Low-frequency phonons in carbon nanotubes: A continuum approach

7 págs.; 6 figs.; 2 apéndices ; PACS number s : 68.65. k, 63.22. mLow-frequency phonons in carbon nanotubes are studied using a continuum model which allows consideration of an arbitrary wall thickness for the nanotube. Phonon dispersion relations are calculated for two archetypal examples of carbon nanotubes, the (5,5) and (10,10) tubes. The dependence of the radial breathing mode frequency at Γ on the inverse nanotube diameter is verified within this model; furthermore, we prove it to hold for all pure modes within the thin-shell approximation. The effect of the nanotube wall thickness on the eigenfrequencies of carbon nanotubes is also analyzed, and a criterion to fix this parameter within a continuum model is presented. We compare our results to other continuum approaches, and show that by choosing the appropriate parameters, excellent agreement with recent first-principles calculations can be achieved. © 2006 The American Physical Society.This work has been partly supported by the Spanish DGES Grant No. MAT 2002-04540-C05-03 and the Vicerrectorado de Relaciones Internacionales UCLM.Peer Reviewe

Squeezing enhancement by competing nonlinearities: Almost perfect squeezing without instabilities

4 págs.; 3 figs.; PACS number~s!: 42.50.Dv, 42.50.Lc, 42.65.KyThe competition between the χ(2) nonlinearity of a resonant second-harmonic-generation (SHG) system and an added χ(3) nonlinearity shifts the Hopf bifurcation of the standard SHG towards higher photon numbers eventually completely stabilizing the system. Remarkably, perfect squeezing survives the stabilization. Two important consequences are discussed, namely, efficient bright squeezing generation and strong suppression of the excess noise which, for parameters corresponding to an experiment reported in the literature, can be reduced by two orders of magnitude without diminishing the squeezing. Possible experimental implementations are finally discussed. ©1997 American Physical SocietyThis work was supported in part by Project No. TIC95- 0563-C05-03 (CICYT, Spain).Peer Reviewe

Collective excitations in liquid D2 confined within the mesoscopic pores of a MCM-41 molecular sieve

We present a comparative study of the excitations in bulk and liquid D2 confined within the pores of MCM-41. The material (Mobile Crystalline Material-41) is a silicate obtained by means of a template that yields a partially crystalline structure composed by arrays of nonintersecting hexagonal channels of controlled width having walls made of amorphous SiO2. Its porosity was characterized by means of adsorption isotherms and found to be composed by a regular array of pores having a narrow distribution of sizes with a most probable value of 2.45 nm. The assessment of the precise location of the sample within the pores is carried out by means of pressure isotherms. The study was conducted at two pressures which correspond to pore fillings above the capillary condensation regime. Within the range of wave vectors where collective excitations can be followed up (0.3<Q<3.0 $\AA$&#8722;1), we found confinement brings forward a large shortening of the excitation lifetimes that shifts the characteristic frequencies to higher energies. In addition, the coherent quasielastic scattering shows signatures of reduced diffusivity.Comment: 6 page

Creation of entangled states of distant atoms by interference

9 págs.; 2 figs.; app.; PACS number~s!: 03.65.Bz, 42.50.VkWe propose a scheme to create distant entangled atomic states. It is based on driving two (or more) atoms with a weak laser pulse, so that the probability that two atoms are excited is negligible. If the subsequent spontaneous emission is detected, the entangled state is created. We have developed a model to analyze the fidelity of the resulting state as a function of the dimensions and location of the detector, and the motional properties of the atoms. ©1999 American Physical SocietyThis work was supported in part by the Acciones Integradas No. HU/997-0030 (Spain-Austria),Grant Nos. TIC95-0563-C05-03 and PB96-00819 from CICYT (Spain), the Comunidad de Madrid under Grant No. 06T/039/96 (Spain), the FWF (Austrian Science Foundation) and TMR Network No. ERB–FMRX–CT96–0087.Peer Reviewe

Molecular Deuterion crystallitation under cuasi-1D confienment

ECNS 2015, Zaragoza (Spain), August 30th-September 4th 2015A particularly interesting aspect of Carbon Nanotubes is their use as nearly one-dimensional nano-containers. Apart of their possibilities for controlled chemistry in nano- fluidics devices new phenomena induced by confinement are also expected, such as liquid like ordered structures or exotic crystalline phases. Here, we present a series of neutron diffraction measurements (instrument D20, ILL, Grenoble) of molecular deuterium confined within Multiple Wall Carbon Nanotubes (MWCTNs). Bulk liquid D2 at its vapour pressure crystallises in an hcp structure at ~18.7 K. At low uptakes we have found a clear depression of the solidification temperature down to ~13.25 K. Interestingly, at the lowest uptake the diffraction pattern is consistent with the minimal fcc lattice compatible with a cylindrical symmetry.Peer Reviewe

Universality of moir\'e physics in collapsed chiral carbon nanotubes

We report the existence of moir\'e patterns and magic angle physics in all families of chiral collapsed carbon nanotubes. A detailed study of the electronic structure of all types of chiral nanotubes, previously collapsed via molecular dynamics, has been performed. We find that each family possesses a unique geometry and moir\'e disposition, as well as a characteristic number of flat bands. Remarkably, all kinds of nanotubes behave the same with respect to magic angle tuning, showing a monotonic behavior that gives rise to magic angles in full agreement with those of twisted bilayer graphene. Therefore, magic angle behavior is universally found in chiral collapsed nanotubes with a small chiral angle, giving rise to moir\'e patterns. Our approach comprises first-principles and semi-empirical calculations of the band structure, density of states and spatial distribution of the localized states signaled by flat bands

One-dimensional moir\'e superlattices and flat bands in collapsed chiral carbon nanotubes

We demonstrate that one-dimensional moir\'e patterns, analogous to those found in twisted bilayer graphene, can arise in collapsed chiral carbon nanotubes. Resorting to a combination of approaches, namely, molecular dynamics to obtain the relaxed geometries and tight-binding calculations validated against ab initio modeling, we find that magic angle physics occur in collapsed carbon nanotubes. Velocity reduction, flat bands and localization in AA regions with diminishing moir\'e angle are revealed, showing a magic angle close to 1$^{\rm o}$. From the spatial extension of the AA regions and the width of the flat bands, we estimate that many-body interactions in these systems are stronger than in twisted bilayer graphene. Chiral collapsed carbon nanotubes stand out as promising candidates to explore many-body effects and superconductivity in low dimensions, emerging as the one-dimensional analogues of twisted bilayer graphene

Anomaly in temperature dependence of thermal transport of two hydrogen-bonded glass-forming liquids

6 págs.; 3 figs.; PACS number s : 66.70. f, 63.50. x, 65.20. w, 65.60. aThe thermal conductivity of two molecular glasses (ethanol and 1-propanol) decrease with increasing temperature up to their glass transitions at Tg 97 and 98 K, respectively. Within their supercooled liquid phases, the conductivity increases with rising temperature up to a maximum which roughly coincides with the liquidus (or melting temperatures Tm 159 K and Tm 149 K, respectively). From there on, the conductivity decreases with increasing temperature, a behavior common to most liquids examined so far, exception made of liquid water. The origin of the rather different dependencies with temperature of thermal transport is understood as a competition between phonon-assisted and diffusive transport effects which are amenable to experiments using high resolution quasielastic neutron scattering and visible and ultraviolet Brillouin light-scattering spectroscopies. © 2007 The American Physical Society.Peer Reviewe

Microscopic structure of Fe-Ni and Fe-Ni-S molten alloys of geophysical interest

9 págs.; 9 figs.; 2 tabs. ; PACS number(s): 61.25.Mv, 62.60.1v, 96.35.2jThe static S(Q) liquid structure factors for binary FexNi 1-x, x=0.90, 0.85, and ternary Fe0.85Ni 0.05S0.15 molten alloys are investigated by means of the concurrent use of neutron diffraction and reverse Monte Carlo simulations. The measured G(r) radial distributions reveal atomic orderings varying from that present in the Fe0.90Ni0.10 alloy, reminiscent of the ideal bcc structure of solid Fe, to a far more open structure found in Fe 0.85Ni0.05S0.15. From data at hand no clear evidence of the clustering of sulfur within the Fe-Ni matrix is found. In contrast, the addition of sulfur leads to significant changes in structural and some dynamical properties that can be inferred from knowledge of the static structure factors. Such dynamical changes seem to arise as a result of a strong decrease of the elastic moduli of the alloys resulting from interactions with a light element rather than from a density effect. ©2004 American Physical SocietyWe thank the Institut Laue Langevin for neutron beam time.We are grateful to Pierre Palleau and to Dr. J. Campo of ILL for his technical support during the diffraction experiments, and Carlos Ayala of Centro Atómico Bariloche for the sample preparationPeer Reviewe

Hypothesis testing for the genetic background of quantitative traits

The testing of Bayesian point null hypotheses on variance component models have resulted in a tough assignment for which no clear and generally accepted method exists. In this work we present what we believe is a succeeding approach to such a task. It is based on a simple reparameterization of the model in terms of the total variance and the proportion of the additive genetic variance with respect to it, as well as on the explicit inclusion on the prior probability of a discrete component at origin. The reparameterization was used to bypass an arbitrariness related to the impropriety of uninformative priors onto unbounded variables while the discrete component was necessary to overcome the zero probability assigned to sets of null measure by the usual continuous variable models. The method was tested against computer simulations with appealing results