Tight-binding Coherent Potential Approximation Research Articles

Tight-binding modeling of interstitial ordering processes in metals: Application to zirconium hydrides

We present here a theoretical study of ordering processes in metal-hydrogen compounds based on a generalized perturbation method and on tight-binding coherent potential approximation. This approach is illustrated for zirconium hydrides, in which case we demonstrate that a cluster expansion of the ordering energy can be limited to effective pair interactions, the leading one being between hydrogen atoms in third-neighbor positions. These results are quantitatively confirmed by comparison to density functional theory calculations and qualitatively interpreted through orbital symmetry analysis. The method is then applied first to draw a preliminary Zr-H phase diagram and then to characterize the effect of lattice deformation on the ordering processes in zirconium hydrides.

Ferromagnetic transition temperature enhancement in (Ga,Mn)As semiconductors by carbon codoping

We present a theoretical study of (Ga,Mn)(As,C) diluted magnetic semiconductors with high C acceptor density that combines insights from phenomenological model and microscopic approaches. A tight-binding coherent-potential approximation is used to describe the electronic structure in the presence of ${\mathrm{Mn}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{As}}$ impurities. We find only a small effect of C on the distribution and coherence of electronic states close to the top of the valence band and on the coupling between Mn moments, even at doping levels of several per cent. These results justify applying the model of ferromagnetic Mn-Mn coupling mediated by itinerant holes in the valence band also to C doped samples. The increase of ferromagnetic transition temperature due to the presence of C acceptors is illustrated by calculations that use the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Kohn-Luttinger description of the GaAs valence band and assume systems where Mn local moment and itinerant hole densities can be varied independently.

Calculation of the densities of states of a-Si 1 − yC y(:H) alloys and comparison with measured X-ray spectra

A simplified model based on the Tight-Binding Coherent-Potential Approximation method that does not account for topological disorder, is employed to calculate the s and p-symmetry densities of states of a-Si 1 − y C y (:H) alloys and crystalline SiC. Valence and conduction partial Si and C densities of states are obtained which allow us to observe that sp 3 hybridization is existing in these compounds. The calculations also show that in a-Si 1 − y C y alloys, the total band gap increases by about 0.7 eV when going from y=0.1 to y=0.287 due to a progressive shift of both valence and conduction edges below and above the top of the valence band, respectively. For crystalline SiC, the gap increases by about 1.5 eV with respect for example to a-Si 0.9C 0.1. In hydrogenated alloys, the calculations display a steepening of the valence and conduction band edges as compared to non-hydrogenated alloys confirming thus the `healing' effect with H incorporation. The soft X-ray Si Kβ emission spectra are simulated by convolution of the occupied Si 3p distribution with appropriate Lorentzian and Gaussian functions which account for the lifetime of the inner level involved in the X-ray transition and the instrumental function, respectively. The calculated spectra are in good agreement with experimental results on samples of similar nominal composition despite the fact that the theoretical model does not account for topological disorder.

Correlation between spin polarization and magnetic moment in ferromagnetic alloys

The correlation between the spin polarization of the tunneling current in ferromagnet/Al2O3/Al tunnel junction experiments and the magnetic moment in ferromagnets has been a mystery [Phys. Rev. B 16, 4907 (1977) and Phys. Rep. 238, 173 (1994)]. In this study, an attempt is made to explain this correlation. By assuming that the s electrons are responsible for the tunneling, the tunneling currents are proportional to the s density of states at the Fermi level [Phys. Rev. B 8, 3252 (1973)]. A tight-binding coherent potential approximation model for itinerant magnetism is applied to calculate the band structure of Fe- and Ni-based alloys [Rev. Mod. Phys. 46, 465 (1974)]. The Slater–Pauling curve for the bulk magnetic moment is recovered, and the spin polarization of the s electrons at the Fermi level is found to correlate well with the magnetic moment. The calculation is carried out in a model where the approximate band structure of an alloy is calculated by using two extra tight-binding parameters besides those of the host. As the average number of electrons is changed by the impurities, the total Coulomb energy and hence the splitting between the up and down spin bands has to be modified self-consistently. The modified band structure is then used to obtain the magnetic moment and the spin polarization. The relation between the magnetic moment and spin polarization can be understood by noting that within approximately 1.5 eV of the Fermi level, the density of states of the s band for either spin is roughly an increasing function of energy and therefore of the number of electrons. For a fixed total number of electrons, the magnetic moment increases with the number of spin-up electrons, corresponding to a larger spin-up density of states at the Fermi level. Thus, the spin polarization increases as the magnetic moment increases.

Tight-binding coherent potential approximation study of ferromagneticLa2/3Ba1/3MnO3

The local-spin-density band structure of a virtual-crystal model of ferromagnetic La${}_{2/3}$Ba${}_{1/3}$MnO${}_{3}$, reported earlier by Pickett and Singh [Phys. Rev. B 53, 1146 (1996)], has been fit to an accurate orthogonal tight-binding (TB) model. This TB Hamiltonian has spin-independent hopping parameters, and the exchange splitting is confined solely to the Mn on-site parameters ${\ensuremath{\varepsilon}}_{{t}_{2g}\ensuremath{\downarrow}}\ensuremath{-}{\ensuremath{\varepsilon}}_{{t}_{2g}\ensuremath{\uparrow}}$=3.44 eV and ${\ensuremath{\varepsilon}}_{{e}_{g}\ensuremath{\downarrow}}\ensuremath{-}{\ensuremath{\varepsilon}}_{{e}_{g}\ensuremath{\uparrow}}$=2.86 eV. The crystal-field splitting, as reflected in the Mn $d$ energies, is 1.77 eV for majority spin and 1.19 eV for minority spin. Oxygen $p\ensuremath{\sigma}$ and $p\ensuremath{\pi}$ states are allowed distinct site energies, and the derived crystal-field splitting is ${\ensuremath{\varepsilon}}_{p\ensuremath{\pi}}\ensuremath{-}{\ensuremath{\varepsilon}}_{p\ensuremath{\sigma}}$ = 1.47 eV. Local Mn $d$ site energy disorder, arising from the random distribution of La${}^{3+}$ and Ba${}^{2+}$ ions, has been treated within the TB coherent potential approximation. The intrinsic resistivity in the fully polarized phase from this charge disorder is estimated to be 15 $\ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}$.

Electronic structure of ordered and disordered Cu-Ag alloys

We present a study of the electronic structure of Cu-Ag alloys. This work includes total-energy evaluations, band structure, and density of states (DOS) determinations using the augmented-plane-wave method for the ${\mathrm{L}1}_{2}$, B1, and B2 structures. We also study the corresponding disordered phases of these alloys by the tight-binding coherent potential approximation. Our results confirm the B2 and ${\mathrm{L}1}_{2}$ (Cu reach) as the stable structures of these systems. We compare the DOS with experiment and other calculations, and confirm the absence of both magnetism and superconductivity in these alloys. Finally we estimate charge transfer for these alloys from Cu to Ag.

Electronic structure and phase equilibria in ternary substitutional alloys

A reliable and consistent scheme for studying phase equilibria in ternary substitutional alloys based on the tight-binding approximation is presented. With the electronic parameters obtained from linear muffin-tin orbital calculations, we show that the computed densities of states and band structures compare well with those obtained from more accurate ab initio calculations. Disordered alloys are studied within the tight-binding coherent-potential approximation formalism extended to multi-component alloys. The energetics of ordered systems is obtained through effective-pair interactions computed with the general perturbation method. Finally, partially ordered alloys are studied with a novel simplification of the molecular coherent-potential approximation combined with the general perturbation method. The formalism is applied to the study of bcc-based ternary Zr - Ru - Pd alloys which are promising candidates for medical implant device applications. Using the energetics obtained with the aforementioned scheme, we apply the cluster-variation method to study phase equilibria for particular pseudo-binary alloys, and show that the results are consistent with the observed behaviour of the electronic specific heat coefficient with composition for the system.

Atomic magnetic moment and X-ray photoelectron spectra of Fe-based crystalline and amorphous alloys

Abstract X-ray photoelectron spectra of the valence band and Fe 2p 3 2 and 3s core levels have been measured both for polycrystalline FeCr alloys and amorphous FeB alloys. The 3s-multiplet splitting has been used to estimate the local magnetic moment of Fe atoms. To explain the observed concentration dependence of the magnetic moment, the calculations of the electronic structure and the magnetic moment of the Fe atoms were performed in the framework of the tight-binding coherent potential approximation. Results of the calculations agree with the experimental concentration dependence of the magnetic moment of Fe atoms. This dependence of the atomic magnetic moment on a boron concentration for amorphous FeB alloys obeys the Slater—Pauling rule on going from 15.5 to 19.5 at.% but shows a slight deviation from this linear behaviour at about 15 at.%. For FeCr, alloys the atomic magnetic moment is almost constant for differences of components concentration in agreement with X-ray photoemission spectroscopy measurements, whereas the average magnetic moment of the alloy decreases almost linearly with increasing Cr concentration. The difference in concentration dependence of the atomic magnetic moment is explained in terms of a charge transfer between s, p and d bands which is found to be larger for the amorphous FeB alloys than for the polycrystalline FeCr alloys.

Tight-binding coherent-potential approximation including off-diagonal disorder.

We present a generalization of the tight-binding coherent-potential approximation (TB-CPA) to substitutionally disordered alloys with both diagonal and off-diagonal disorder (ODD), taking into account all relevant {ital s}, {ital p}, and {ital d}, orbitals. This generalization is based on the work of Blackman, Esterling, and Berk (Phys. Rev. B 4, 2412 (1971)) which treated ODD within a single-band model of a disordered alloy. The method is illustrated with calculations of the density of states of disordered Cu{sub 0.75}Pd{sub 0.25} alloys. We find that inclusion of ODD improves the agreement with specific-heat data, but it does not resolve a discrepancy with the results of first-principles, muffin-tin CPA calculations regarding the Pd component of the density of states.

Calculations of the electronic structure of La2-xBaxCuO4-y by the coherent-potential approximation.

We present the first calculations of the electronic structure of the high-temperature superconductors in the disordered phase using the tight-binding coherent-potential approximation. These calculations are used to assess the influence of increasing oxygen vacancies when La is alloyed with $M$ (=Sr or Ba) in the system ${\mathrm{La}}_{2\ensuremath{-}x}{M}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}y}$. Contrary to simple doping arguments, it is found that deoxygenation does not raise the Fermi level (${E}_{F}$). The increase in ${T}_{c}$ in the range $0.05<x<0.15$ and the decrease in ${T}_{c}$ beyond the peak at $x=0.15$ follow the density of states at ${E}_{F}$.

A Tight-Binding Hamiltonian for the High-Temperature Superconductor YBa2Cu3O7

AbstractWe present a highly accurate tight-binding parametrization of the LAPW band structure of the high-temperature superconductor YBa2Cu3O7, discuss the methodology used in obtaining this fit, and its potential application to a Tight-Binding Coherent-Potential Approximation (TB-CPA) calculation of the effects of oxygen vacancies on the electronic structure.

Tight-binding coherent-potential-approximation study of the electronic states of palladium-noble-metal alloys.

We have performed calculations of the electronic states of palladium--noble-metal disordered alloys using an improved version of the tight-binding coherent-potential-approximation (TB-CPA) formalism. Our results for Pd-Ag are in excellent agreement with both experiment and the Korringa-Kohn-Rostoker (KKR) CPA. For Pd-Au we also obtain good agreement with specific-heat measurements. For Pd-Cu, probably due to charge-transfer effects, we do not obtain satisfactory agreement with experiment at the Cu-rich end. Our calculations indicate that if (a) highly accurate Slater-Koster parameters are used and (b) all the orbitals are included in the CPA conditions (i.e., not using a one-level TB model), the TB-CPA gives results that are equivalent to those of the KRR-CPA except in cases where off-diagonal disorder might be important.

Calculations of the electronic structure of disordered transition metal dihydrides

Employing the tight binding coherent potential approximation method, we investigated the electronic structure of the substitutionally disordered transition metal dihydride series Ti 1- y Nb y H x , Ti 1- y V y H x (0 ⩽ y ⩽ 1; 1.5 ⩽ x ⩽ 2.0). These alloys contain disorder on both the metal and hydrogen sublattices. We found that trends in the region of the Fermi level display rigid band-like characteristics for the substitution of niobium or vanadium for titanium, and for the hydrogen disorder as well. The calculations are compared with electronic specific heat measurements and predictions based on nuclear magnetic resonance experiments.

Theory of superconductivity in palladium-noble-metal hydrides.

The tight-binding coherent-potential approximation is used to calculate the electron densities of states for various concentrations of palladium--noble-metal hydrides. Our results are used to give a satisfactory explanation of the mechanism of superconductivity in these systems and to predict the variation of the phonon moments as a function of noble-metal concentration.

Superconductivity in PdAg alloys

The tight binding coherent potential approximation is used to calculate the electron densities of states (DOS) for various concentrations of the Pd 1− y Ag y H x system. The DOS are used in the evaluation of the electron-phonon coupling constant λ and the superconducting temperature T c. Our results reproduce the experimentally determined behavior of the superconducting transition temperature's dependency on Ag concentration, and provide a satisfactory explanation of the mechanism of superconductivity in this system.

Electronic structure of substoichiometric Ti-Fe hydride

We applied the tight-binding coherent potential approximation method to a study of the electronic structure of TiFeH x . The starting point of these calculations was a Slater-Koster fit to the band structure of TiFe at one end of the compositional spectrum and of TiFeH 1.0 at the other end. We obtained the densities of states, decomposed per site and also per angular momentum component, as a function of the hydrogen content. These densities of states were used to examine the effect of hydrogenation on both the bonding and antibonding levels as well as on the position of the Fermi level.

On the tight binding theory of the heats of formation

The accuracy of a recently proposed simple analytic theory of the heats of formation ΔH of transition metal alloys is demonstrated by comparison with the tight binding coherent potential approximation calculations of Van der Rest et al. The d charge transfer is evaluated within a model that includes explicitly the deformation of the pure metal bands on alloying.