6 research outputs found
Mode-coupling theory of lattice dynamics for classical and quantum crystals
The dynamical properties of nuclei, carried by the concept of phonon
quasiparticles (QP), are central to the field of condensed matter. While the
harmonic approximation can reproduce a number of properties observed in real
crystals, the inclusion of anharmonicity in lattice dynamics is essential to
accurately predict properties such as heat transport or thermal expansion. For
highly anharmonic systems, non perturbative approaches are needed, which result
in renormalized theories of lattice dynamics. In this article, we apply the
Mori-Zwanzig projector formalism to derive an exact generalized Langevin
equation describing the quantum dynamics of nuclei in a crystal. By projecting
this equation on quasiparticles in reciprocal space, and with results from
linear response theory, we obtain a formulation of vibrational spectra that
fully accounts for the anharmonicity. Using a mode-coupling approach, we
construct a systematic perturbative expansion in which each new order is built
to minimize the following ones. With a truncation to the lowest order, we show
how to obtain a set of self-consistent equations that can describe the
lineshapes of quasiparticles. The only inputs needed for the resulting set of
equations are the static Kubo correlation functions, which can be computed
using (fully quantum) path-integral molecular dynamics or approximated with
(classical or ab initio) molecular dynamics. We illustrate the theory with an
application on fcc 4He, an archetypal quantum crystal with very strong
anharmonicity
Ătude des effets de la tempĂ©rature sur les combustibles nuclĂ©aires par une approche ab initio
To ensure the security of nuclear electricity production, an understanding of the behavior of nuclear fuel materials is necessary. This work aims at making a contribution to the study of the effects of temperature on nuclear fuels, by using an ab initio approach through density functional theory and ab initio molecular dynamics (AIMD). To explicity take account of the temperature, a non-perturbative lattice dynamics method is formalised, allowing to study the evolution of phonons and thermodynamic properties with temperature. In order to reduce the important computational cost of AIMD, a machine-learning based sampling method is developped, which allows to accelerate the simulation of materials at finite temperature. Those different methods are applied to describe the stabilisation of uranium-molybdenum alloy at high temperature, as well as the lattice dynamics of uranium and plutonium dioxides.Pour assurer la sĂ©curitĂ© de la production dâĂ©lectricitĂ© par lâĂ©nergie nuclĂ©aire, une comprĂ©hension du comportement des matĂ©riaux servant de combustibles est nĂ©cessaire. Ce travail apporte une contribution Ă lâĂ©tude des effets de la tempĂ©rature sur les combustibles nuclĂ©aires, en utilisant une approche ab initio Ă travers la thĂ©orie de la fonctionnelle de la densitĂ© et la dynamique molĂ©culaire ab initio (AIMD). Pour prendre en compte explicitement les effets de la tempĂ©rature, une mĂ©thode non perturbative de dynamique des rĂ©seaux est formalisĂ©e, permettant ainsi dâĂ©tudier lâĂ©volution des phonons et des propriĂ©tĂ©s thermodynamiques avec la tempĂ©rature. Afin de rĂ©duire le coĂ»t en temps de calcul important de lâAIMD, une mĂ©thode dâĂ©chantillonnage utilisant le machine-learning est dĂ©veloppĂ©e, ce qui permet dâaccĂ©lĂ©rer les simulations de matĂ©riaux Ă tempĂ©rature finie. Ces diffĂ©rentes mĂ©thodes sont appliquĂ©es pour dĂ©crire la stabilisation de lâalliage uranium-molybdĂšne Ă haute tempĂ©rature, ainsi que la dynamique des rĂ©seaux des dioxydes dâuranium et de plutonium
Ătude des effets de la tempĂ©rature sur les combustibles nuclĂ©aires par une approche ab initio
To ensure the security of nuclear electricity production, an understanding of the behavior of nuclear fuel materials is necessary. This work aims at making a contribution to the study of the effects of temperature on nuclear fuels, by using an ab initio approach through density functional theory and ab initio molecular dynamics (AIMD). To explicity take account of the temperature, a non-perturbative lattice dynamics method is formalised, allowing to study the evolution of phonons and thermodynamic properties with temperature. In order to reduce the important computational cost of AIMD, a machine-learning based sampling method is developped, which allows to accelerate the simulation of materials at finite temperature. Those different methods are applied to describe the stabilisation of uranium-molybdenum alloy at high temperature, as well as the lattice dynamics of uranium and plutonium dioxides.Pour assurer la sĂ©curitĂ© de la production dâĂ©lectricitĂ© par lâĂ©nergie nuclĂ©aire, une comprĂ©hension du comportement des matĂ©riaux servant de combustibles est nĂ©cessaire. Ce travail apporte une contribution Ă lâĂ©tude des effets de la tempĂ©rature sur les combustibles nuclĂ©aires, en utilisant une approche ab initio Ă travers la thĂ©orie de la fonctionnelle de la densitĂ© et la dynamique molĂ©culaire ab initio (AIMD). Pour prendre en compte explicitement les effets de la tempĂ©rature, une mĂ©thode non perturbative de dynamique des rĂ©seaux est formalisĂ©e, permettant ainsi dâĂ©tudier lâĂ©volution des phonons et des propriĂ©tĂ©s thermodynamiques avec la tempĂ©rature. Afin de rĂ©duire le coĂ»t en temps de calcul important de lâAIMD, une mĂ©thode dâĂ©chantillonnage utilisant le machine-learning est dĂ©veloppĂ©e, ce qui permet dâaccĂ©lĂ©rer les simulations de matĂ©riaux Ă tempĂ©rature finie. Ces diffĂ©rentes mĂ©thodes sont appliquĂ©es pour dĂ©crire la stabilisation de lâalliage uranium-molybdĂšne Ă haute tempĂ©rature, ainsi que la dynamique des rĂ©seaux des dioxydes dâuranium et de plutonium
Détermination expérimentale des constantes élastiques de FeRh en fonction de la température et de la phase magnétique
The elastic constants of an epitaxial film of FeRh have been determined experimentally in both ferromagnetic (FM) and antiferromagnetic (AF) phases, using a combination of Brillouin light scattering and picosecond acoustics experiments. The C11 constant is noticeably larger in the FM phase than in the AF phase, while C12 and C44 are both lower, leading to larger Rayleigh wave velocities in the FM phase than in the AF phase. The elastic constants were calculated numerically using first principles anharmonic modeling and machine-learned interatomic potentials. We find that using a temperature-dependent effective potential is indispensable to correctly reproduce the experimental values to within 80 to 100%. The accurate knowledge of the temperature- and phase-dependencies of the elastic constants of crystalline FeRh are valuable ingredients for the predictive modeling of the acoustic and magneto-acoustic properties of this magnetostrictive material