Showing papers on "Coherent potential approximation published in 2017"

A Review of Solid-Solution Models of High-Entropy Alloys Based on Ab Initio Calculations

Abstract: Similar to the importance of XRD in experiments, ab initio calculations, as a powerful tool, have been applied to predict the new potential materials and investigate the intrinsic properties of materials in theory. As a typical solid-solution material, the large degree of uncertainty of high-entropy alloys (HEAs) results in the difficulty of ab initio calculations application to HEAs. The present review focuses on the available ab initio based solid-solution models (virtual lattice approximation, coherent potential approximation, special quasi-random structure, similar local atomic environment, maximum-entropy method, and hybrid Monte Carlo/molecular dynamics) and their applications and limits in single phase HEAs.

Thermal Expansion, Elastic and Magnetic Properties of FeCoNiCu-Based High-Entropy Alloys Using First-Principle Theory

Abstract: The effects of V, Cr, and Mn on the magnetic, elastic, and thermal properties of FeCoNiCu high-entropy alloy are studied by using the exact muffin-tin orbitals method in combination with the coherent potential approximation. The calculated lattice parameters and Curie temperatures in the face-centered-cubic structure are in line with the available experimental and theoretical data. A significant change in the magnetic behavior is revealed when adding equimolar V, Cr, and Mn to the host composition. The three independent single-crystal elastic constants are computed using a finite strain technique, and the polycrystalline elasticity parameters including shear modulus, Young’s modulus, Pugh ratio, Poisson’s ratio, and elastic anisotropy are derived and discussed. The effects of temperature on the structural parameters are determined by making use of the Debye–Gruneisen model. It is found that FeCoNiCuCr possesses a slightly larger thermal expansion coefficient than do the other alloys considered here.

Composition dependent band offsets of ZnO and its ternary alloys

Abstract: We report the calculated fundamental band gaps of wurtzite ternary alloys Zn1−xMxO (M = Mg, Cd) and the band offsets of the ZnO/Zn1−xMxO heterojunctions, these II-VI materials are important for electronics and optoelectronics. Our calculation is based on density functional theory within the linear muffin-tin orbital (LMTO) approach where the modified Becke-Johnson (MBJ) semi-local exchange is used to accurately produce the band gaps, and the coherent potential approximation (CPA) is applied to deal with configurational average for the ternary alloys. The combined LMTO-MBJ-CPA approach allows one to simultaneously determine both the conduction band and valence band offsets of the heterojunctions. The calculated band gap data of the ZnO alloys scale as Eg = 3.35 + 2.33x and Eg = 3.36 − 2.33x + 1.77x2 for Zn1−xMgxO and Zn1−xCdxO, respectively, where x being the impurity concentration. These scaling as well as the composition dependent band offsets are quantitatively compared to the available experimental data. The capability of predicting the band parameters and band alignments of ZnO and its ternary alloys with the LMTO-CPA-MBJ approach indicate the promising application of this method in the design of emerging electronics and optoelectronics.

Composition dependent band offsets of ZnO and its ternary alloys

Abstract: We report the calculated fundamental band gaps of \emph{wurtzite} ternary alloys Zn$_{1-x}$M$_x$O (M=Mg, Cd) and the band offsets of the ZnO/Zn$_{1-x}$M$_x$O heterojunctions, these II-VI materials are important for electronics and optoelectronics. Our calculation is based on density functional theory within the linear muffin-tin orbital (LMTO) approach where the modified Becke-Johnson (MBJ) semi-local exchange is used to accurately produce the band gaps, and the coherent potential approximation (CPA) is applied to deal with configurational average for the ternary alloys. The combined LMTO-MBJ-CPA approach allows one to simultaneously determine both the conduction band and valence band offsets of the heterojunctions. The calculated band gap data of the ZnO alloys scale as $E_g=3.35+2.33x$ and $E_g=3.36-2.33x+1.77x^2$ for Zn$_{1-x}$Mg$_x$O and Zn$_{1-x}$Cd$_x$O, respectively, where $x$ being the impurity concentration. These scaling as well as the composition dependent band offsets are quantitatively compared to the available experimental data. The capability of predicting the band parameters and band alignments of ZnO and its ternary alloys with the LMTO-CPA-MBJ approach indicate the promising application of this method in the design of emerging electronics and optoelectronics.

Rare-earth/transition-metal magnetic interactions in pristine and (Ni,Fe)-doped YCo 5 and GdCo 5

Abstract: We present an investigation into the intrinsic magnetic properties of the compounds YCo5 and GdCo5, members of the RETM5 class of permanent magnets (RE = rare earth, TM = transition metal). Focusing on Y and Gd provides direct insight into both the TM magnetization and RE-TM interactions without the complication of strong crystal field effects. We synthesize single crystals of YCo5 and GdCo5 using the optical floating zone technique and measure the magnetization from liquid helium temperatures up to 800 K. These measurements are interpreted through calculations based on a Green’s function formulation of density-functional theory, treating the thermal disorder of the local magnetic moments within the coherent potential approximation. The rise in magnetization with temperature of GdCo5 is shown to arise from a faster disordering of the Gd magnetic moments compared to the antiferromagnetically-aligned Co sublattice. We use the calculations to analyze the different Curie temperatures of the compounds and also compare the molecular (Weiss) fields at the RE site with previously-published neutron scattering experiments. To gain further insight into the RE-TM interaction we perform substitutional doping on the TM site, studying the compounds RECo4.5Ni0.5, RECo4Ni and RECo4.5Fe0.5. Both our calculations and experiments on powdered samples find an increased/decreased magnetization with Fe/Ni-doping respectively. The calculations further reveal a pronounced dependence on the location of the dopant atoms of both the Curie temperatures and the Weiss field at the RE site

Rare-earth/transition-metal magnetic interactions in pristine and (Ni,Fe)-doped YCo5 and GdCo5

Abstract: We present an investigation into the intrinsic magnetic properties of the compounds YCo5 and GdCo5, members of the RETM5 class of permanent magnets (RE = rare earth, TM = transition metal). Focusing on Y and Gd provides direct insight into both the TM magnetization and RE-TM interactions without the complication of strong crystal field effects. We synthesize single crystals of YCo5 and GdCo5 using the optical floating zone technique and measure the magnetization from liquid helium temperatures up to 800 K. These measurements are interpreted through calculations based on a Green's function formulation of density-functional theory, treating the thermal disorder of the local magnetic moments within the coherent potential approximation. The rise in the magnetization of GdCo5 with temperature is shown to arise from a faster disordering of the Gd magnetic moments compared to the antiferromagnetically aligned Co sublattice. We use the calculations to analyze the different Curie temperatures of the compounds and also compare the molecular (Weiss) fields at the RE site with previously published neutron scattering experiments. To gain further insight into the RE-TM interactions, we perform substitutional doping on the TM site, studying the compounds RECo4.5Ni0.5, RECo4Ni, and RECo4.5Fe0.5. Both our calculations and experiments on powdered samples find an increased/decreased magnetization with Fe/Ni doping, respectively. The calculations further reveal a pronounced dependence on the location of the dopant atoms of both the Curie temperatures and the Weiss field at the RE site.

Impact of aluminum doping on the thermo-physical properties of refractory medium-entropy alloys

Abstract: We investigate the elastic moduli, ideal tensile strength, and thermodynamic properties of TiVNb and AlTiVNb refractory medium-entropy alloys (HEAs) by using ab initio alloy theories: the coherent potential approximation (CPA), the special quasi-random supercell (SQS), and a 432-atom supercell (SC). We find that with increasing number of alloy components, the SQS elastic constants become sensitive to the supercell size. The predicted elastic moduli are consistent with the available experiments. Aluminum doping decreases the stability of the body centered cubic phase. The ideal tensile strength calculation indicates that adding equiatomic Al to TiVNb random solid solution increases the intrinsic strength (ideal strain increase from 9.6% to 11.8%) and decreases the intrinsic strength (from 9.6 to 5.7 GPa). Based on the equation of states calculated by the CPA and SC methods, the thermodynamic properties obtained by the two ab initio methods are assessed. The L21 AlTiVNb (Ti-Al-V-Nb) alloy is predicted to be...

Ab initio calculations of the magnetic properties of TM (Ti, V)-doped zinc-blende ZnO

Abstract: In order to promote suitable material to be used in spintronics devices, this study purposes to evaluate the magnetic properties of the titanium and vanadium-doped zinc-blende ZnO from first-principles. The calculations of these properties are based on the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA), using the local density approximation (LDA). We have calculated and discussed the density of states (DOSs) in the energy phase diagrams for different concentration values, of the dopants. We have also investigated the magnetic and half-metallic properties of this doped compound. Additionally, we showed the mechanism of the exchange coupling interaction. Finally, we estimated and studied the Curie temperature for different concentrations.

First-principles quantum transport method for disordered nanoelectronics: Disorder-averaged transmission, shot noise, and device-to-device variability

Abstract: Because disorders are inevitable in realistic nanodevices, the capability to quantitatively simulate the disorder effects on electron transport is indispensable for quantum transport theory. Here, we report a unified and effective first-principles quantum transport method for analyzing effects of chemical or substitutional disorder on transport properties of nanoelectronics, including averaged transmission coefficient, shot noise, and disorder-induced device-to-device variability. All our theoretical formulations and numerical implementations are worked out within the framework of the tight-binding linear muffin tin orbital method. In this method, we carry out the electronic structure calculation with the density functional theory, treat the nonequilibrium statistics by the nonequilbrium Green's function method, and include the effects of multiple impurity scattering with the generalized nonequilibrium vertex correction (NVC) method in coherent potential approximation (CPA). The generalized NVC equations are solved from first principles to obtain various disorder-averaged two-Green's-function correlators. This method provides a unified way to obtain different disorder-averaged transport properties of disordered nanoelectronics from first principles. To test our implementation, we apply the method to investigate the shot noise in the disordered copper conductor, and find all our results for different disorder concentrations approach a universal Fano factor $1/3$. As the second test, we calculate the device-to-device variability in the spin-dependent transport through the disordered Cu/Co interface and find the conductance fluctuation is very large in the minority spin channel and negligible in the majority spin channel. Our results agree well with experimental measurements and other theories. In both applications, we show the generalized nonequilibrium vertex corrections play a determinant role in electron transport simulation. Our results demonstrate the effectiveness of the first-principles generalized CPA-NVC for atomistic analysis of disordered nanoelectronics, extending the capability of quantum transport simulation.

Electronic structure and magnetic properties of two-dimensional nonstoichiometric rutile

Abstract: The coherent potential approximation is applied to study the influence of vacancies in the oxygen lattice on the electronic structure and magnetic properties of the TiO1.99 rutile (110) surface. Stoichiometric two-dimensional rutile is found to be a nonmagnetic semiconductor. Vacancies in the oxygen positions on the surface lead to the metallic type of the electronic spectrum. Additionally, they result in the appearance of spin magnetic moments on titanium atoms surrounded by only five oxygen atoms due to the surface formation. The vacancies in all the other oxygen positions except of the surface cause a nonmagnetic semiconducting character of the energy spectrum of two-dimensional TiO1.99. A mechanism that underlies the formation of spin magnetic moments of the titanium atoms, namely Stoner ferromagnetism of a defect related impurity band, is discussed.

Interplay of electronic, structural and magnetic properties as the driving feature of high-entropy CoCrFeNiPd alloys

Abstract: The structural and magnetic properties of CoCrFe y Ni and CoCrFeNi-Pd x alloys earlier investigated experimentally by x-ray and neutron diffraction techniques and magnetometry have been theoretically reproduced using two complementary approaches for electronic structure calculations, i.e. the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA) and implemented in the ab initio framework of density functional theory and the Vienna ab initio simulation package (VASP) for supercell models of high-entropy alloy (HEA) structures. The comparison between experimental results and calculations of the lattice constants by both calculation methods indicate that the structure of CoCrFe y Ni is well described by ordered fcc configurations. The values of local magnetic moments on Fe, Co, Cr, and Ni atoms depend not only on the Pd concentration but on chemical disordering. In the case of the CoCrFeNi-Pd x alloys, the KKR-CPA and the VASP calculations of disordered configurations reproduce the experimental values at 5 K up to equimolar composition and at 300 K above. The experimental values above the equimolar composition at 5 K are not satisfactorily reproduced by any of the calculations. The divergence between the experimental and calculated values is related to the variation of the ferromagnetic to paramagnetic transition temperature as a function of palladium content and to the existence of several phases, FeCoCr-rich above room temperature and FeCrPd-rich below, observed by diffraction and detected by microscopy and atom probe investigations. VASP calculations of a FeCrPd-rich phase effectively reproduced both the lattice constant and magnetization of the alloy above equimolar composition. An important conclusion of this work is that the combined analysis of the electronic, structural, and magnetic properties plays an important role in understanding the complexity of magnetic HEAs.

Theoretical band structure of the superconducting antiperovskite oxide Sr3−xSnO

Abstract: In order to investigate the position of the strontium deficiency in superconductive S r 3 − x SnO, we synthesized and measured X-ray-diffraction patterns of S r 3 − x SnO ( x ∼ 0.5 ). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on S r 3 − x SnO ( x = 0, 0.5 ) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of S r 3 − x SnO in case of x ∼ 0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr 3 SnO, where the mixing of the valence p and conduction d orbitals are considered to be small.

Correlation between electronic structure, transport and electrochemical properties of a LiNi1-y-zCoyMnzO2 cathode material.

Abstract: Herein, the correlation between electronic structure, transport and electrochemical properties of layered LixNi1−y−zCoyMnzO2 cathode material is revealed. Comprehensive experimental studies of physicochemical properties of LixNi1−y−zCoyMnzO2 cathode material (XRD, electrical conductivity, thermoelectric power) are supported by electronic structure calculations performed using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA) to account for the chemical disorder. It is found that even small O defects (∼1%) could significantly modify electronic density of states DOS characteristics via the formation of extra broad peaks inside the former band gap leading to its substantial narrowing. The calculated DOS values and their changes near EF tend to support experimental findings with irregular changes in the sign of thermoelectric power as well as the behavior of electrical conductivity curves as a function of Li content. Furthermore, the variations of the electromotive force of the Li/Li+/LixNi1−y−zCoyMnzO2 cell (for 0 < x < 1) remains in a quite good agreement with the relative variation of EF on DOS calculated from the KKR-CPA method.

Synthesis and magnetic properties of Mg doped SnO2 thin films: experimental and Ab-initio study

Abstract: Mg-doped tin oxide (SnO2) thin films were deposited using spray pyrolysis technique with an aqueous solution of SnCl2 and magnesium sulfate (Mg (SO4)·7H2O) on a heated glass substrate. In this work, the effect of Mg doping on the structural, optical and electrical properties of SnO2 was investigated in some detail by X-ray diffraction, UV–Vis spectroscopy and Hall Effect measurements. The XRD diffractograms demonstrate that SnO2 crystallized in tetragonal rutile structure with preferential orientation along (110) plane. The average transmittance in the visible range was increased from 65 to 78% and the values of energy band gap were found in the range of 3.62–3.87 eV. The lowest resistivity [1.021 × 101 (Ω.cm)] was obtained for the film doped with 5 at% Mg. The electronic structure and optical properties of the rutile structure Sn1−xMgxO2 were obtained by ab initio calculations using the Korringa-Kohn-Rostoker method (KKR) combined with the Coherent Potential Approximation (CPA), as well as CPA confirms our results.

Fully Relativistic Temperature-Dependent Electronic Transport Properties of Magnetic Alloys From the First Principles

Abstract: Ab initio calculations based on the fully relativistic Dirac approach and incorporating chemical disorder and temperature-induced atomic displacements (phonons) are presented. The tight-binding linear muffin-tin orbital method is used, and the multicomponent coherent potential approximation deals with the chemical disorder and phonons on the same level. Electrical resistivities calculated without and with spin–orbit interaction are compared, and the anomalous Hall effect for magnetic alloys is investigated. The developed technique is tested on pure nickel and on random binary Cu–Ni alloys. The calculated results are found to be in good agreement with experimental and other theoretical data. The combined effect of phonons and spin disorder, simulated within the disordered local moment state, has also been studied. The results confirm the validity of Matthiessen’s rule in a wide range of temperatures for the electrical resistivity of pure nickel.

First-principles and experimental characterization of the electronic properties of CaGaSiN3 and CaAlSiN3: the impact of chemical disorder

Abstract: We report a detailed investigation of the electronic, mechanical and optical properties of the recently discovered nitridogallosilicate CaGaSiN3 which has potential as a LED-phosphor host material. We focus on chemical disorder effects, originating from the Ga/Si site, and compared them to those of isostructural CaAlSiN3. We calculate the elastic moduli and the Debye temperature in terms of quasi harmonical approximation. Spectral properties like the joint density of states (JDOS) are evaluated and the absorption, reflectance and energy loss function are obtained from the dielectric function. The optical band gap of CaGaSiN3 from experiment is compared to the electronic band gap in terms of electronic DOS and band structure calculations. All properties are evaluated for different ordering models of Ga/Si while the experimentally observed substitutional disorder is accounted for by utilizing the Coherent Potential Approximation (CPA). We conclude a shrinking of the band gap for both CaGaSiN3 and CaAlSiN3 due to atomic disorder, which is unfavorable for potential phosphor applications. This study contributes to materials design considerations, and provides a close look on the electronic impact of substitutional disorder. Moreover, we open the scope for future investigations on solid solutions and phosphor host materials with low doping concentrations.

Phase transitions in the Haldane-Hubbard model within coherent potential approximation

Abstract: Within the coherent potential approximation we study the two-dimensional Haldane-Hubbard model, in which an interplay between topology and correlation effects is realized. The model essentially describes correlated electrons moving in a honeycomb lattice with zero net magnetic flux. The influence of the next-nearest-neighbor hopping and electron correlations on the metal-insulator transitions are investigated by monitoring the density of states at the Fermi level and the energy gap. The topological properties of the insulators is determined by the Chern number. With a given next-nearest-neighbor hopping, electron correlations drive the system from the topological Chern insulator to a metal, and then to the topologically trivial Mott insulator.

Spin-dependent electrical transport at finite temperatures from the first principles

Abstract: The finite-temperature electrical transport properties depending on the spin are essential for spintronics research focused on developing devices that should operate not only in the conditions of low temperatures. In this study we present a theoretical approach incorporating both chemical and temperature-induced disorder within the coherent potential approximation and the tight-binding linear muffin-tin orbital method, and the linear response theory is used to obtain spin-resolved electrical conductivity. Both nonmagnetic and magnetic materials are studied from the first principles in a wide temperature range. It was found, with neglected magnetic disorder, that vertex corrections to the total conductivity and spin-flip contributions to the conductivity are small; therefore, the spin-resolved coherent conductivities can be used to describe spin-dependent electrical transport. The developed formalism is applied to pure nonmagnetic platinum and to ferromagnetic random Cu-Ni alloys. For the latter system, the spin polarization of the current is nearly constant in the examined temperature range.

Full counting statistics of conductance for disordered systems

Abstract: Quantum transport is a stochastic process in nature. As a result, the conductance is fully characterized by its average value and fluctuations, i.e., characterized by full counting statistics (FCS). Since disorders are inevitable in nanoelectronic devices, it is important to understand how FCS behaves in disordered systems. The traditional approach dealing with fluctuations or cumulants of conductance uses diagrammatic perturbation expansion of the Green's function within coherent potential approximation (CPA), which is extremely complicated especially for high order cumulants. In this paper, we develop a theoretical formalism based on nonequilibrium Green's function by directly taking the disorder average on the generating function of FCS of conductance within CPA. This is done by mapping the problem into higher dimensions so that the functional dependence of generating a function on the Green's function becomes linear and the diagrammatic perturbation expansion is not needed anymore. Our theory is very simple and allows us to calculate cumulants of conductance at any desired order efficiently. As an application of our theory, we calculate the cumulants of conductance up to fifth order for disordered systems in the presence of Anderson and binary disorders. Our numerical results of cumulants of conductance show remarkable agreement with that obtained by the brute force calculation.

Ab Initio Study of Electronic and Magnetic Properties of Ga1-xCoxN (Doped) and Ga1-x-yCoxCryN (Co-doped)

Abstract: Using the Korring–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA) we study the electronic structure and magnetic properties of the doped and co-doped of Ga1-x Co x N and Ga1-x-y Co x Cr y N materials with single and double impurities, respectively. The total and local density of state (DOS) in two systems are obtained We investigate the effect of the different values of dilution in Ga1-x Co x N and Ga1-x-y Co x Cr y N. On the other hand, the magnetic moment and the gap energies are deduced for different values of concentrations.