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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Spin–spin interactions in defects in solids from mixed all-electron and pseudopotential first-principles calculations

Abstract Understanding the quantum dynamics of spin defects and their coherence properties requires an accurate modeling of spin-spin interaction in solids and molecules, for example by using spin Hamiltonians with parameters obtained from first principles calculations. We present a real-space approach based on density functional theory for the calculation of spin-Hamiltonian parameters, where only selected atoms are treated at the all-electron level, while the rest of the system is described with the pseudopotential approximation. Our approach permits calculations for systems containing more than 1000 atoms, as demonstrated for defects in diamond and silicon carbide. We show that only a small number of atoms surrounding the defect needs to be treated at the all-electron level, in order to obtain an overall all-electron accuracy for hyperfine and zero-field splitting tensors. We also present results for coherence times, computed with the cluster correlation expansion method, highlighting the importance of accurate spin-Hamiltonian parameters for quantitative predictions of spin dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electronic and magnetic properties of armchair MoS{sub 2} nanoribbons under both external strain and electric field, studied by first principles calculations

The electronic and magnetic properties of armchair edge MoS{sub 2} nanoribbons (MoS{sub 2}-ANRs) underboth the external strain and transverse electric field (E{sub t}) have been systematically investigated by using the first-principles calculations. It is found that: (1) If no electric field is applied, an interesting structural phase transition would appear under a large tensile strain, leading to a new phase MoS{sub 2}-A'NR, and inducing a big jump peak of the band gap in the transition region. But, the band gap response to compressive strains is much different from that to tensile strain, showing no the structural phase transition. (2) Under the small tensile strains (<10%), the combined E{sub t} and tensile strain give rise to a positive superposition (resonant) effect on the band gap reduction at low E{sub t} (<3 V/nm), and oppositely a negative superposition (antiresonant) one at high E{sub t} (>4 V/nm). On the other hand, the external compressive strains have always presented the resonant effect on the band gap reduction, induced by the electric field. (3) After the structural phase transition, an external large tensile strain could greatly reduce the critical field E{sub tc} causing the band gap closure, and make the system become a ferromagnetic (FM) metal at a relative low E{sub t} (e.g., <4 V/nm), which is very helpful for its promising applications in nano-mechanical spintronics devices. (4) At high E{sub t} (>10 V/nm), the magnetic moments of both the MoS{sub 2}-ANR and MoS{sub 2}-A'NR in their FM states could be enhanced greatly by a tensile strain. Our numerical results of effectively tuning physical properties of MoS{sub 2}-ANRs by combined external strain and electric field may open their new potential applications in nanoelectronics and spintronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First principles calculation of two dimensional antimony and antimony arsenide

This work focuses on the strain dependence of the electronic properties of two dimensional antimony (Sb) material and its alloy with As (SbAs) using density functional theory based first principles calculations. Both systems show indirect bandgap semiconducting character which can be transformed into a direct bandgap material with the application of relatively small strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Structural Polymorphism in “Kesterite” Cu 2 ZnSnS 4 : Raman Spectroscopy and First-Principles Calculations Analysis

This work presents detailed structural and vibrational characterization of different Cu 2 ZnSnS 4 (CZTS) polymorphs (space groups: I$\bar{4}$, P$\bar{4}$2c, and P$\bar{4}$2m), using Raman spectroscopy and first-principles calculations. Multiwavelength Raman measurements on bulk crystalline CZTS samples permitted determination of the vibrational modes of each polymorph structure, with frequencies matching well with the results obtained from simulations. Lastly, the results present Raman spectra fingerprints as well as experimental references for the different polymorph modifications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Study of Pb-doped Ge{sub 2}Sb{sub 2}Te{sub 5} in crystalline phase using first principle calculations

To improve the phase change characteristics of Ge{sub 2}Sb{sub 2}Te{sub 5} (GST), doping is used as one of the effective methods. 4.4 atomic % of Pb doped GST has been studied using first principle calculations. No effect of doping on Te-Ge and Te-Sb bond length has been observed, but the Te-Te bond gets shrink with Pb doping. Due to which the Sb{sub 2}Te{sub 3} segregates as a second phase, with increased doping concentration of Pb in GST alloy. Using such type of calculation, we can calculate the desirable concentration of dopant atoms to prepare the desired material. We can control any segregation in required material with pre-theoretical calculations. The metallic nature of Pd doped GST has been discussed with band structure plots. The metallic character of alloys calculated as in this paper will be helpful to understand the tuning of conductivity of phase change materials, which helps to enhance the phase change properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

First-Principles Calculations of van der Waals and Spin Orbit Effects on the Two-Dimensional Topological Insulator Stanene and Stanene on Ge(111) Substrate

The structural and electronic properties of the monolayer and bilayer stanene structures have been studied using first-principles calculations. For the monolayer, the buckled structure is more stable than the flat one, with an opening of the band gap when spin-orbit coupling is taken into account, as mentioned in recent studies. For the bilayer, three types of stacking are considered: parallel layers, anti-parallel layers, and parallel layers where the first layer is shifted from the second one. These three configurations are named AA1, AA2, and AB, respectively. The two layers are separated by the distance d. The interactions between two layers of stanene are strong for a short distance, while the van der Waals bonding appears for a longer distance. Furthermore, stanene was fabricated experimentally on a substrate; thus, we proposed another study of electronic properties of stanene deposited on Ge(111) to reveal other behavior as a topological insulator and show the existence of the quantum spin Hall effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

First-Principles Calculations of Acoustic and Anharmonic Properties of Ferromagnetic Cu{sub 2}MnZ (Z = Al and In) Heusler Alloys

The acoustic and anharmonic properties of ferromagnetic Cu{sub 2}MnZ (Z = Al and In) Heusler alloys have been investigated by first-principles calculations method. The obtained lattice parameters, magnetic moments, elastic properties are found to be in good agreement with the available experimental values. From the elastic properties, the sound velocities and the related Debye temperatures and minimum thermal conductivities have been obtained for the two alloys. The sound velocities and thermal conductivities of both alloys are found to show significant anisotropy, and the Cu{sub 2}MnAl with small density and large elastic constants has large sound velocities, high Debye temperature, and large thermal conductivities. Also, the effective second-order elastic constants, generalized Grüneisen constants, and nonlinear ultrasonic parameters to characterize anharmonic properties have been studied for the two alloys. These predictions on the acoustic and anharmonic properties can provide a valuable guidance or reference for further related investigations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electronic Structures, Magnetic Properties and Half-Metallicity of Heusler Compounds Hf{sub 2}VZ (Z = Ga, In, Tl, Si, Ge, Sn and Pb): First-Principle Calculations

We investigate the electronic structures, magnetic properties and half-metallicity of Heusler compounds Hf{sub 2}VZ (Z = Ga, In, Tl, Si, Ge, Sn and Pb) with Hg{sub 2}CuTi-type structure by using the first-principle calculations based on density-functional theory. It is found that the equilibrium lattice constants of Hf{sub 2}VZ (Z = Ga, In, Tl, Si, Ge, Sn and Pb) compounds are 6.68, 6.88, 6.91, 6.57, 6.63, 6.85 and 6.94 Å, respectively. Hf{sub 2}VZ (Z = Ga, In, Tl, Si and Sn) compounds exhibit half-metallicity at their respective equilibrium lattice constants. Hf{sub 2}VGe and Hf{sub 2}VPb do not exhibit half-metallicity at their equilibrium lattice constants but can present the half-metallicity when the lattice constant decreases to 6.51 and 6.71 Å, respectively. In case of the lattice thermal expansion, the half-metallicity can be kept in the range of 6.30∼ 7.27, 6.36∼ 7.80, 6.46∼ 7.35, 6.00∼ 6.70 and 6.10∼ 6.95 Å (lattice constant) for Hf{sub 2}VZ (Z = Ga, In, Tl, Si and Sn). Under the unaxial strain, the half-metallicity can still be kept for Hf{sub 2}VZ (Z = Ga, In, Tl) when the lattice distortion is up to ± 10% (c/a) and be kept for Hf{sub 2}VZ (Z = Si, Sn) when the lattice distortion is 0.88∼ 1.09, 0.91∼ 1.09 (c/a). For the Hf{sub 2}VZ compounds with half-metallicity, the total magnetic moment is 2 μ{sub B} when Z is the third main group elements and is 1 μ{sub B} when Z is the fourth main group elements. The Hf{sub 2}VZ compounds with half-metallicity obey the Slater-Pauling formula: M{sub t} = 18 − Z{sub t} rule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Structural and physical properties of 99 complex bulk chalcogenides crystals using first-principles calculations

Chalcogenide semiconductors and glasses have many applications in the civil and military fields, especially in relation to their electronic, optical and mechanical properties for energy conversion and in enviormental materials. However, they are much less systemically studied and their fundamental physical properties for a large class chalcogenide semiconductors are rather scattered and incomplete. Here, we present a detailed study using well defined first-principles calculations on the electronic structure, interatomic bonding, optical, and mechanical properties for 99 bulk chalcogenides including thirteen of these crytals which have never been calculated. Due to their unique composition and structures, these 99 bulk chalcogenides are divided into two main groups. The first group contains 54 quaternary crystals with the structure composition (A2BCQ4) (A = Ag, Cu; B = Zn, Cd, Hg, Mg, Sr, Ba; C = Si, Ge, Sn; Q = S, Se, Te), while the second group contains scattered ternary and quaternary chalcogenide crystals with a more diverse composition (AxByCzQn) (A = Ag, Cu, Ba, Cs, Li, Tl, K, Lu, Sr; B = Zn, Cd, Hg, Al, Ga, In, P, As, La, Lu, Pb, Cu, Ag; C = Si, Ge, Sn, As, Sb, Bi, Zr, Hf, Ga, In; Q = S, Se, Te; x=1, 2, 3; y=0, 1, 2, 5; z=0, 1, 2 and n=3, 4, 5, 6, 9). Moreover, the total bond order density (TBOD) is used as a single quantum mechanical metric to characterize the internal cohesion of these crystals enabling us to correlate them with the calculated properties, especially their mechanical properties. This work provides a very large database for bulk chalcogenides crucial for the future theoretical and experimental studies, opening opportunities for study the properties and potential application of a wide variety of chalcogenides.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Adsorption Properties of p -Methyl Red Monomeric-to-Pentameric Dye Aggregates on Anatase (101) Titania Surfaces: First-Principles Calculations of Dye/TiO 2 Photoanode Interfaces for Dye-Sensitized Solar Cells

The optical and electronic properties of dye aggregates of p-methyl red on a TiO2 anatase (101) surface were modeled as a function of aggregation order (monomer to pentameric dye) via first principles calculations. A progressive red-shifting and intensity increase toward the visible region in UV/vis absorption spectra is observed from monomeric-to-tetrameric dyes, with each molecule in a given aggregate binding to one of the four possible TiO2 (101) adsorption sites. The pentamer exhibits a blue-shifted peak wave- length in the UV/vis absorption spectra and less absorption intensity in the visible region in comparison; a corresponding manifestation of H-aggregation occurs since one of these five molecules cannot occupy an adsorption site. This finding is consistent with experiment. Calculated density of states (DOS) and partial DOS spectra reveal similar dye…TiO2 nanocomposite conduction band characteristics but different valence band features. Associated molecular orbital distributions reveal dye-to-TiO2 interfacial charge transfer in all five differing aggregate orders; meanwhile, the level of intramolecular charge transfer in the dye becomes progressively localized around its azo- and electron-donating groups, up to the tetrameric dye/TiO2 species. Dye adsorption energies and dye coverage levels are calculated and compared with experiment. Overall, the findings of this case study serve to aid the molecular design of azo dyes towards better performing DSSC devices wherein they are incorporated. In addition, they provide a helpful example reference for understanding the effects of dye aggregation on the adsorbate…TiO2 interfacial optical and electronic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles calculations of thermal, electrical, and thermoelectric transport properties of semiconductors

The transport properties of semiconductors are key to the performance of many solid-state devices (transistors, data storage, thermoelectric cooling and power generation devices, etc). An understanding of the transport details can lead to material designs with better performances. In recent years simulation tools based on first-principles calculations have been greatly improved, being able to obtain the fundamental ground-state properties of materials (such as band structure and phonon dispersion) accurately. Accordingly, methods have been developed to calculate the transport properties based on an ab initio approach. We focus on the thermal, electrical, and thermoelectric transport properties of semiconductors, which represent the basic transport characteristics of the two degrees of freedom in solids—electronic and lattice degrees of freedom. Starting from the coupled electron-phonon Boltzmann transport equations, we illustrate different scattering mechanisms that change the transport features and review the first-principles approaches that solve the transport equations. We then present the first-principles results on the thermal and electrical transport properties of semiconductors. The discussions are grouped based on different scattering mechanisms including phonon-phonon scattering, phonon scattering by equilibrium electrons, carrier scattering by equilibrium phonons, carrier scattering by polar optical phonons, scatterings due to impurities, alloying and doping, and the phonon drag effect. We show how the first-principles methods allow one to investigate transport properties with unprecedented detail and also offer new insights into the electron and phonon transport. The current status of the simulation is mentioned when appropriate and some of the future directions are also discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

First-Principles Calculations of Thermoelectric Transport Properties of Quaternary and Ternary Bulk Chalcogenide Crystals

Chalcogenide crystals have a wide range of applications, especially as thermoelectric materials for energy conversion. Thermoelectric materials can be used to generate an electric current from a temperature gradient based on the Seebeck effect and based on the Peltier effect, and they can be used in cooling applications. Using first-principles calculations and semiclassical Boltzmann theory, we have computed the Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor, and figure of merit of 30 chalcogenide crystals. A Quantum Espresso package is used to calculate the electronic properties and locate the Fermi level. The transport properties are then calculated using the BoltzTraP code. The 30 crystals are divided into two groups. The first group has four crystals with quaternary composition (A 2 BCQ 4 ) (A = Tl; B = Cd, Hg; C = Si, Ge, Sn; Q = S, Se, Te). The second group contains 26 crystals with the ternary composition (A’B’Q 2 ) (A’ = Ag, Cu, Au, Na; B’ = B, Al, Ga, In; Q = S, Se, Te). Among these 30 chalcogenide crystals, the results for 11 crystals: Tl 2 CdGeSe 4 , Tl 2 CdSnSe 4 , Tl 2 HgSiSe 4 , Tl 2 HgSnS 4 , AuBSe 2 , AuBTe 2 , AuAlTe 2 , AuGaTe 2 , AuInTe 2 , AgAlSe 2 , and AgAlTe 2 are revealed for the first time. In addition, temperature-dependent transport properties of pure and doped AgSbSe 2 and AgSbTe 2 crystals with dopant compositions of AgSb 0.94 Cd 0.06 Te 2 and AgSbTe 1.85 Se 0.15 were explored. These results provide an excellent database for bulk chalcogenides crucial for a wide range of potential applications in renewable energy fields.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

First-Principle Calculations on the Structural, Mechanical, and Electronic Properties of Mn{sub 2}RuSi and Mn{sub 2}RuGe Under Pressure

The second- and third-order elastic constants of the new Inverse Heusler compounds Mn{sub 2}RuZ (Z = Si, Ge) are determined from the first-principle calculations combined with homogeneous deformation methods. In terms of the second- and third- order elastic constants, the effective second-order elastic constants of Mn{sub 2}RuZ (Z = Si, Ge) are also presented. Besides, the pressure dependence of the brittle/ductile properties, elastic anisotropy factor, Young’s modulus, and Debye temperature is further investigated. It is found that both Mn{sub 2}RuSi and Mn{sub 2}RuGe display ductile nature at ground state and Mn{sub 2}RuGe has better ductility than Mn{sub 2}RuSi under different pressures. This phenomenon is improved by the Young’s modulus. Finally, the density of states of Mn{sub 2}RuZ (Z = Si, Ge) under different pressures is also calculated in order to reveal the origin of the effect of pressure on the the ductility of Mn{sub 2}RuZ (Z = Si, Ge).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Monte Carlo analysis of transient electron transport in wurtzite Zn{sub 1−x}Mg{sub x}O combined with first principles calculations

Transient characteristics of wurtzite Zn{sub 1−x}Mg{sub x}O are investigated using a three-valley Ensemble Monte Carlo model verified by the agreement between the simulated low-field mobility and the experiment result reported. The electronic structures are obtained by first principles calculations with density functional theory. The results show that the peak electron drift velocities of Zn{sub 1−x}Mg{sub x}O (x = 11.1%, 16.7%, 19.4%, 25%) at 3000 kV/cm are 3.735 × 10{sup 7}, 2.133 × 10{sup 7}, 1.889 × 10{sup 7}, 1.295 × 10{sup 7} cm/s, respectively. With the increase of Mg concentration, a higher electric field is required for the onset of velocity overshoot. When the applied field exceeds 2000 kV/cm and 2500 kV/cm, a phenomena of velocity undershoot is observed in Zn{sub 0.889}Mg{sub 0.111}O and Zn{sub 0.833}Mg{sub 0.167}O respectively, while it is not observed for Zn{sub 0.806}Mg{sub 0.194}O and Zn{sub 0.75}Mg{sub 0.25}O even at 3000 kV/cm which is especially important for high frequency devices.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structures and magnetic properties of Ni n (n = 36-40) clusters from first-principles calculations

A genetic algorithm (GA) coupled with a tight-binding (TB) interatomic potential is used to search for the low-energy structures of medium-sized Ni n (n = 36-40) clusters. Structural candidates obtained from our GA search are further optimized with first-principles calculations. The medium-sized nickel clusters ranging from 36 to 40 atoms are found to favor the double-icosahedron-based structures with a Ni7 core (a pentagonal bipyramidal structure) except Ni38 cluster. The lowest-energy structure of Ni38 can be considered to be a magic cluster, which is a typical face-centered cubic structure with large stability and magnetic moment.

Song, W.↗
DOE OSTI2014

First-principle calculation and assignment for vibrational spectra of Ba(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} microwave dielectric ceramic

1:2 B-site cation ordered Ba(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} ceramic was synthesized using conventional solid-state reaction at 1600 °C for 12 h. The structure parameters were obtained through Rietveld refinement of X-ray diffraction data. The Raman peak frequencies were obtained by Lorenz fitting on Raman spectrum. Four-parameter semiquantum model was used to fit the infrared (IR) reflectivity spectrum, and the fitted parameters were used to calculate the dielectric permittivity ε and dielectric loss tanδ. A total of 9 active Raman and 16 active IR modes were obtained using first-principle calculations based on density functional theory with local density approximation. All of the vibrational modes were assigned and represented by linear combinations of the symmetry coordinates deduced using group theory analysis. The Raman mode with the highest frequency A{sub 1g}{sup (4)} (789 cm{sup −1}) can be described as the breathing vibration of NbO{sub 6}. The IR modes E{sub u}{sup (1)} (149 cm{sup −1}) and A{sub 2u}{sup (2)} (212 cm{sup −1}), which can be described as the twisting vibrations of Ba–MgO{sub 6}/Ba–NbO{sub 6} on the a–b plane and the stretching vibrations of Ba–MgO{sub 6}/Ba–NbO{sub 6} along the c direction, respectively, are the dominant contributing modes to ε and tanδ. The dielectric property parameters obtained using IR spectrum fittings, first-principal calculations, and microwave measurements were compared.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Elastic properties of perovskite ATiO{sub 3} (A = Be, Mg, Ca, Sr, and Ba) and PbBO{sub 3} (B = Ti, Zr, and Hf): First principles calculations

The mechanical properties of perovskite oxides depend on two metal oxide lattices that are intercalated. This provides an opportunity for separate tuning of hardness, Poisson's ratio (transverse expansion in response to the compression), and shear strength. The elastic constants of series of perovskite oxides were studied by first principles approach. Both A-site and B-site cations were systematically varied in order to see their effects on the elastic parameters. To study the effects of A-site cations, we studied the elastic properties of perovskite ATiO{sub 3} for A being Be, Mg, Ca, Sr, or Ba, one at a time. Similarly, for B-site cations, we studied the elastic properties of PbBO{sub 3} for B being Ti, Zr, or Hf, one at a time. The density functional first principles calculations with local density approximation (LDA) and generalized gradient approximation (GGA) were employed. It is found that the maximum C{sub 11} elastic constant is achieved when the atomic size of the cations at A-site and B-site are comparable. We also found that C{sub 12} elastic constant is sensitive to B-site cations while C{sub 44} elastic constant is more sensitive to A-site cations. Details and explanations for such dependencies are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Effects of alloying elements on the elastic properties of bcc Ti-X alloys from first-principles calculations

Titanium alloys are great implant materials due to their mechanical properties and biocompatibility. However, a large difference in Young’s modulus between bone (~10–40 GPa) and common implant mate-rials (ie. Ti-6Al-4V alloy ~ 110 GPa) leads to stress shielding and possible implant failure. Here, the present work predicts the single crystal elastic stiffness coefficients (c ij ’s) for five binary systems with the body centered cubic lattice of Ti-X (X = Mo, Nb, Ta, Zr, Sn) using first-principles calculations based on Density Functional Theory. In addition, the polycrystalline aggregate properties of bulk modulus, shear modulus, Young’s modulus, and Poisson ratio are calculated. It is shown that the lower Young’s modulus of these Ti-alloys stems from the unstable bcc Ti with a negative value of (c 11 –c 12 ). The data gathered from these efforts are compared with available experimental and other first-principles results in the literature, which set a foundation to design biocompatible Ti alloys for desired elastic properties.

36 MATERIALS SCIENCE↗