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The role of carbon in the formation of alpha boron at high pressure and high temperature (HPHT) has been investigated by combining HPHT experiments and density functional theory (DFT) calculations. Starting from beta rhombohedral or amorphous boron and amorphous carbon at 5 GPa, the alpha boron phase has been repeatedly observed between 1473 K and 2273 K, at temperatures that are much higher than those reported in the phase diagram of boron. The DFT investigation of the effect of carbon insertion into the alpha rhombohedral boron atomic structure on the formation enthalpy, volume and optical properties shows that only the (B11C) substituted icosahedron defect accounts for the slight volume contraction observed in {\alpha} boron. It is also compatible with the observed red colour of {\alpha} boron crystals, and consistently has a very low formation energy. Our calculations also provide a calibration tool to evaluate the carbon concentration in the samples. Finally, the synthesis temperature is found to be an easy means to tune the carbon concentration on demand.

International audience; In this work, we revisit the DFT-based results for the electron-phonon scattering in highly excited silicon. Using state-of-the-art ab initio methods, we examine the main scattering channels which contribute to the total electron-phonon scattering rate and to the energy loss rate of photoexcited electrons in silicon as well as their temperature dependence. Both temperature dependence and the main scattering channels are shown to strongly differ for the total electron-phonon scattering rate and for the energy loss rate of photoexcited electrons. Whereas the total electron-phonon scattering rate increases strongly with temperature, the temperature dependence of the energy loss rate is negligible. Also, while acoustic phonons dominate the total electron-phonon scattering rate at 300 K, the main contribution to the energy loss rate comes from optical modes.

Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas–Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high- [Formula: see text] dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

International audience; We report a combined experimental and theoretical study of boron carbide under stress/deformation. A special rotating anvil press, the rotating tomography Paris Edinburgh cell (RoToPEC), has been used to apply torsional deformation to boron carbide under a pressure of 5 GPa at ambient temperature. Subsequent damages and point defects have been analysed at ambient pressure by energy dispersive X-ray microdiffraction at the synchrotron and by Raman spectroscopy, combined with calculations based on the density functional theory (DFT). We show that apart from the signals due to B4C, new peaks appear in both characterisation methods. The DFT calculations of atomic structures and phonon frequencies enable us to attribute most of the new peaks to boron vacancies in the intericosahedral chains of boron carbide. Some of the Raman spectra also show three peaks that have been attributed to amorphous boron carbide in the literature. Deformed boron carbide thus shows small inclusions of clusters of boron carbide with chain vacancies, and/or small zones interpreted as amorphous zones

International audience; In this work we present our recent results of the ab initio calculations of anharmonic coupling in cubic semiconductors and bismuth. Our results allow us to explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy in the subterahertz domain, which shows a plateau between 0.6 and 1 THz at low temperatures. The plateau is explained by the competition between different phonon-phonon scattering processes such as Herring's mechanism, which dominates at low frequencies, saturates, and disappears at higher frequencies. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. We predict that the same phenomenon should occur in other cubic semiconductors. In the case of bismuth, we discuss the occurrence of the hydrodynamic heat transport regime at low temperatures, in consistency with the experimental observations. Bismuth is one of the rare materials in which second sound has been experimentally observed. Our calculations predict the occurrence of the Poiseuille phonon flow in Bi between 1.5 K and 3.5 K for sample size of 3.86 mm and 9.06 mm, in consistency with the experimental observations. We will also discuss a Gedanken experiment allowing to assess the occurrence of the hydrodynamic regime in any bulk material.

International audience; Ultrafast relaxation dynamics of hot electrons with excess energies exceeding 1 eV in Si is studied using time-resolved photoemission spectroscopy and ab initio calculations. Experimentally, the photoemission peaks from hot electrons excited in bulk electronic states along the Γ−L and Γ−X directions with excess energy (Eex) 1.1–3.2 eV with respect to the conduction band minimum are identified, and the time constants that characterize the decay of transient populations are determined. The decay time, which is 30±3fs at Eex=3.0eV and increases to 115±5fs at Eex=1.1eV, has the same scaling with Eex irrespective of the location of hot electrons in the Brillouin zone. The calculations show that the momentum scattering time due to electron-phonon coupling is shorter than 10 fs for Eex larger than 1.5 eV, being too short to be measured. The combination of theoretical and experimental results reveals that hot electrons with high excess energy in Si are transformed into hot-electron ensembles quasiequilibrated only in momentum space by the ultrafast momentum scattering, and that the experimentally determined time constant of population decay corresponds to the energy relaxation taking place as a whole on a time scale ten times longer than that of the momentum relaxation. The detailed methodology of the analysis of experimental data which we provide in this work, as well as our conclusions which concern the relaxation dynamics of electrons with Eex exceeding 1 eV in Si, can be applied to interpret hot-carrier relaxation phenomena in a wide range of semiconducting materials.

International audience; The rapid development of the computational methods based on density functional theory, on the one hand, and of the time-energy-and momentum-resolved spectroscopy, on the other hand, allows today an unprecedently detailed insight into the processes governing hot electron relaxation dynamics, and, in particular, into the role of the electron-phonon coupling [1]. Recently, we have developed a computational method, based on density functional theory and on interpolation of the electron-phonon matrix elements in Wannier space, for the calculation of the electron-phonon coupling in polar materials [2]. This method allowed us to successfully interpret the dynamics of hot electron relaxation in bulk GaAs, in excellent agreement with time-and angle-resolved photoemission experiments. We have demonstrated, for the relaxation of hot carriers in GaAs, the existence of two distinct relaxation regimes, one related with the momentum, and the other with energy relaxation [3]. Interestingly, the energy relaxation times become faster at lower energies [4]. In this work, we will present our new results, both experimental and theoretical, on hot electron relaxation in silicon. Numerous additional experiments were performed with respect to the work of [5], and a new interpretation of the measured relaxation times is provided, based on our ab initio calculations and on the concept of hot electron ensembles proposed recently in [3]. Moreover, we will present our recent results, both experimental and theoretical, on the hot electron relaxation and cooling in InSe. InSe is a quasi-2D material which was shown recently to have potential interest for optoelectronics [6]. In this work, we will discuss our new results on the relaxation and cooling dynamics in doped InSe.

International audience; The phase diagram of boron carbide is calculated within the density functional theory as a function of temperature and pressure up to 80 GPa, accounting for icosahedral, graphitelike, and diamondlike atomic structures. Only some icosahedral phases turn out to be thermodynamically stable with atomic carbon concentrations (c) of 8.7% (B10.5C), 13.0% (B6.7C), 20% (B4C), and 28.6% (B2.5C), respectively. Their respective ranges of stability under pressure and temperature are calculated, and the theoretical T-P-c phase diagram boundaries are discussed. At ambient conditions, the introduction in the phase diagram of the new phase B10.5C with an ordered crystalline motif of 414 atoms is shown to bring the theoretical solubility range of carbon in boron close to the experimental one. The link with the experimental phase diagram consisting of one single phase having the R3⎯⎯⎯mspace group is discussed, and the concept of partial occupation of Wyckoff’s site is introduced. At high pressure, the phase diagram is defined by a new carbon-rich phase B2.5C, which is stabilized by both pressure and temperature in our calculations. All of the other diamond and graphite phases reported previously turn out to be thermodynamically unstable in our calculations, although some of them are observed in high pressure experiments.

In the present work we explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy $\ensuremath{\omega}$ in the subterahertz domain. These attenuations along the [100] direction show a plateau between 0.6 and 1 GHz at low temperatures. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. The formation of the plateau is explained by the competition between different phonon-phonon scattering processes such as Herring's mechanism, which dominates at low frequencies, saturates, and disappears. The plateau is shown to be determined by the phononic final-state phase space available at a given temperature. We predict that a change of scattering mechanism should also show up in the attenuation of silicon around 1.2--1.7 THz, and argue that the attenuation plateau is a general feature of cubic semiconductors.

International audience; The rapid development of the computational methods based on density functional theory, on the one hand, and of time-, energy-, and momentum-resolved spectroscopy, on the other hand, allows today an unprecedently detailed insight into the processes governing hot-electron relaxation dynamics, and, in particular, into the role of the electron–phonon coupling. Instead of focusing on the development of a particular method, theoretical or experimental, this review aims to treat the progress in the understanding of the electron–phonon coupling which can be gained from both, on the basis of recently obtained results.We start by defining several regimes of hot electron relaxation via electron–phonon coupling, with respect to the electron excitation energy. We distinguish between energy and momentum relaxation of hot electrons, and summarize, for several semiconductors of the IV and III–V groups, the orders of magnitude of different relaxation times in different regimes, on the basis of known experimental and numerical data. Momentum relaxation times of hot electrons become very short around 1 eV above the bottom of the conduction band, and such ultrafast relaxation mechanisms are measurable only in the most recent pump-probe experiments.Then, we give an overview of the recent progress in the experimental techniques allowing to obtain detailed information on the hot-electron relaxation dynamics, with the main focus on time-, energy-, and momentum-resolved photoemission experiments. The particularities of the experimental approach developed by one of us, which allows to capture time-, energy-, and momentum-resolved hot-electron distributions, as well as to measure momentum relaxation times of the order of 10 fs, are discussed.We further discuss the main advances in the calculation of the electron–phonon scattering times from first principles over the past ten years, in semiconducting materials. Ab initio techniques and efficient interpolation methods provide the possibility to calculate electron–phonon scattering times with high precision at reasonable numerical cost. We highlight the methods of analysis of the obtained numerical results, which allow to give insight into the details of the electron–phonon scattering mechanisms.Finally, we discuss the concept of hot electron ensemble which has been proposed recently to describe the hot-electron relaxation dynamics in GaAs, the applicability of this concept to other materials, and its limitations. We also mention some open problems.

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the $\overline{M}$ valley. Subsequently, the electronic cooling is dictated by Fr\"ohlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\overline{M}$ valley could travel through a multilayer flake of InSe with a lateral size of 1 $\ensuremath{\mu}\mathrm{m}$. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

International audience; We have recently demonstrated that the fast momentum relaxation due to electron-phonon scattering of hot electrons excited into the conduction band of GaAs leads to the formation of hot-electron ensembles spread over the Brillouin zone. In the present work, we study the energy relaxation of hot-electron ensembles in GaAs, by means of ab initio calculations and time-, energy-, and momentum-resolved spectroscopy. We theoretically show that when the temperature decreases, the energy relaxation time ascribed to electron-phonon interaction becomes faster than at ambient temperature and prove that this is indeed the case in the experimental results.

In this work, we examine the pressure and biaxial tensile strain dependence of the intervalley electron–phonon matrix elements in the lowest conduction band of GaAs, GaP, Si and Ge within the density functional perturbation theory. We study both individual transitions and average deformation potential values which can be used as parameters in transport simulations. In the case of a hydrostatic pressure, we draw the general conclusion that the hydrostatic pressure dependence of the intervalley electron–phonon matrix elements can be safely neglected in the interpretation of most commonly studied phenomena. In contrast, the case of a biaxial strain in silicon shows that strain-dependence should be taken into account in calculations of the intervalley electron–phonon matrix elements.

International audience

We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the \ensuremath{\Gamma} valley of the conduction band of GaAs, using time- and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the \ensuremath{\Gamma} valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction.

International audience; Hand-on routes to reduce lattice thermal conductivity (LTC) in bismuth have been explored by employing a combination of Boltzmann's transport equation and ab initio calculations of phonon-phonon interaction within the density functional perturbation theory. We have first obtained the temperature dependence of the bulk LTC in excellent agreement with available experiments. A very accurate microscopic description of heat transport has been achieved and the electronic contribution to thermal conductivity has been determined. By controlling the interplay between phonon-phonon interaction and phonon scattering by sample boundaries, we predict the effect of size reduction for various temperatures and nanostructure shapes. The largest heat transport reduction is obtained in polycrystals with grain sizes smaller than 100 nm.

We generalize the Wannier interpolation of the electron-phonon matrix elements to the case of polar-optical coupling in polar semiconductors. We verify our methodological developments against experiments, by calculating the widths of the electronic bands due to electron-phonon scattering in GaAs, the prototype polar semiconductor. The calculated widths are then used to estimate the broadenings of excitons at critical points in GaAs and the electron-phonon relaxation times of hot electrons. Our findings are in good agreement with available experimental data. Finally, we demonstrate that while the Fr\"ohlich interaction is the dominant scattering process for electrons/holes close to the valley minima, in agreement with low-field transport results, at higher energies, the intervalley scattering dominates the relaxation dynamics of hot electrons or holes. The capability of interpolating the polar-optical coupling opens new perspectives in the calculation of optical absorption and transport properties in semiconductors and thermoelectrics., Comment: To appear on Phys. Rev. B

We use the wave packet propagation approach to study the dynamics of resonant charge transfer in hydrogen back-scattering from Cs adsorbates on Cu(1 1 1). The affinity level of the projectile strongly interacts with an adsorbate-localized state leading to an important modification of the projectile–substrate resonant charge transfer. At very low collision energies (

We present a 'virtual detector' method which allows the extraction of the momentum distributions in continuum states from time-dependent wave packet propagation studies with explicitly time-dependent Hamiltonians. It is based on the approach developed by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741) for quantum chemistry applications and consists in measuring a signal in the asymptotic region. In that sense, it is close to the semiclassical method proposed recently in this journal (Feuerstein and Thumm 2003 J. Phys. B: At. Mol. Opt. Phys. 36 707). The advantage of the present approach is that it is based on a rigorous quantum mechanical treatment, robust, easy to handle and requires a much smaller grid size. The method is illustrated with the 3D example of electron detachment from the H− ion colliding with a free-electron metal surface.

The charge transfer between an H− ion and a free-electron metal surface with a single alkali adsorbate (Li and Cs) is studied with the wave packet propagation approach in the back-scattering geometry. Both the static problem for a fixed projectile–surface distance and the problem of charge transfer during a collision are considered. The three body (projectile–adsorbate–surface) aspect of the charge transfer process in this case results in an avoided crossing between the projectile and adsorbate-localized quasi-stationary states. We analyze and discuss the local effect of the adsorbate on the resonant charge transfer and the applicability of the rate equation approach.

International audience; The carbon-rich boron-carbide (B11C)C-C has been designed from first principles within the density functional theory. With respect to the most common boron carbide at 20% carbon concentration B4C, the structural modification consists in removing boron atoms from the chains linking (B11C) icosahedra. With C-C instead of C-B-C chains, the formation of vacancies is shown to be hindered, leading to enhanced mechanical strength with respect to B4C. The phonon frequencies and elastic constants turn out to prove the stability of the carbon-rich phase, and important fingerprints for its characterization have been identified.

Two kinds of chain defects have been studied in icosahedral boron carbide, with the density functional theory: (i) The boron chain vacancy at the center of the chain, modeled by (B 11 C p )C–V–C, is found to be metastable at low pressure, and to transform to (B 11 C p ) C–C with a discontinuous variation of volume at high pressure. (ii) The formation of a carbon–carbon bond in the defective chain, modeled by (B 11 C p )C–C, is found to be metastable both at low and high pressure. The energy barrier for the formation of a carbon–carbon bond in presence of an interstitial boron atom is shown to be very high.

International audience; An exhaustive study of the neutral structural defects of icosahedral B4C has been performed with the density functional theory. Vacancies have been determined to be boron vacancies in the C-B-C chains. Their presence is shown to yield a discontinuous variation of crystal volume upon increasing pressure, when the formation of a C-C bond occurs in the chains. The dynamical failure of shocked B4C is attributed to the formation of these C-C bonds.

The intervalley scattering time in n-type germanium from thevalley to the L, � ,a ndX valleys, has been computed ab initio with a method based on the density functional perturbation theory. We demonstrate that the pressure dependence of the lifetime of the exciton limited by the electron-phonon interaction is well described. Moreover, we discuss relaxation times measured by various pump-probe experiments at low and ambient temperatures. The contributions of the various phonons to the scattering are computed. Relaxation times due to the electron-phonon coupling are provided for each intervalley transition, as well as their behavior under pressure.

We have combined the Boltzmann transport equation with an {\it ab initio} approach to compute the thermoelectric coefficients of semiconductors. Electron-phonon, ionized impurity, and electron-plasmon scattering rates have been taken into account. The electronic band structure and average intervalley deformation potentials for the electron-phonon coupling are obtained from the density functional theory. The linearized Boltzmann equation has then been solved numerically beyond the relaxation time approximation. Our approach has been applied to crystalline silicon. We present results for the mobility, Seebeck coefficient, and electronic contribution to the thermal conductivity, as a function of the carrier concentration and temperature. The calculated coefficients are in good quantitative agreement with experimental results., 4 figures, 15 pages

International audience; The intrinsic lifetime of the 2p(0) shallow impurity state in silicon doped with P, As, Sb, or Bi has been computed by combining matrix elements of the electron-phonon coupling within the density-functional perturbation theory, with the envelope function approximation for the impurity wave functions. The theoretical lifetime due to the electronic interaction with intervalley phonons has been found to be 1.1 ns for P-doped silicon, and this theoretical limit is much longer than has been previously believed for the last five decades.

International audience; In this work, we focus on the understanding gained from the investigation of the physical properties of boron carbides with theoretical methods based on density functional theory (DFT). Together with the examination of the DFT total energies of various atomic configurations in the unit cell, comparison with the experiments of the theoretical vibrational or NMR spectra has led to the determination of the atomic structure of B4C as C-B-C chains linking mostly B11C icosahedra, and a few percents of B10C2 icosahedra. In the icosahedron, the carbon atom is found to be in the polar site (B4Cp). When there are two carbon atoms, they are found to be in antipodal polar positions. At carbon concentrations other than 20%, we find that only four structural models have a negative formation energy with respect to a formation from alpha-boron + diamond. Moreover, they all have a positive formation energy with respect to B4Cp, showing a tendency to decompose into B4Cp + alpha-boron or B4Cp + diamond. This metastability explains actual difficulties in the synthesis of clean samples, in particular for B13C2. Finally, the idea of combining high hardness and superconductivity in the same material by doping boron-rich solids has emerged. We show results on the strength of the electron-phonon coupling constant obtained with DFT-based methods in B13C2.

International audience; Interactions between excited electrons and short-wavelength (intervalley) phonons are the major source for the broadening of the excitonic line in GaAs under pressure and in GaP. We have studied the intervalley scattering deformation potential using density functional theory for the conduction bands, and density functional perturbation theory for phonon frequencies and for the matrix elements of the electronphonon interaction. We have calculated the electronphonon scattering time due to intervalley scattering t, integrating the scattering probabilities over all possible final states in the Brillouin zone without any ad hoc assumption about the behavior of the electronphonon matrix elements nor the topology of the conduction band. We compare our theoretical results with time-resolved photoluminescence experiment in GaAs, and with spectroellipsometry measurements in GaAs under pressure and in GaP. We show that our ab initio method also allows the evaluation of effective deformation potentials

We propose a fully ab initio approach to calculate electron-phonon scattering times for excited electrons interacting with short-wavelength (intervalley) phonons in semiconductors. Our approach is based on density functional perturbation theory and on the direct integration of electronic scattering probabilities over all possible final states with no ad hoc assumptions. We apply it to the deexcitation of hot electrons in GaAs, and calculate the lifetime of the direct exciton in GaP, both in excellent agreement with experiments. Matrix elements of the electron-phonon coupling, and their dependence on the wave vector of the final state and on the phonon modes, are shown to be crucial ingredients of the evaluation of electron-phonon scattering times.

Interactions between excited electrons and short-wavelength (intervalley) phonons in GaAs are studied using density functional theory for the conduction bands, and density functional perturbation theory for phonon frequencies and matrix elements of the electron–phonon interaction. We have calculated the deformation potentials (DPs) and the average intervalley scattering time 〈τ〉. The integration of the scattering probabilities over all possible final states in the Brillouin zone has been performed without any ad hoc assumption about the behavior of the electron–phonon matrix elements nor the topology of the conduction band. For transitions from the L point to Γ valley (within the first conduction band), we find 〈τ〉L to be 1.5 ps at 300 K, in good agreement with time-resolved photoluminescence experiment. We discuss the difference between our calculated DPs, and effective parameters used in Monte Carlo simulations of optical and transport properties of semiconductors. The latter are based on Conwell’s model, in which electron–phonon interaction is described by one single constant and a parabolic model is used for conduction bands. We deduce the effective DP from our 〈τ〉, and compare it to our calculated DPs. We conclude that only effective DPs obtained from a full calculation of 〈τ〉 are relevant parameters for Monte Carlo simulations.

State-of-the-art ab initio methods have been used to describe the evolution of the $\ensuremath{\Gamma}\text{\ensuremath{-}}X$ intervalley scattering deformation potential (IDP) in gallium arsenide under pressure. We show that both our IDP dispersion and its pressure dependence lead to a decrease of the scattering within the first conduction band. ${D}_{\ensuremath{\Gamma}X}=4.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{\AA{}}$ is our theoretical value of the average IDP for transitions at the direct-to-indirect band gap crossover pressure. We estimate the theoretical average IDP to be $3.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{\AA{}}$ at the pressure where $\ensuremath{\Gamma}\text{\ensuremath{-}}L$ scattering becomes allowed. We propose a model beyond Conwell's one that includes the pressure dependence of the IDP, which we have applied to the broadening of the excitonic line under pressure. Fitting the experimental results of Go\~ni et al. [Phys. Rev. B 41, 10111 (1990)] to our model, we found a satisfactory agreement between the experimental IDP and our theoretical values.

We report on a theoretical study of the ionization process of Xe* Rydberg atoms colliding with a metal surface, in the presence of an external electric field. The evolution of the Xe* outer electron is studied by a wave packet propagation approach, allowing to include all dynamical aspects of the collision, in particular nonadiabatic inter-Rydberg transitions. We investigate how the different Xe* Stark states formed in the external field couple together and ionize on the surface and how the different polarizations of the electronic cloud in the Xe* states are reflected in their ionization properties. We show that the presence of the external electric field can significantly perturb the dynamics of the ionization process. Our results account for recent results from Dunning et al. [Nucl. Inst. Meth. B 203, 69 (2003)]. In particular, it is explained how the external electric field present in the experimental procedure of Dunning et al. leads to the apparent absence of a polarization effect in the ionization process.

Charge transfer during back scattering of a H - ion from a Cu(111) metal surface with Cs adsorbates is studied theoretically within a wave packet propagation approach. We show that the long lifetime of the Cs-localized state in the Cs/Cu(111) system deeply modifies the nature of projectile-surface charge transfer, suppressing its irreversible character. Back and forth electron transfer between the projectile and the adsorbate during the collision results in characteristic oscillations in the H - yield as a function of projectile energy.

The aim of this thesis is to study the influence of the pressure on the optical transitions between multi-determinant ground state and excited states of the NV center from the first-principles.In this work, I study both the neutral NV0 and negatively charged NV- centers.Long-range interactions have a crucial effect in such defects: first, elastic deformations have a long range and need to be accounted for; second, when the defect has a charge, it is important to avoid spurious charge-charge interactions between neighboring supercells caused by the use of periodic boundary conditions. Thus, I study the atomic structure of defect with large supercells by the density functional theory (DFT).The NV center is a deep-center defect, its optical and magnetic properties are related with localized levels in the electronic band-gap. These levels are believed to be built out of the localized orbitals of dangling bonds pointing towards the vacancy, providing strongly correlated electronic states. Thus, an accurate quantum mechanical treatment is needed.DFT is a powerful approach for the calculation of the ground state properties of defects. However, the single Slater determinant nature of the DFT wave function lacks the non-dynamical correlations, that characterize such defects, and does not allow for the calculation of many-body levels. Moreover, exchange and correlation (XC) functionals used in DFT add have a limited accuracy.Therefore, in this PhD work, I first develop a combined DFT + Hubbard model technique. I study the triplet-triplet transition both with the PBE XC functional and the HSE06 one. I confirm that the use of the hybrid XC functional HSE06 improves the description of correlations beyond DFT-PBE and allows for more accurate prediction of optical transitions.An exact diagonalization (or in quantum chemistry language full Configuration Interaction calculations) of the Hubbard Hamiltonian in the many-electron basis constructed of in-gap localized levels, allows to get access to multi-determinant ground and excited states. I benchmark this technique comparing it to the recent state of the art methods.Finally, I apply the developed technique in order to study the effect of the hydrostatic pressure on NV- and NV0 centers. Among many results of my work, I discovered a very interesting effect related to the singlet-singlet transition in the NV-center under hydrostatic pressure. The results I have obtained during my PhD have never been calculated nor observed experimentally. In order to validate the theoretical model, I have compared our results with the measurements that have been obtained by our experimental collaborators for the optical transition in the NV- and NV0. Last but not least, the effect of the electron-phonon interaction was discussed.As a perspective, I developed a new code that can be applied to study other defect systems of interest in the quantum technologies., Ce travail de doctorat a pour objet l’étude, dans le diamant, de l'influence de la pression sur les transitions optiques entres l’état fondamental et les états excités du centre « azote-lacune de carbone » NV, sans paramètre ajustable. Le centre neutre NV0 et le centre chargé négativement NV- ont chacun été étudiés. L’étude a nécessité le développement d’un modèle de Hubbard où les valeurs des interactions sont obtenues sans ajustement sur l’expérience, par une méthode de calcul à partir des principes premiers.Le centre NV est un défaut à niveaux profonds, ses propriétés optiques et magnétiques sont liées aux niveaux sans dispersion dans la bande interdite électronique associés à des états électroniques fortement localisés. Ces niveaux proviennent de combinaisons linéaires d’orbitales localisées correspondant aux quatre liaisons pendantes pointant vers le centre de la lacune et issues des corrélations électroniques fortes. C'est pourquoi un traitement rigoureux, à l’échelle quantique, est nécessaire. La DFT est une approche puissante pour les calculs des propriétés de l'état fondamental des défauts ponctuels. Cependant, les états électroniques en DFT ont un caractère mono-déterminantal : un seul déterminant de Slater intervient, auquel il manque les corrélations non dynamiques. La DFT seule ne permet pas de calculer certains états électroniques à N - corps qui caractérisent les défauts profonds. De plus, les fonctionnelles d'échange et corrélation (FXC) utilisées en DFT ont une précision limitée.C'est pourquoi j’ai d’abord développé une approche combinée modèle d'Hubbard + DFT. La transition triplet-triplet entre l’état fondamental et le premier état triplet excité est étudié à la fois avec la FXC standard GGA-PBE, et la FXC hybride HSE06. Il est montré que l'utilisation de cette dernière améliore la description des corrélations au-delà de la DFT-PB, et permet la prédiction des transitions optiques plus précise. De plus, les interactions à longue portée ont un effet crucial dans la modélisation des défauts profonds: premièrement, les déformations élastiques, dues à la présence d’un atome de nature différente (N) de ceux de la matrice (C), sont à longue portée et doivent être prises en compte; ensuite, quand le défaut est chargé, il est important d'éviter l'interaction non-physique charge-charge entre supermailles voisines, causée par l'utilisation des conditions périodiques aux limites. Par conséquent, j’ai étudié la structure atomique d’un défaut dans de grandes supermailles.La diagonalisation exacte soit, en termes de chimie quantique, le calcul d'interaction de configurations, du Hamiltonian de Hubbard dans la base à plusieurs électrons, construite à partir des niveaux localisé dans la bande interdite, permet d’accéder aux états fondamental et excités multi-configurationels. Cette technique a été comparée aux méthodes récentes de l'état de l'art.La méthode développée est appliquée à l’étude de l'effet de la pression hydrostatique sur les niveaux triplets et singulets du centre NV-, et sur les niveaux doublets et quadruplets du centre NV0. Parmi les nombreux résultats, j'ai découvert un effet très intéressant lié à la transition singulet-singulet sous pression hydrostatique dans le centre NV-. Les résultats obtenus dans ce travail n'ont jamais été ni calculé ni mesurés expérimentalement. Pour valider mon modèle théorique, j'ai comparé mes résultats avec des mesures expérimentales disponibles pour les transitions optiques du centre NV- et du centre NV0 obtenus par nos collaborateurs. Enfin, l'effet de couplage électron-phonon a été discuté.En perspective, j'ai développé un nouveau code de calcul qui peut être utilisé pour étudier d'autres défauts d'interêt dans les technologies quantiques.