

This paper reports on two studies that investigate empirically how privacy preferences about the audience and emphasis of Facebook posts change over time. In a 63-participant longitudinal study, participants gave their audience and emphasis preferences for up to ten of their Facebook posts in the week they were posted, again one week later, and again one month later. In a 234-participant retrospective study, participants expressed their preferences about posts made in the past week, as well as one year prior. We found that participants did not want content to fade away wholesale with age; the audience participants wanted to be able to access posts remained relatively constant over time. However, participants did want a handful of posts to become more private over time, as well as others to become more visible. Participants ’ predictions about how their preferences would change correlated poorly with their actual changes in preferences over time, casting doubt on ideas for setting an expiration date for content. Although older posts were seen as less relevant and had often been forgotten, participants found value in these posts for reminiscence. Surprisingly, we observed few concerns about privacy or self-presentation for older posts. We discuss our findings ’ implications for retrospective privacy mechanisms