Effect Of Electronic Excitation Research Articles

Phase stability and mechanical response of tantalum nitrides to electronic excitation effect

Based on an analysis ofelectronic structures and phonon spectra calculationsas a function of electronic temperature Te using the density functional theory, we investigated the phase stability and mechanical responseto electronic excitation effect on variousphases of tantalum nitrides TaNx with fully or partially occupied nitrogen sublattices. The results suggest that the cubic phase exhibits lattice instability at room temperature, while the hexagonal phases are dynamically stable. The phase stability increases in the sequence TaN (δ-type), TaN0.43, TaN0.5, TaN (AsNi type), TaN0.83, TaN (θ-type), and TaN (ε-type). The nonmetal vacancies exhibit enhanced latticestability with hexagonal phasescompared to the cubicphase. The electronic excitation effect providedextra stability for cubic δ-TaNwith the increase inelectronic temperature, whilethe excitation results in a lower degree of stability for hexagonal phases.The physical origin of electronic excitation effects on both the cubic and hexagonal phase stability of TaN can be attributed to the peculiarities of their electronicdensities of states near the Fermi level and a combination of three different types of bonding characteristics.The counterintuitivebehavior oflattice stabilities appears or disappears as a response to mechanical properties under electronic excitation.

Channel-closing effects of electronic excitation in solids.

We investigate the electronic excitation of solids in strong fields by solving the time-dependent Schrödinger equation. The excitation probability exhibits a strong modulation as a function of laser intensity when the initial states fill in the whole valence band. To have a clear insight into the modulation, we further study the electronic excitation from a single eigenstate in solids. A series of resonance-like enhancements of excitation probability are produced by changing the laser intensity and wavelength. We attribute the resonance-like enhancements to the channel-closing effects in solids. It is shown that the excitation probability exhibits enhancements when the value of channel is odd for intracycle interference and an integer for intercycle interference. This is different from the atom that the enhancement occur in the integer channels. We also reveal that the channel-closing effects can be observed by solid high-order harmonic generation.

Kohn anomaly and van Hove singularity in IVB and VB group transition metals nitrides and carbides

The superconducting behavior in IVB-VB group transition metal nitrides and carbides has generally been associated with the phonon anomaly and Fermi surface nesting. However, the origin of phonon anomaly has remained ambiguous (i.e. longitudinal acoustic or transverse acoustic modes). We performed first-principles calculations to investigate the phononic properties of these materials and theoretically confirmed that the Kohn anomaly originates from the lower transverse acoustic mode along the ГX direction, thereby revealing the frequency derivative discontinuity of the mode. In particular, the Kohn anomaly region is found to move from the interior to the boundary X point of the Brillouin zone with increasing number of valence electrons. We deduced that the Kohn anomaly originated from the electrons of the filled energy level near the van Hove singularity. These results suggest that the screening of the ionic electric field decreases, while the coupling of conduction electrons with the highly degenerate modes between the TA∥ and LA via Umklapp scattering process increases. The Fermi surface nesting also plays a role in enhancing the superconductivity. The electronic excitation effect induces a stabilization of the V2 group transition metal nitrides.

Applying Tersoff-potential and bond-softening models in a molecular dynamics study of femtosecond laser processing

In the molecular dynamics study of short-pulsed laser processing of semiconductors, potential models capable of describing the atomistic behavior during high electronic excitations is the most critical issue at the current stage. This study of the molecular dynamics adopts the Tersoff-potential model to analyze the ultrafast laser processing of silicon. The model was modified to include electronic excitation effects by reducing the attraction of the antibonding state by half. It offers an excellent description of the experimental behavior during nonthermal melting. Subpicosecond melting is achieved above certain threshold levels of superheating and carrier density as required in experiments. Energy conservation is demonstrated with a bandgap energy of the order obtained in experiments. The modification of the potential mimics an absorption of bandgap energy and a subsequent lattice heating on a time scale within 0.3 ps. The melting kinetics establishes a correlation between nonthermal melting and thermal bulk melting. For superheating of less than two, the electronic melting of bond softening proceeds via homogeneous nucleation. The associated thermal theory, corrected with a limit on the nucleus radius to bond length, is still valid for the higher superheating regime. The original Tersoff model shows that this superheating by a factor of two is isothermal for spallation—the lowest-energy ablative mechanism. Its proximity to the evaporating point suggests the role of thermal boiling during spallation.

Electronically Nonadiabatic Structural Transformations Promoted by Electron Beams

AbstractThe use of scanning transmission electron microscopes to manipulate substitutional defects in graphene has recently been demonstrated and modeled using ground state molecular dynamics, but the role of electronic excitations induced through inelastic electron scattering in promoting these transformations has so‐far remained unexplored. Here, probed are the effects of electronic excitation on the structural dynamics of graphene quantum dots of differing edge morphologies that are substitutionally doped with silicon or phosphorous. The ground and excited state potential energy barriers for pyramidal inversion of these nonplanar doped species are evaluated using time‐dependent density functional theory. Optically bright excited states in which the barrier is decreased are identified in the low energy region of the electronic spectrum, suggesting that photoexcitation can modulate the reactivity of defects in graphene under electron beam irradiation. Coupling matrix elements between these inversion‐favoring excited states and the ground state and time‐domain simulations of the material's response to a point charge impulse indicate that focusing an electron beam near the defect can also lead to population of these states, suggesting that beam electrons incident on a defect can both excite the material to an inversion‐favoring state and transfer momentum to the defect to initiate the inversion.

An alternative approach to predict Seebeck coefficients: Application to La3−xTe4

A thermodynamic understanding of Seebeck coefficient was demonstrated in terms of electrochemical potential. It divided the contributions to the Seebeck coefficient into two contributions: the effect of thermal electronic excitations due to Fermi distribution and the effect of charge carrier gradient due to thermal expansion. The procedure is illustrated within the rigid band approximation in terms of the electronic density-of-states and the quasiharmonic approximation in terms of the phonon density-of-states. Numerical results were given using the n-type high temperature thermoelectric material La3-xTe4 at x = 0, 0.25, and 0.33 as the prototype at a variety of carrier concentrations.

Theoretical study of the electron correlation and excitation effects on energy distribution in photon impact ionization

We performed a detailed theoretical study of the electron correlation and core excitation effects on the energy distribution of the ejected electrons in the process of photon impact tunnel ionization. We used the Landau-Dykhne approach to obtain analytical formulas for the transition rate and the energy distribution with included these effects. We have limited ourselves to a non-relativistic domain, in which the rate and distribution are determined by electrical component of the laser field while the influence of magnetic can be neglected. We observed helium and helium like atoms. We have shown that the tunneling ionization mechanism may be understood as the combination of mentioned processes. We considered the case of a monochromatic wave with an elliptically polarized laser field. We compared our results with experimental and shown that ellipticity plays an important role and that inclusion of additional processes significantly influences the transition rate, as well as the energy distribution of the ejected photoelectrons.

Effective depth of electronic sputtering of WO3 films by high-energy ions

We have measured the electronic sputtering yields of WO3 as a function of the film thickness. WO3 films have been prepared by thermal oxidation of W deposited on MgO substrate and the film thickness is obtained by Rutherford backscattering spectrometry (RBS). It is found that the electronic sputtering yields by 100 MeV Xe and 90 MeV Ni ions steeply increase with increasing the film thickness and the yields reach the maximum value of 2.4 × 104 and 4.7 × 103 at the film thickness (effective depth contributing to the sputtering) of 40 nm for both ions. Assuming that cylindrical volume is eroded, the effective radius is obtained to be 1.6 and 0.7 nm, respectively. One of the mechanisms for the electronic excitation effect, the exciton model will be examined.

Low Temperature Crystallization of Amorphous Materials by Electron Excitation Effects

Low Temperature Crystallization of Amorphous Materials by Electron Excitation Effects

External irradiation with heavy ions of neodymium silicate apatite ceramics and glass-ceramics

This work aims at comparing the damage induced in the Nd silicate apatite ceramic (Ca2Nd8(SiO4)6O2) by medium energy (ME) and swift heavy ion (SHI) irradiations to evaluate the effects of nuclear collisions and intense electronic excitations for ME ions and SHI ions respectively. The macroscopic induced changes were studied as a function of the fluence by swelling and hardness measurements, whereas structural modifications were followed by X-ray diffraction, Raman spectroscopy and grazing incidence EXAFS (Nd L3-edge). At ME (1.9–6.75 MeV Au ions), radiation-induced amorphization occurred above 6.22 × 1013 Au/cm2 associated with a volume expansion of about 8% and a drop of 37% in hardness. At SHI (90 MeV Xe ions or 35 MeV Ar ions), similar macroscopic and structural changes were observed. The electronic stopping power threshold of Nd silicate apatite for amorphization was assessed at about Se = 5 keV/nm. As apatite crystals containing actinides could be present in rare-earths rich nuclear glasses, SHI irradiation with Xe (995 MeV) ions was also used to damage the Nd silicate apatite crystals dispersed in a soda-lime aluminoborosilicate simplified nuclear glass over a depth of about 60 μm, to evaluate the possible formation of cracks in the residual glass due to crystals swelling. In spite of apatite crystals amorphization under SHI irradiation in the glass-ceramics, no cracking was observed in the glassy phase even close to the biggest crystals which could be explained by strain relaxation in the glass due to plastic deformation (creep) induced by SHI ion beam.

Properties of the ground state of electronic excitations in carbon-like nanocones

On the basis of the continuum model for long-wavelength charge carriers, originating in the tight-binding approximation for the nearest-neighbour interaction of atoms in the crystalline lattice, we consider quantum ground-state effects of electronic excitations in Dirac materials with two-dimensional monolayer honeycomb structures warped into nanocones by a disclination; the nonzero size of the disclination is taken into account, and a boundary condition at the edge of the disclination is chosen to ensure self-adjointness of the Dirac–Weyl Hamiltonian operator. We show that the quantum ground-state effects are independent of the disclination size and find circumstances when they are independent of a parameter of the boundary condition. The magnetic flux circulating in the angular direction around the nanocone apex and the pseudomagnetic flux directed orthogonally to the nanocone surface are shown to be induced in the ground state.

Photo-induced lattice contraction in layered materials

Structural and electronic changes induced by optical excitation is a promising technique for functionalization of 2D crystals. Characterizing the effect of excited electronic states on the in-plane covalent bonding network as well as the relatively weaker out-of-plane dispersion interactions is necessary to tune photo-response in these highly anisotropic crystal structures. In-plane atom dynamics was measured using pump-probe experiments and characterized using ab initio simulations, but the effect of electronic excitation on weak out-of-plane van der Waals bonds is less well-studied. We use non-adiabatic quantum molecular dynamics to investigate atomic motion in photoexcited MoS2 bilayers. We observe a strong athermal reduction in the lattice parameter along the out-of-plane direction within 100 fs after electronic excitation, resulting from redistribution of electrons to excited states that have lesser anti-bonding character between layers. This non-trivial behavior of weakly bonded interactions during photoexcitation could have potential applications for modulating properties in materials systems containing non-covalent interactions like layered materials and polymers.

The effect of electronic excitation on London dispersion

Atomic and molecular dispersion coefficients can now be calculated routinely using density-functional theory. In this work, we present the first determination of how electronic excitation affects molecular C6 London dispersion coefficients from the exchange-hole dipole moment (XDM) dispersion model. Excited states are typically found to have larger dispersion coefficients than the corresponding ground states, due to their more diffuse electron densities. A particular focus is both intramolecular and intermolecular charge-transfer excitations, which have high absorbance intensities and are important in organic dyes, light-emitting diodes, and photovoltaics. In these classes of molecules, the increase in C6 for the electron-accepting moiety is largely offset by a decrease in C6 for the electron-donating moiety. As a result, the change in dispersion energy for a chromophore interacting with neighbouring molecules in the condensed phase is minimal.

Electronic excitation induced modifications in the ferroelectric polarization of BiFeO3 thin films

We report the effect of electronic excitations induced modifications in the ferroelectric polarization in BiFeO3 (BFO) multiferroic films grown on 0.2% Nb doped SrTiO3 (SNTO) substrates by pulsed laser deposition. The BFO/SNTO films were irradiated with 200 MeV Ag+15 ions with ion fluences of 5 × 1010 to 5 × 1012 ions/cm2 and characterized by using X–ray diffraction (XRD), atomic force microscopy (AFM), ferroelectric polarization and near-edge X-ray absorption fine structure (NEXAFS) measurements. The XRD and AFM results show the creation of structural defects and oxygen vacancies by ion irradiation. Such defects induced enhancement in structural strain and reduction in mobility of charge carriers leading to the improvement in ferroelectric behaviour in BFO films. In addition, the local electronic structures investigated using NEXAFS studies reveal the significant change in spectral features suggesting the role of Bi-O and Fe-O hybridizations that modifies the ferroelectric behaviour of BFO films. These results are used to understand the mechanism.

Effect of electron excitation in nonsequential double ionization by intense laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 61475168, 11674231, and 61575124), Natural Science Foundation of Shanghai, China (Grant No. 15ZR1430600), and Shanghai Gaofeng & Gaoyuan Project for University Academic Program Development, China.

We study the double ionization process of atoms in intense laser fields. The momentum distributions of the correlated electrons are calculated. Contrary to the general expectation, we show an increasing proportion of the electrons ionized via excitation with the increasing laser intensity. These electrons generally have small energy thus they concentratedly distribute on the central region of the momentum diagram. Consequently, the central part of the momentum diagram becomes more notable in higher intensity laser fields. Further study suggests that this phenomenon is general in double ionization.

Electronic excitation effects on Fe2O3 films by high-energy ions

For better understanding of electronic excitation effects on materials with small bandgap, we have extended measurements to Fe2O3 (hematite) films. Applying carbon-foil (100 nm)-collector method, we have measured amounts of Fe in the C-collector as a function of fluence of 198 MeV Xe, 99 MeV Xe and 89 MeV Ni ions with the equilibrium charge. From the slope of Fe amount vs ion fluence with the efficiency fc of C-collector (0.3), sputtering yields Y are obtained to be 81.7, 57.9 and 38.3 by these ions, assuming stoichiometric sputtering. It appears that the sputtering yields are much larger than the calculated sputtering yields based on the elastic collisions. We have obtained Y = BSen with n = 1.25 and B = 1.2, Se being the electronic stopping powers (keV/nm). Optical direct bandgap is obtained to be 2.6 eV. The present Fe2O3 results are found to be within the bandgap scheme. Also, we have studied modifications of atomic structure using X-ray diffraction and optical absorption. Electronic excitation effects on modifications of these properties are described and compared with low-energy ion, 100 keV Ne ion impact. A mechanism is discussed for electron-lattice coupling.

Sculpting Nanoscale Functional Channels in Complex Oxides Using Energetic Ions and Electrons.

The formation of metastable phases has attracted significant attention because of their unique properties and potential functionalities. In the present study, we demonstrate the phase conversion of energetic-ion-induced amorphous nanochannels/tracks into a metastable defect fluorite in A2B2O7 structured complex oxides by electron irradiation. Through in situ electron irradiation experiments in a scanning transmission electron microscope, we observe electron-induced epitaxial crystallization of the amorphous nanochannels in Yb2Ti2O7 into the defect fluorite. This energetic-electron-induced phase transformation is attributed to the coupled effect of ionization-induced electronic excitations and local heating, along with subthreshold elastic energy transfers. We also show the role of ionic radii of A-site cations (A = Yb, Gd, and Sm) and B-site cations (Ti and Zr) in facilitating the electron-beam-induced crystallization of the amorphous phase to the defect-fluorite structure. The formation of the defect-fluorite structure is eased by the decrease in the difference between ionic radii of A- and B-site cations in the lattice. Molecular dynamics simulations of thermal annealing of the amorphous phase nanochannels in A2B2O7 draw parallels to the electron-irradiation-induced crystallization and confirm the role of ionic radii in lowering the barrier for crystallization. These results suggest that employing guided electron irradiation with atomic precision is a useful technique for selected area phase formation in nanoscale printed devices.

Effect of elastic collisions and electronic excitation on lattice structure of NiTi bulk intermetallic compound irradiated with energetic ions

Abstract NiTi bulk intermetallic compound with the B19′ structure was irradiated with 1 MeV He, 5 MeV Al, 16 MeV Au and 200 MeV Xe ions, and the change in lattice structure near the surface by the ion bombardment was investigated by using the grazing incidence X-ray diffraction (GIXD) and the extended X-ray absorption fine structure (EXAFS). The lattice structure transformation by the irradiation strongly depends on ion species and/or energies. For the 1 MeV He irradiation, the lattice structure changed from B19′ to the A2 structure, but did not show an amorphization even after the high fluence irradiation. For the 5 MeV Al irradiation, the samples are partially amorphized. For the 16 MeV Au irradiation, the lattice structure of the NiTi samples changed nearly completely from the B19′ structure to the amorphous state via the A2 structure. The value of dpa (displacement per atom) which is needed for the amorphization is, however, much smaller than the case of the Al ion irradiation. For the 200 MeV Xe ion irradiation, the lattice structure completely changed to the A2 structure even by a small ion fluence. The dependence of the lattice structure transformation on elastic collisions (dpa), the spectrum of the primary knock-on (PKA) atoms and the density of energy deposited through electronic excitation was discussed.

Modelling radiation effects in solids with two-temperature molecular dynamics

The ability to predict the structural modifications of materials resulting from a broad range of irradiation scenarios would have a positive impact on many fields of science and technology. Established techniques for modelling large atomic systems, such as classical molecular dynamics, are limited by the neglect of the electronic degrees of freedom which restricts their application to irradiation events that primarily interact with atomic nuclei. Ab initio methods, on the other hand, include electronic degrees of freedom, but the requisite computational costs restrict their application to relatively small systems. Recent methodological developments aimed at overcoming some of these limitations are based on methods that couple atomistic models to a continuum model for the electronic energy, where energy is exchanged between the nuclei and electrons via electronic stopping and electron-phonon coupling mechanisms. Such two-temperature molecular dynamics models, as they are known, make it practicable to simulate the effects of electronic excitations on systems with millions, or even hundreds of millions, of atoms. They have been used to study laser irradiation of metallic films, swift heavy ion irradiation of metals and semiconductors, and moderately high ion irradiation of metals.In this review we describe the two-temperature molecular dynamics methodology and the various practical considerations required for its implementation. We provide example applications of the model to multiple irradiation scenarios that accommodate electronic excitations. We also describe the challenges of including the effects of the modification of the interatomic interactions, due to the excitation of electrons, in the simulations and how these challenges can be overcome.

Ion irradiation used as surrogate of neutron irradiation in graphite: Consequences on 14C and 36Cl behavior and structural evolution

Ion irradiation used as surrogate of neutron irradiation in graphite: Consequences on 14C and 36Cl behavior and structural evolution