Terms Of First-principles Calculations Research Articles

Prediction of crossing nodal-lines and large intrinsic spin Hall conductivity in topological Dirac semimetal Ta3As family

Topological insulators (TIs) are considered as ideal platforms for generating large spin Hall conductivity (SHC), however, the bulk carrier problem, which is unavoidable in TIs, hinders their practical applications. Recently, topological semimetals (TSMs) have been proposed to achieve large SHC to replace TIs. However, the ideal TSM candidates with large SHC are still lacking. In terms of first-principles calculations, we predict that Ta3As family compounds exhibit complex crossing nodal-lines (CNL) properties in absence of the spin-orbit coupling (SOC). However, they transfer to Dirac TSMs under the influence of strong SOC, and present large SHC around Fermi level in particular. Remarkably, the SHC value of Ta3Y (Y = As, Sb, Bi) is around 1500–1700 (hbar /e)({mathrm{{Omega}}} cdot {mathrm{cm}})^{ - 1}, which is comparable to noble metal Pt and much larger than TIs, Weyl TSMs, and 4d/5d transition metals. Our work not only suggests a kind of TSM family with interesting Dirac CNL around Fermi level, but also paves the way for searching large intrinsic SHC materials in complex CNL TSM systems.

Editors' Choice—Review—Theory and Characterization of Doping and Defects in β-Ga2O3

Gallium oxide (β-Ga2O3) is an emerging semiconductor with relevant properties for power electronics, solar-blind photodetectors, and some sensor applications due to its ultra-wide bandgap and developing technology base for high quality, melt-based substrate growth and thick, low-doped homoepitaxial layers. Of critical importance for the commercialization of this potentially important material is understanding of doping mechanisms in the monoclinic lattice, where two types of Ga sites and three types of O sites have been identified. A critical literature review of doping and defects of the monoclinic β-phase of gallium oxide is provided in this work. Theoretical fundamentals of both donor and acceptor doping in Ga2O3 are reviewed. Advances in doping of epitaxial Ga2O3 with a focus on molecular beam epitaxy and ion implantation are critically examined. As doping is fundamentally related to defects, particularly in this material, a review of defect characterization by optical and electrical spectroscopic methods is provided as well. P-type doping, one of the fundamental challenges for Ga2O3, is discussed in terms of first-principles calculations and ion implantation of known acceptors such as Mg and N.

Functionalized Single-Atom-Embedded Bilayer Graphene and Hexagonal Boron Nitride

Single-atom-embedded bilayer graphene and two-dimensional hexagonal boron nitride are proposed in terms of first-principles calculations. In particular, a series of 68 different single atoms are em...

Strain-Modulated Band Engineering in Two-Dimensional Black Phosphorus/MoS2 van der Waals Heterojunction.

We investigate the band shift and band alignment of two-dimensional (2D) black phosphorus (BP)/MoS2 van der Waals heterojunction (vdW HJ) via uniaxial strain in terms of first-principles calculations and atomic-bond-relaxation method. We find that the band gap of 2D BP/MoS2 HJ decreases linearly with applied tensile strain and Mo–S bond breaks down at 10% tensile strain. Meanwhile, the band gap slightly increases and then monotonically decreases under compressive strain and there appears a semiconductor-to-metal transition at −11 and −12% strain in the y and x directions, respectively. Moreover, 2D BP/MoS2 HJ maintains type-II band alignment for strain applied in the y direction whereas type-II/I band transition appears at −5% strain in the x direction. Moreover, we propose an analytical model to address the strain-modulated band engineering of 2D BP/MoS2 vdW HJ at the atomic level. Our results suggest a promising way to explain the intrinsic mechanism of strain engineering and manipulate the electronic properties of 2D vdW HJs.

Role of descriptors in predicting the dissolution energy of embedded oxides and the bulk modulus of oxide-embedded iron

Oxide-embedded bulk iron is investigated in terms of first principles calculations and data mining. Twenty-nine oxides are embedded into a vacancy site of iron where first principles calculations are performed and the resulting calculations are stored as a data set. A prediction of the dissolution energy of oxides within iron and the bulk modulus of oxide-embedded iron is performed using machine learning. In particular, support vector machine (SVM) and linear regression (LR) are implemented where descriptors for determining the dissolution energy and bulk modulus are revealed. With trained SVM and LR, the prediction of the dissolution energy for different oxides in iron and the inverse problem---deriving the corresponding descriptor variables from a desired bulk modulus---are achieved. The physical origin behind the chosen descriptors is also revealed where manipulating each individual descriptor within a multidimensional space allows for the prediction of the dissolution energy and bulk modulus. Thus, predictions of physical phenomena are, in principle, achievable if the appropriate descriptors are determined.

New family of graphene-based organic semiconductors: An investigation of photon-induced electronic structure manipulation in half-fluorinated graphene

The application of graphene to electronic and optoelectronic devices is limited by the absence of reliable semiconducting variants of this material. A promising candidate in this respect is graphene oxide, with a band gap on the order of $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, however, this has a finite density of states at the Fermi level. Here, we examine the electronic structure of three variants of half -fluorinated carbon on Sic(0001), i.e., the $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})$ $R{30}^{\ensuremath{\circ}}$ C/SiC ``buffer layer,'' graphene on this $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})$ $R{30}^{\ensuremath{\circ}}$ C/SiC buffer layer, and graphene decoupled from the SiC substrate by hydrogen intercalation. Using angle-resolved photoemission, core level photoemission, and x-ray absorption, we show that the electronic, chemical, and physical structure of all three variants is remarkably similar, exhibiting a large band gap and a vanishing density of states at the Fermi level. These results are explained in terms of first-principles calculations. This material thus appears very suitable for applications, even more so since it is prepared on a processing-friendly substrate. We also investigate two separate UV photon-induced modifications of the electronic structure that transform the insulating samples (6.2-eV band gap) into semiconducting ($\ensuremath{\sim}2.5$-eV band gap) and metallic regions, respectively.

Charge density wave transition in single-layer titanium diselenide.

A single molecular layer of titanium diselenide (TiSe2) is a promising material for advanced electronics beyond graphene—a strong focus of current research. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe2 exhibits a charge density wave (CDW) transition at critical temperature TC=232±5 K, which is higher than the bulk TC=200±5 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering. The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The observed Bardeen-Cooper-Schrieffer (BCS) behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.

A first-principles study of structure, elasticity and thermal decomposition of Ti1 − xTMxN alloys (TM = Y, Zr, Nb, Hf, and Ta)

A systematic investigation concerning the effects of transition metals (TM=Y, Zr, Nb, Hf, and Ta) on the structure, elasticity and thermal decomposition of the TiN-based nitride coatings with a cubic rock-salt structure has been performed in terms of first-principles calculations. Calculated lattice parameters of Ti1−xTMxN as a function of alloying concentration show positive derivations from the linearized Vegard's law, agreeing well with experimental and theoretical results. Positive enthalpies of mixing of Ti1−xTMxN (TM=Y, Zr, and Hf) indicate that the formation of these alloys is energetically unfavored with respect to the mixing of the cubic phases. The predicted consolute temperature of Ti1−xZrxN agrees reasonably well with previous theoretical findings. The miscibility gaps disappear in the case of alloying TiN with NbN and TaN. Predicted elastic stiffness constants C11, C12, and C44 together with the aggregate polycrystalline properties of Ti1−xTMxN are determined by an efficient strain–stress method. The present results indicate that the above nitride alloys are mechanically stable and addition of Nb and Ta increases the ductility, with Ta possessing the largest effect.

Vacancy mechanism of oxygen diffusivity in bcc Fe: A first-principles study

Diffusivity of interstitial oxygen (O) in bcc iron (Fe) with and without the effect of vacancy has been investigated in terms of first-principles calculations within the framework of transition state theory. Examination of migration pathway and phonon results indicates that O in octahedral interstice is always energetically favorable (minimum energy) with and without vacancy. It is found that vacancy possesses an extremely high affinity for O in bcc Fe, increasing dramatically the energy barrier (∼80%) for O migration, and in turn, making the predicted diffusion coefficient of O in bcc Fe in favorable accord with experiments.

First-principles investigation of electronic, mechanical and thermodynamic properties of L12 ordered Co3(M, W) (M = Al, Ge, Ga) phases

Studies were carried out on the equilibrium structural, temperature-dependent mechanical and thermodynamic properties of the Co3(M, W) (M=Al, Ge, Ga) phases in terms of first-principles calculations. The results of the ground-state elastic constants revealed that Co3(M, W) phases are mechanically stable and possess intrinsic ductility. It was found that the elastic heat-resistant properties of Co3(Ge, W) phase are inferior to those of Co3(Al, W) and Co3(Ga, W). Analyzing the charge density difference provides the explanation that the sharp decrease in mechanical properties is mainly due to the weakening of Co–Ge bonding at elevated temperatures for Co3(Ge, W). The elastic anisotropy as a function of temperature is discussed using a universal index. It is observed that Co3(M, W) phases show a high degree of elastic anisotropy. The degree of elastic anisotropy could be significantly decreased by an increase in temperature for Co3(M, W). The lattice vibration is treated with the quasiharmonic phonon approach, considering both the vibrational and thermal electronic contributions. The thermodynamic properties as a function of temperature are computed without any adjustable parameters, including heat capacity, entropy, enthalpy and thermal expansion coefficient.

Interplay of localized and itinerant character of Ru ions: Tl2Ru2O7versus Hg2Ru2O7

We carry out a comparative study of the electronic structure of two pyrochlore ruthenate compounds, Tl2Ru2O7 and Hg2Ru2O7, in terms of first principles calculations. Our study reveals the Ru d electrons in Hg2Ru2O7 to be much more delocalized compared to that in Tl2Ru2O7. The subtle change in the Ru-d bandwidths in the two compounds, triggered by the differences in Hg 5d-Ru 4d hybridization compared to that of Tl 5d-Ru 4d, bring in the observed differences in behavior. Our study further shows that the development of long range noncollinear antiferromagnetic structure at low temperature is sufficient to produce the insulating solution in Hg2Ru2O7, in line with the prediction from recent nuclear magnetic resonance study.

Temperature-dependent ideal strength and stacking fault energy of fcc Ni: a first-principles study of shear deformation

Variations of energy, stress, and magnetic moment of fcc Ni as a response to shear deformation and the associated ideal shear strength (τIS), intrinsic (γSF) and unstable (γUS) stacking fault energies have been studied in terms of first-principles calculations under both the alias and affine shear regimes within the {111} slip plane along the and directions. It is found that (i) the intrinsic stacking fault energy γSF is nearly independent of the shear deformation regimes used, albeit a slightly smaller value is predicted by pure shear (with relaxation) compared to the one from simple shear (without relaxation); (ii) the minimum ideal shear strength τIS is obtained by pure alias shear of ; and (iii) the dissociation of the dislocation into two partial Shockley dislocations () is observed under pure alias shear of . Based on the quasiharmonic approach from first-principles phonon calculations, the predicted γSF has been extended to finite temperatures. In particular, using a proposed quasistatic approach on the basis of the predicted volume versus temperature relation, the temperature dependence of τIS is also obtained. Both the γSF and the τIS of fcc Ni decrease with increasing temperature. The computed ideal shear strengths as well as the intrinsic and unstable stacking fault energies are in favorable accord with experiments and other predictions in the literature.

Predicting the spin-lattice order of frustrated systems from first principles

A novel general method of describing the spin-lattice interactions in magnetic solids was proposed in terms of first principles calculations. The spin exchange and Dzyaloshinskii-Moriya interactions as well as their derivatives with respect to atomic displacements can be evaluated efficiently on the basis of density functional calculations for four ordered spin states. By taking into consideration the spin-spin interactions, the phonons, and the coupling between them, we show that the ground state structure of a representative spin-frustrated spinel, MgCr2 O4, is tetragonally distorted, in agreement with experiments. However, our calculations find the lowest energy for the collinear spin ground state, in contrast to previously suggested non-collinear models.

Elastic properties of cubic, tetragonal and monoclinic ZrO 2 from first-principles calculations

In terms of first-principles calculations, elastic stiffness constants C ij ’s as well as the polycrystalline aggregates including the bulk, shear, Young’s moduli, Possion’s ratio, and anisotropy factors have been predicted for three technologically important polymorphs of ZrO 2, i.e., monoclinic m-ZrO 2, tetragonal t-ZrO 2, and cubic c-ZrO 2. Here, both the strain vs. stress (S–S) and the strain vs. strain energy (S–E) methods are adopted. In the first-principles calculations, both the local density approximation (LDA) and the generalized gradient approximation (GGA) are utilized. It is found that the more accurate structural and elastic properties are determined by LDA in comparison with experimental results and the S–S method is more effective than the S–E method although the two methods predict the similar results. The predicted negative values for C 16, C 36, and C 45 of m-ZrO 2 suggest that the certain normal or shear stress corresponds to an opposite shear strain for reducing the total energy. Small differences of shear and Young’s modulus between m-ZrO 2 and t-ZrO 2 suggest that their mechanical properties are comparable.

Effect of pressure and mechanical stress on the electronic properties of AlN and GaN

The fundamental properties of the AlN and GaN compounds with a wurtzite structure under external hydrostatic pressure, uniaxial mechanical stress σ⊥ along the hexagonal axis, and biaxial mechanical stress σ⊥ in the basal plane of the unit cell have been considered in terms of first-principles calculations in the frame-work of the density functional theory. The pressures of the phase transitions from the structures of wurtzite and zinc blende to the structure of rock salt have been obtained. The behavior of the structural parameters, interband transitions, and positions of the charge neutrality level has been investigated. The calculated pressure coefficients of the band gap are as follows: ∂E g /∂p = 40.9 meV/GPa, −∂E g /∂σ | = −4.2 meV/GPa, and −∂E g /∂σ ⊥ = 45.2 meV/GPa for AlN and ∂E g /∂p = 33.0 meV/GPa, −∂E g /∂σ | = 23.6 meV/GPa, and −∂E g /∂σ ⊥ = 9.6 meV/GPa for GaN. The pressure coefficients of the charge neutrality level in almost all cases are substantially smaller than the corresponding values obtained for the band gap E g .

Theoretical investigation of nitrogen-doping effect on vacancy aggregation processes in Si

The nitrogen-doping effect on vacancy aggregation in Si is studied by comparing total energies of various complexes of nitrogen atoms and Si vacancies in terms of first-principles calculations. Two nitrogen atoms are found to form a stable complex with two Si vacancies, strongly suggesting that a supersaturation of “isolated” Si vacancies in growth cooling can be suppressed. The delayed void formation observed in N-doped Czochralski Si indeed supports this suggestion.

Overlayer-dependent magnetic moment and anisotropy of a Co monolayer on Cu(100)

An extensive study of the magnetic moment and of the magnetic anisotropy of a Co monolayer on a Cu(100) substrate capped by an additional $3d$, $4d,$ or $5d$ monolayer is presented in terms of first-principles calculations. While for the magnetic moment of the Co and the cap layer systematic trends can be traced that seem to be consistent with the Stoner model, the dependence of the magnetic anisotropy energy on the type of overlayer is found to be less straightforward. However, in some selected cases a correlation of the magnetic anisotropy with the specific features in the electronic structure of the Co and the cap layer can be pinpointed.