Showing papers on "Coherent potential approximation published in 2018"

Thermopower of thermoelectric materials with resonant levels: PbTe:Tl versus PbTe:Na and Cu 1 − x Ni x

Abstract: Electronic transport properties of thermoelectric materials containing resonant levels are discussed by analyzing the two best known examples: copper-nickel metallic alloy (Cu-Ni, constantan) and thallium-doped lead telluride (PbTe:Tl). As a contrasting example of a material with a nonresonant impurity, sodium-doped PbTe is considered. Theoretical calculations of the electronic structure, Bloch spectral functions, and energy-dependent electrical conductivity at $T=0$ K are done using the Korringa-Kohn-Rostoker method with the coherent potential approximation and the Kubo-Greenwood formalism. The effect of a resonance on the residual resistivity and electronic lifetimes in PbTe is analyzed. By using the full Fermi integrals, room-temperature thermopower is calculated, confirming its increase in PbTe:Tl versus PbTe:Na, due to the presence of the resonant level. In addition, our calculations support the self-compensation model, in which the experimentally observed reduction of carrier concentration in PbTe:Tl against the nominal one is explained by the presence of $n$-type Te vacancies.

Impurity Resistivity of fcc and hcp Fe-Based Alloys: Thermal Stratification at the Top of the Core of Super-Earths

Abstract: It is widely known that the Earth's Fe dominant core contains a certain amount of light elements such as H, C, N, O, Si, and S. We report the results of first-principles calculations on the band structure and the impurity resistivity of substitutionally disordered hcp and fcc Fe based alloys. The calculation was conducted by using the AkaiKKR (machikaneyama) package, which employed the Korringa-Kohn-Rostoker (KKR) method with the atomic sphere approximation (ASA). The local density approximation (LDA) was adopted for the exchange-correlation potential. The coherent potential approximation (CPA) was used to treat substitutional disorder effect. The impurity resistivity is calculated from the Kubo-Greenwood formula with the vertex correction. In dilute alloys with 1 at. % impurity concentration, calculated impurity resistivities of C, N, O, S are comparable to that of Si. On the other hand, in concentrated alloys up to 30 at. %, Si impurity resistivity is the highest followed by C impurity resistivity. Ni impurity resistivity is the smallest. N, O, and S impurity resistivities lie between Si and Ni. Impurity resistivities of hcp-based alloys show systematically higher values than fcc alloys. We also calculated the electronic specific heat from the density of states (DOS). For pure Fe, the results show the deviation from the Sommerfeld value at high temperature, which is consistent with previous calculation. However, the degree of deviation becomes smaller with increasing impurity concentration. The violation of the Sommerfeld expansion is one of the possible sources of the violation of the Wiedemann-Franz law, but the present results could not resolve the inconsistency between recent electrical resistivity and thermal conductivity measurements. Based on the present thermal conductivity model, we calculated the conductive heat flux at the top of terrestrial cores, which is comparable to the heat flux across the thermal boundary layer at the bottom of the mantle. This indicates that the thermal stratification may develop at the top of the liquid core of super-Earths, and hence, chemical buoyancies associated with the inner core growth and/or precipitations are required to generate the global magnetic field through the geodynamo.

High surface conductivity of Fermi-arc electrons in Weyl semimetals

Abstract: Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semiquantized anomalous Hall effect, and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder-tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi-arc surface states. Here, we analyze electron-phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight-binding model with the presence of quenched surface vacancies using the coherent potential approximation and Kubo-Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is one to two orders of magnitude larger than a comparable setup with surface states of TI. This is primarily attributed to a significantly different hybridization strength of the surface states with the remaining electrons in two systems. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

High Surface Conductivity of Fermi Arc Electrons in Weyl semimetals

Abstract: Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semiquantized anomalous Hall effect, and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder-tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi-arc surface states. Here, we analyze electron-phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight-binding model with the presence of quenched surface vacancies using the coherent potential approximation and Kubo-Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is one to two orders of magnitude larger than a comparable setup with surface states of TI. This is primarily attributed to a significantly different hybridization strength of the surface states with the remaining electrons in two systems. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

The effects of ferromagnetism and interstitial hydrogen on the equation of states of hcp and dhcp FeHx: Implications for the Earth's inner core age

Abstract: Hydrogen has been considered as an important candidate of light elements in the Earth's core. Because iron hydrides are unquenchable, hydrogen content is usually estimated from in situ X-ray diffraction measurements that assume the following linear relation: x = (V-FeHx - V-Fe)/Delta V-H, where x is the hydrogen content, Delta V-H is the volume expansion caused by unit concentration of hydrogen, and V-FeHx and V-Fe are volumes of FeHx and pure iron, respectively. To verify the linear relationship, we computed the equation of states of hexagonal iron with interstitial hydrogen by using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). The results indicate a discontinuous volume change at the magnetic transition and almost no compositional (x) dependence in the ferromagnetic phase at 20 GPa, whereas the linearity is confirmed in the non-magnetic phase. In addition to their effect on the density-composition relationship in the Fe-FeHx system, which is important for estimating the hydrogen incorporation in planetary cores, the magnetism and interstitial hydrogen also affect the electrical resistivity of FeHx. The thermal conductivity can be calculated from the electrical resistivity by using the Wiedemann-Franz law, which is a critical parameter for modeling the thermal evolution of the Earth. Assuming an Fe1-ySiyHx ternary outer core model (0.0 <= x <= 0.7), we calculated the thermal conductivity and the age of the inner core. The resultant thermal conductivity is similar to 100 W/m/K and the maximum inner core age ranges from 0.49 to 0.86 Gyr.

Spin waves in disordered materials.

Abstract: We present an efficient methodology to study spin waves in disordered materials. The approach is based on a Heisenberg model and enables calculations of magnon properties in spin systems with disorder of an arbitrary kind and concentration of impurities. Disorder effects are taken into account within two complementary approaches. Magnons in systems with substitutional (uncorrelated) disorder can be efficiently calculated within a single-site coherent potential approximation for the Heisenberg model. From the computation point of view the method is inexpensive and directly applicable to systems like alloys and doped materials. It is shown that it performs exceedingly well across all concentrations and wave vectors. Another way is the direct numerical simulation of large supercells using a configurational average over possible samples. This approach is applicable to systems with an arbitrary kind of disorder. The effective interaction between magnetic moments entering the Heisenberg model can be obtained from first-principles using a self-consistent Green function method within the density functional theory. Thus, our method can be viewed as an ab initio approach and can be used for calculations of magnons in real materials.

Semiconducting behavior of substitutionally doped bilayer graphene

Abstract: In the framework of the Green's functions approach, random tight-binding model and using the coherent potential approximation, electronic characteristics of the bilayer graphene are investigated by exploring various forms of substitutional doping of a single or both layers of the system by either boron and (or) nitrogen atoms. The results for displacement of the Fermi level resemble the behavior of acceptor or donor doping in a conventional semiconductor, dependent on the impurity type and concentration. The particular pattern of doping of just one layer with one impurity type is most efficient for opening a gap within the energy bands which could be tuned directly by impurity concentration. Doping both layers at the same time, each with one impurity type, leads to an anomaly whereby the gap decreases with increasing impurity concentration.

Systematic Quantum Cluster Typical Medium Method for the Study of Localization in Strongly Disordered Electronic Systems

Abstract: Great progress has been made in recent years towards understanding the properties of disordered electronic systems. In part, this is made possible by recent advances in quantum effective medium methods which enable the study of disorder and electron-electronic interactions on equal footing. They include dynamical mean-field theory and the Coherent Potential Approximation, and their cluster extension, the dynamical cluster approximation. Despite their successes, these methods do not enable the first-principles study of the strongly disordered regime, including the effects of electronic localization. The main focus of this review is the recently developed typical medium dynamical cluster approximation for disordered electronic systems. This method has been constructed to capture disorder-induced localization and is based on a mapping of a lattice onto a quantum cluster embedded in an effective typical medium, which is determined self-consistently. Unlike the average effective medium-based methods mentioned above, typical medium-based methods properly capture the states localized by disorder. The typical medium dynamical cluster approximation not only provides the proper order parameter for Anderson localized states, but it can also incorporate the full complexity of Density-Functional Theory (DFT)-derived potentials into the analysis, including the effect of multiple bands, non-local disorder, and electron-electron interactions. After a brief historical review of other numerical methods for disordered systems, we discuss coarse-graining as a unifying principle for the development of translationally invariant quantum cluster methods. Together, the Coherent Potential Approximation, the Dynamical Mean-Field Theory and the Dynamical Cluster Approximation may be viewed as a single class of approximations with a much-needed small parameter of the inverse cluster size which may be used to control the approximation. We then present an overview of various recent applications of the typical medium dynamical cluster approximation to a variety of models and systems, including single and multiband Anderson model, and models with local and off-diagonal disorder. We then present the application of the method to realistic systems in the framework of the DFT and demonstrate that the resulting method can provide a systematic first-principles method validated by experiment and capable of making experimentally relevant predictions. We also discuss the application of the typical medium dynamical cluster approximation to systems with disorder and electron-electron interactions. Most significantly, we show that in the limits of strong disorder and weak interactions treated perturbatively, that the phenomena of 3D localization, including a mobility edge, remains intact. However, the metal-insulator transition is pushed to larger disorder values by the local interactions. We also study the limits of strong disorder and strong interactions capable of producing moment formation and screening, with a non-perturbative local approximation. Here, we find that the Anderson localization quantum phase transition is accompanied by a quantum-critical fan in the energy-disorder phase diagram.

First-principles study of ZnSnAs2-based dilute magnetic semiconductors

Abstract: The electronic structure and magnetic properties of chalcopyrite Zn(Sn,TM)As2 and (Zn,TM)SnAs2 have been investigated by the Korringa–Kohn–Rostoker method combined with the coherent potential approximation within the local spin density approximation, where TM denotes a 3d transition metal element. We find that the half-metallic and high-spin ferromagnetic state can be obtained in Zn(Sn,V)As2, Zn(Sn,Cr)As2, Zn(Sn,Mn)As2, (Zn,V)SnAs2, and (Zn,Cr)SnAs2. The calculated result of Zn(Sn,Mn)As2 is in good agreement with the experimentally observed room-temperature ferromagnetism if we can control selective Mn doping at Sn sites. In addition, (Zn,V)SnAs2 and (Zn,Cr)SnAs2 are predicted to exhibit high-Curie-temperature ferromagnetism.

Systematic Quantum Cluster Typical Medium Method For the Study of Localization in Strongly Disordered Electronic Systems

Abstract: Great progress has been made in the last several years towards understanding the properties of disordered electronic systems. In part, this is made possible by recent advances in quantum effective medium methods which enable the study of disorder and electron-electronic interactions on equal footing. They include dynamical mean field theory and the coherent potential approximation, and their cluster extension, the dynamical cluster approximation. Despite their successes, these methods do not enable the first-principles study of the strongly disordered regime, including the effects of electronic localization. The main focus of this review is the recently developed typical medium dynamical cluster approximation for disordered electronic systems. This method has been constructed to capture disorder-induced localization, and is based on a mapping of a lattice onto a quantum cluster embedded in an effective typical medium, which is determined self-consistently. Here we provide an overview of various recent applications of the typical medium dynamical cluster approximation to a variety of models and systems, including single and multi-band Anderson model, and models with local and off-diagonal disorder. We then present the application of the method to realistic systems in the framework of the density functional theory.

Magnetic Compton profiles of disordered Fe 0.5 Ni 0.5 and ordered FeNi alloys

Abstract: We study the magnetic Compton profile (MCP) of the disordered Fe0.5Ni0.5 and of the ordered FeNi alloys and discuss the interplay between structural disorder and electronic correlations. The coherent potential approximation is employed to model the substitutional disorder within the single-site approximation, while local electronic correlations are captured with the dynamical mean field theory. Comparison with the experimental data reveals the limitation of local spin-density approximation in the low momentum region, where we show that including local but dynamic correlations the experimental spectra is excellently described. We further show that using local spin-density approximation no significant difference is seen between the MCP spectra of the disordered Fe0.5Ni0.5 and a hypothetical FeNi alloy having the ordered CuAu L1(0) structure. Only by including the electronic correlations, the spectra significantly separate, from the second Brillouin zone boundary down to zero momenta. The difference between the MCP spectra of ordered and disordered alloys is discussed also in terms of the atomic-type decompositions. Finally based on the presented calculations we predict the shape of the MCP profile for the ordered FeNi alloy along the [111] direction.

Origin of Spin Reorientation Transitions in Antiferromagnetic MnPt-Based Alloys

Abstract: Antiferromagnetic MnPt exhibits a spin reorientation transition (SRT) as a function of temperature, and off-stoichiometric Mn-Pt alloys also display SRTs as a function of concentration. The magnetocrystalline anisotropy in these alloys is studied using first-principles calculations based on the coherent potential approximation and the disordered local moment method. The anisotropy is fairly small and sensitive to the variations in composition and temperature due to the cancellation of large contributions from different parts of the Brillouin zone. Concentration and temperature-driven SRTs are found in reasonable agreement with experimental data. Contributions from specific band-structure features are identified and used to explain the origin of the SRTs.

Effect of Tuning the Structure on the Optical and Magnetic Properties by Various Transition Metal Doping in ZnO/TM (TM = Fe, FeCo, Cr, and Mn) Thin Films

Abstract: We have investigated in this work the physical, optical, electronic, and the magnetic behavior (Curie temperature, magnetic moment) of Zn.1-x.MxO (M = Fe 5%, Co 1%, Cr 5%, and Mn 5%), diluted magnetic semiconductors (DMSs). The samples were deposited on glass substrate by the spray pyrolysis technique, and the results of the x-ray diffraction (XRD) of the prepared substrates was used to prove the incorporation of the dopants into the ZnO lattice host; the ferromagnetic and the antiferromagnetic state competitions and their effects on the physical, magnetic, and optical properties, were investigated. The electronic structure and magnetic properties of transition metal (TM) defects, were investigated in detail, by using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA). As a result, doping by TM impurities induce the ferromagnetism with different competitions between the ferromagnetic and antiferromagnetic states, which affects the physical properties of the TM (Fe, Fe/Co, Cr, and Mn)-doped ZnO, with a Curie temperature closer to room-temperature ferromagnetic.

Spin-disorder resistivity of random fcc-NiFe alloys

Abstract: The spin-disorder resistivity (SDR) of a disordered fcc-(${\mathrm{Ni}}_{1\ensuremath{-}x},{\mathrm{Fe}}_{x}$) alloy is determined from first principles. We identify the SDR at and above the critical temperature with the residual resistivity of the corresponding paramagnetic state evaluated in the framework of the disordered local moment (DLM) model. The underlying electronic structure is determined by means of the tight-binding linear muffin-tin orbital method, which employs the coherent potential approximation (CPA) to describe both the DLM state and the chemical disorder in alloys. An extension of the DLM fixed-spin moment method for two independent magnetic moments is used and combined with the paramagnetic lattice gas entropy to determine local moments by minimizing the corresponding free energy. The effect of phonon scattering is included through the mapping of static atomic displacements into a multicomponent random alloy which is then treated in the CPA. Finally, the Kubo-Greenwood-CPA approach is employed to estimate the SDR. We also address the problem of the validity of the Matthiessen rule at the Curie point. Good agreement of calculated and measured SDR is obtained over the whole studied concentration range; the results point to the importance of nonzero Ni magnetic moments in the limit of pure nickel.

First principle calculations with SIC correction of Fe-doped CuO compound

Abstract: In this work the electronic properties of Fe doped CuO thin films are studied by using a standard density functional theory. This approach is based on the abinitio calculations under the Korringa Kohn Rostoker coherent potential approximation. We carried out our investigations in the framework of the general gradient approximation and self interaction corrected. The density of states in the energy diagrams are presented and discussed. The computed electronic properties of the studied compound confirm the half metalicity nature of this material. In addition, the absorption spectra of the studied compound within the Generalized Gradient Approximation, as proposed by Perdew Burke Ernzerhof approximations are examined. When compared with the pure CuO, the Fermi levels of doped structures are found to move to the higher energy directions. To complete this study, the effect of Fe doping method in CuO has transformed the material to half metallic one. We found a high wide impurity band in two cases of approximations methods.

Electronic, Magnetic and Optical Properties of TM (Ti, V, Cr, Mn and Co)-Doped Rare-Earth Nitride GdN: First-Principles Theory

Abstract: The half-metallic ferromagnetic behavior of rare-earth nitride Gd0.95 TM0.05N (TM = Ti, V, Cr, Mn and Co), based on diluted magnetic semiconductors (DMSs), is investigated using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA) within a framework of density functional theory (DFT). The energy difference between the ferromagnetic and disorder local moment states has been evaluated. The exchange interactions obtained from first-principles calculations resulted in ferromagnetic states with Curie temperatures within the ambient conditions. Moreover, the optical absorption spectra obtained by ab initio calculations confirm the ferromagnetic stability based on the charge state of magnetic impurities.

Breakdown of coherence in Kondo alloys: crucial role of concentration vs band filling

Abstract: We study the low energy states of the Kondo alloy model (KAM) as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, nc. In previous works, two different Fermi liquid regimes had been identified at strong Kondo coupling JK, that may be separated by a transition at x=nc. Here, we analyze the KAM for finite JK on a Bethe lattice structure. First, using the mean-field coherent potential approximation (DMFT-CPA) which is exact at lattice coordination Z=infty, we show that the real part of the local potential scattering may be located outside the conduction electron band, revealing a possible breakdown of Luttinger 'theorem' for intermediate values of impurity concentration x. Unusual physical signatures are expected, e.g., in ARPES experiments. In order to take into account fluctuations associated with finite dimensionality,i.e., finite Z, we extend this analyze by also studying the KAM with an adaptation of the statistical-DMFT method that was developped elsewhere. We review the distributions of local potential scattering and their evolution with model parameters: concentration, strength of Kondo coupling, coordination number, local site neighborhood, connection with percolation issue. Relevence for Kondo alloys material with f-electrons is also discussed.

Ab Initio Study of Electronic and Magnetic Properties in ZnO-Doped and Co-doped by Vanadium and Silver

Abstract: Based upon the ab initio of spin density functional calculation, the electronic and magnetic properties of Zn1−x M x O (M = Ag or V) and Zn1−x−y A x B y O (A = V; B = Ag) are investigated, using the Korringa-Kohn-Rostoker (KKR) method coupled with the coherent potential approximation (CPA). The total and partial densities of state are calculated. The effect of concentration values in Zn1−x V x O and Zn1−x−y V x Ag y O are deduced. Moreover, the magnetic disorder local moment (DLM) and the total energy are obtained for different concentration values of doped and co-doped zinc oxide (ZnO).

Calculation of Ferromagnetic State and Critical Temperature in Transition Metal Doped III-V Wurtzite Semiconductors

Abstract: The magnetic properties and electronic states of the transition metal doped III-V wurtzite compounds ( A 1 − x M x )N are calculated using Korringa-Kohn-Rostoker Green’s function method combined with the coherent potential approximation, where A = Al, Ga and M = 3 d transition metal atoms namely V, Cr, Mn, Fe, Co, Ni and x is the fractional concentration of M . The positive value of the energy difference between ferromagnetic (FM) state and disordered local magnetic moment (DLM) state per unit cell denotes the magnetic phase stability. The total energy difference (E DLM E FM ) is used to estimate the Curie temperature (T C ) within the mean-field approximation. The calculated T C of V and Cr doped nitrides increases rapidly at lower concentrations and is found to be above the room temperature in the concentration range of x = 0.05 – 0.20. The FM behavior in Mn doped (Al 1− x Mn x ) N is suppressed at the concentration range of x = 0.01 – 0.10. A clear phase transition from DLM to FM state occurs at concentrations x > 0.10. The energy difference in Fe, Co and Ni doped materials, results in lower values of the DLM states, where the super-exchange interaction dominates over the FM one. The FM materials exhibiting T C above room temperature have applications in the field of spintronics. Journal of Bangladesh Academy of Sciences, Vol. 41, No. 2, 217-225, 2017

First-Principles Calculations of the Electronic Structure of Imperfect Crystals in the Coherent Potential Approximation

Abstract: The results of electronic structure calculations of imperfect crystals—nonstoichiometric metal oxides (TiO2 – x, TiO1 – x, Al2O3 – x) are reviewed. All calculations were performed within the density functional theory in the coherent potential approximation with vacancies randomly distributed in metal and oxygen sublattices. It is found that the deviations from stoichiometric composition are accompanied by appearance of vacancy induced electronic states inside the band gap of initial insulating oxides.