arXiv subjects

Rafael Ramirez

Publications and source records attributed to Rafael Ramirez.

40 records · Page 3Linked to original sources

arXiv2006

Rare-gas solids under pressure: A path-integral Monte Carlo simulation

Rare-gas solids (Ne, Ar, Kr, and Xe) under hydrostatic pressure up to 30 kbar have been studied by path-integral Monte Carlo simulations in the isothermal-isobaric ensemble. Results of these simulations have been compared with available experimental data and with those obtained from a quasiharmonic approximation (QHA). This comparison allows us to quantify the overall anharmonicity of the lattice vibrations and its influence on several structural and thermodynamic properties of rare-gas solids. The vibrational energy increases with pressure, but this increase is slower than that of the elastic energy, which dominates at high pressures. In the PIMC simulations, the vibrational kinetic energy is found to be larger than the corresponding potential energy, and the relative difference between both energies decreases as the applied pressure is raised. The accuracy of the QHA increases for rising pressure.

cond-mat.mtrl-sci↗

arXiv2006

Hydrogen and muonium in diamond: A path-integral molecular dynamics simulation

Isolated hydrogen, deuterium, and muonium in diamond have been studied by path-integral molecular dynamics simulations in the canonical ensemble. Finite-temperature properties of these point defects were analyzed in the range from 100 to 800 K. Interatomic interactions were modeled by a tight-binding potential fitted to density-functional calculations. The most stable position for these hydrogenic impurities is found at the C-C bond center. Vibrational frequencies have been obtained from a linear-response approach, based on correlations of atom displacements at finite temperatures. The results show a large anharmonic effect in impurity vibrations at the bond center site, which hardens the vibrational modes with respect to a harmonic approximation. Zero-point motion causes an appreciable shift of the defect level in the electronic gap, as a consequence of electron-phonon interaction. This defect level goes down by 70 meV when replacing hydrogen by muonium.

cond-mat.mtrl-sci↗

arXiv2006

A path-integral molecular dynamics simulation of diamond

Diamond is studied by path integral molecular dynamics simulations of the atomic nuclei in combination with a tight-binding Hamiltonian to describe its electronic structure and total energy. This approach allows us to quantify the influence of quantum zero-point vibrations and finite temperatures on both the electronic and vibrational properties of diamond. The electron-phonon coupling mediated by the zero-point vibration reduces the direct electronic gap of diamond by 10 %. The calculated decrease of the direct gap with temperature shows good agreement with the experimental data available up to 700 K. Anharmonic vibrational frequencies of the crystal have been obtained from a linear-response approach based on the path integral formalism. In particular, the temperature dependence of the zone-center optical phonon has been derived from the simulations. The anharmonicity of the interatomic potential produces a red shift of this phonon frequency.At temperatures above 500 K, this shift is overestimated in comparison to available experimental data. The predicted temperature shift of the elastic constant c_{44} displays reasonable agreement with the available experimental results.

cond-mat.mtrl-sci↗

arXiv1996

Low-energy quantum dynamics of atoms at defects. Interstitial oxygen in silicon

The problem of the low-energy highly-anharmonic quantum dynamics of isolated impurities in solids is addressed by using path-integral Monte Carlo simulations. Interstitial oxygen in silicon is studied as a prototypical example showing such a behavior. The assignment of a "geometry" to the defect is discussed. Depending on the potential (or on the impurity mass), there is a "classical" regime, where the maximum probability-density for the oxygen nucleus is at the potential minimum. There is another regime, associated to highly anharmonic potentials, where this is not the case. Both regimes are separated by a sharp transition. Also, the decoupling of the many-nuclei problem into a one-body Hamiltonian to describe the low-energy dynamics is studied. The adiabatic potential obtained from the relaxation of all the other degrees of freedom at each value of the coordinate associated to the low-energy motion, gives the best approximation to the full many-nuclei problem.

cond-mat↗