Eduardo Hernández

ELASTIC PROPERTIES OF C AND BX CY NZ COMPOSITE NANOTUBES

We present a comparative study of the energetic, structural, and elastic properties of carbon and composite single-wall nanotubes, including BN,

New boron based nanostructured materials

{\mathrm{BC}}_{3}

Reversible bending of carbon nanotubes using a transmission electron microscope

, and

NUCLEAR QUANTUM EFFECTS IN THE ELECTRONIC STRUCTURE OF C2H4 : A COMBINED FEYNMAN PATH INTEGRAL-AB INITIO APPROACH

{\mathrm{BC}}_{2}\mathrm{N}

Electrons and nuclei of ethylene isomers; a Feynman path integral – ab initio study

nanotubes, using a nonorthogonal tight-binding formalism. Our calculations predict that carbon nanotubes have a higher Young modulus than any of the studied composite nanotubes, and of the same order as that found for defect-free graphene sheets. We obtain good agreement with the available experimental results.

Isotope effect in the Mott transition; a prediction on the basis of molecular all-quantum simulations

Based on a series of ab initio studies we have pointed out the remarkable structural stability of nanotubular and quasiplanar boron clusters, and postulated the existence of novel layered, tubular, and quasicrystalline boron solids built from elemental subunits. The present study illustrates and predicts qualitative structural and electronic properties for various models of nanotubular and layered boron solids, and compares them to well-known tubular and layered forms of pure carbon and mixed boron compounds.

Two-dimensional Bloch electrons in perpendicular magnetic fields: An exact calculation of the Hofstadter butterfly spectrum

Multiwall carbon nanotubes can be bent by changing the current density of the electron beam in a transmission electron microscope. The effect could be observed in a small fraction of nanotubes in the investigated samples. The bending can be varied continuously, is reversible, and highly reproducible. On removing the force which makes them bend, they relax to their originally straight shape without any damage, thus exhibiting spring-like behavior. Possible mechanisms for this effect are discussed.

A comparison between the opto-thermo-mechanical model and lab measurements for CHEOPS

Abstract A tight-binding equipped Feynman path integral Monte Carlo formalism has been linked to a Hartree–Fock Hamiltonian to derive the electronic properties of C 2 H 4 considering the quantum character of the nuclei. Configurationally averaged electronic quantities are compared with single-configuration results. The potential energy of the vibrational problem is caused by an energetic up-shift of the electron-nuclear interaction of the electronic Hamiltonian under the influence of nuclear quantum fluctuations. Relative to the values optimized by bare electronic Hamiltonians, calculated bond lengths are elongated by nuclear quantum effects. This elongation becomes more pronounced with decreasing atomic masses. Nuclear quantum properties are discussed via the radial distribution function, projected probability distributions and spatial fluctuations.

HAND-SAND: A tool for simulating handover techniques in cellular networks

Abstract The finite temperature properties of the ethylene isomers C 2 H 4 , C 2 D 4 , C 2 13 H 4 and C 2 13 D 4 have been studied by a Feynman path integral quantum Monte Carlo (PIMC) approach which has been combined with different electronic Hamiltonians. The nuclear potential V ( R ) in the PIMC step of the present formalism has been modeled by an efficient tight-binding one-electron Hamiltonian. Electronic expectation values in thermal equilibrium have been evaluated by ab initio Hartree–Fock and Moller–Plesset calculations. The quantum degrees of freedom of the ethylene nuclei as well as the anharmonicities in V ( R ) cause sizable elongations of the bond lengths relative to the hypothetical vibrationless values at the minimum of the potential energy surface. The PIMC results demonstrate impressively the wave-packet character of the nuclear wave function. This effect is neglected in the crude Born–Oppenheimer approximation which forms the basis of the large majority of electronic structure calculations of molecules. The nuclear degrees of freedom have a strong influence on the expectation values of the electronic Hamiltonian. The isotope and temperature dependence of these quantities has been analyzed. The nuclear fluctuations attenuate the nuclear–nuclear and electron–electron repulsions and lower the electronic kinetic energy. These stabilizing shifts in thermal equilibrium compete with a destabilization of the electron–nuclear attraction. The analysis of the ensemble averaged electronic quantities offers insight into the modifications of covalent bonding under the conditions of thermal equilibrium. Conceptual problems of classical Monte Carlo simulations as well as the shortcomings of electronic structure calculations on the basis of a single nuclear configuration in molecules with light atoms are emphasized. It is demonstrated that the nuclear degrees of freedom up to room temperature of the ethylene isomers studied are caused by quantum tunneling. Physical implications which follow from the present PIMC – ab initio investigation are mentioned concisely.

Tight-binding calculation of the elastic properties of single-wall nanotubes

We have linked an ab initio Hartree-Fock Hamiltonian to the Feynman path integral quantum Monte Carlo formalism in order to derive electronic expectation values under consideration of nuclear degrees of freedom. This approach yields electronic expectation values which depend on the atomic masses. On the basis of combined path integral – ab initio calculations we predict an isotope effect in the correlation driven Mott transition. The nuclear degrees of freedom lead to an enhancement in the electronic correlation strength, an effect which supports the transition conditions. This enhancement is negatively correlated with the atomic masses. Implications of an isotope effect in the Mott transition for the explanation of the superconducting pairing are mentioned concisely.