Electronic Excitation Contributions Research Articles

α-SpecTrim: A new application designed for reproducing alpha-particle spectra

In this work, we introduce a new software application developed to reproduce α-particle spectra measured with silicon detectors. It takes into consideration the main physical processes involved in the interaction of alpha particles with the source and the detector. This application is considerably more user-friendly than multipurpose codes, being also the runtimes to obtain a spectrum much lower. It makes use of the TRIM simulation code for tracking alpha particles in the source and the detector entrance window. Concerning to the response function of a silicon detector, the contribution of electronic excitation and ionization produced by the alpha particles is also obtained by simulation, while the electron-hole pair statistics and the broadening produced by the amplifier system (electronic chain) are included as convolutions of theoretical functions. The code α-SpecTrim is written in Python 3 language using Flask framework, so it can be run in every PC or laptop. It is free and can be provided if requested to the authors.

Quasi-harmonic thermoelasticity of palladium, platinum, copper, and gold from first principles

We calculate the temperature-dependent elastic constants (ECs) of palladium, platinum, copper and gold within the quasi-harmonic approximation using a first-principles approach and evaluating numerically the second derivatives of the Helmholtz free-energy with respect to strain at the minimum of the free-energy itself. We find an overall good agreement with the experimental data although the anomalies of palladium and platinum reported at room temperature are not reproduced. The contribution of electronic excitations is also investigated: we find that it is non-negligible for the C 44 ECs of palladium and platinum while it is irrelevant in the other cases. Its effect is not sufficient to explain the details of the anomalies found by experiments, not even when, in the case of platinum, we take into account the electron–phonon interaction. Lastly, the effect of the exchange and correlation functional is addressed and it is found that it is important at T = 0 K, while all functionals give similar temperature dependencies.

Plasmons in graphene traversed by relativistic electronic beam with arbitrary trajectories

SynopsisIn this work we study the energy loss of a beam of relativistic electrons impinging on a graphene sheet under oblique incidence. Using several models for the electric conductivity of graphene, we cover a wide range of frequencies of the energy loss spectrum, from the terahertz to the ultraviolet. To describe this spectrum, we consider the contribution of electronic excitations as well as that of transition radiation.

In-situ kinetics of modifications induced by swift heavy ions in Al2O3: Colour centre formation, structural modification and amorphization

This paper details in-situ studies of modifications induced by swift heavy ion irradiation in α-Al2O3. This complex behaviour is intermediary between the behaviour of amorphizable and non-amorphizable materials, respectively. A unique combination of irradiation experiments was performed at the IRRSUD beam line of the GANIL facility, with three different characterisation techniques: in-situ UV–Vis absorption, in-situ grazing incidence X-Ray diffraction and ex-situ transmission electron microscopy. This allows a complete study of point defects, and by depth profile of structural and microstructural modifications created on the trajectory of the incident ion. The α-Al2O3 crystals have been irradiated by 92 MeV Xenon and 74 MeV Krypton ions, the irradiation conditions have been chosen rather similar with an energy range where the ratio between electronic and nuclear stopping power changes dramatically as function of depth penetration. The main contribution of electronic excitation, above the threshold for track formation, is present beneath the surface to finally get almost only elastic collisions at the end of the projected range. Amorphization kinetics by the overlapping of multiple ion tracks is observed. In the crystalline matrix, long range strains, unit-cell swelling, local microstrain, domain size decrease, disordering of oxygen sublattice as well as colour centre formation are found. This study highlights the relationship between ion energy losses into a material and its response. While amorphization requires electronic stopping values above a certain threshold, point defects are predominantly induced by elastic collisions, while some structural modifications of the crystalline matrix, such as unit-cell swelling, are due to contribution of both electronic and nuclear processes.

Influence of phonon and electron excitations on the free energy of defect clusters in solids: A first-principles study

Although many processes of nanostructure evolution in solids occur at elevated temperatures, basic data obtained from ground state energetics are used in the modeling of these phenomena. In order to illustrate the effect of phonon and electron excitations on the free binding energy of defect clusters, first-principles calculations are performed for vacancy-solute pairs as well as vacancy and Cu dimers, trimers, and quadromers in bcc Fe. Based on the equilibrium atomic positions determined by the relaxation of the supercell with the defect in the ground state under constant volume (CV) as well as zero pressure (ZP) conditions, the contribution of phonon excitations to the free binding energy is calculated within the framework of the harmonic approximation. The contribution of electron excitations is obtained using the corresponding ground state data for the electronic density of states. Quasi-harmonic corrections to the ZP-based results do not yield significant changes in the temperature range relevant for applications. At 1000K the maximum decrease/increase of the ZP-based data for the absolute value of the free binding energy with respect to the corresponding ground state value is found for the vacancy-W (43%)/vacancy-Mn (35%) pair. These results clearly demonstrate that contributions of phonon and electron excitation to the free binding energy of the defect clusters are generally not negligible. The general behavior of the free binding energy of vacancy and Cu dimers, trimers and quadromers is similar to that of the vacancy-solute pairs. The results obtained in this work are of general importance for studies on the thermodynamics and kinetics of defect clusters in solids.

Contribution of electronic excitation to the structural evolution of ultrafast laser-irradiated tungsten nanofilms

The redistribution of the electron density in a material during laser irradiation can have a significant impact on its structural dynamics. This electronic excitation can be incorporated into two temperature molecular dynamics (2T-MD) simulations through the use of electronic temperature dependent potentials. Here, we study the structural dynamics of laser irradiated tungsten nanofilms using 2T-MD simulations with an electronic temperature dependent potential and compare the results to equivalent simulations that employ a ground-state interatomic potential. Electronic excitation leads to an expansion of the crystal and a decrease in the melting point of tungsten. During laser irradiation these factors ensure that the threshold fluences to the different melting regimes are reduced. Furthermore, both heterogenous and homogeneous melting are predicted to occur more rapidly due to excitation and oscillations in the film thickness will be accentuated.

MeV-energy Xe ion-induced damage in LiF: The contribution of electronic and nuclear stopping mechanisms

The contribution of electronic and nuclear damage mechanisms in the modification of structure and micromechanical properties of LiF crystals irradiated with 52, 224, and 450 MeV Xe ions at fluences 1010–1014 ions cm−2 has been studied. The ion-induced formation of dislocations and hardening in LiF at fluences above 1010 ions cm−2 has been observed. The depth profiles of nanoindentation show a joint contribution of electronic excitation and nuclear (impact) mechanisms to the ion-induced hardening. The electronic excitation mechanism dominates in the major part of the ion range while the impact mechanism prevails in a narrow zone at the end of the ion range. The efficiency of hardening produced by electronic excitations is by one order of magnitude higher than that produced by the impact mechanism. Ion-induced prismatic dislocation loops are the main cause of the hardening. At an electronic energy loss above the threshold of 10 keV nm−1, an ordered dislocation structure is created. HRXRD measurements on heavily irradiated LiF crystals (at fluences above 1012 Xe cm−2) reveal a mosaic type substructure composed of nanosize domains tilted under low angles.

First-principles calculation of defect free energies: General aspects illustrated in the case of bcc Fe

The first-principles calculation of contributions of phonon and electron excitations to free formation, binding, and migration energies of defects is illustrated in the case of bcc-Fe. First of all, the ground state properties of the vacancy, the foreign atoms Cu, Y, Ti, Cr, Mn, Ni, V, Mo, Si, Al, Co, O, and the O-vacancy pair are determined under constant volume (CV) as well as zero pressure (ZP) conditions. Second, the phonon contribution to defect free energies is calculated within the harmonic approximation using the equilibrium atomic positions determined in the ground state under CV and ZP conditions. Additionally, quasi-harmonic corrections are applied to the ZP-based data. A simple transformation similar to the quasi-harmonic approach is found between the CV- and ZP-based frequencies. Therefore, it is not necessary to calculate these quantities and the corresponding defect free energies separately. In contrast to ground state energetics the CV- and ZP-based defect free energies do not become equal with increasing supercell size. Third, it was found that the contribution of electron excitations to the defect free energy can lead to an additional deviation of the total free energy from the ground state value or can compensate the deviation caused by the phonon contribution. Finally, self-diffusion via the vacancy mechanism is investigated. The ratio of the respective CV- and ZP-based results for the vacancy diffusivity is nearly equal to the reciprocal of that for the equilibrium concentration. This behaviour leads to almost identical CV- and ZP-based values for the self-diffusion coefficient. Obviously, this agreement is accidental. The consideration of the temperature dependence of the magnetization yields self-diffusion data in very good agreement with experiments.

Depth profiles of indentation hardness and dislocation mobility in MgO single crystals irradiated with swift 84Kr and 14N ions

The depth dependence of damage and modification of micromechanical properties in MgO single crystals irradiated with 150 MeV 84Kr and 24.5 MeV 14N ions (specific energy 1.75 MeV/u) at fluences up to 1015 ions/cm2 has been studied. The effects of ion-induced increase in hardness and reduction in dislocation mobility, magnitude of which varies along the ion range, were observed. These effects are related to ion-induced dislocations which were revealed by chemical etching. The results confirm a joint contribution of electronic excitations and atomic displacements by elastic collisions in the structural damage of MgO. The excitation mechanism in hardening dominates in the incoming part of ion range (up to the Bragg’s maximum), while the role of impact mechanism becomes dominant only at the end of ion range.

Electron transport and nonlinear optical properties of substituted aryldimesityl boranes: a DFT study.

A comprehensive theoretical study was carried out on a series of aryldimesityl borane (DMB) derivatives using Density Functional theory. Optimized geometries and electronic parameters like electron affinity, reorganization energy, frontiers molecular contours, polarizability and hyperpolarizability have been calculated by employing B3PW91/6-311++G (d, p) level of theory. Our results show that the Hammett function and geometrical parameters correlates well with the reorganization energies and hyperpolarizability for the series of DMB derivatives studied in this work. The orbital energy study reveals that the electron releasing substituents increase the LUMO energies and electron withdrawing substituents decrease the LUMO energies, reflecting the electron transport character of aryldimesityl borane derivatives. From frontier molecular orbitals diagram it is evident that mesityl rings act as the donor, while the phenylene and Boron atom appear as acceptors in these systems. The calculated hyperpolarizability of secondary amine derivative of DMB is 40 times higher than DMB (1). The electronic excitation contributions to the hyperpolarizability studied by using TDDFT calculation shows that hyperpolarizability correlates well with dipole moment in ground and excited state and excitation energy in terms of the two-level model. Thus the results of these calculations can be helpful in designing the DMB derivatives for efficient electron transport and nonlinear optical material by appropriate substitution with electron releasing or withdrawing substituents on phenyl ring of DMB system.

First-principles thermal equation of state and thermoelasticity of hcp Fe at high pressures

We investigate the equation of state and elastic properties of hcp iron at high pressures and high temperatures using first principles linear response linear-muffin-tin-orbital method in the generalized-gradient approximation. We calculate the Helmholtz free energy as a function of volume, temperature, and volume-conserving strains, including the electronic excitation contributions from band structures and lattice vibrational contributions from quasi-harmonic lattice dynamics. We perform detailed investigations on the behavior of elastic moduli and equation of state properties as functions of temperature and pressure, including the pressure-volume equation of state, bulk modulus, the thermal expansion coefficient, the Gruneisen ratio, and the shock Hugoniot. Detailed comparison has been made with available experimental measurements and theoretical predictions.

Role played by the organometallic fragment on the first hyperpolarizability of iron–acetylide complexes: A TD-DFT study

The static first hyperpolarizabilities ( β) for a series of both substituted thiophene-acetylide ligands and the corresponding η 5–monocyclopentadienyliron(II) complexes were determined by density functional theory (DFT) calculations. The effect on the hyperpolarizabilities by various donor and acceptor substituents in the thiophene-acetylide ligands was studied. The nature and role of the electronic excitation contributions to the first hyperpolarizability, using time-dependent DFT (TD-DFT) calculations, are rationalized in terms of the two-level model. Our calculations show that the organometallic fragment can form a very effective push-pull system in combination with electron-withdrawing substituents in the thiophene-acetylide moiety, leading to enhanced static first hyperpolarizabilities. Also, an improvement of the magnitude of β is expected if solvation effects are taken into account.

First-principles thermoelasticity of bcc iron under pressure

We investigate the elastic and isotropic aggregate properties of ferromagnetic bcc iron as a function of temperature and pressure by computing the Helmholtz free energies for the volume-conserving strained structures using the first-principles linear response linear-muffin-tin-orbital method and the generalized-gradient approximation. We include the electronic excitation contributions to the free energy from the band structures, and phonon contributions from quasiharmonic lattice dynamics. We make detailed comparisons between our calculated elastic moduli and their temperature and pressure dependences with available experimental and theoretical data. The isotropic aggregate sound velocities obtained based on the calculated elastic moduli agree with available ultrasonic and diamond-anvil-cell data. Birch's law, which assumes a linear increase in sound velocity with increasing atomic density, fails for bcc Fe under extreme conditions. First-principles linear-response lattice dynamics is shown to provide a tractable approach to examine the elasticity of transition metals at high pressures and high temperatures.

NLO Properties of Metallabenzene-Based Chromophores: A Time-Dependent Density Functional Study

The static and dynamic first hyperpolarizabilities for a series of substituted metallabenzene-based nonlinear optical (NLO) chromophores were determined by time-dependent density functional theory (TDDFT). The electronic excitation contributions to the first hyperpolarizability are rationalized in terms of the two-level model. The effects on the hyperpolarizabilities of (a) the metal center (Os, Ir, Pt); (b) the ligand environment (PH3, CO, Cl); (c) various donor and acceptor substituents (NH2, OH, Me, H, Cl, Br, I, COOMe, COOH, CN, NO2); and (d) the length of pi-conjugation were studied. Our calculations predict that metallabenzenes have significant second-order NLO susceptibilities, ranging from = 1.0 x 10(-29) to 5.6 x 10(-28) esu and from mu = 3.0 x 10(-47) to 1.1 x 10(-44) esu, that can be tuned by changing the metal center and/or ligand environment.

Equation of state of condensed matter in laser-induced high-pressure regime

We have simulated the shock Hugoniot of copper and uranium based on the results of first principles electronic structure calculations. The room temperature isotherm has been obtained by evaluating the accurate ground state total energies at various compressions, and the thermal and electronic excitation contributions were obtained by adopting isotropic models using the results obtained by the band structure calculations. Our calculations ensure smooth consideration of pressure ionization effects as the relevant core states are treated in the semi-core form at the ambient pressure. The pressure variation of the electronic Grüneisen parameter was estimated for copper using the band structure results, which leads to good agreement of the simulated shock Hugoniot with the measured shock data. The simulation results obtained for U are also compared with the experimental data available in literature and with our own data.

Defect production induced by electronic excitation in iron

The contribution of high-density electronic excitation to defect production in iron has been studied. Irradiations of iron thin films with several kinds of particles in the wide energy and mass ranges are performed at ∼77 K systematically, and the electrical resistivity change is measured in situ as a function of particle fluence. For each irradiation, defect production cross-section is derived from the resistivity change rate. From the experimental data, we extract a contribution of electronic excitation to the defect production cross-section. The primary ionization rate well describes the defect production in iron induced by high-density electronic excitation.

High-pressure thermoelasticity of body-centered-cubic tantalum

We have investigated the thermoelasticity of body-centered cubic (bcc) tantalum from first principles by using the linearized augmented plane wave (LAPW) and mixed--basis pseudopotential methods for pressures up to 400 GPa and temperatures up to 10000 K. Electronic excitation contributions to the free energy were included from the band structures, and phonon contributions were included using the particle-in-a-cell (PIC) model. The computed elastic constants agree well with available ultrasonic and diamond anvil cell data at low pressures, and shock data at high pressures. The shear modulus $c_{44}$ and the anisotropy change behavior with increasing pressure around 150 GPa because of an electronic topological transition. We find that the main contribution of temperature to the elastic constants is from the thermal expansivity. The PIC model in conjunction with fast self-consistent techniques is shown to be a tractable approach to studying thermoelasticity.

Electronic and phonon properties of six crystalline phases of Pu metal

In the light actinides Th to Np, and in the low-temperature {alpha} phase of Pu, the 5f electrons are itinerant, while in the five higher-temperature phases of Pu, and in the heavy actinides from Am on, the 5f electrons are localized to varying degrees. To understand the physics of 5f localization is a current problem in electronic-structure theory. The kind of experimental data that can help in solving this problem is not available. In this paper, thermodynamic data for Pu metal are analyzed to extract the quantities important for electronic-structure theory. For each of the six crystal phases, the following quantities are evaluated: (a) the static lattice potential, which is the ground-state energy of electronic-structure calculations, (b) central moments of the phonon distribution, which can be compared to any theoretical calculation of phonon frequencies, and (c) a measure of combined anharmonic and electronic excitation contributions to entropy, which contains information on the electronic density of states. Though at present we are not able to separate anharmonic and electronic excitation effects, the last named quantity will help to do so in the future. {copyright} {ital 1998} {ital The American Physical Society}

Contribution of electronic excitations to the entropy of crystals: The relative stabilities of the hcp, fcc, and bcc structures among the transition metals

The entropy contributions to the relative stability of one crystal structure versus another due to electronic excitations are calculated in lowest order. While the electronic contribution is but a small part of the total entropy of a crystal, the change in these terms with change in structure is as large as the entropy changes inferred from experiment. One will not have a complete accounting of such entropy effects until there are detailed estimates of the phonon contributions. However, the present results offer the possibility of dividing the experimental values into electronic and phonon terms and attributing to each the temperature dependence of their leading terms when doing thermodynamic calculations.

Thermodynamic properties of fluid sodium from molecular dynamics

The high-temperature thermodynamic properties of metallic fluid sodium were calculated from melting to a temperature $T=2200$ K. A pseudopotential model was used that consisted of a large volume-dependent potential plus a small effective two-body potential. Using molecular-dynamics calculations for the ionic motions and including electronic excitation contributions, we have determined the zero-pressure volume versus temperature curve, the adiabatic bulk modulus, the heat capacities at constant pressure and volume, and the thermodynamic Gr\uneisen parameter. We find our theory to be in excellent agreement with experiment up to $T=1200$ K, but note a significant deviation at higher temperatures.