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U. Gerstmann - One of the best experts on this subject based on the ideXlab platform.
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Nitrogen split Interstitial center (N-N)(N) in GaN: High frequency EPR and ENDOR study
Physical Review B: Condensed Matter and Materials Physics, 2014Co-Authors: H. J. Bardeleben, E. Rauls, J. L. Cantin, H. Vrielinck, F. Callens, L. Binet, U. GerstmannAbstract:The nitrogen split Interstitial Defect introduced by high-energy particle irradiation in n-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and Q-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the g-tensor anisotropy, which is not resolved at X or Q band. The good agreement of the principal values g(xx) = 1.9966, g(yy) = 2.0016, and g(zz) = 2.0036 with the theoretically predicted g tensor confirm the (N-N)(N)(0) Defect model. The hyperfine interactions of this Defect have been studied byQ-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20-90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split Interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon Interstitials is also the split Interstitial configuration.
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Identification of the Nitrogen Split Interstitial
Physical Review Letters, 2012Co-Authors: H. J. Von Bardeleben, J. L. Cantin, U. Gerstmann, A. Scholle, S. Greulich-weber, E. Rauls, M. Landmann, W. G. Schmidt, A. Gentils, J. BotsoaAbstract:Combining electron paramagnetic resonance, density functional theory, and positron annihilation spectroscopy (PAS), we identify the nitrogen Interstitial Defect in GaN. The isolated Interstitial is unstable and transforms into a split Interstitial configuration (N-N)N. It is generated by particle irradiation with an introduction rate of a primary Defect, pins the Fermi level at EC-1.0 eV for high fluences, and anneals out at 400 °C. The associated Defect, the nitrogen vacancy, is observed by PAS only in the initial stage of irradiation.
Adam Gali - One of the best experts on this subject based on the ideXlab platform.
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characterization of the nitrogen split Interstitial Defect in wurtzite aluminum nitride using density functional theory
Journal of Applied Physics, 2014Co-Authors: Attila Szallas, Krisztian Szasz, Xuan Thang Trinh, N T Son, Erik Janzen, Adam GaliAbstract:We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane wave supercell calculations in wurtzite aluminum nitride in order to characterize the geometry, formation energies, transition levels, and hyperfine tensors of the nitrogen split Interstitial Defect. The calculated hyperfine tensors may provide useful fingerprint of this Defect for electron paramagnetic resonance measurement.
Sergei L. Dudarev - One of the best experts on this subject based on the ideXlab platform.
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Free energy of a ⟨110⟩ dumbbell Interstitial Defect in bcc Fe: Harmonic and anharmonic contributions
Physical Review B, 2009Co-Authors: S. Chiesa, Peter M. Derlet, Sergei L. DudarevAbstract:The stability of Interstitial Defect and dislocation structures in bcc Fe as a function of temperature is believed to play a crucial role in determining Defect evolution under irradiation conditions. The vibrational properties of Defects constitute one contribution to the corresponding energetics and much work has been done within the harmonic approximation to determine the vibrational formation free energy and formation entropy of such Defects. Defects can however exhibit strong local strain fields that break the cubic symmetry of the bcc lattice leading to large anharmonicities and a breakdown of the harmonic approximation as an accurate means to calculate vibrational thermodynamic quantities. Moreover, if Defect diffusion is active at a time scale comparable to an atomic vibration, strong anharmonicities will always be present at any finite temperature. The current work investigates the vibrational free energy and entropy of the $⟨110⟩$ self-Interstitial dumbbell Defect in bcc Fe using both harmonic and anharmonic free-energy calculation methods for a range of modern empirical potentials. It is found that depending on the empirical potential and for temperatures where diffusion is limited, the harmonic approximation is justified especially for empirical potentials that have been fitted to third-order elastic constants. The unique applicability range of such calculations for bcc Fe is also discussed given that with rising temperature spin fluctuations become increasingly important ultimately leading to a softening of the 110 shear modulus and to the $\ensuremath{\alpha}\text{-bcc}/\ensuremath{\gamma}\text{-bcc}$ structural phase transformation.
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free energy of a 110 dumbbell Interstitial Defect in bcc fe harmonic and anharmonic contributions
Physical Review B, 2009Co-Authors: S. Chiesa, Peter M. Derlet, Sergei L. DudarevAbstract:The stability of Interstitial Defect and dislocation structures in bcc Fe as a function of temperature is believed to play a crucial role in determining Defect evolution under irradiation conditions. The vibrational properties of Defects constitute one contribution to the corresponding energetics and much work has been done within the harmonic approximation to determine the vibrational formation free energy and formation entropy of such Defects. Defects can however exhibit strong local strain fields that break the cubic symmetry of the bcc lattice leading to large anharmonicities and a breakdown of the harmonic approximation as an accurate means to calculate vibrational thermodynamic quantities. Moreover, if Defect diffusion is active at a time scale comparable to an atomic vibration, strong anharmonicities will always be present at any finite temperature. The current work investigates the vibrational free energy and entropy of the $⟨110⟩$ self-Interstitial dumbbell Defect in bcc Fe using both harmonic and anharmonic free-energy calculation methods for a range of modern empirical potentials. It is found that depending on the empirical potential and for temperatures where diffusion is limited, the harmonic approximation is justified especially for empirical potentials that have been fitted to third-order elastic constants. The unique applicability range of such calculations for bcc Fe is also discussed given that with rising temperature spin fluctuations become increasingly important ultimately leading to a softening of the 110 shear modulus and to the $\ensuremath{\alpha}\text{-bcc}/\ensuremath{\gamma}\text{-bcc}$ structural phase transformation.
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The non-Arrhenius migration of Interstitial Defects in bcc transition metals
Comptes Rendus Physique, 2008Co-Authors: Sergei L. DudarevAbstract:Abstract Thermally activated migration of Defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-Interstitial Defects, and particularly self-Interstitial atom clusters, is very low. In this case a trajectory of a Defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a Defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual Interstitial Defects, as well as clusters of Interstitial Defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of Interstitial Defects and Interstitial Defect clusters is associated with the interaction between a Defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).
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Materials subjected to fast neutron irradiation/Matériaux soumis à irradiation par neutrons rapides The non-Arrhenius migration of Interstitial Defects in bcc transition metals
2008Co-Authors: Sergei L. DudarevAbstract:Thermally activated migration of Defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-Interstitial Defects, and particularly self-Interstitial atom clusters, is very low. In this case a trajectory of a Defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a Defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual Interstitial Defects, as well as clusters of Interstitial Defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of Interstitial Defects and Interstitial Defect clusters is associated with the interaction between a Defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).
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Thermal mobility of Interstitial Defects in irradiated materials
Physical Review B, 2002Co-Authors: Sergei L. DudarevAbstract:Thermally activated mobility of clusters of Interstitial atoms is an important factor driving microstructural evolution of materials under irradiation. Molecular dynamics simulations show that the statistics of one-dimensional Brownian motion of clusters is characterized by unusual correlated jumps spanning many interatomic distances. We use the Frenkel-Kontorova model to investigate the dynamics of one-dimensional Brownian motion of a spatially delocalized Interstitial Defect interacting with acoustic phonon excitations. Using a quantum-mechanical approach, we evaluate the coefficient of dissipative friction characterizing the stochastic motion of the Defect. We show that the origin of unusual features observed in atomistic simulations is associated with low friction experienced by an Interstitial Defect propagating through the crystal lattice in the presence of thermal fluctuations. We also find that the coefficient of dissipative friction is highly sensitive to the character of interatomic bonding in the material.
S. Dudarev - One of the best experts on this subject based on the ideXlab platform.
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Multiscale modelling of the interaction of hydrogen with Interstitial Defects and dislocations in BCC tungsten
Nuclear Fusion, 2018Co-Authors: A. De Backer, M.-c. Marinica, D. Mason, C. Domain, D. Nguyen-manh, L. Ventelon, C.s. Becquart, S. DudarevAbstract:In a fusion tokamak, the plasma of hydrogen isotopes is in contact with tungsten at the surface of a divertor. In the bulk of the material, the hydrogen concentration profile tends towards dynamic equilibrium between the flux of incident ions and their trapping and release from Defects, either native or produced by ion and neutron irradiation. The dynamics of hydrogen exchange between the plasma and the material is controlled by pressure, temperature, and also by the energy barriers characterizing hydrogen diffusion in the material, trapping and de-trapping from Defects. In this work, we extend the treatment of interaction of hydrogen with vacancy-type Defects, and investigate how hydrogen is trapped by self-Interstitial atom Defects and dislocations. The accumulation of hydrogen on dislocation loops and dislocations is assessed using a combination of density functional theory (DFT), molecular dynamics with empirical potentials, and linear elasticity theory. The equilibrium configurations adopted by hydrogen atoms in the core of dislocations as well as in the elastic fields of Defects, are modelled by DFT. The structure of the resulting configurations can be rationalised assuming that hydrogen atoms interact elastically with lattice distortions and that they interact between themselves through short-range repulsion. We formulate a two-shell model for hydrogen interaction with an Interstitial Defect of any size, which predicts how hydrogen accumulates at Defects, dislocation loops and line dislocations at a finite temperature. We derive analytical formulae for the number of hydrogen atoms forming the Cottrell atmosphere of a mesoscopic dislocation loop or an edge dislocation. The solubility of hydrogen as a function of temperature, pressure and the density of dislocations exhibits three physically distinct regimes, dominated by the solubility of hydrogen in a perfect lattice, its retention at dislocation cores, and trapping by long-range elastic fields of dislocations.
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ab initio scaling laws for the formation energy of nanosized Interstitial Defect clusters in iron tungsten and vanadium
Physical Review B, 2016Co-Authors: R. Alexander, Mihaicosmin Marinica, L. Proville, F. Willaime, K. Arakawa, M R Gilbert, S. DudarevAbstract:The size limitation of ab initio calculations impedes first-principles simulations of crystal Defects at nanometer sizes. Considering clusters of self-Interstitial atoms as a paradigm for such crystal Defects, we have developed an ab initio\char21{}accuracy model to predict formation energies of Defect clusters with various geometries and sizes. Our discrete-continuum model combines the discrete nature of energetics of Interstitial clusters and continuum elasticity for a crystalline solid matrix. The model is then applied to Interstitial dislocation loops with $\ensuremath{\langle}100\ensuremath{\rangle}$ and $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ Burgers vectors, and to C15 clusters in body-centered-cubic crystals Fe, W, and V, to determine their relative stabilities as a function of size. We find that in Fe the C15 clusters were more stable than dislocation loops if the number of self-Interstitial atoms involved was fewer than 51, which corresponds to a C15 cluster with a diameter of $1.5$ nm. In V and W, the $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ loops represent the most stable configurations for all Defect sizes, which is at odds with predictions derived from simulations performed using some empirical interatomic potentials. Further, the formation energies predicted by the discrete-continuum model are reparametrized by a simple analytical expression giving the formation energy of self-Interstitial clusters as a function of their size. The analytical scaling laws are valid over a very broad range of Defect sizes, and they can be used in multiscale techniques including kinetic Monte Carlo simulations and cluster dynamics or dislocation dynamics studies.
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Ab initio scaling laws for the formation energy of nanosized Interstitial Defect clusters in iron, tungsten, and vanadium
Physical Review B: Condensed Matter and Materials Physics, 2016Co-Authors: R. Alexander, M.-c. Marinica, L. Proville, F. Willaime, K. Arakawa, M. Gilbert, S. DudarevAbstract:The size limitation of ab initio calculations impedes first-principles simulations of crystal Defects at nanometer sizes. Considering clusters of self-Interstitial atoms as a paradigm for such crystal Defects, we have developed an ab initio–accuracy model to predict formation energies of Defect clusters with various geometries and sizes. Our discrete-continuum model combines the discrete nature of energetics of Interstitial clusters and continuum elasticity for a crystalline solid matrix. The model is then applied to Interstitial dislocation loops with (100) and 1/2(111) Burgers vectors, and to C15 clusters in body-centered-cubic crystals Fe, W, and V, to determine their relative stabilities as a function of size. We find that in Fe the C15 clusters were more stable than dislocation loops if the number of self-Interstitial atoms involved was fewer than 51, which corresponds to a C15 cluster with a diameter of 1.5nm. In V and W, the 1/2(111) loops represent the most stable configurations for all Defect sizes, which is at odds with predictions derived from simulations performed using some empirical interatomic potentials. Further, the formation energies predicted by the discrete-continuum model are reparametrized by a simple analytical expression giving the formation energy of self-Interstitial clusters as a function of their size. The analytical scaling laws are valid over a very broad range of Defect sizes, and they can be used in multiscale techniques including kinetic Monte Carlo simulations and cluster dynamics or dislocation dynamics studies.
E. Rauls - One of the best experts on this subject based on the ideXlab platform.
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Nitrogen split Interstitial center (N-N)(N) in GaN: High frequency EPR and ENDOR study
Physical Review B: Condensed Matter and Materials Physics, 2014Co-Authors: H. J. Bardeleben, E. Rauls, J. L. Cantin, H. Vrielinck, F. Callens, L. Binet, U. GerstmannAbstract:The nitrogen split Interstitial Defect introduced by high-energy particle irradiation in n-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and Q-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the g-tensor anisotropy, which is not resolved at X or Q band. The good agreement of the principal values g(xx) = 1.9966, g(yy) = 2.0016, and g(zz) = 2.0036 with the theoretically predicted g tensor confirm the (N-N)(N)(0) Defect model. The hyperfine interactions of this Defect have been studied byQ-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20-90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split Interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon Interstitials is also the split Interstitial configuration.
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Identification of the Nitrogen Split Interstitial
Physical Review Letters, 2012Co-Authors: H. J. Von Bardeleben, J. L. Cantin, U. Gerstmann, A. Scholle, S. Greulich-weber, E. Rauls, M. Landmann, W. G. Schmidt, A. Gentils, J. BotsoaAbstract:Combining electron paramagnetic resonance, density functional theory, and positron annihilation spectroscopy (PAS), we identify the nitrogen Interstitial Defect in GaN. The isolated Interstitial is unstable and transforms into a split Interstitial configuration (N-N)N. It is generated by particle irradiation with an introduction rate of a primary Defect, pins the Fermi level at EC-1.0 eV for high fluences, and anneals out at 400 °C. The associated Defect, the nitrogen vacancy, is observed by PAS only in the initial stage of irradiation.
