WORKSHOP PROGRAM and ABSTRACTS (click for a group picture)

 
Friday, September 24, 2004 

9:00 to 9:40	Welcome by the organizers -Ole K. Andersen,  Stuttgart, Germany Downfolding, NMTO, and Wannier functions
9:40 to 10:20	-Peter Dederichs,  Juelich , Germany Exchange interactions and Curie Temperatures of Diluted Magnetic Semiconductors
10:20	Coffee break
10:40 to 11:10	-Olle Eriksson,  Uppsala, Sweden Electronic structure, magnetism and critical temperatures of diluted magnetic semiconductors
11:10 to 11:40	-Rudy Zeller,  Juelich, Germany Progress of the KKR Green-function method for complex electronic systems
11:40 to 12:10	-Jens Kortus, Strasbourg, France Electron transport through a single Co-complex and Co-grids
12:10 to 12:40	-Tanusri Saha-Dasgupta, S.N . Bose Centre, Kolkata, India Na2V3O7: a low-dimensional quantum spin system with nano-tubular structure
12:40	Lunch
14:00 to 14:40	-Antoine Georges,  Polytechnique, Paris, France Electronic Structure of Correlated Materials: an overview of the Dynamical Mean-Field Theory approach
14:40 to 15:20	-Misha Katsnelson,  Nijmegen, The Netherlands Nonquasiparticle states in half-metallic ferromagnets
15:20 to 16:00	-Karsten Held,  Stuttgart, Germany Filling of the Mott-Hubbard Gap in (V0.97 Cr0.03)2O3 : Theory and Experiment
16:00	Coffee break
16:30 to 17:10	-Walter Metzner, Stuttgart, Germany Transport through interacting quantum wires: functional renormalization group computation
17:10 to 17:50	-Olle Gunnarsson, Stuttgart, Germany Electron-phonon interaction in strongly correlated materials
17:50 to 18:30	.-Vaclav Drchal, Praha, Czech Republic Electron correlations in diluted magnetic semiconductors
19:00	Diner

Saturday, September 25, 2004 

8:30 to 9:10	-Walter Temmerman,  Daresbury, UK Single-Site Self-Interaction Correction in the KKR-CPA
9:10 to 9:50	-Stefan Bluegel,  Juelich, Germany Aspects of correlation in low-dimensional magnetic systems
9:50 to 10:20	-Dzidka Szotek,  Daresbury, UK Electronic Structure of Half-Metallic Ferromagnets and Ferromagnetic Insulators from SIC-LSD
10:20	Coffee break
10:40 to 11:10	-Marcelo Rozenberg, Buenos Aires, Argentina Modelling resistance random access memory
11:10 to 11:40	-Frank Lechermann, Polytechnique, France Importance of inter-orbital charge transfers on the metal-insulator transition in BaVS3
11:40 to 12:10 12:10 to 12:30	-Eva Pavarini, Pavia, Italy Mott transition in t2g GdFeO3-type perovskites -Alexander Poteryaev, Polytechnique, Paris, France Non-local Coulomb interactions and metal-insulator transition in lighter transition metal oxides: a cluster LDA+DMFT approach
12:30	Lunch
14:00 to 14:40	-Ferdi Aryasetiawan, JRCAT, Japan Towards an ab initio scheme for strongly correlated materials
14:40 to 15:15	-Brice Arnaud, Rennes, France Quasiparticle’s lifetimes from first principles
15:15 to 15:50	-Arno Schindlmayr,  Juelich, Germany Quasiparticle Electronic Structure and Energetics of Point Defects on Semiconductor Surfaces
15:50 to 16:30	-Mechael Rohlfing, Augsburg, Germany Time dynamics of excited electronic states
16:30	Coffee Break
17:00 to 17:30	-Fabien Bruneval, Polytechnique, Paris, France Electronic Excitations of Cu2O within GW approximation
17:30 to 18:10	Andrea Marini, San Sebastien, Spain Quasiparticles and excitons in extended systems: Many-Body versus time-dependent density-functional approach
19:00	Diner

Sunday, September 26, 2004 

8:30 to 9:10	-Erik Koch, Juelich, Germany Quasiparticles from quantum Monte Carlo: Effective mass and Wilson ratio for K3C60
9:10 to 9:50	-Martin Feldbacher, MPI FKF, Stuttgart, Germany Projective Quantum Monte Carlo for DMFT
9:50 to 10:30	-Alexy Rubtsov, Moscow University, Russia Continuous-time QMC for fermions: state of art and perspectives
10:30	Coffee break
10:50 to 11:20	- Peter Kratzer, MPI Berlin, Germany Theoretical investigations of MnSi and Co2MnSi thin films as spin injectors: Structural, electronic and magnetic properties
11:20 to 12:00	- Arthur Ernst, MPI Halle, Germany Beyond LDA: GW and SIC implementation in KKR
12:00	Lunch
14:00 to 18:00	Organized Hike (see access for the trail)

Quasiparticle's lifetimes from first principles

Brice Arnaud1, S. Lebègue2, M. Alouani3, V. Olevano4

1GMCM, campus de Beaulieu, 35042 Rennes Cedex, France

2Uppsala university, Sweden

3IPCMS, 23 rue du Loess, 67034 Strasbourg, France

4Laboratoire des solides irradiés, UMR 7642, Ecole Polytechnique, 91128 Palaiseau, France



Today, different techniques like angular resolved photoemission spectroscopy, time resolved two-photon photoemission or scanning tunnelling spectroscopy are used to investigate excitation lifetimes. Up to now, these techniques have been widely used to study quasiparticle's lifetimes in metals and experimental results have been successfully compared with ab initio calculations performed within the GW approximation [1,2,3]. By contrast, no experimental results are available for insulators and very few calculations have been reported. We present our new implementation [4] of the GW approximation based on an all-electron Projector-Augmented-Wave method, where no plasmon-pole model is used to mimic the frequency dependence of the dynamically screened interaction W. Therefore, both the hermitian and non-hermitian part of the self-energy are accessible and spectral functions can be easily computed. Thus, we show the calculated spectral functions of different insulators and discuss the lifetimes of quasiparticles in terms of Auger decay channels, neglecting the electron-phonon scattering mechanisms.





1. P. Echenique, J. Pitarke, E. Chulkov, A. Rubio, Chem. Phys. 251, 1 (2000)

2. R. Keyling, W.-D. Schöne and W. Ekardt, Phys Rev B 61, 1670 (2000)

3. C. D. Spataru, M. A. Cazalilla, A. Rubio and L. X. Benedict, Phys. Rev. Lett. 87, 246405 (2001)

4. S. Lebègue, B. Arnaud, M. Alouani and P.E. Bloechl, Phys Rev B 67, 155208 (2003)

Towards an ab-initio scheme for strongly correlated materials

F. Aryasetiawan1,*, S. Biermann2 and A. Georges2
1Research Institute for Computational Sciences, AIST, Tsukuba, Japan
2Ecole Polytechnique, CPHT, Palaiseau, France


In the first part, a recently proposed scheme for treating correlated materials combining the GW approximation (GWA) and dynamical mean-field theory into a self-consistent scheme is described [1]. The problems with the GWA and what can be expected from the new scheme will be discussed. The feasibility of the scheme is demonstrated by applying it to calculate the electronic structure of nickel. 

In the second part, a scheme for calculating the Hubbard U from realistic electronic structure calculations is proposed. The energy-dependent effective interaction among electrons living in a narrow band around the Fermi level can be derived and calculated from first-principles. The scheme allows for calculations of the full U matrix. Results for the 3d transition metal series, as well as for correlated metals Ca/SrVO3, obtained within the random-phase approximation will be presented.

 [1] S. Biermann, F. Aryasetiawan, and A. Georges, Phys. Rev. Lett. 90, 086402 (2003)

 [2] F. Aryasetiawan, M. Imada, A. Georges, G. Kotliar, S. Biermann, and A. I. Lichtenstein,
 cond-mat/0401620, (to appear in Phys. Rev. B).

*f-aryasetiawan@aist.go.jp

Aspects of correlation in low-dimensional magnetic systems

Gustav Bihlmayer and Stefan Blügel
Institut für Festkörperforschung, Forschungszentrum Jülich
D-52425 Jülich, Germany 

Lacking orbital-dependent terms, the local density approximation (LDA) as well as the generalized gradient approximation (GGA) to the density functional theory (DFT) is unable to reproduce correctly orbital moments and magnetocrystalline anisotropy energies (MAE) in low dimensional systems. In this talk systematic studies to overcome this failure of conventional DFT calculations employing the LDA+U method for atoms, adatoms, wires and deposited wires are presented. Including a Hubbard-U, the system no longer possesses a single minimum for the total energy and it is possible to obtain a couple of solutions. To find the global minimum, obtain reasonable values for U, and the extreme sensitivity of the MAE on relaxations still pose serious problems for these calculations. Nevertheless, it seems that the underlying physics in these systems is correctly captured in this model.

Electronic Excitations of Cu2O within GW approximation 
 
Fabien Bruneval, N. Vast, and L. Reining.
Laboratoire des Solides Irradiés, CNRS, CEA, Ecole Polytechnique, Palaiseau, France 


Cuprous oxide has been extensively studied during the last decades, mainly because of its exciton series in the optical range. Cu$_2$O is a good starting point to address the fundamental issue of {\it 3d} electrons of metals in oxides. This semiconductor material has indeed a cubic structure, a closed d shell, and is non-magnetic. This is now a very important topic: it is known that density functional theory fails to predict a gap in various insulating oxides like CoO, CuO.We performed band structure calculations on Cu$_2$O within Density Functional Theory and GW approximation We carefully studied the role of semicore states (3s23p6). Though deep in energy, these states have a large overlap with valence bands. Their influence is slight on the Kohn-Sham band structure. However, we state that the semicore states have to be included in the GW calculation to get meaningful results. Even a GW calculation including semicore states largely underestimates the quasiparticle gap. Further approximations are usually used to perform a ``standard" GW calculation: the use of a plasmon pole model to describe the dynamical screening, a first-order perturbation scheme, an assumption that LDA and GW wavefunctions are equal... We extensively discuss many of them and conclude that they are reliable.
Then the failure of GW may lie in the neglect of the vertex part of the self-energy. We therefore propose a scheme to include some local vertex corrections, thanks to Time-Dependent DFT.

Exchange interactions and Curie temperatures in dilute magnetic semiconductors

P. H. Dederichs 
IFF, Forschungszentrum Juelich, D-52425 Juelich, Germany;
K. Sato 
ISIR, Osaka Univ., Ibaraki, Osaka 567-0047, Japan

The magnetic properties of dilute magnetic semiconductors (DMS) are calculated from first-principles by mapping the ab-initio results on a Heisenberg model. The electronic structure of DMS is calculated by using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method within the local density approximation. Effective exchange coupling constants Jij’s are evaluated by embedding two impurities i and j in the CPA medium and using the Jij formula of Liechtenstein et al. [1]. First, the Curie temperatures (Tc) of (Ga, Mn)N, (Ga, Mn)P, (Ga, Mn)As and 
(Ga, Mn)Sb are estimated from the calculated exchange coupling constants by using the mean-field approximation (MFA). It is found that DMS show different concentration dependences in calculated Tc depending on their electronic structures and are divided into two classes. In one extreme case of (Ga, Mn)N, impurity bands appear in the band gap and due to the broadening of the impurity bands the ferromagnetic state is stabilized (double exchange [2]). In this case, Tc is proportional to the square root of Mn concentration c and the exchange interaction is short ranged since the wave functionin the gap is exponentially damped and well localized. In the other extreme case of (Ga, Mn)Sb, Mn 3d-states are located deep in the valence band. The valence band polarization due to the hybridization between localized d-states and valence bands gives the effective magnetic field that stabilizes the ferromagnetic states (p-d exchange [3]). In this case, Tc is linear to c and the exchange interaction is long ranged since the extended valence hole states mediate the ferromagnetic coupling. Since the experimentally realistic concentration is about 10% or lower, the percolation effect, which the MFA cannot describe, could be important in particular in double exchange systems where the magnetic coupling is short ranged. To take the percolation effect into account, we perform Monte Carlo simulations for the dilute Heisenberg model with the exchange coupling constants which we calculate from first-principles. It is found that exact Tc value of (Ga, Mn)N is very low due to the percolation effect and the MFA estimations are qualitatively wrong and unreliable in this system. Even in (Ga, Mn)As, where long ranged p-d exchange dominates, the percolation effect is still serious and the MFA overestimates Tc very much.

[1] A. I. Liechtenstein et al., J. Magn. Magn. Mater. 67 (1987) 65. 
[2] H. Akai, Phys. Rev. Lett. 81 (1998) 3002.
[3] T. Dietl et al., Science 287 (2000) 1019.

Electron correlations in diluted magnetic semiconductors

Vaclav Drchal, J. Kudrnovsky, and A. B. Shick 
Institute of Physics, AS CR, Praha, Czech Republic
 
We examine the effect of Coulomb correlations in diluted ferromagnetic III-V semiconductors using the LSDA+U correlated band theory method. We show that Hubbard U plays an important role in determination of Mn d states' interaction with the semiconductor host valence band. We present the results of supercell calculations by the FP-LAPW and TB-LMTO-CPA methods for 
Ga1-xMnx As and Ga1-xMnx N alloys. For Ga1-xMnxAs, we find a reasonable increase of d-Mn binding energy in agreement with the experiment. For Ga1-xMnxN alloy, an account of the Hubbard U is even more important, moving the sharp d peak from the top into the body of the valence band. Based on the calculated electronic structure, we determine exchange interactions between magnetic moments and discuss the role of electron correlations on Curie temperature. We also discuss the energetics connected with possible aggregation of Mn atoms and its dependence on electron correlations. 

Electronic structure, magnetism and critical temperatures of dilute magnetic semiconductors

Olle Eriksson 
Department of Physics, Uppsala University, Sweden

The electronic structure and magnetic properties of dilute magnetic semiconductors (DMS) are analysed theoretically and compared to experimental data. It is argued that the effect of defects influences the magnetic properties strongly, where in particular the effect of As anti-sites is to stabilize a disordered local moment (DLM) state. In addition it is shown that the observed critical temperature can be reproduced only when considering a random distribution of Mn (Cr) atoms in GaAs (ZnTe) and it is argued that magnetic percolation is important in DMS systems. The measured spectroscopic properties are analysed by combining ab-initio theory with an atomic multiplet calculation in the Hubbard I approximation.

GW and SIC implementation in the KKR method

A. Ernst, V. Dugaev, P. Bruno 
 Max-Planck-Institut für Mikrostrukturphysik Halle(Saale), 06120, Germany \

 M. Lüderes, W.M. Temmerman, Z. Szotek

 Daresbury Laboratory, Warrington WA4 4AD, United Kingdom 
 B. Györfy 

 HH Wills Physics Laboratory, University of Bristol

The ab-initio study of semiconductors and insulators as well as systems with strong localized electrons entails great difficulties involved by the treatment of excitation energies and many-body effects. The most successful first principles method, the density functional theory (DFT) within the local spin density approximation (LSDA), is designed for ground state properties and can not provide proper description of band structure of semiconductors and insulators. If some localized electrons are present in the system, like 3d-electrons in the transition metal oxides, the local

density approximation can be essentially improved by so-called self-interaction correction (SIC). In this approximation the self-iterations of single particle charges, which are present in the LSDA, can be canceled out for the localized electrons. However, the self-interaction correction within the LSDA is still not sufficient for proper description of the excitation energies and band-gaps. It is possible to do accurately from first principles by solving the Hedin's set of equations for the full Green's function. The implementation of this formalism is very difficult, one neglects commonly the vertex correction (the random-phase approximation), and the self-energy is calculated in this case within so-called GW approximation. The non-self-consistent GW approximation was successfully implemented within several first-principle methods, but most of existing implementations are generally designed for systems with delocalized (fast) electrons. 
Here we present a general ab-initio approach designed for the study of electronic properties of solids, in which on base of the Korringa-Kohn-Rostoker (KKR) method we implemented the self-interaction correction for strong-localized electrons and the non-self-consistent GW approximation for the inclusion of many-body effects. We illustrate our approach on the electronic structure study of some semiconductors and transitions metal oxides.


Projective Quantum Monte Carlo for DMFT 



Martin Feldbacher 

Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany



In recent years, there has been a revival of interest in Kondo-like physics, in particular in quantum dot systems and in connection with the dynamical mean field theory (DMFT). The numerical solution of the underlying Anderson impurity models is, however, limited: In the Numerical Renormalization Group treatment the effort grows exponentially with the number of orbitals, allowing not more than two interacting orbitals; the Hirsch-Fye Quantum Monte Carlo (QMC) algorithm on the other hand scales like T-3 (T: temperature) and quickly becomes too expensive in CPU time. This limitation is especially severe when DMFT is used to model materials with strong electron correlations where, in order to observe the physics of interest, low temperatures need to be achieved. We propose a projective QMC algorithm for the Anderson impurity model, which converges rapidly to the ground state. With this new impurity solver, we study the Mott-Hubbard metal-insulator ransition in the Hubbard model, demonstrating that it gives reliable “T=0'” DMFT results.

Electronic Structure of Correlated Materials: 
an overview of the Dynamical Mean-Field Theory approach

Antoine Georges
Centre de Physique Théorique, Ecole Polytechnique,
91128 Palaiseau Cedex, France

In recent years, Dynamical Mean Field Theory (DMFT) has been combined with electronic structure calculation methods in order to provide a quantitative description of strong correlation effects in a realistic setting.


I will review the basic principles of this approach. Functionals of the local density and the local Green's function can be constructed in order to provide a convenient formal framework for LDA+DMFT. These functionals can be extended to the local screened interaction, opening the way to a first-principle treatment of the Coulomb interaction, which brings together the GW approximation and DMFT.

Early successes of the DMFT approach at describing physical effects of correlations (such as the Mott transition and the properties of transition metal oxides) will also be briefly reviewed, and new challenges will be emphasized.

For a recent review and references, see e.g:
A.~Georges, Strongly Correlated Electron Materials: Dynamical Mean-Field Theory and Electronic Structure,
 cond-mat/0403123 (to be published in the Proceedings of the VIII Training Course in the Physics of Correlated Electron Systems and High-Tc Superconductors, American Institute of Physics).

georges@cpht.polytechnique.fr


Electron-phonon interaction in strongly correlated materials

Olle Gunnarsson and O. Rösch
Max-Planck-Institut für Festkörperforschung, D-70506 Stuttgart, Germany

Recent photoemission work suggests a strong electron-phonon coupling in cuprates. These experiments were interpreted in terms of a strong coupling to a half-breathing Cu-O bond-stretching phonon. Neutron scattering experiments show that this phonon has an anomalous softening upon doping, suggesting an appreciable electron-phonon coupling. It is interesting, however, that photoemission experiments were interpreted in terms of a rather strong coupling, λ ~ 1, while neutron scattering suggest a modest coupling, λ ~ 0.2-0.3, and LDA calculations a very weak coupling, λ ~0.01. 

To address this issue, we start from the three-band model of a CuO2 layer and derive a t-J model with electron-phonon interaction, taking into account the modulation of both hopping and Coulomb integrals by phonons. The former is found to be dominant, but the modulation of the Coulomb integrals cannot be neglected. The model is studied using exact diagonalization. It explains the anomalous softening of the half-breathing mode upon doping and a weaker renormalization of the breathing mode. We find that the creation of low-energy excitations in the t-J model is an important reason for the difference from LDA calculations.

We next study more generally the electron-phonon interaction in a doped insulator and compare with results for non-interacting electrons. Using exact sum-rules, we find that the effect of the electron-phonon interaction on the phonon self-energy is strongly reduced by correlation effects, while there is no corresponding reduction for the electron self-energy or the phonon induced electron-electron interaction. 

Filling of the Mott-Hubbard Gap in (V0.97Cr0.03) 2O3 : Theory and Experiment 


Karsten Held
MPI-FKF, Stuttgart, Germany


 The combination of band structure theory in the local density approximation (LDA) with dynamical mean field theory (DMFT) [1] was recently applied successfully to V2O3 [2] -- a material that undergoes the famous Mott-Hubbard metal-insulator transition upon Cr doping. We will emphasize two aspects of our recent results: (i) The filling of the Mott-Hubbard gap with increasing temperature, a genuine feature of the Mott-Hubbard insulator, was recently observed in photoemission experiments [3], ascertaining the DMFT prediction. (ii) The Mott-Hubbard transition in V2O3 shows peculiarities due to inequivalent orbitals. In particular, the effective mass of the  a1g-orbital does not diverge at the Mott-Hubbard transition [2]. 



[1] For reviews see K. Held et al., Psi-k Newsletter \#56 (2003), p. 5 [psi-k.dl.ac.uk/newsletters/News\_56/Highlight\_56.pdf]; 

A. I. Lichtenstein, M. I. Katsnelson, and G.\ Kotliar, to be published in Electron Correlations and Materials Properties 2, ed. A. Gonis (Kluwer, NY). 



[2] K. Held et al. Phys. Rev.Lett. 86, 5345 (2001); G. Keller et al., cond-mat/0402133. 



[3] S.-K. Mo  et al.,  Phys.Rev. Lett. in print [cond-mat/0403094]. ANOQUANTA (NOE 500198-2).

Nonquasiparticle states in half-metallic ferromagnets

Misha Katsnelson
University of Nijmegen, The Netherlands

Anomalous magnetic and electronic properties of the half-metallic ferromagnets (HMF) are discussed. A general concept of the HMF electronic structure hich take into account the most important correlation effects from electron-magnon interactions, that is, the spin-polaron effects, is presented Special attention is paid to the so called non-quasiparticle (NQP), or incoherent, states which are present in the gap near the Fermi level. Manifestations f NQP in electronic density of states, tunneling transport, nuclear magnetic relaxation, core-level spectra and other properties of HMF are considered. irst-principle calculations of the NQP-states for various half-metallic ferromagnets within the local-density approximation plus dynamical mean field theory LDA+DMFT) are reviewed.
Quasiparticles from quantum Monte Carlo:  Effective mass and Wilson ratio for K3C60
Erik Koch
FKF, Juelich, Germany 



Taking advantage of the fixed-node approximation to stabilize excited states, we calculate quasiparticle energies by uantum Monte Carlo. We apply this approach to a model describing K3C60 and show that it works surprisingly well. We determine how the quasiparticle dispersion changes with increasing correlation and compare to the many-body nhancement of the Pauli susceptibility. While the self-energies for our model seem to depend only weakly on the omentum (and band-index), we find a Wilson ration that appears to behave qualitatively different from what is expected
from dynamical mean-field theory. 

Electron transport through a transition metal complexes and grids

Jens Kortus
IPCMS, Strasbourg, France

Metal organic complexes offer a promising route to the design of magnetic materials, because intramolecular magnetic interactions can be modified through the choice of the bridging ligands and variation of the metal-ions. The [2x2]-grid structures containing 4 metal ions belong to this class, where the transition metal ions are positioned by perpendicular rrays of ligands.

We study the electronic and spin degrees of freedom of Fe(2+) and Co(2+) single ions and grids using first-principles ll-electron density-functional calculations. One goal is to identify the orbitals important for transport. Experimentally single Co2+/3+ ion can be fixed between a left and right terpyridine ligand, which can be coupled by functionalized inker molecules to electrodes. It was found experimentally [1] that depending on the type of linker molecules, a strong ondo effect in the current appears. We have investigated the conjecture that the tunnelling is dominated by single atom-like d-orbitals of the ion by electronic structure calculations. We show that microscopically the ligands of this articular complex act as a tunnel barrier due to different symmetry of the highest ligand pi-orbital and Co2+/3+-ion igma-orbitals. The role of the linker molecules connecting the ligands to the electrodes is therefore crucial. For the ow-spin Fe(2+) [2x2] grid we show, that the electrons can move between the ligands through an empty orbital on the etal bridge. This is compared to the Co(2+) [2x2] grid, where the metal bridge contains a localized electron which can nteract with the transported charge.

[1] J. Park et al, Nature 417, 722 (2002)

In collaboration with M.R. Wegewijs, C. Romeike and H. Schoeller Inst. for Theoretical Physics A, RWTH Aachen, Germany

Importance of inter-orbital charge transfers on the metal-insulator transition in BaVS3 

Frank Lechermann, Silke Biermann, and Antoine Georges
Centre de Physique Theorique, Ecole Polytechnique, Palaiseau, France

An understanding of the physics of strongly correlated multi-orbital electron systems is one of the key ingredients in order to describe a wide range of novel solid-state compounds. The complex interplay of the crystal structure with the competition between the localized and the itinerant character of the electrons in a manifestly multi-orbital case is giving rise to highly interesting physical phenomena. By combining the Dynamical Mean Field Theory (DMFT) with the Local Density Approximation (LDA) to Density Functional Theory, a powerful many-body approach is provided to tackle the given problem on a realistic level. Within the LDA + DMFT method, we investigated the multi-orbital system BaVS3 that is well known for undergoing three distinct continuous phase transitions with decreasing temperature. In BaVS3, a first structural transition from a hexagonal to an orthorhombic crystal structure at 240 K is followed by a metal-to-insulator transition (MIT) at around 70 K. Close to 30 K a final magnetic transition to an incommensurate antiferromagnetic ordered state seems to occur. The highlighted MIT seems to be accompanied by a charge density wave instability in this partially one-dimensional sulfide [1]. In our study, we concentrate on the MIT and reveal the importance of the inter-orbital charge transfer between the 3d-orbitals of the V atoms induced by electronic correlations. Using the LDA density of states as an input, with our DMFT investigations we are able to shed light on the underlying mechanism that drives BaVS3 into the insulating regime.

[1] S. Fagot, P. Foury-Leylekian, S. Ravy, J.-P. Pouget, and H. Berger, Phys. Rev. Lett. 90, 196401 (2003).
 

Theoretical investigations of MnSi and Co2MnSi thin films as candidates for spin injectors: Structural, electronic and magnetic properties

P. Kratzer, H. Wu, S.J. Hashemifar, M. Hortamani and M. Scheffler
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

We investigate thin films of Mn intermetallic compounds with the goal to identify materials systems which could be of possible use as spin injectors on a Si(001) semiconductor substrate. In ordered metallic compounds containing both Mn and Si, the Mn atoms often have large spin moments (depending on their local chemical environment), and show various types of magnetic ordering. All calculations presented here have been performed using the spin-polarized generalized gradient approximation to density-functional theory and the all-electron code WIEN2k, using the full-potential LAPW or APW+lo method.

One promising candidate for spintronics applications is the ferromagnetic half-metal Co2MnSi, which, in its bulk phase, has a high Curie temperature of 985 K and displays a gap at the Fermi level in the minority spin channel. At surfaces and interfaces, however, it is conceivable that electronic states in the gap arise which modify or destroy the half-metallic properties of this material. To investigate this possibility, we perform electronic structure calculations for the Co2MnSi(001) surface. Analysis of the surface energies of different surface terminations shows that terminations by a layer of only Mn atoms, only Si atoms, or a mixture of both are all thermodynamically stable, depending on the chemical environ­ment used for surface preparation. We find that the surface electronic structure is strongly affected by the type of surface termination; in particular, terminating the surface by a monolayer of Mn atoms preserves the half-metallic gap, while surface states occur for other terminations. We surmise that such details of the atomic structure may as well be crucial for the electronic properties of Co2MnSi semi­conductor interfaces.

Next, we investigate the properties of ultra-thin films (1-3 monolayers [ML]) grown epitaxially on the Si(001) surface, focussing on the intermetallics MnSi and Co2MnSi. In its bulk P213 phase, MnSi is known to be a weak helical magnet below 30K. However, from our total-energy calculations, we conclude that a novel, CsCl-like crystalline phase of MnSi should be producible as a metastable epi­taxial structure on the Si(001) substrate. We find that the Mn atoms in thin films with CsCl-like sand­wich structure of alternating Mn and Si layers have sizeable magnetic moments in the range of 1-2μB. Moreover, these films show a layered magnetic ground state, with pronounced ferromagnetic coupling between the Mn magnetic moments within the layers. The interlayer coupling is much weaker and leads to a ferromagnetic ground state for the 2 ML film, or a ferrimagnetic ground state for the 3 ML film. For ultra-thin Co2MnSi films on Si(001) we find that the Mn atoms possess magnetic moments in the range of 2.7-3.5μB, similar in size as in bulk Co2MnSi, and strong ferromagnetic coupling is found for Mn magnetic moments both within the same layer and between layers. Moreover, we learn from the calculations that the Co2MnSi thin films are stable against decomposition into elementary silicides. Both for the MnSi and the Co2MnSi films, our calculations predict values of 20% to 50% for the spin polarization at the Fermi level. 

In summary, our results suggest that Mn-Si-compounds are potentially useful as spin-polarized electrodes on Si(001), and thus deserve further exploration by theoretical and experimental work. 
 

Quasiparticles and excitons in extended systems: Many-body versus time-dependent density-functional approach 



Andrea Marini1,2, D. Varsano1, R. Del Sole2, and A. Rubio1

(1) Departamento de F\'\i sica de Materiales, UPV/EHU,  Centro Mixto CSIC--UPV/EHU  and  Donostia International Physics Center. E--20018 San Sebasti\'an, Spain.

(2) Istituto Nazionale per la Fisica della Materia e Dipartimento di Fisica dell'Universit à di Roma ``Tor Vergata'', Via della Ricerca Scientifica, I--00133 Roma, Italy. 



The correct description of the physical properties of quasiparticles  and excitons in extended and low--dimensional systems constitute an important test for modern Ab-Initio theories. The commonly used approximation for the DFT exchange--correlation potential and TDDFT kernel based on the Local--Density Approximation, (LDA) fail to describe, among other things, the quasiparticle band structure properties and the excitonic effects in the optical and energy-loss spectra of semiconductors and insulators. 

In this framework Many--Body perturbation theory has provided successful tools (GW approximation, Bethe--Salpeter equation) to improve the results of the, (simpler) calculations performed within DFT. However, only recently the first principle description of excitons in the optical absorption has been achieved, due to the high complexity and large computational requirements of many--body calculations. Furthermore, to--date, calculations of the absorption spectra of solids beyond time-dependent LDA were performed in semiconductors characterized by weak continuous excitonic effects. Consequently, it remains open whether or not strong electron-hole effects (e.g. bound excitons) in the optical and energy-loss spectra can be described within TDDFT.  We present a robust, efficient, frequency dependent and non-local exchange-correlation derived by imposing TDDFT to reproduce the many-body diagrammatic expansion of the Bethe-Salpeter polarization function [1]. As illustration, we show the calculated optical spectra of solids [1] and nanostructures [2] characterized by moderate, (diamond) and strong, (LiF and SiO2) electron-hole interaction. In the second part of the talk we will investigate the relation between this novel fxc and the self--energy vertex function fxc Г. As the polarization function can be easily expressed in terms of both Г and fxc we will show how to derive a consistent expression of Г in terms of fxc. We will show that it is possible to obtain a practical and general expression for the three-point vertex function based on the two--point exchange--correlation kernel [3]. Taking LiF as a test case we will study the quasiparticle dynamics of the low--energy conduction states beyond the GW approximation. We will show that excitonic effects in LiF strongly modifies the electronic lifetimes leading to linear scaling with quasiparticle energy [3]. As consequence, we will find that, in contrast to previous results for the electron gas, simple metals and semiconductors, vertex corrections in the self--energy and in the screening function do not compensate each other. 



[1] A. Marini, R. Del Sole, and A. Rubio,     Phys. Rev. Lett. 91, 256402 (2003). 

[2] D. Varsano, A. Rubio, and A. Marini. Work in progress. 

[3] A. Marini, and A. Rubio. Submitted. cond-mat/0405590 
Transport through interacting quantum wires: functional renormalization group computation

Walter Metzner

Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany



The interplay of electron correlations and impurities in one-dimensional metals leads to striking effects at low energy scales. 

We compute the conductance of an interacting quantum wire with isolated static impurities by using a recently developed functional renormalization group (fRG) method [1]. The calculation is perturbative in the interaction, but non-perturbative in the impurity strength. Moreover, the method can deal with arbitrary shapes of the impurity potential. A comparison with exact numerical results for finite systems with up to 1000 lattice sites shows that the fRG is surprisingly accurate even for moderate interaction strength [1].

The agreement with exact asymptotic results, such as the scaling function for the conductance through a wire with a single short-ranged impurity [2], is also remarkable. The flexibility of the fRG in dealing with arbitrary impurity potentials and also with complex crossover phenomena becomes particularly useful in the case of resonant tunneling through a double barrier potential. Results for the peak conductance Gp(T) as a function of temperature exhibit non-monotonous behavior for suitable choices of the barrier parameters and distinctive power-laws at different energy scales [3]. 



1. V. Meden, W. Metzner, U. Schollw\"ock, and K. Schönhammer, Phys. Rev. B 65, 045318 (2002); S. Andergassen, T. Enss, V Meden, W. Metzner, U. Schollwöck, and K. Schönhammer, cond-mat/0403517.

2. V. Meden, S. Andergassen, W. Metzner, U. Schollwöck, and K. Schönhammer, Europhys. Lett. 64, 769 (2003).

3. V. Meden, T. Enss, S. Andergassen, W. Metzner, and K. Sch\"onhammer, cond-mat/0403655.

Mott transition in t2g GdFeO3-type perovksites

Eva Pavarini
Volta Institute, Pavia, Italy

Using a first principle downfolding method, we derive a low-energy Hubbard Hamiltonian for strongly correlated t2g  Perovskites. We treat Coulomb repulsion effects by means of dynamical mean-field theory and calculate photoemission spectra. We show that cation covalency (GdFeO3-type distortions) is the most important material dependent mechanism. We study the Mott transition for different fillings and show that the interplay of Coulomb repulsion effects and cation covalency control the electronic properties of these materials.

Non-local Coulomb interactions and metal-insulator transition in lighter transition metal oxides: a cluster LDA+DMFT approach
Alexander .I. Poteryaev, 

Centre de Physique Theorique de l'Ecole Polytechnique, F91128 PALAISEAU CEDEX, FRANCE



 We present an ab initio quantum theory of the metal-insulator transition in Ti2O3 and VO2. There is a strong competition between local Coulomb interaction and chemical bonding in a M-M pair which results in a small insulating gap of a low temperature phase. The state of the art calculations in the local density approximation (LDA) show that the bonding-antibonding splitting is not enough to open a gap and correlation effects are important. The conventional single site dynamical mean filed theory (DMFT) cannot reproduce an insulating phase for any reasonable values of the local Coulomb interaction since it leads to the reduction of the bonding-antibonding splitting. The new cluster LDA+DMFT scheme is applied to describe the many-body features of these compounds. We have investigated the metal-insulator transition in Ti2O3 and VO2 and have shown that the many body effects related to the non-local Coulomb interactions are essential for a simultaneous description of the low-temperature insulating and high-temperature metallic states. 

Time dynamics of excited electronic states

Michael Rohlfing
International University, Bremen, Germany

Ab-initio many-body perturbation theory (MBPT) provides a rigorous approach to excited electronic states. Within MBPT, single-particle excitations (electron states and hole states) are obtained from a GW calculation, which includes electron exchange and correlation effects in terms of the self-energy operator. In addition, coupled electron-hole excitations are obtained from the Bethe-Salpeter equation (BSE), which includes electron-hole interaction. These techniques can be employed to discuss the Spectra of a wide range of systems, from bulk crystals (like silicon or MgO) to molecules and polymers (like PPV or PPP), and from weakly correlated systems to Mott-Hubbard insulators like the one on the SiC(0001) surface. In addition to the spectra obtained from MBPT, recent developments concern the femtosecond dynamics of excited states. As an example, we discuss the initial decay of a molecular excitation (first excited state of CO) due to adsorption of the molecule on an insulator surface (MgO), which leads to a spatial separation of the excited electron and hole. 

Modelling resistance random access memory 

Marcelo J. Rozenberg 
CPHT - Ecole Polytechnique, France 
 
There is a current upsurge in research on nonvolatile two-terminal resistance random access memory (RRAM) for next generationelectronic applications. The RRAM is composed of a simple sandwich of a semiconductor (including strongly correlated electron compounds) with two metal electrodes. In this talk we shall: (i) review some of the essential features associated with RRAM behavior, such as hysteresys and multilevel resistance switching. (ii) introduce a basic model for RRAM with the made assumption that the semiconducting part has a non-percolating domain structure. (iii) present and discuss the behavior of the model investigated with numerical simulations that allows to understand the carrier transfer mechanisms in detail, and see how our model captures three key features observed in experiments: multilevel switchability of the resistance, its memory retention and hysteretic behaviour in the current-voltage curve. A very interesting aspect of our investigations is that they suggest that strong correlation effects are crucial for important resistance switching features.
 
Continuous-time QMC for fermions: state of art and perspectives

 A.N. Rubtsov
Moscow State University, Russia

Novel continuous-time algorithm for QMC simulations of the fermionic systems is surveyed. The algorithm is based on the stochastic calculation of the interaction-representation series for quantum mechanical averages. Action is divided into Gaussian and interation part in a special way to minimize the sign problem and the complexity of calculation. Also, special re-weighting procedures, particularly Wang-Landau sampling, are invoked for the same purpose. No time discretisation is required, therefore there is no any systematic errors in the scheme. No auxiliary fields are introduced, therefore the algorithm operates natively with an interaction nonlocal in time, space, spin or orbital indices. Several test examples are presented, as well as new results for systems with the dynamical screening and rotationally-invariant exchange interaction.

Na2V3O7: a low-dimensional quantum spins system with nano-tubular structure 

Tanusri Saha-Dasgupta, R. Valenti, F. Capraro and C. Gros
S. N. Bose. National Center for Basic Sciences JD Block, Salt Lake, Kolkata 700 098, India


Following the recent discussion on the nature of the interactions in the tubular system Na2V3O7, we present a detailed ab-initio microscopic analysis of its electronic and magnetic properties. We show by means of a downfolding study that, due to the special geometry of this material, the edge-sharing V-V hopping interactions are of the same order of magnitude as the corner-sharing paths within a ring and an order of magnitude bigger than the hopping interactions between rings in a tube. We propose an effective model in terms of weakly coupled partially frustrated nine-site rings and calculate the susceptibility behavior by exact diagonalization of the model. Good agreement with experimental observations is obtained.

Quasiparticle Electronic Structure and Energetics of Point Defects on Semiconductor Surfaces

Arno Schindlmayr
Institut für Festkörperforschung, Forschungszentrum Jülich, 52425 Jülich, Germany

Point defects strongly influence the transport, electronic, and optical properties of semiconductors in technological applications. In particular, they act as compensation centers and are responsible for Fermi-level pinning. The energetics of transitions between different charge states is hence an important material characteristic. The relevant charge-transition levels contain two energy contributions due to the transfer of electrons between the Fermi level and the defect state, and due to the lattice relaxation. While the latter is accessible in density- functional theory (DFT), the underestimation of the band gap translates into a significant error in the first, dominant term. We therefore use the GW approximation for the electronic self-energy to accurately
 describe the electronic properties of localized defects. As specific examples we consider anion vacancies on the GaAs(110) and InP(110) surfaces. Concomitant with the opening of the band gap, the charge-transition levels are raised in comparison to DFT-LDA predictions and show improved agreement with the available experimental data.


Electronic Structure of Half-Metallic Ferromagnets and Ferromagnetic Insulators from SIC-LSD

Dzidka Szotek
Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

We discuss electronic structure of materials of relevance to spintronics and among them half-metallic ferromagnets and ferromagnetic insulators, as obtained from the SIC-LSD calculations. Apart from materials containing transition metal elements, we consider also rare earth compounds. In addition, diluted magnetic semiconductors will be briefly discussed.

Self-interaction correction in multiple scattering theory

Walter M. Temmerman
Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

 We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LS method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of  physical quantities averaged over ensembles of such potential using the coherent potential approximation (CPA). This facilitate applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well intermediate valences. As demonstration of the method, we study the well-known α-γ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

Progress of the KKR Green-function method for complex electronic systems

Rudolf Zeller
Institut fuer Festkoerperforschung, Forschungszentrum Juelich, Germany

The KKR Green-function method is a powerful and accurate tool to solve the Kohn-Sham density-functional equations for spatially complex systems like impurities in bulk crystals and at surfaces and interfaces. The method is based on the concept of a referencessystem and solves the integral Dyson equation for the Green function by use of multiple scattering theory. In the talk, the basicfeatures of the method will be explained and some recent examples for the calculated electronic structure of impurities at surfaces will be presented. It will be further discussed that the method is not necessarily restricted to local potentials, as used in LDA density-functional calculations, and that generalization to certain types of non-local energy-dependent potentials is possible.