Pressure Induced Charge Disproportionation in LaMnO3_{3}

We present a total energy study as a function of volume in the cubic phase of LaMnO$_{3}$. A charge disproportionated state into planes of Mn$^{3+}$O$_{2}$/Mn$^{4+}$O$_{2}$ was found. It is argued that the pressure driven localisation/delocalisation transition might go smoothly through a region of Mn$^{3+}$ and Mn$^{4+}$ coexistence.Comment: 3 pages, 1 figure, Conference Proceedings: Nanospintronics: Design and Realization (Kyoto, Japan 24-28 May, 2004

Comment on "Systematics of the Induced Magnetic Moments in 5d Layers and the Violation of the Third Hund's Rule"

Comment on F. Wilhelm et al., Phys. Rev. Lett. 87, 207202 (2001)Comment: 1 pag

Study of the volume and spin collapse in orthoferrite LuFeO_3 using LDA+U

Rare earth (R) orthoferrites RFeO_3 exhibit large volume transitions associated with a spin collapse. We present here ab initio calculations on LuFeO_3. We show that taking into account the strong correlation among the Fe-3d electrons is necessary. Indeed, with the LDA+U method in the Projector Augmented Wave (PAW), we are able to describe the isostructural phase transition at 50 GPa, as well as a volume discontinuity of 6.0% at the transition and the considerable reduction of the magnetic moment on the Fe ions. We further investigate the effect of the variation of U and J and find a linear dependence of the transition pressure on these parameters. We give an interpretation for the non-intuitive effect of J. This emphasizes the need for a correct determination of these parameters especially when the LDA+U is applied to systems (e.g in geophysical investigations) where the transition pressure is a priori unknown

Cluster coherent potential approximation for electronic structure of disordered alloys

We extend the single-site coherent potential approximation (CPA) to include the effects of non-local disorder correlations (alloy short-range order) on the electronic structure of random alloy systems. This is achieved by mapping the original Anderson disorder problem to that of a selfconsistently embedded cluster. This cluster problem is then solved using the equations of motion technique. The CPA is recovered for cluster size $N_{c}=1$, and the disorder averaged density-of-states (DOS) is always positive definite. Various new features, compared to those observed in CPA, and related to repeated scattering on pairs of sites, reflecting the effect of SRO are clearly visible in the DOS. It is explicitly shown that the cluster-CPA method always yields positive-definite DOS. Anderson localization effects have been investigated within this approach. In general, we find that Anderson localization sets in before band splitting occurs, and that increasing partial order drives a continuous transition from an Anderson insulator to an incoherent metal.Comment: 7 pages, 6 figures. submitted to PR

NON-LOCAL ELECTRON-POSITRON ENHANCEMENT FACTORS IN SOLIDS

Non-local electron-positron correlation effects in solids are studied. The weighted density approximation is applied to calculations of the non-local electron-positron correlation functions. The calculated weighted density approximation electron-positron enhancement factors for the core electrons are compared with those obtained within the local density approximation. Also, differences in the electron-positron enhancement factors due to the s, p, d and f angular momentum channels of the electron charge density are studied. The formalism is applied to ab initio calculations of positron lifetimes in a variety of metals and silicon. The influence of various approximations to the electron-positron interaction on the positron lifetimes is also presented. The weighted density approximation results are compared to those calculated within the local density approximation, the recent generalized gradient approximation and with experimental data. PACS numbers: 78.70. Bj, The positron lifetime, τ, is an important characteristic of electronic properties of solids. This parameter provides information on the electron density distribution in the host material, thus yielding also useful information on defects in metals and semiconductors. The positron annihilation rate, A = 1/r, is calculated as where nei(re ) is the electron density in the host material, φ+(r p ) is the wave function of a thermalised positron and g(re , rp ) denotes the correlation function of the positron at rp and electrons at re , and r0 and c are the classical electron radius and velocity of light, respectively