First-principles calculations of heat capacities of ultrafast laser-excited electrons in metals

Ultrafast laser excitation can induce fast increases of the electronic subsystem temperature. The subsequent electronic evolutions in terms of band structure and energy distribution can determine the change of several thermodynamic properties, including one essential for energy deposition; the electronic heat capacity. Using density functional calculations performed at finite electronic temperatures, the electronic heat capacities dependent on electronic temperatures are obtained for a series of metals, including free electron like, transition and noble metals. The effect of exchange and correlation functionals and the presence of semicore electrons on electronic heat capacities are first evaluated and found to be negligible in most cases. Then, we tested the validity of the free electron approaches, varying the number of free electrons per atom. This shows that only simple metals can be correctly fitted with these approaches. For transition metals, the presence of localized d electrons produces a strong deviation toward high energies of the electronic heat capacities, implying that more energy is needed to thermally excite them, compared to free sp electrons. This is attributed to collective excitation effects strengthened by a change of the electronic screening at high temperature

Plastic ablator and hydrodynamic instabilities: A first-principles set of microscopic coefficients

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Free-electron properties of metals under ultrafast laser-induced electron-phonon nonequilibrium: A first-principles study

International audienceThe electronic behavior of various solid metals (Al, Ni, Cu, Au, Ti, and W) under ultrashort laser irradiation is investigated by means of density functional theory. Successive stages of extreme nonequilibrium on picosecond time scale impact the excited material properties in terms of optical coupling and transport characteristics. As these are generally modelled based on the free-electron classical theory, the free-electron number is a key parameter. However, this parameter remains unclearly defined and dependencies on the electronic temperature are not considered. Here, from first-principles calculations, density of states are obtained with respect to electronic temperatures varying from 10^-2 to 10^5 K within a cold lattice. Based on the concept of localized or delocalized electronic states, temperature dependent free-electron numbers are evaluated for a series of metals covering a large range of electronic configurations. With the increase of the electronic temperature we observe strong adjustments of the electronic structures of transition metals. These are related to variations of electronic occupation in localized d bands, via change in electronic screening and electron-ion effective potential. The electronic temperature dependence of nonequilibrium density of states has consequences on electronic chemical potentials, free-electron numbers, electronic heat capacities, and electronic pressures. Thus electronic thermodynamic properties are computed and discussed, serving as a base to derive energetic and transport properties allowing the description of excitation and relaxation phenomena caused by rapid laser action

Dynamics of femtosecond heated warm dense copper with time-resolved L3-edge XANES

International audienceCombining experimental set up and ab initio molecular dynamics simulations, we were able to follow the time evolution of the X-ray absorption near edge spectrum (XANES) of a dense copper plasma. This provides a deep insight into femtosecond laser interaction with a metallic copper target. This paper presents a review of the experimental developments we made to reduce the X-ray probe duration, from approximately 10 ps to fs duration with table-top laser systems. Moreover, we present microscopic scale simulations, performed with Density Functional Theory, as well as macroscopic simulations considering the Two-Temperature Model. These tools allow us to get a complete picture of the evolution of the target at a microscopic level, from the heating process to the melting and expansion stages, with a clear view of the physics involved during these processes. This article is part of the theme issue ‘Dynamic and transient processes in warm dense matter’

Ab initio determination of transient electronic properties of an ultrafast laser irradiated metal surface. Consequences for LIPSS formation

International audienceNanostructuring features under ultrafast laser excitation of metallic surfaces are strongly influenced by light coupling and the associated material response under conditions of electron-phonon nonequilibrium. This is nowadays imperfectly described with uncertainties on the transient variation of optical and electronic properties during irradiation. In that context, dedicated ab initio calculations were carried out in the framework of the Density Functional Theory to elucidate some of the primary aspects of material response. Ground-state calculations and molecular dynamic simulations have been thus conducted to derive electronic structure and associated transport properties under nonequilibrium conditions. We observe that electronic temperature leads to strong modifications of the electronic screening. This displaces in turn the electronic structure, affecting transient electronic properties such as free electron number, specific heat and thermal pressure. Finally, we evaluate the optical index under different electronic temperatures based on the Kubo-Greenwood formalism. In addition to providing insights into the dynamics of optical response of a metallic surface, these transport properties also shines a new light on a recurring problem concerning periodicity variations of LIPSS (ripples) under ultrashort excitation. Accordingly, the consequences of thermal nonequilibrium on inhomogeneous electric field distribution on a rough metallic surface will be also addressed

Thermodynamic and transport properties of metals under ultrafast laser-induced electron-phonon nonequilibrium

International audienceFemtosecond laser irradiation is able to generate a fast heating of the electronic subsystem, away from ion matrix equilibrium. The induced nonequilibrium state modifies the material response to laser irradiations. Under specific conditions, it governs the nanostructurating of materials surfaces either by localized ablation or by triggering ripples formation,1) with many possible applications in tribology, wettability or anti-counterfeiting domains. Optical properties driving the material responses under intense laser irradiations, directly rely on the electronic structure. Here, based on first-principles calculations performed at finite electronic temperatures, we show how electronic structures react to intense laser irradiations. These are modelled by considering thermalized states of electrons at given finite electronic temperatures. For a series of metals (Al, Ni, Cu, Au, Ti and W), evolution of electronic structures are explained in terms of population or depopulations of localized d bands with consequences on the spatial localization of the charge density that modifies the electronic screening and thus the density of electronic states.2,3)Modifications of the electronic structures impact thermodynamic, optical and transport properties. The electronic chemical potential, free electron numbers, electronic pressure and electron heat capacity are strongly affected by nonequilibrium conditions and consequences will be discussed3). Changes of optical properties are studied on tungsten metal from ab initio molecular dynamic simulations coupled to the Kubo-Greenwood formalism. An increase of the intraband component and an attenuation of the interband signal are observed and lead to modification of plasmon properties, with an important increase of its existence domain, in a way that reconciles experimental observations and theoretical predictions based on electrodynamic models.4)1) J.P. Colombier, F. Garrelie, N. Faure, S. Reynaud, M. Bounhalli, E. Audouard, R. Stoian, and F. Pigeon, J. Appl . Phys. 111, 024902 (2012)2) V. Recoules, J. Clérouin, G. Z ́erah, P.M. Anglade, and S. Mazevet, Phys. Rev. Lett., 96 055503 (2006)3) E. Bévillon, J.P. Colombier, V. Recoules, and R. Stoian, Phys. Rev. B, 89, 115117 (2014)4) A. Y. Vorobyev, and C. Guo, J. Appl. Phys. 104, 063523 (2008

Ab Initio Calculations of Transient Optical Properties of Metals under Femtosecond Laser Irradiation

International audienceUltrashort pulse laser irradiation can generate, under particular conditions, structuring of materials on the nanoscale. In this respect, localized ablation on subdiffraction limit scales or ripples formation have found a particular interest in tribology, wettability or anti-counterfeiting domains. The achievement of nanostructuring is governed by material response under thermal nonequilibrium [1] which still have to be clearly understood. The present work is a first-principles study of simple metal properties (Al, Ni, Cu and Ti) in nonequilibrium conditions. Density functional theory is used in order to perform ground states calculations, response-function calculations and ab initio molecular dynamic simulations. From these calculations, we derive nonequilibrium material properties for optical and thermal models. We also evaluate the stability of materials under irradiation [2] or compute optical index and optical conductivities from the resolution of time-resolved Kubo-Greenwood formalism. [1] J. P. Colombier, et al., J. Appl. Phys. 111, 024902 (2012) [2] V. Recoules, J. Clérouin, G. Zérah, P.M. Anglade and S. Mazevet, PRL 96, 055503, (2006

First-principles calculations of metal properties under ultrashort laser irradiation

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Book of Abstracts,16th International Conference on the Physics of Non-Ideal Plasmas (PNP16)

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