Showing papers on "Coherent potential approximation published in 2016"

Calculation of the magnetic anisotropy with projected-augmented-wave methodology and the case study of disordered Fe 1 -x Co x alloys

Abstract: The magnetic anisotropy energy of tetragonally distorted disordered alloys Fe ${}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$ is calculated by two different virtual crystal approximation methods and an averaged supercell method within the projected-augmented-wave (PAW) methodology and the magnetic force theorem. The details of the spin-orbit coupling implementation in the PAW methodology are given. We compare our results to the recent coherent potential approximation (CPA) studies, results of full potential calculations, and to the available experiments.

Atomistic clustering-ordering and high-strain deformation of an Al0.1CrCoFeNi high-entropy alloy.

Abstract: Computational investigations of structural, chemical, and deformation behavior in high-entropy alloys (HEAs), which possess notable mechanical strength, have been limited due to the absence of applicable force fields. To extend investigations, we propose a set of intermolecular potential parameters for a quinary Al-Cr-Co-Fe-Ni alloy, using the available ternary Embedded Atom Method and Lennard-Jones potential in classical molecular-dynamics simulations. The simulation results are validated by a comparison to first-principles Korringa-Kohn-Rostoker (KKR) - Coherent Potential Approximation (CPA) [KKR-CPA] calculations for the HEA structural properties (lattice constants and bulk moduli), relative stability, pair probabilities, and high-temperature short-range ordering. The simulation (MD)-derived properties are in quantitative agreement with KKR-CPA calculations (first-principles) and experiments. We study AlxCrCoFeNi for Al ranging from 0 ≤ x ≤2 mole fractions, and find that the HEA shows large chemical clustering over a wide temperature range for x < 0.5. At various temperatures high-strain compression promotes atomistic rearrangements in Al0.1CrCoFeNi, resulting in a clustering-to-ordering transition that is absent for tensile loading. Large fluctuations under stress, and at higher temperatures, are attributed to the thermo-plastic instability in Al0.1CrCoFeNi.

Microscopic interpretation of the Dynes formula for the tunneling density of states

Abstract: Excellent fits of the tunneling density of states in disordered superconductors can be often achieved making use of the phenomenological Dynes formula. However, no consistent derivation of this formula has been available so far. The Dynes formula can be interpreted by the simplest causal frequency-dependent gap function $\mathrm{\ensuremath{\Delta}}(\ensuremath{\omega})$ with a vanishing gap at the Fermi level. Here we show, within the coherent potential approximation, that precisely such a gap function describes superconductors with a Lorentzian distribution of pair-breaking fields and arbitrary potential disorder. We predict spectral and thermodynamic properties of such superconductors.

Electron?phonon coupling in Ni-based binary alloys with application to displacement cascade modeling

Abstract: Energy transfer between lattice atoms and electrons is an important channel of energy dissipation during displacement cascade evolution in irradiated materials. On the assumption of small atomic displacements, the intensity of this transfer is controlled by the strength of electron-phonon (el-ph) coupling. The el-ph coupling in concentrated Ni-based alloys was calculated using electronic structure results obtained within the coherent potential approximation. It was found that Ni0.5Fe0.5, Ni0.5Co0.5 and Ni0.5Pd0.5 are ordered ferromagnetically, whereas Ni0.5Cr0.5 is nonmagnetic. Since the magnetism in these alloys has a Stoner-type origin, the magnetic ordering is accompanied by a decrease of electronic density of states at the Fermi level, which in turn reduces the el-ph coupling. Thus, the el-ph coupling values for all alloys are approximately 50% smaller in the magnetic state than for the same alloy in a nonmagnetic state. As the temperature increases, the calculated coupling initially increases. After passing the Curie temperature, the coupling decreases. The rate of decrease is controlled by the shape of the density of states above the Fermi level. Introducing a two-temperature model based on these parameters in 10 keV molecular dynamics cascade simulation increases defect production by 10-20% in the alloys under consideration.

Fully relativistic description of spin-orbit torques by means of linear response theory

Abstract: Symmetry and magnitude of spin-orbit torques (SOT), i.e., current-induced torques on the magnetization of systems lacking inversion symmetry, are investigated in a fully relativistic linear response framework based on the Kubo formalism. By applying all space-time symmetry operations contained in the magnetic point group of a solid to the relevant response coefficient, the torkance expressed as torque-current correlation function, restrictions to the shape of the direct and inverse response tensors are obtained. These are shown to apply to the corresponding thermal analogs as well, namely the direct and inverse thermal SOT in response to a temperature gradient or heat current. Using an implementation of the Kubo-Bastin formula for the torkance into a first-principles multiple-scattering Green function framework and accounting for disorder effects via the so-called coherent potential approximation, all contributions to the SOT in pure systems, dilute as well as concentrated alloys can be treated on equal footing. This way, material specific values for all torkance tensor elements in the fcc (111) trilayer alloy system $\text{Pt}|{\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}|\mathrm{Cu}$ are obtained over a wide concentration range and discussed in comparison to results for electrical and spin conductivity, as well as to previous work---in particular concerning symmetry with respect to magnetization reversal and the nature of the various contributions.

Statistical physics of multicomponent alloys using KKR-CPA

Abstract: We apply variational principles from statistical physics and the Landau theory of phase transitions to multicomponent alloys using the multiple-scattering theory of Korringa-Kohn-Rostoker (KKR) and the coherent potential approximation (CPA). This theory is a multicomponent generalization of the S(2) theory of binary alloys developed by G. M. Stocks, J. B. Staunton, D. D. Johnson and others. It is highly relevant to the chemical phase stability of high-entropy alloys as it predicts the kind and size of finite-temperature chemical fluctuations. In doing so it includes effects of rearranging charge and other electronics due to changing site occupancies. When chemical fluctuations grow without bound an absolute instability occurs and a second-order order-disorder phase transition may be inferred. The S(2) theory is predicated on the fluctuation-dissipation theorem; thus we derive the linear response of the CPA medium to perturbations in site-dependent chemical potentials in great detail. The theory lends itself to a natural interpretation in terms of competing effects: entropy driving disorder and favorable pair interactions driving atomic ordering. Moreover, to further clarify interpretation we present results for representative ternary alloys CuAgAu, NiPdPt, RhPdAg, and CoNiCu within a frozen charge (or band-only) approximation. These results include the so-called Onsager mean field correction thatmore » extends the temperature range for which the theory is valid.« less

Thermal vacancies in random alloys in the single-site mean-field approximation

Abstract: A formalism for the vacancy formation energies in random alloys within the single-site mean-filed approximation, where vacancy-vacancy interaction is neglected, is outlined. It is shown that the alloy configurational entropy can substantially reduce the concentration of vacancies at high temperatures. The energetics of vacancies in random ${\mathrm{Cu}}_{0.5}{\mathrm{Ni}}_{0.5}$ alloy is considered as a numerical example illustrating the developed formalism. It is shown that the effective formation energy increases with temperature, however, in this particular system it is still below the mean value of the vacancy formation energy, which would correspond to the vacancy formation energy in a homogeneous model of a random alloy, such as given by the coherent potential approximation.

Effect of thermal lattice expansion on the stacking fault energies of fcc Fe and Fe 75 Mn 25 alloy

Abstract: Temperature dependent stacking fault energies in fcc Fe and the ${\mathrm{Fe}}_{75}{\mathrm{Mn}}_{25}$ random alloy are calculated within density functional theory. The high temperature paramagnetic state of Fe is modeled by the spin wave (SW) method within a Hamiltonian formalism and by the disordered local moment (DLM) approach in the Green's function technique using the coherent potential approximation (CPA). To determine the stacking fault energy, the supercell approach is used in the case of the SW method, while the axial Ising model is used in both the SW method and CPA-DLM calculations. The SW and CPA-DLM results are in very good agreement with each other, and they also accurately reproduce the existing experimental data. In both cases, fcc Fe and the ${\mathrm{Fe}}_{75}{\mathrm{Mn}}_{25}$ alloy, the SFE increases with temperature. This increase is almost entirely due to thermal lattice expansion, in contrast to earlier claims connecting such a dependence with magnetic entropy. Additionally, we check the convergence of the SW method with respect to the number of spin waves in the calculations of the phonon spectrum and the vacancy formation energy of paramagnetic fcc Fe.

Theoretical impurity-limited carrier mobility of monolayer black phosphorus

Abstract: Monolayer black phosphorus (MBP) is a strong candidate for applications in emerging electronic devices. In this work, we report theoretical calculations of impurity limited carrier mobility of MBP using a state-of-the-art first principles quantum transport method where density functional theory is carried out within nonequilibrium Green's function formalism and multiple impurity scattering is calculated by coherent potential approximation. We predict mobilities of both hole and electron carriers due to carbon (C) and sulfur (S) impurity atoms. For impurities concentrations ranging from 0.6% to very high 2.0%, the mobilities drop from several hundreds (in cm2/Vs) to less than 100 in the armchair direction (AC) and show less variation in the zigzag (ZZ) one. The mobilities at smaller impurity concentration range are consistent with the various experimentally reported values. For the entire range, hole mobility is slightly larger than electron mobility in the AC direction and an order of magnitude smaller in the ZZ direction.

Generalized nonequilibrium vertex correction method in coherent medium theory for quantum transport simulation of disordered nanoelectronics

Abstract: Electron transport properties of nanoelectronics can be significantly influenced by the inevitable and randomly distributed impurities/defects. For theoretical simulation of disordered nanoscale electronics, one is interested in both the configurationally averaged transport property and its statistical fluctuation that tells device-to-device variability induced by disorder. However, due to the lack of an effective method to do disorder averaging under the nonequilibrium condition, the important effects of disorders on electron transport remain largely unexplored or poorly understood. In this work, we report a general formalism of Green's function based nonequilibrium effective medium theory to calculate the disordered nanoelectronics. In this method, based on a generalized coherent potential approximation for the Keldysh nonequilibrium Green's function, we developed a generalized nonequilibrium vertex correction method to calculate the average of a two-Keldysh-Green's-function correlator. We obtain nine nonequilibrium vertex correction terms, as a complete family, to express the average of any two-Green's-function correlator and find they can be solved by a set of linear equations. As an important result, the averaged nonequilibrium density matrix, averaged current, disorder-induced current fluctuation, and averaged shot noise, which involve different two-Green's-function correlators, can all be derived and computed in an effective and unified way. To test the general applicability of this method, we applied it to compute the transmission coefficient and its fluctuation with a square-lattice tight-binding model and compared with the exact results and other previously proposed approximations. Our results show very good agreement with the exact results for a wide range of disorder concentrations and energies. In addition, to incorporate with density functional theory to realize first-principles quantum transport simulation, we have also derived a general form of conditionally averaged nonequilibrium Green's function for multicomponent disorders.

Applications of coherent potential approximation to HEAs

Abstract: This chapter details the coherent potential approximation (CPA) to describe the chemically and magnetically disordered phases for systems of arbitrary number of components. Two widely used CPA implementations, namely, the exact muffin-tin orbitals (EMTO) and the Korringa–Kohn–Rostoker (KKR) methods, are briefly reviewed. Applications to predict lattice stability, electronic and magnetic structure, elasticity properties, and stacking fault energies of single-phase HEAs are presented.

Flat band degeneracy and near-zero refractive index materials in acoustic crystals

Abstract: A Dirac-like cone is formed by utilizing the flat bands associated with localized modes in an acoustic crystal (AC) composed of a square array of core-shell-structure cylinders in a water host. Although the triply-degeneracy seems to arise from two almost-overlapping flat bands touching another curved band, the enlarged view of the band structure around the degenerate point reveals that there are actually two linear bands intersecting each other at the Brillouin zone center, with another flat band passing through the same crossing point. The linearity of dispersion relations is achieved by tuning the geometrical parameters of the cylindrical scatterers. A perturbation method is used to not only accurately predict the linear slopes of the dispersions, but also confirm the linearity of the bands from first principles. An effective medium theory based on coherent potential approximation is developed, and it shows that a slab made of the AC carries a near-zero refractive index around the Dirac-like point. Full-wave simulations are performed to unambiguously demonstrate the wave manipulating properties of the AC structures such as perfect transmission, unidirectional transmission and wave front shaping.

Resonant impurity states in chemically disordered half-Heusler Dirac semimetals

Abstract: We address the electron transport characteristics in bulk half-Heusler alloys with their compositions tuned to the borderline between topologically nontrivial semimetallic and trivial semiconducting phases. Accurate first-principles calculations based on the coherent potential approximation (CPA) reveal that all the studied systems exhibit sets of dispersionless impurity-like resonant levels, with one of them being located at the Dirac point. By means of the Kubo-Bastin formalism we reveal that the residual conductivity of these alloys is strongly suppressed by impurity scattering, whereas the spin Hall conductivity exhibits a rather complex behavior induced by the resonant states. In particular for ${\mathrm{LaPt}}_{0.5}{\mathrm{Pd}}_{0.5}\mathrm{Bi}$ we find that the total spin Hall conductivity is strongly suppressed by two large and opposite contributions: the negative Fermi-surface contribution produced by the resonant impurity and the positive Fermi-sea term stemming from the occupied states. At the same time, we identify no conductivity contributions from the conical states.

Magnetic properties of Fe 1 − x Ni x alloy from CPA+DMFT perspectives

Abstract: We use a combination of the coherent potential approximation and dynamical mean field theory to study magnetic properties of the ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ alloy from first principles. Calculated uniform magnetic susceptibilities have a Curie-Weiss-like behavior, and extracted effective temperatures are in agreement with the experimental results. The individual squared magnetic moments obtained as a function of nickel concentration follow the same trends as experimental data. An analysis of the ionic and spin weights shows a possibility of high-spin to intermediate- and low-spin state transitions at high temperatures.

Defect-induced magnetic structure of CuMnSb

Abstract: The observed ground state for the CuMnSb alloy is the antiferromagnetic (111) phase as confirmed by neutron diffraction experiments. Ab initio total energy calculations for ideal, defect-free CuMnSb contradict this result and indicate that other magnetic structures can have their total energies lower. It is known that Heusler alloys usually contain various defects depending on the sample preparation. We have therefore investigated magnetic phases of CuMnSb assuming the most common defects which exist in real experimental conditions. The full-potential supercell approach and a Heisenberg model approach using the coherent potential approximation are adopted. The results of the total energy supercell calculations indicate that defects that bring Mn atoms close together promote the antiferromagnetic (111) structure already for a low critical defect concentrations $(\ensuremath{\approx}3$%). A detailed study of exchange interactions between Mn moments further supports the above stabilization mechanism. Finally, the stability of the antiferromagnetic (111) order is enhanced by inclusion of electron correlations in narrow Mn bands. The present refinement structure analysis of the neutron scattering experiment supports theoretical conclusions.

First-principles evaluation of intrinsic, side-jump, and skew-scattering parts of anomalous Hall conductivities in disordered alloys

Abstract: We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $\sigma_{xy}$. This method is based on the different microscopic conductive processes of each origin of $\sigma_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value of each origin of $\sigma_{xy}$ exhibits reasonable dependencies on the electron scattering in these disordered alloys.

Theory of quantum transport in disordered systems driven by voltage pulse

Abstract: Predicting time-dependent quantum transport in the transient regime is important for understanding the intrinsic dynamic response of a nanodevice and for predicting the limit of how such a device can switch on or off a current. Theoretically, this problem becomes quite difficult to solve when the nanodevice contains disorder because the calculated transient current must be averaged over many disorder configurations. In this work, we present a theoretical formalism to calculate the configuration averaged time-dependent current flowing through a phase coherent device containing disorder sites where the transient current is driven by sharply turning on and off the external bias voltage. Our theory is based on the Keldysh nonequilibrium Green's function (NEGF) formalism and is applicable in the far from equilibrium nonlinear response quantum transport regime. The effects of disorder scattering are dealt with by the coherent potential approximation (CPA) extended in the time domain. We show that after approximations such as CPA and vertex corrections for calculating the multiple impurity scattering in the transient regime, the derived NEGFs perfectly satisfy a Ward identity. The theory is quantitatively verified by comparing its predictions to the exact solution for a tight-binding model of a disordered two-probe transport junction.

Theoretical study on the anisotropic electronic structure of antiferromagnetic BaFe2As2 and Co-doped Ba(Fe1-xCox)(2)As-2 as seen by angle-resolved photoemission

Abstract: By means of one-step model calculations the strong in-plane anisotropy seen in angle-resolved photoemission of the well-known iron pnictide prototype compounds BaFe2As2 and Ba(Fe1-xCox)(2)As-2 in their low-temperature antiferromagnetic phases is investigated. The fully relativistic calculations are based on the Korringa-Kohn-Rostoker-Green function approach combined with the coherent potential approximation alloy theory to account for the disorder induced by Co substitution on Fe sites in a reliable way. The results of the calculations can be compared directly to experimental spectra of detwinned single crystals. One finds very good agreement with experiment and can reveal all features of the electronic structure contributing to the in-plane anisotropy. In particular the local density approximation can capture most of the correlation effects for the investigated system without the need for more advanced techniques. In addition, the evolution of the anisotropy for increasing Co concentration x in Ba(Fe1-xCox)(2)As-2 can be tracked almost continuously. The results are also used to discuss surface effects and it is possible to identify clear signatures to make conclusions about different types of surface termination.

Effect of Grain Size on Magnetic Properties of ZnO Doped with Nickel Single Impurities

Abstract: The purpose of our study is to find a suitable material to be used in spintronic applications and to find the relation between the parameters of deposition by spray pyrolysis technic (temperature, concentration of Ni doping) and the ferromagnetic properties (Curie temperature, magnetic moment). The nickel-doped zinc oxide, Zn.1−x.NixO (x = 0.01, 0.03, 0.05), and diluted magnetic semiconductors (DMSs) are synthesized by the spray pyrolysis technic. The results of The X-ray diffraction (XRD) of prepared substrates confirm the incorporation of the dopants into the ZnO lattice structure. The spin-polarized electronic properties has been found and was investigated in detail by using the density-functional theory (DFT), the local density approximation (LDA), and The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). As result, nickel doping brings up a half-metallic appearance due to the hybridization between the 3d state of Nickel impurities and the oxygen 2p state. The mechanism of the interatomic exchange has been explained as being a p-d double exchange.

Electronic structure of UO 2.12 calculated in the coherent potential approximation taking into account strong electron correlations and spin-orbit coupling

Abstract: Based on the coherent potential approximation, the method of calculating the electronic structure of nonstoichiometric and hyperstoichiometric compounds with strong electron correlations and spin-orbit coupling has been developed. This method can be used to study both substitutional and interstitial impurities, which is demonstrated based on the example of the hyperstoichiometric UO2.12 compound. The influence of the coherent potential on the electronic structure of compounds has been shown for the nonstoichiometric UO1.87 containing vacancies in the oxygen sublattice as substitutional impurities, for stoichiometric UO2 containing vacancies in the oxygen sublattice and oxygen as an interstitial impurity, and for hyperstoichiometric UO2.12 with excess oxygen also as interstitial impurity. In the model of the uniform distribution of impurities, which forms the basis of the coherent potential approximation, the energy spectrum of UO2.12 has a metal-like character.

Charge doping versus impurity scattering in chemically substituted iron pnictides

Abstract: To reveal the relative importance of charge doping and defect scattering in substitutionally modified 122 iron pnictides, we perform a systematic first-principles study on selected bands at the Fermi level. Disorder effects are induced by various substitutions using an orbital-based coherent potential approximation. Pronounced level shifts of individual bands suggest that transition-metal substitutions introduce mobile charge carriers into the system. However, important deviations from such a rigid-band scenario as well as spectral broadenings due to impurity scattering correlate with the band character. Finally, a $T$-matrix analysis exhibits a larger intraband than interband scattering consistent with an ${s}^{+\ensuremath{-}}$ pairing state. Comparing different substitutions reveals an increase of pair breaking along the transition-metal series.

Explanation of ferromagnetism origin in N-doped ZnO by first-principle calculations

Abstract: By ab-initio calculations, the possible source of ferromagnetism in N-doped ZnO compound was systematically studied. The electronic structure and magnetic properties of N-doped ZnO with/without ZnO host and N defects were investigated using the Korringa–Kohn–Rostoker method combined with coherent potential approximation. It was shown that Zn vacancy and the presence of N defects (substitutional, interstitial or combination of both) induce the ferromagnetism in N-doped ZnO. From density of state analysis, it was shown that p–p interaction between 2p-elements (N,O) is the mechanism of ferromagnetic coupling in N-doped ZnO.

Paramagnetic properties of Fe-Mn and Fe-V alloys: a DMFT study.

Abstract: We calculate magnetic susceptibility of paramagnetic bcc Fe-Mn and Fe-V alloys by two different approaches. The first approach employs the coherent potential approximation (CPA) combined with the dynamical mean-field theory (DMFT). The material-specific Hamiltonians in the Wannier function basis are obtained by density functional theory. In the second approach, we construct supercells modeling the binary alloys and study them using DMFT. Both approaches lead to a qualitative agreement with experimental data. In particular, the decrease of Curie temperature with Mn content and a maximum at about 10 at.% V are well described in units of the Curie temperature of pure iron. In contrast to the Mn impurities, the V ones are found to be antiferromagnetically coupled to Fe atoms. Our calculations for the two-band Anderson-Hubbard model indicate that the antiferromagnetic coupling is responsible for a maximum in the concentration dependence of Curie temperature in Fe-V alloys.