Showing papers on "Coherent potential approximation published in 2019"

First principle study of magnetism and vacancy energetics in a near equimolar NiFeMnCr high entropy alloy

Abstract: We report the results of ab initio calculations of a novel NiFeMnCr high entropy alloy (HEA) with potential applications as a high performance structural material. The bulk and defect property variations due to chemical disordering and magnetic frustration have been studied using both supercell and coherent potential approximation-based techniques. While magnetic frustration due to the presence of multiple 3d transition metals can severely affect the accuracy of vacancy formation energy in first-principles calculations, this effect should be suppressed at intermediate and high temperatures. An efficient approach to evaluate the chemical potential in HEA is constructed and implemented. Vacancy formation energies are computed based on the chemical potential. The statistical distribution of formation energies is weakly dependent upon the chemical identity of the vacancy. On the other hand, the calculated vacancy migration energies show that Fe is more likely to have a large migration barrier than Cr, Mn, or Ni. Finally, atomic-level stresses are computed. A qualitative model to explain the elemental segregation trend in HEA is built upon the atomic-level stress calculation results and provides a reasonable qualitative agreement with ion irradiation experimental data of a NiFeMnCr HEA.

Electron Transport in Carbon Nanotubes with Adsorbed Chromium Impurities.

Abstract: We employ Green’s function method for describing multiband models with magnetic impurities and apply the formalism to the problem of chromium impurities adsorbed onto a carbon nanotube. Density functional theory is used to determine the bandstructure, which is then fit to a tight-binding model to allow for the subsequent Green’s function description. Electron–electron interactions, electron–phonon coupling, and disorder scattering are all taken into account (perturbatively) with a theory that involves a cluster extension of the coherent potential approximation. We show how increasing the cluster size produces more accurate results and how the final calculations converge as a function of the cluster size. We examine the spin-polarized electrical current on the nanotube generated by the magnetic impurities adsorbed onto the nanotube surface. The spin polarization increases with both increasing concentration of chromium impurities and with increasing magnetic field. Its origin arises from the strong electron correlations generated by the Cr impurities.

Hidden Mn magnetic-moment disorder and its influence on the physical properties of medium-entropy NiCoMn solid solution alloys

Abstract: The ab initio Korringa-Kohn-Rostoker method combined with the coherent potential approximation (CPA) was employed to investigate the electronic, magnetic, and transport properties of medium-entropy face-centered-cubic (fcc) NiCoMn solid solution alloys. By comparing the CPA electronic structure with that from supercell calculations, we uncovered an unconventional CPA ground state, which correctly distinguishes two equally populated Mn CPA components---with large spin moments but opposite orientations. Using the spin spiral calculations, we further demonstrated that this ground state is most energetically favorable in the presence of spin noncollinearity, and no significant longitudinal spin fluctuation is observed, justifying the applicability of the Heisenberg model. The finite-temperature magnetism was further studied using different approximations based on the Heisenberg model, and we found the Mn moments to be fully disordered at low temperature due to a small net effective Weiss field on Mn. In addition, the magnetic effect on the electron scattering at finite temperatures was evaluated and compared with other scattering mechanisms. Since the magnetization-induced electron scattering is almost saturated in the ground state, (full) spin disorder only yields a small addition to the resistivity, whereas the thermal displacements increase it modestly. Finally, we elucidate the role of hydrostatic pressure on the magnetic and transport properties. These findings reflect the importance of the magnetic signatures on the physical properties of alloys, and they provide a window into magnetism-controlled electronic structure and energy dissipation.

Exact muffin tin orbital based first-principles method for electronic-structure and electron-transport simulation of device materials

Abstract: The exact muffin tin orbital (EMTO) method features high efficiency and accuracy for first-principles simulations with density functional theory. In this paper we report our implementation of the EMTO method for electronic-structure and quantum transport simulation of device materials. We consider a device-material structure with a central device region in contact with different semi-infinite electrodes. Based on the Green's function method, the infinite device, nonperiodic in transport direction, is transformed into a calculable finite material system by treating the semi-infinite electrodes with electrode self-energies, and the Green's function of the device region is calculated with an efficient recursive technique. In the present implementation we adopt the spherical cell approximation to treat the electrostatics, and we solve the electrostatic potential of the finite device region by enforcing the boundary conditions to the known potential of electrode materials. The coherent potential approximation is incorporated for treating the atomic disorders inevitable in realistic materials, and the effects of multiple disorder scattering on electron transport are accounted for by vertex correction for simulating disordered electronic devices. To demonstrate the capability of the present implementation, we calculate the monolayer two-dimensional material MoS2 and black phosphorus, and study the spin-dependent tunneling in the Fe/MgO/Fe magnetic tunneling junction. We find the EMTO electronic structures of the calculated systems agree well with the results of the projector augmented wave method. The EMTO transport simulation produces the important spin-filtering effect of the Fe/MgO/Fe junction and the important influence of the interfacial disorders on the spin-dependent tunneling, agreeing well with previous theoretical and experimental studies. The implementation of the EMTO based device simulator provides an effective simulation tool for simulating both ordered and disordered device materials, extending the capability for theoretical design of electronic devices from first principles.

Coulomb drag between quantum wires: A nonequilibrium many-body approach

Abstract: We present a real-space theoretical formalism and its numerical implementation for investigating nonequilibrium quantum Coulomb drag between parallel two-terminal transport structures in quasi-one-dimension. In addition to the Coulomb interaction and the finite external potential bias, our formalism takes into account the effects of impurity disorder. The theory is formulated in the nonequilibrium Green's function formalism, with the long-range Coulomb interaction treated at the many-body GW approximation level and the disorder-average carried out at the coherent potential approximation (CPA) level. The coupled GW and CPA equations are solved self-consistently so that the fundamental conservation laws are ensured. The effects related to the electron-hole symmetry in the Coulomb drag physics have been generalized to the nonlinear transport regime. A set of symmetry-induced relations linking physical quantities with the particle distribution were established on generic footing and, remarkably, they are found robust against uniformly distributed impurities. As an application, the theoretical formalism is employed to analyze the Coulomb drag transport physics in quasi-one-dimensional systems. The dependencies of the drag current on external bias, chemical potential, temperature, and the system sizes are predicted.

Atomic ordering, magnetic properties, and electronic structure of Mn 2 CoGa Heusler alloy

Abstract: The magnetic properties and atomic arrangement of Mn2CoGa Heusler alloy were investigated experimentally and by theoretical calculations. The magnetic moment derived from spontaneous magnetization at 5 K was 2.06 μ B/f.u. and was close to the integer number of the expected value from theoretical calculation and the Slater-Pauling rule predicted by Galanakis et al. The Curie temperature and L21-B2 order-disorder phase transition temperature were 741 and 1047 K, respectively. Powder neutron diffraction experiment results suggested that the atomic arrangement prefers an L21b-type structure rather than that of Hg2CuTi, being consistent with our previous results of high-angle annular dark-field-scanning transmission electron microscopic observations. The magnetic moments obtained were in good agreement with the theoretical values in the model of the L21b-type structure. The density of states obtained by the first-principles calculation combined with the coherent potential approximation in Mn2CoGa with the L21b-type crystal structure maintained the half-metallic character, even though disordering by Mn and Co atoms was introduced.

First principle calculations of electronic and magnetic properties of Cr doped HgSe

Abstract: Ab-initio calculations have been performed based on the Korringa-Kohn-Rostoker (KKR) Green's function method combined with coherent potential approximation (CPA) in order to investigate the electronic and magnetic properties of HgSe doped by chromium (Cr). The lattice parameter has been optimized to be 6.5 A which is close to the experimental value of 6.085 A. The pure HgSe compound is found to be a semimetal, whereas the Cr dopant introduces an itinerant ferromagnetic moment with the spin polarization at the Fermi surface of around 90%. The Curie temperature (TC) has been estimated using the mean field theory and is found to vary from 326/313 to 485/585 (K) for 8% and 24% of Cr concentration, against the lattice parameter of 6.5/6.085 (A), respectively. The double exchange is suggested to be the most responsible interaction of ferromagnetism in the system.

Auxiliary coherent medium theory for lattice vibrations in random binary alloys with mass and force-constant disorders

Abstract: The inevitable and random impurities or defects can significantly influence the lattice-vibrational properties of materials and devices. Thus, the capability of effectively treating disorder effects is indispensable for theoretical simulations. In this paper, we report an auxiliary coherent medium theory, in the framework of multiple scattering theory, to simulate disordered vibrational systems containing both mass and force-constant disorders. In this method, the physical Green's function is related to an auxiliary Green's function by introducing a separable force-constant model to describe disordered systems. As an important result, the force-constant disorder can be transformed to a diagonal-like disorder in the auxiliary Hamiltonian while maintaining the important force-constant sum rule. In combination with the single-site and cluster coherent potential approximation, the configurational average over the auxiliary Green's function can be performed to obtain the configuration-averaged physical properties. To demonstrate the effectiveness of this method, we apply it to a one-dimensional harmonic chain with atomic disorders and find our calculations agree very well with the exact results for a wide range of mass and force constants. Moreover, we show that the phonon transport property of disordered devices can be derived based on the auxiliary Green's function formalism in combination with vertex corrections. The auxiliary coherent medium theory features easy implementation and feasible incorporation with diagrammatic technique in many-body perturbation and various cluster approximations, providing an important approach to analyze disorder effects on the vibrational properties. Moreover, it is also straightforward to apply the present formalism to treat the general atomic disorder in electronic systems.

Temperature-dependent resistivity and anomalous Hall effect in NiMnSb from first principles

Abstract: We present implementation of the alloy analogy model within fully relativistic density-functional theory with the coherent potential approximation for a treatment of nonzero temperatures. We calculate contributions of phonons and magnetic and chemical disorder to the temperature-dependent resistivity, anomalous Hall conductivity (AHC), and spin-resolved conductivity in ferromagnetic half-Heusler NiMnSb. Our electrical transport calculations with combined scattering effects agree well with experimental literature for Ni-rich NiMnSb with 1--2% Ni impurities on Mn sublattice. The calculated AHC is dominated by the Fermi surface term in the Kubo-Bastin formula. Moreover, the AHC as a function of longitudinal conductivity consists of two linear parts in the Ni-rich alloy, while it is nonmonotonic for Mn impurities. We obtain the spin polarization of the electrical current $Pg90%$ at room temperature and we show that $P$ may be tuned by chemical composition. The presented results demonstrate the applicability of an efficient first-principles scheme to calculate temperature dependence of linear transport coefficients in multisublattice bulk magnetic alloys.

Effect of proton uptake on the structure of energy levels in the band-gap of Sr-doped LaScO3: diffuse reflectance spectroscopy and coherent potential approximation calculations.

Abstract: Features of the energy levels in the band-gap of La1−xSrxScO3−x/2 and the effect on those levels of proton uptake from H2 and H2O atmospheres were studied by diffuse reflectance spectroscopy and coherent potential approximation (CPA) calculations. It was shown that oxygen vacancies appearing due to acceptor doping with Sr form energy levels near the bottom of the conduction band that are strongly hybridized with the states of the nearest atoms. Excitation of electrons from the valence band to these vacancy levels gives rise to an additional absorption band which overlaps with the fundamental absorption edge. Proton incorporation from both H2 and H2O atmospheres leads to formation of proton levels below the valence band. However, during H2 uptake, electrons from hydrogen atoms occupy oxygen vacancy levels, and as a result additional absorption in the red-IR range appears due to electronic transitions from these levels to the conduction band. On the other hand, H2O uptake leads to the disappearance of oxygen vacancy levels. Experimental results obtained are similar to literature data on the optical absorption properties of some other proton-conducting perovskites, allowing the conclusion that findings from these CPA calculations on the nature of the energy levels could be extrapolated to some extent to those oxides.

Electronic Structure and Conductivity of Off-stoichiometric and Si-doped Ge 2 Sb 2 Te 5 Crystals from Multiple-Scattering Theory

Abstract: The transport properties of the phase-change material Ge2Sb2Te5 can be tuned by controlling its atomic structure and concentration of charge carriers. Moving away from the "225" stoichiometry or doping with atoms of different chemical species are major methods to reach this aim. The transport properties of these doped samples are challenging to study experimentally, since their crystalline phase generally possesses a complicated microstructure, consisting of grains with different compositions. They are also challenging to investigate by first-principles methods based on the calculation of Kohn-Sham wave functions, as larger supercells are needed to describe the unavoidable chemical disorder among Ge, Sb, dopant atoms, and vacancies. In this work, we perform first-principles calculations of the electronic structure and electrical conductivity of off-stoichiometric or Si-doped cubic Ge2Sb2Te5 crystals, using the spin polarized relativistic Korringa-Kohn-Rostoker (KKR) method based on the multiple-scattering theory. The doped crystals have all been described with a rock-salt unit cell, in which the chemical disorder is taken into account through the coherent potential approximation (CPA). The accuracy of the results obtained using this method is verified by comparing, for several crystal compositions, the density of electronic states calculated with this method and with a method that uses Kohn-Sham wave functions and big supercells. We calculated the Bloch spectral function, which shows the dispersion of the electron states and its modification with the deviation from the 225 stoichiometry, silicon doping, and chemical disorder. We describe the composition dependence of the electrical conductivity, which we discuss in terms of the concentration of charge carriers and of the modification of their scattering by the intrinsic chemical disorder in the crystal. These results can be used to model real samples, the microstructure of which consists of grains with different concentrations of Ge, Sb, or Si atoms, each grain being described by a conductivity that depends on its composition.

Nonequilibrium dual-fermion approach to electronic transport in disordered nanostructures

Abstract: First-principles transport modeling of disordered nanostructures commonly resorts to certain effective medium theory constructed with single-site methods. These methods are essentially approximations that account for the diffusive aspect of the problem, but missing the interference-induced long-range effects such as localization, which also plays an important role in the mesoscopic regime. In this work, we report a real-space implementation of the dual-fermion method in the nonequilibrium Keldysh formalism for correcting the transport coefficients given by single-site methods. A mapping between the Keldysh Green's function and its dual counterpart is established, and a diagrammatic perturbation technique is used in the dual space, whereby the long-range Cooperon is taken into account. When treated at the zeroth order, this theory reproduces the nonequilibrium coherent potential approximation. We require self-consistency on two aspects: the dual Green's function is solved consistently with its self-energy, and the medium Green's function must have its real-space diagonal equal to that of the local impurity. The method is applied to a quasi-one-dimensional hopping lattice which mimics a disordered transport structure. The numerically computed transmission coefficient shows quantitative agreement with the exact solution in the weak-localization regime, significantly correcting the single-site results. In addition, we find that the dual-fermion method leads to a power-law dependency of resistance on the channel length, instead of the classical Ohm's law, and the exponent is insensitive to disorder strengths. We also show that the negative magnetoresistance effect, a phenomenon associated with weak localization, is obtained by our numerical model when a perpendicular magnetic field is introduced. The method presented here paves the way for an ab initio atomistic implementation to simulate disordered quantum transport in real nanostructures.

Magnetic Properties of Carbon Co-Doped (Zn,Mn)O Using LDA and LDA-SIC Approximations

Abstract: In this paper, we used the ab-initio calculations, based on the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA), to simulate the magnetic properties of ZnO, doped and co-doped with manganese and carbon, respectively. For this purpose, we have used two different approximations: the Local Density Approximation (LDA) and the Local Density Approximation-Self-Interaction Correction (LDA-SIC). Numerical results are presented for the compound Zn1 − 0.06Mn0.06O1−xCx when doping and co-doping is performed with Mn and C as doping elements. Total and partial DOSs are given for different concentrations using the two approximations, LDA and LDA-SIC. It is found that for 6% with doping by Mn the system becomes magnetic. The co-doping with carbon changes the behavior of the system : it becomes also magnetic for 4, 6 and 10% concentrations within both, LDA and LDA-SIC approximations. Furthermore, we have discussed the type of mechanism of exchange interaction and found that the double exchange is responsible for the appearing magnetism in the system, within the LDA and p-d interaction for LDA-SIC approximation. For 10% of carbon, we have found that the critical temperature approaches 280 K in the LDA approximation solely; and is about 305 K in the LDA-SIC approximation.

Ab initio theory of the spin-dependent conductivity tensor and the spin Hall effect in random alloys

Abstract: We present an extension of the relativistic electron transport theory for the standard (charge) conductivity tensor of random alloys within the tight-binding linear muffin-tin orbital method to the so-called spin-dependent conductivity tensor, which describes the Kubo linear response of spin currents to external electric fields. The approach is based on effective charge- and spin-current operators that correspond to intersite electron transport and that are nonrandom, which simplifies the configuration averaging by means of the coherent potential approximation. Special attention is paid to the Fermi sea term of the spin-dependent conductivity tensor, which contains a nonzero incoherent part, in contrast to the standard conductivity tensor. The developed formalism is applied to the spin Hall effect in binary random nonmagnetic alloys, both on a model level and for Pt-based alloys with an fcc structure. We show that the spin Hall conductivity consists of three contributions (one intrinsic and two extrinsic) which exhibit different concentration dependences in the dilute limit of an alloy. Results for selected Pt alloys (Pt-Re, Pt-Ta) lead to the spin Hall angles around 0.2; these sizable values are obtained for compositions that belong to thermodynamically equilibrium phases. These alloys can thus be considered as an alternative to other systems for efficient charge to spin conversion, which are often metastable crystalline or amorphous alloys.

Frequency-dependent transport properties in disordered systems: A generalized coherent potential approximation approach

Abstract: To predict transport properties of disordered systems, especially ac transport properties, one has to calculate the disorder average of the correlation of the multiple Green's function at different energies. To avoid brute force calculation, diagrammatic perturbation expansion must be used along with the coherent potential approximation (CPA). In this paper, we develop a theoretical formalism based on the nonequilibrium Green's function that maps the average of the correlation of the multiple Green's function into an average over a single generalized Green's function. After the mapping, this formalism is structurally very similar to the CPA and completely eliminates the need to perform diagrammatic expansion. As a demonstration of our theory, the dynamic conductance, frequency-dependent shot noise under dc bias, and frequency-dependent noise spectrum under ac bias in the presence of Anderson disorder are calculated by directly taking the disorder average of the generating function of full counting statistics (FCS) within the CPA. Our numerical results on dynamic conductance, frequency-dependent shot noise under dc bias, and frequency-dependent noise spectrum under ac bias show remarkable agreement with that obtained by the brute force calculation. The phase diagram in the frequency versus disorder strength plane has been efficiently calculated using the generalized FCS-CPA method.

Phonon dispersion of binary alloys with auxiliary coherent potential approximation

Abstract: We report the auxiliary coherent potential approximation (ACPA) for calculating the phonon dispersion of three-dimensional alloys with both mass and force-constant disorders. To obtain the average spectra function of disordered alloys, the average coherent scattering structure factors are derived from the auxiliary coherent medium in single-site approximation. We provide an analytical proof of the sum rule in the auxiliary coherent medium, which ensures the analyticity of physical properties. To demonstrate the accuracy and applicability of the ACPA method, we apply the ACPA to calculate phonon dispersion of several alloys, including CuPd CuAu, PdFe, and NiPt with different disorder concentrations. We find the ACPA phonon dispersion results agree very well with the itinerant coherent potential approximation calculations and experimental measurements. The approximate separable force-constant model used in the ACPA can very well represent the first-principles disordered force constants, presenting minor or even negligible influence on the phonon dispersion of the alloy. The ACPA model features easy implementation and high computational efficiency, providing an effective method for simulating vibrational properties of realistic alloys.

Low dimensions electron localization in the beyond real space super cell approximation

Abstract: Metal to insulator phase transition due to electron localization in disordered alloys (Anderson transition) and interacting electrons (Mott transition) systems is one of major problem in these fields. Multi site electron scattering is responsible for localization which can't be seen by single site approximations such as coherent potential approximation (CPA) and dynamical mean field theory (DMFT). Here we develop a multi site technique to calculate multi site electron scattering for observation of phenomenons such as electron localization especially in low dimension systems. Our self-energy in first Brillouin zone (FBZ) is casual, in contrast to previous approximation fully crystal electron wave vector, q, dependent and continuous with respect to q. It recovers coherent potential approximation in the single site approximation and is exact when the number of sites in the super cell approaches to the total number of lattice sites. We illustrate that this approximation undertakes electrons localization for one and two dimensional alloy systems which isn't observed by previous multi site approximations such as dynamical cluster approximation (DCA).

Half Metallic Ferromagnetic Character in ZnXP2 (X = Ge, Si) Chalcopyrites Doped with Mn

Abstract: The electronic and the magnetic properties of ZnXP2 (X = Ge, Si) were studied using the Korring-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA). The total and partial density of state (DOS) are computed for different Mn concentrations. The total magnetic moment, FM and DLM energies, and their variation as well as Curie temperature are also given. It is shown that the substitution of X cations (X = Ge, Si) by Mn atoms in ZnXP2 chalcopyrites leads to half metallic ferromagnetic character with double exchange mechanism. Thus, the critical temperature can be controlled by varying the concentration of manganese impurity.

Effective medium approach in the renormalization group theory of phase transitions

Abstract: An effective medium approach similar to the coherent potential approximation (CPA) in the theory of disordered alloys and to the DMFT has been extended to the renormalization group equations in the local potential approximation (LPA). Non-universal characteristics of the second order phase transitions such as the critical temperatures and critical amplitudes have been calculated in good agreement with the best known estimates. A possibility of cluster extension of the LPA to improve its accuracy and to make the approach systematic and self-contained has been discussed. A qualitative explanation of the discrepancy between theoretical value of the critical exponent $\beta$ and recent experimental data on ordering in beta brass by the influence of non-universal contributions has been suggested.

Breakdown of coherence in Kondo alloys: crucial role of concentration versus 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, n c . In previous works, two different Fermi liquid regimes had been identified at strong Kondo coupling [Formula: see text], that may be separated by a transition at x = n c . Here, we analyze the KAM for finite [Formula: see text] on a Bethe lattice structure. First, using the mean-field coherent potential approximation (DMFT-CPA) which is exact at lattice coordination [Formula: see text], 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 analysis by 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.

Thermoelectric properties of CeNi 2 Al 3 compound: an experimental and theoretical study

Abstract: We present thermoelectric properties of the CeNi2Al3 compound in the temperature range from 4 to 300 K. The electrical resistance (ρ) exhibits a metallic-like character reaching approximately 50 μΩ cm at room temperature. The temperature dependence of the Seebeck coefficient (S) is typical for mixed valence compounds having positive values with a broad maximum (~ 46 μV/K) over a wide temperature range from 200 to 300 K. The thermal conductivity (κ) value reaches 15 W/(m K) at T = 300 K. The power factor (PF = S2/ρ) at 150 K is high (~ 70 μW/cm K2), larger than for conventional thermoelectric materials based on Bi2Te3. The dimensionless figure of merit (ZT) has a broad maximum over a wide temperature range, which reaches the value of 0.1 around 220 K. The experimental results are supported by calculations within the density functional theory (DFT) performed on the basis of the full-potential local-orbital minimum-basis scheme (FPLO). The coherent potential approximation (CPA) is used to simulate the chemical disorder. The calculations are focused on the site preference of Ni and Al atoms. Investigations of the energetic stability have shown that in CeNi2Al3 the aluminum atoms prefer the 3g sites and the nickel ones the 2c sites.

First principles calculation of effect of graphene coating on transmission coefficient of Cu thin film with low surface roughness

Abstract: In this paper, we calculate the effect of a graphene coating on the transmission coefficient of a Cu thin film with surface disorder. The nonequilibrium coherent potential approximation combined with the linear muffin-tin orbital formulation, which is based on first principles, is applied by assuming that there is surface disorder. The graphene coating mitigates the effect of Cu surface scattering on the transmission coefficient. The weak interaction between Cu and graphene and the upward shift of the Fermi level with respect to the Dirac point improve the transport characteristics by offering more conduction bands. Moreover, graphene-coated Cu with a perfect surface has a completely specular transmission coefficient. The surface disorder decreases the transmission coefficient due to the nonconserved transverse momentum ( k) of the scattering wave through the central area of the two-probe system. However, for a graphene coating on a Cu thin film with surface disorder x < 3.90 %, length l < 5.09 nm, width 0.25 nm, and thickness 1.23 nm, the transmission coefficient is higher. The increased transmission coefficient due to graphene coating can overcome the diffusive scattering originating from the surface disorder. The coherent potential approximation band structure shows that graphene bands are less affected by Cu surface disorder than Cu bands, which enhances the total conduction by offering additional channels for electrons. Our results demonstrate that graphene is a potential liner material for a Cu thin film with low surface disorder.

First-principles theoretical study on carrier doping effects induced by Zn vacancies in Mn-doped in ZnSnAs 2

Abstract: We investigate the dependence of the Curie temperature (T C), electronic structure and magnetic properties on hole doping by Zn vacancy (VZn) generations in Zn(Sn,Mn)As2 and (Zn,Mn)SnAs2 using Korringa-Kohn-Rostoker method incorporated with the coherent potential approximation and a local spin density approximation (KKR-CPA-LSDA) within density functional theory (DFT). We find that T C of (Zn,VZn)(Sn,Mn)As2 is strongly reduced by hole doping due to the reduced double-exchange interaction. In contrast to this, (Zn,VZn,Mn)SnAs2 shows ferromagnetism because of the introduction of holes into paramagnetic (Zn,Mn)SnAs2. In this case, T C is significantly enhanced by the hole doping because of the increased double exchange interaction.

Electron Transport in Graphene-Versus Al/Pd-Coated Thin Cu Films With Low-Surface Roughness: A First Principles Study

Abstract: Surface scattering is a major issue in thin Cu films at reduced scales. The rise in the diffusive scattering due to the surface roughness causes the electrical resistance to increase remarkably. In this paper, graphene, as opposed to Al and Pd, is considered as a liner layer for thin Cu film with low-surface roughness using first principles calculation. The surface roughness is simulated using the nonequilibrium coherent potential approximation combined with the linear muffin-tin orbital formulation. The coherent potential approximation band structure shows that the graphene $\pi $ -bands is not significantly affected by the surface disorder at the Cu surface and that graphene acts as a parallel path to the electrons. On the other hand, the bands of Cu–Al/Pd around the Fermi level are substantially broadened due to the surface disorder. Moreover, the graphene-coated Cu shows less electrical resistance than Al/Pd-coated Cu for surface disorder $x\lessapprox 5$ % for thin films with 0.245 nm in width, and 1.23 nm in thickness. The enhancement in the transport properties in Cu–Gr is attributed to the weak electronic interaction at the interface. The obtained results suggest that graphene is better than Al and Pd as a liner material for thin Cu films.