Typical-medium, multiple-scattering theory for disordered systems with Anderson localization

The typical medium dynamical cluster approximation (TMDCA) is reformulated in the language of multiple scattering theory to make possible first principles calculations of the electronic structure of substitutionally disordered alloys including the effect of Anderson localization. The TMDCA allows for a systematic inclusion of non-local multi-site correlations and at same time provides an order parameter, the typical density of states, for the Anderson localization transition. The relation between the dynamical cluster approximation and the multiple scattering theory is analyzed, and is illustrated for a tight-binding model.Comment: 15 pages, 11 figure

Multiple scattering formalism for correlated systems: A KKR+DMFT approach

We present a charge and self-energy self-consistent computational scheme for correlated systems based on the Korringa-Kohn-Rostoker (KKR) multiple scattering theory with the many-body effects described by the means of dynamical mean field theory (DMFT). The corresponding local multi-orbital and energy dependent self-energy is included into the set of radial differential equations for the single-site wave functions. The KKR Green's function is written in terms of the multiple scattering path operator, the later one being evaluated using the single-site solution for the $t$-matrix that in turn is determined by the wave functions. An appealing feature of this approach is that it allows to consider local quantum and disorder fluctuations on the same footing. Within the Coherent Potential Approximation (CPA) the correlated atoms are placed into a combined effective medium determined by the dynamical mean field theory (DMFT) self-consistency condition. Results of corresponding calculations for pure Fe, Ni and Fe$_{x}$Ni$_{1-x}$ alloys are presented.Comment: 25 pages, 5 fig. acepted PR

Half-Metallic Ferromagnetism and the spin polarization in CrO2_2

We present electronic structure calculations in combination with local and non-local many-body correlation effects for the half-metallic ferromagnet CrO$_2$. Finite-temperature Dynamical Mean Field Theory results show the existence of non-quasiparticle states, which were recently observed as almost currentless minority spin states near the Fermi energy in resonant scattering experients. At zero temperatures, Variational Cluster Approach calculations support the half-metallic nature of CrO$_2$ as seen in superconducting point contact spectroscopy. The combination of these two techniques allowed us to qualitatively describe the spin-polarization in CrO$_2$.Comment: 5 pages, 3 figure

Half-metallic ferromagnets: From band structure to many-body effects

A review of new developments in theoretical and experimental electronic structure investigations of half-metallic ferromagnets (HMF) is presented. Being semiconductors for one spin projection and metals for another ones, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMF by the peculiarities of their electronic structure and chemical bonding is discussed. Effects of electron-magnon interaction in HMF and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Especial attention is paid to appearance of non-quasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-art electronic calculations for correlated $d$-systems is discussed, and results for specific HMF (Heusler alloys, zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.Comment: to be published in Reviews of Modern Physics, vol 80, issue

Lattice dynamics of palladium in the presence of electronic correlations

We compute the phonon dispersion, density of states, and the Gr\"uneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures $T \lesssim 20~K$ the phonon frequency in the vicinity of the Kohn anomaly, $\omega_{T1}({\bf q}_{K})$, strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.Comment: minor revision

Non-quasiparticle states in Co2_2MnSi evidenced through magnetic tunnel junction spectroscopy measurements

We investigate the effects of electronic correlations in the full-Heusler Co$_2$MnSi, by combining a theoretical analysis of the spin-resolved density of states with tunneling-conductance spectroscopy measurements using Co$_2$MnSi as electrode. Both experimental and theoretical results confirm the existence of so-called non-quasiparticle states and their crucial contribution to the finite-temperature spin polarisation in this material.Comment: Repalced Fig. 1. of PRL, 100, 086402 (2008), better k-space resolution for DOS around Fermi energ

Half-metallicity in NiMnSb: a Variational Cluster Approach with ab-initio parameters

Electron correlation effects in the half-metallic ferromagnet NiMnSb are investigated within a combined density functional and many-body approach. Starting from a realistic multi-orbital Hubbard-model including Mn and Ni-d orbitals, the many-body problem is addressed via the Variational Cluster Approach. The density of states obtained in the calculation shows a strong spectral weight transfer towards the Fermi level in the occupied conducting majority spin channel with respect to the uncorrelated case, as well as states with vanishing quasiparticle weight in the minority spin gap. Although the two features produce competing effects, the overall outcome is a strong reduction of the spin polarisation at the Fermi level with respect to the uncorrelated case. This result emphasizes the importance of correlation in this material.Comment: 8 pages, 6 figure

Transmission through correlated Cun_nCoCun_n heterostructures

The effects of local electronic interactions and finite temperatures upon the transmission across the Cu$_4$CoCu$_4$ metallic heterostructure are studied in a combined density functional and dynamical mean field theory. It is shown that, as the electronic correlations are taken into account via a local but dynamic self-energy, the total transmission at the Fermi level gets reduced (predominantly in the minority spin channel), whereby the spin polarization of the transmission increases. The latter is due to a more significant $d$-electrons contribution, as compared to the non-correlated case in which the transport is dominated by $s$ and $p$ electrons.Comment: 29 pages, 7 figures, submited to PR

Half-metallic ferromagnetism induced by dynamic electron correlations in VAs

The electronic structure of the VAs compound in the zinc-blende structure is investigated using a combined density-functional and dynamical mean-field theory approach. Contrary to predictions of a ferromagnetic semiconducting ground state obtained by density-functional calculations, dynamical correlations induce a closing of the gap and produce a half-metallic ferromagnetic state. These results emphasize the importance of dynamic correlations in materials suitable for spintronics.Comment: Published in Phys. Rev. Lett. 96, 197203 (2006