Magnetism and spin entropy in Ru doped Na0.5CoO2.

The effect of Ru doping on the magnetic coupling among Co ions and on the Seebeck effect in Na0.5CoO2 was systematically studied using density functional theory. It was found that the Ru dopant takes the 4+ oxidation state and replaces a Co4+ ion. In addition, the remaining Co4+ ions in Na0.5CoO2:Ru were stabilized in a low spin state. Magnetically, the Ru dopants couple in a ferrimagnetic manner with Co ions in the host lattice. Due to the higher electronic degeneracy of Ru4+ dopants, the Seebeck coefficient in Na0.5CoO2 is predicted to be higher than that of the pristine compound.

Dopant incorporation site in sodium cobaltate's host lattice: a critical factor for thermoelectric performance.

NaxCoO2 that comprises alternating Na and CoO layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped NaxCoO2 (x = 0.50, 0.625, 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA + U method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid-like Na layer and, therefore, improve the crystallinity and decrease the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.

First principles studies of GeTe based dilute magnetic semiconductors.

The electronic structure and magnetic properties of GeTe-based dilute magnetic semiconductors (DMS) are investigated by the Korringa-Kohn-Rostoker Green's function method and the projector augmented wave method. Our calculations for the formation energies of transition metal impurities (TM) in GeTe indicate that the solubilities of TM are quite high compared to typical III-V and II-VI based DMS and that the TM doped GeTe has a possibility of room temperature ferromagnetism with high impurity concentrations. The high solubilities originate from the fact that the top of the valence bands of GeTe consists of the Te-5p anti-bonding states which are favorable to acceptor doping. (Ge, Cr)Te system shows strong ferromagnetic interaction by the double exchange mechanism and is a good candidate for DMS with high Curie temperature. Additionally, in the case of (Ge, Mn)Te with the d(5) configuration, by introducing the Ge vacancies the p-d exchange interaction is activated and it dominates the antiferromagnetic superexchange, resulting in ferromagnetic exchange interactions between Mn. This explains recent experimental results reasonably. Based on the accurate estimation of the Curie temperatures by Monte Carlo simulation for the classical Heisenberg model with the calculated exchange coupling constants, we discuss the relevance of the TM doped GeTe for semiconductor spintronics.

Computational materials design of negative effective U system in hole-doped chalcopyrite CuFeS2.

A general rule of negative effective U(U(eff)) system caused by (i) exchange correlation and (ii) charge excitation mechanisms is proposed. Based on the general rule, we perform ab initio electronic structure calculations by generalized gradient approximation (GGA) + U method for hole-doped chalcopyrite CuFeS2 [Cu(+)(d(10))Fe(3+)(d(5))S(2-)(s(2)p(6))2]. It is found from our calculations that the hole-doped CuFeS2 has the negative U(eff) = -0.44 eV, where U(eff) ≡ E(N + 1) + E(N - 1) - 2E(N) < 0 and E(N) is the total energy of the hole-doped CuFeS2. The negative U(eff) is caused by the charge-excitation in the hole-doped Cu(2+)(d(9)) and S(-)(s(2)p(5)), and also caused by the exchange-correlation in the hole-doped Fe(4+)(d(4)). The strong attractive electron-electron interaction (U(eff) = -0.44 eV ∼ -5000 K) originates from the purely electronic mechanism. The closed shell of the d(10) electronic configuration is more stable than the d(9) electronic configuration, since the first excited state with the d(9)s(1) electronic configuration and the ground state with the d(10) electronic configuration are very close, then these two states repel very strongly through the second order perturbation. Therefore, the spin-polarized total energy curve for the hole-doped CuFeS2 shows the strong upward convexity with N - 1, N and N + 1 electronic configurations leading to the negative U(eff). The hole-doped paramagnetic and metallic CuFeS2 with the negative U(eff) may cause a possible high-Tc superconductor (Tc ∼ 1000 K, if 2Δ/kBTc ≈ 10 by assuming a strong coupling regime) because of the strong attractive electron-electron interactions (superconducting gap Δ ≈ |U(eff|) ∼ 5000 K). Finally, we propose a new computational materials design methodology to design ultra high-Tc superconductors by using three steps starting from the atomic number only.

Hubbard U calculations for gap states in dilute magnetic semiconductors.

On the basis of constrained density functional theory, we present ab initio calculations for the Hubbard U parameter of transition metal impurities in dilute magnetic semiconductors, choosing Mn in GaN as an example. The calculations are performed by two methods: (i) the Korringa-Kohn-Rostoker (KKR) Green function method for a single Mn impurity in GaN and (ii) the full-potential linearized augmented plane-wave (FLAPW) method for a large supercell of GaN with a single Mn impurity in each cell. By changing the occupancy of the majority t2 gap state of Mn, we determine the U parameter either from the total energy differences E(N + 1) and E(N - 1) of the (N ± 1)-electron excited states with respect to the ground state energy E(N), or by using the single-particle energies for n(0) ± 1/2 occupancies around the charge-neutral occupancy n0 (Janak's transition state model). The two methods give nearly identical results. Moreover the values calculated by the supercell method agree quite well with the Green function values. We point out an important difference between the 'global' U parameter calculated using Janak's theorem and the 'local' U of the Hubbard model.

A new structure of Cu complex in Si and its photoluminescence.

A recent discovery of complicated isotope shifts of the PL 1014 meV line in Cu-containing silicon, which was made by Thewalt's group, indicates that existing models of Cu pairs are not appropriate for accounting for the luminescence center. A new structural model has been studied. First-principles calculations show that the most probable form of the complex is a four-membered complex which is composed of a substitutional Cu associated with three neighboring interstitial Cu atoms. The symmetry is C(3v). The formation mechanism of this complex is discussed. For interpreting the complicated splitting for the Γ(4) exciton peak, involvement of a non-totally symmetric mode is proposed. The pattern of splitting obtained by this model is almost in agreement with the experiment. Selection rules beyond the usual treatments are necessary to connect a specific exciton peak to the corresponding phonon.

Control of impurity diffusion in silicon by IR laser excitation.

Control of the diffusion of a specific impurity species is desirable in Si device processes. IR laser excitation matching the impurity vibration mode is a promising method for this purpose. To illustrate the effectiveness of this method, first-principles molecular dynamics simulation has been applied. Technical issues of the simulation are described in detail. It is seen that resonant effects can be reproduced in adiabatic molecular dynamics simulations by applying an external force on the impurity only. The present study forms the basis for further developments of this approach.

Ferromagnetism and spinodal decomposition in dilute magnetic nitride semiconductors.

We propose a materials design for dilute magnetic semiconductors (DMS) based on first-principles calculations by using the Korringa-Kohn-Rostoker coherent potential approximation method. We develop an accurate method for calculations of the Curie temperature (T(C)) of DMS and show that the mean field approximation completely fails to predict T(C) for DMS, in particular for wide gap nitride DMS where the exchange interaction is short ranged. The T(C) calculated for homogeneous DMS by using the present method agrees very well with available experimental values. For more realistic material design, we simulate spinodal nanodecomposition by applying the Monte Carlo method to the Ising model with ab initio chemical pair interactions between magnetic impurities in DMS. It is found that by controlling the dimensionality of the decomposition, various characteristic phases occur in DMS. It is suggested that superparamagnetic blocking phenomena should be important for understanding the magnetism of wide gap DMS.