Coherent Potential Approximation Research Articles

Structure and magnetism in metastable bcc Co[formula omitted]Mn[formula omitted] epitaxial films

Co-rich Co1−xMnx alloys have hcp or fcc disordered phase and those ferromagnetic orderings are significantly deteriorated with increasing Mn concentration x in bulk. On the other hand, those metastable bcc phases show properties attractive to magnetics and spintronics, as studied recently. Here, we report systematic study of structure and magnetism for epitaxial thin films. The single phase bcc Co1−xMnx(001) films were pseudomorphically grown on Cr(001) for 0.14<x<0.50 with a sputtering technique. The saturation magnetization was larger than that of pure Co for the films with x= 0.14 and 0.25 and smaller for the x= 0.34 films, disappearing completely for the x= 0.5 films. X-ray magnetic circular dichroism unveiled that this behavior stemmed from the composition dependence of elemental magnetic moments. Slight enhancement of magnetic moment of Co was observed in bcc alloy films. We also found that magnetic moments of Mn exceeded 2 μB at the maximum and were not as high as theoretical values, approximately 3 μB, obtained from ab-initio calculation with coherent potential approximation. This difference is qualitatively understood in terms of microscopic Mn atom pairing with antiferromagnetic coupling, being suggested from the ab-initio band structure calculations based on special quasi-random structure of bcc Co1−xMnx.

Vibrational disorder and densification-induced homogenization of local elasticity in silicate glasses

We report the effect of structural compaction on the statistics of elastic disorder in a silicate glass, using heterogeneous elasticity theory with the coherent potential approximation (HET-CPA) and a log-normal distribution of the spatial fluctuations of the shear modulus. The object of our study, a soda lime magnesia silicate glass, is compacted by hot-compression up to 2 GPa (corresponding to a permanent densification of ~ 5%). Using THz vibrational spectroscopic data and bulk mechanical properties as inputs, HET-CPA evaluates the degree of disorder in terms of the length-scale of elastic fluctuations and the non-affine part of the shear modulus. Permanent densification decreases the extent of non-affine elasticity, resulting in a more homogeneous distribution of strain energy, while also decreasing the correlation length of elastic heterogeneity. Complementary 29Si magic angle spinning NMR spectroscopic data provide a short-range rationale for the effect of compression on glass structure in terms of a narrowing of the Si–O–Si bond-angle and the Si–Si distance.

Machine-learning potentials enable predictive and tractable high-throughput screening of random alloys

We present an automated procedure for computing stacking fault energies in random alloys from large-scale simulations using moment tensor potentials (MTPs) with the accuracy of density functional theory (DFT). To that end, we develop an algorithm for training MTPs on random alloys. In the first step, our algorithm constructs a set of $\ensuremath{\sim}10$ 000 or more training candidate configurations with 50--100 atoms that are representative for the atomic neighborhoods occurring in the large-scale simulation. In the second step, we use active learning to reduce this set to $\ensuremath{\sim}100$ most distinct configurations---for which DFT energies and forces are computed and on which the potential is ultimately trained. We validate our algorithm for the MoNbTa medium-entropy alloy by showing that the MTP reproduces the DFT $\frac{1}{4}[111]$ unstable stacking fault energy over the entire compositional space up to a few percent. Contrary to state-of-the-art methods, e.g., the coherent potential approximation (CPA) or special quasi-random structures (SQSs), our algorithm naturally accounts for relaxation, is not limited by DFT cell sizes, and opens opportunities to efficiently investigate follow-up problems, such as chemical ordering. In a broader sense, our algorithm can be easily modified to compute related properties of random alloys, for instance, misfit volumes, or grain boundary energies. Moreover, it forms the basis for an efficient construction of MTPs to be used in large-scale simulations of multicomponent systems.

Electronic correlations and Fermi liquid behavior of intermediate-band states in titanium-doped silicon

We study the nature of the electronic states in the intermediate band formed by interstitial titanium in silicon. Our single-site description combines effects of electronic correlations, captured by dynamical mean-field theory, and disorder, modeled using the coherent potential approximation and the typical medium mean-field theory. For all studied concentrations an extended metallic state with a strongly depleted density of states at the Fermi level is obtained. The self-energy is characteristic to Fermi-liquids and for certain temperatures reveals the existence of coherent quasi-particles.

Theoretical Study of Temperature Dependence of Spin Susceptibility in Anisotropic Itinerant Ferromagnets

We developed a framework for directly calculating the spin susceptibility of anisotropic itinerant ferromagnets in the full temperature range within the coherent potential approximation in the disordered local moment picture. As a test of our formulation, we demonstrate the computation for the temperature ($T$) dependence of the spin susceptibility for Rashba-type ferromagnets with the Curie temperature $T_\mathrm{C}$. In a certain parameter, we found that the inverse transverse susceptibility $1/\chi_\perp\simeq\mathrm{const.}$ for $T<T_\mathrm{C}$ and $1/\chi_\perp\propto T-T_\mathrm{C}$ for $T>T_\mathrm{C}$, which reflects characteristics of itinerant ferromagnets including a spin-orbit interaction.

The effect of Ni doping on the electronic structure and superconductivity in the noncentrosymmetric ThCoC[formula omitted

ThCoC2 is a non-BCS, noncentrosymmetric superconductor with Tc≃2.5 K, in which pairing mechanism was suggested to be either spin fluctuations or the electron–phonon coupling. Earlier experimental work revealed that Tc can be greatly enhanced upon Ni doping in ThCo1−xNixC2, up to 12 K for x=0.4. In this work, using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA), the evolution of the electronic structure upon Ni doping was studied and the increase of the density of states at the Fermi level was found. The electron–phonon coupling constant λ was calculated using the rigid muffin tin approximation (RMTA) and the increase in λ(x) was found. This indirectly confirms the electron–phonon coupling scenario as the trend in λ(x) is in qualitative agreement with the experiment when the electron–phonon pairing is assumed.

Magnetic properties of CrFeCoNi based high entory alloy

Monte Carlo simulations are performed on three high entropy alloys: Cr0.25Fe0.25Co0.25Ni0.25, Cr0.2Fe0.2Co0.2Ni0.2Pd0.2, and Cr0.2Mn0.2Fe0.2Co0.2Ni0.2, with exchange interactions extracted from The ab initio Korringa-Kohn-Rostoker method combined with the coherent potential approximation calculations. Using finite size scaling analyses, we estimate the magnetic phase transition temperature for the four component alloy to be 108(2) K, and although the individual critical exponents are different from 3D Heisenberg universality class, the reduced exponent follows Suzuki weak universality. With the additional Palladium component, the transition temperature elevates to about 200 K. In contrast, we find no magnetic order for the five component alloy with Manganese at any finite temperatures.

Structural stability and mechanical properties of TiAl+Mo alloys: A comprehensive ab initio study

Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc βo phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the βo and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.

Disorder classification of the vibrational spectra of modern glasses

Using the coherent-potential approximation in heterogeneous-elasticity theory with a log-normal distribution of elastic constants for the description of the Raman spectrum and the temperature dependence of the specific heat, we are able to reconstruct the vibrational density of states and characteristic descriptors of the elastic heterogeneity of a wide range of glassy materials. These descriptors are the nonaffine contribution to the shear modulus, the mean-square fluctuation of the local elasticity, and its correlation length. They enable a physical classification scheme for disorder in modern, industrially relevant glass materials. We apply our procedure to a broad range of real-world glass compositions, including metallic, oxide, chalcogenide, hybrid, and polymer glasses. Universal relationships between the descriptors on the one side, and the height and frequency position of the boson peak, the Poisson ratio and the liquid fragility index on the other side are established.

Energy Gap Decrease in Cation Multidoped Aluminum Oxide

The electronic structure of metal‐doped α‐Al2O3 with the disordered distribution of multi‐d‐ (Zr, Nb or/and Mo) and s‐ (Mg) impurities in the aluminum sublattice under the variation of their concentrations have been calculated using the coherent potential approximation method. Herein, an energy gap reduction due to appearance of new isolated occupied or empty Zr, Nb, and Mo d bands near the conduction band bottom of undoped α‐Al2O3 is shown. New multi d‐ and s‐doped α‐Al2O3 dielectrics with energy gap values of 7.1, 5.6, and 4.3 eV are found. Four multi‐d‐doped α‐Al2O3 semiconductors , , , and with energy gap values of 0.2–0.4 eV can be considered as promising materials for optoelectronic devices.

Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution

Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties.

Oxygen-vacancy enhanced tunnel electroresistance in LaNiO3/BaTiO3/LaNiO3 ferroelectric tunnel junctions

Oxygen vacancies (OVs) usually exist in perovskite oxides in ferroelectric tunnel junctions (FTJs), which significantly influence electron transport properties of FTJ. However, the role of OVs is currently not well understood since the OVs concentration is difficult to detect in experiments or to simulate using traditional first-principles methods. Here, using the density functional theory combined with nonequilibrium Green's function and coherent potential approximation (NECPA-DFT), we investigate electron transport properties of the LaNiO3/BaTiO3/LaNiO3 FTJ, which has a low concentration OVs in the left LaNiO3/BaTiO3 interface. The tunnel barrier height monotonously decreases with the increase in the OVs concentration for the rightward polarization in BaTiO3, leading to an increased electron tunneling coefficient. In contrast, for a leftward polarization, the barrier height only slightly decreases with the increasing OVs concentration, leading to a nearly invariant electron tunneling coefficient. The tunnel electroresistance ratio, therefore, increases monotonously with the OVs concentration and reaches to 5898% for a OVs concentration of 9%. Our results show that OVs play a critical role in determining electron transport properties of an FTJ as well as provide an alternative avenue to realize a natural asymmetric FTJ to improve its performance.

An investigation of high entropy alloy conductivity using first-principles calculations

The Kubo–Greenwood equation, in combination with the first-principles Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) can be used to calculate the DC residual resistivity of random alloys at T = 0 K. We implemented this method in a multiple scattering theory based ab initio package, MuST, and applied it to the ab initio study of the residual resistivity of the high entropy alloy AlxCoCrFeNi as a function of x. The calculated resistivities are compared with experimental data. We also predict the residual resistivity of refractory high entropy alloy MoNbTaVxW. The calculated resistivity trends are also explained using theoretical arguments.

DFT study, Mean-Field Approximation and Monte-Carlo simulation of physical properties of Fe and Cr doped and co-doped GaN

The present study is carried out for investigating the half-metallic ferromagnetic behavior of Cr and Fe doped and co-doped GaN, respectively. To this end, ab-initio calculations using the Korringa-Kohn-Rostoker Green's function method coupled with the coherent potential approximation have been employed. In the background to make the study more useful, the first-principles computations were added and merged to the mean-field approximation and the Monte-Carlo simulation for the co-doped compound. Several variations were perceived in the results as in the bandgap energy, the energetic location of the transition-metal 3d band and the X-ray absorption spectra. Besides, the stability of the system between the ferromagnetic and the spin-glass states is studied. Finally, we have integrated as input parameters in the classical Ising model by Monte-Carlo simulation, the exchange interactions obtained from ab-initio calculation, in order to confirm the half-metallic ferromagnetic states with high Néel temperature.

Martensitic Transformation in Fe_{x}Mn_{80-x}Co_{10}Cr_{10} High-Entropy Alloy.

High-entropy alloys and even medium-entropy alloys are an intriguing class of materials in that structure and property relations can be controlled via alloying and chemical disorder over wide ranges in the composition space. Employing density-functional theory combined with the coherent-potential approximation to average over all chemical configurations, we tune free energies between face-centered-cubic and hexagonal-close-packed phases in Fe_{x}Mn_{80-x}Co_{10}Cr_{10} systems. Within Fe-Mn-based alloys, we show that the martensitic transformation and chemical short-range order directly correlate with the face-centered-cubic and hexagonal-close-packed energy difference and stacking-fault energies, which are in quantitative agreement with recent observation of two phase region (face-centered cubic and hexagonal closed pack) in a polycrystalline high-entropy alloy sample at x=40 at.%. Our predictions are further confirmed by single-crystal measurements on a x=40 at.% using transmission-electron microscopy, selective-area diffraction, and electron-backscattered-diffraction mapping. The results herein offer an understanding of transformation-induced or twinning-induced plasticity in this class of high-entropy alloys and a design guide for controlling the physics at the electronic level.

First-principles calculations for studying effect of Tellurium (Te) doping on electronic properties, half-metallic ferromagnetism and “Curie temperature of V-doped AlSb DMS compound

The electronic structure and magnetic features of intrinsic Al1-xVxSb, AlSb1-yTey and Al1-xVxSb1-yTey Tey diluted magnetic semiconductors (DMSs) compounds at various doping concentrations of x and y have been calculated in this study using Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA), using a coupling local-density-functional approx-imation (LDA). Our structural parameters results showed generally agreement with the previous experimental and theoretical findings. The results of DOS demonstrate that incorporation of V-3d develops a half-metallic (HM) property in our compounds with an engendered bandgap at the Fermi energy in the down-spin state. In these materials, the stability of ferromagnetism (FM) is explained by contribution of both p-d interaction and Zener's double-exchange interaction mechanisms. Accordingly, it has been shown in our studied systems that ferromagnetic (FM) ground states are easily attainable, and that the measured Curie temperature (TC) of the (V, Te)-codoped AlSb system reaches a maximum of 1142.48K (Tcmax) and is far above room temperature (RT) and to that measured of V-doped AlSb compound (Tcmax = 364.18K) This fact and the very large Curie temperature of (V, Te)-codoped AlSb compound make inevitably this material a serious candidate for future electronic applications, especially for spintronics devices.

Determination of spin-wave stiffness in the Fe-Si system using first-principles calculations

The behavior of magnetic materials can be simulated at the macroscale using the micromagnetic model whose key parameters, such as exchange stiffness constants and magnetic anisotropies, can be derived from first-principles electronic structure calculations. In this work we employed the Korringa-Kohn-Rostoker (KKR) Green's function method with the coherent potential approximation (CPA) to investigate the dependence of the spin-wave stiffness on the Si concentration for the three magnetic phases of FeSi, namely A2, B2, and $\mathrm{D}{0}_{3}$. Based on the structural, magnetic, and electronic structure analysis using the KKR-CPA methodology, the changes in the spin-wave stiffness caused by the addition of Si are primarily governed by the variations in the electronic structure.

Anomalous Hall and Nernst Conductivities in Co2NbGa: A first principles study

The ground state electronic, magnetic and elastic properties of Co2NbGa are studied using first-principles calculations. The investigated system is mechanically and dynamically stable which is confirmed through Born mechanical stability criteria and positive phonon modes. Two kinds of disorders B2 and D03-types are studied to ensure the experimental magnetic moment in Co2NbGa within coherent potential approximation(CPA). The intrinsic anomalous Hall effect and anomalous Nernst effect are analysed from the Berry curvature point of view. The values of anomalous Hall conductivity and anomalous Nernst conductivity are 214.2 S/cm and −2.53 Am-1K-1 within the range of ±300meV around Fermi level respectively, and the same are comparable with the values of other Heusler alloys. The Curie temperature (TC) is estimated up to 548.7 K through the Heisenberg magnetic exchange interactions within mean-field approximation. Half-metal to a metal transition under pressure is evidenced through band structure and Fermi surface topology. Also the magnetic, Heisenberg exchange, elastic properties are probed under pressure.

Direct comparison of auxiliary and itinerant coherent potential approximations for disordered lattice vibration: Phonon spectral and transport properties

The auxiliary and itinerant coherent potential approximations (ACPA and ICPA) have been proposed as effective methods for treating both mass and force-constant disorder in lattice vibration. In this work we make a direct comparison of ACPA and ICPA to identify the differences between these two methods for simulating vibrational properties of disordered materials and devices. We investigate the major approximations in the disorder self-energies of these two methods by using the diagrammatic method. The single-site ACPA neglects the crossing diagrams describing nonlocal correlations, while ICPA utilizes only the single-fluctuation states in the self-consistency and presents extra errors due to a change in the terms with nearest-neighbor fluctuation. To further demonstrate the important differences between the ACPA and ICPA in the phonon spectral and transport properties, we extend ICPA in combination with the Keldysh nonequilibrium Green's function technique to simulate quantum transport and introduce a Green's-function-based spectral unfolding technique to use with molecular ACPA and the supercell methods for comparison. By studying the disordered one- and three-dimensional alloys, we find that ACPA and ICPA agree very well in the weak scattering regime due to weak force-constant disorder or low phonon frequency. However, for the strong scattering of force-constant disorder, it is found that the two methods present important deviations in the linewidth (and height) of disorder-averaged phonon spectra. Compared to the ACPA and exact results, the problematic treatment of the nearest-neighbor fluctuation in ICPA can present unphysical scattering and induce large errors in the phonon transport, presenting important limitations of ICPA for transport simulation.

Spin waves in alloys at finite temperatures: Application to the FeCo magnonic crystal

We study theoretically the influence of the temperature and disorder on the spin wave spectrum of the magnonic crystal Fe$_{1-c}$Co$_{c}$. Our formalism is based on the analysis of a Heisenberg Hamiltonian by means of the wave vector and frequency dependent transverse magnetic susceptibility. The exchange integrals entering the model are obtained from the \emph{ab initio} magnetic force theorem. The coherent potential approximation is employed to treat the disorder and random phase approximation in order to account for the softening of the magnon spectrum at finite temperatures. The alloy turns out to exhibit many advantageous properties for spintronic applications. Apart from high Curie temperature, its magnonic bandgap remains stable at elevated temperatures and is largely unaffected by the disorder. We pay particular attention to the attenuation of magnons introduced by the alloying. The damping turns out to be a non-monotonic function of the impurity concentration due to the non-trivial evolution of the value of exchange integrals with the Co concentration. The disorder induced damping of magnons is estimated to be much smaller than their Landau damping.