80 research outputs found
Low-temperature anharmonicity of barium titanate: a path-integral molecular dynamics study
We investigate the influence of quantum effects on the dielectric and
piezoelectric properties of barium titanate in its (low-temperature)
rhombohedral phase, and show the strongly anharmonic character of this system
even at low temperature. For this purpose, we perform path-integral
molecular-dynamics simulations under fixed pressure and fixed temperature,
using an efficient Langevin thermostat-barostat, and an effective hamiltonian
derived from first-principles calculations. The quantum fluctuations are shown
to significantly enhance the static dielectric susceptibility (~ by a factor 2)
and the piezoelectric constants, reflecting the strong anharmonicity of this
ferroelectric system even at very low temperature. The slow
temperature-evolution of the dielectric properties observed below ~ 100 K is
attributed (i) to zero-point energy contributions and (ii) to harmonic behavior
if quantum effects are turned off.Comment: submitted to Phys. Rev.
Dielectric characteristic of nanocrystalline Na0.5K0.5NbO3 ceramic green body
International audienceDielectric spectroscopy was applied to porous nanocrystalline Na0.5K0.5NbO3 (NKN) ceramic green body, wherein influences of percolation effect and water adsorption at pore surface of the ceramic green body on dielectric response were examined over wide temperature (150 to 450 K) and frequency (100 Hz to 1 MHz) ranges. Dielectric permittivity of the ceramic green body is about 2-3 orders of magnitude higher than that of pure NKN powder or NKN ceramic. Furthermore, the high dielectric permittivity and high humidity sensitivity of the ceramic green body can appear again with aging a period of time in air. The data from this investigation make potential applications for NKN as a giant dielectric material or a humidity sensing material
Quantum thermal bath for molecular dynamics simulation
International audienceMolecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected
Radiative heat transfer from a black body to dielectric nanoparticles
International audienceHeating of dielectric nanoparticles by black-body radiation is investigated by using molecular-dynamics simulation. The thermal interaction with the radiation is modeled by coupling the ions with a random electric field and including a radiation reaction force. This approach shows that the heat is absorbed by the polariton mode. Its subsequent redistribution among other vibration modes strongly depends on the particle size and on temperature.We observe energy trapping in a finite subset of vibrational mode
On the Use of Quantum Thermal Bath in Unimolecular Fragmentation Simulation
International audienc
On the Use of Quantum Thermal Bath in Unimolecular Fragmentation Simulations
In the present work we have investigated the possibility of using the Quantum
Thermal Bath (QTB) method in molecular simulations of unimolecular dissociation
processes. Notably, QTB is aimed in introducing quantum nuclear effects with a com-
putational time which is basically the same as in newtonian simulations. At this end
we have considered the model fragmentation of CH4 for which an analytical function
is present in the literature. Moreover, based on the same model a microcanonical
algorithm which monitor zero-point energy of products, and eventually modifies tra-
jectories, was recently proposed. We have thus compared classical and quantum rate
constant with these different models. QTB seems to correctly reproduce some quantum
features, in particular the difference between classical and quantum activation energies,
making it a promising method to study unimolecular fragmentation of much complex
systems with molecular simulations. The role of QTB thermostat on rotational degrees
of freedom is also analyzed and discussed.
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Contribution des effets quantiques nucléaires au coefficient de diffusion de l’hydrogène dans BaZrO3
International audienc
Contribution des effets quantiques nucléaires au coefficient de diffusion de l’hydrogène dans BaZrO3
International audienc
Contribution des effets quantiques nucléaires au coefficient de diffusion de l’hydrogène dans BaZrO3
International audienc
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