We use the Gutzwiller variational theory to calculate the ground-state phase diagram and quasiparticle bands of LaOFeAs. The Fe3d-As4p Wannier-orbital basis obtained from density-functional theory defines the band part of our eight-band Hubbard model. The full atomic interaction between the electrons in the iron orbitals is parametrized by the Hubbard interaction U and an average Hund's-rule interaction J. We reproduce the experimentally observed small ordered magnetic moment over a large region of (U,J) parameter space. The magnetically ordered phase is a stripe spin-density wave of quasiparticles.

We present a study of the magnetic order and the structural stability of two-dimensional quantum spin systems in the presence of spin-lattice coupling. For a square lattice it is shown that the plaquette formation is the most favourable form of static two-dimensional dimerization. We also demonstrate that such distortions may coexist with long range magnetic order, in contrast to the one-dimensional case. Similarly, the coupling to Einstein phonons is found to reduce, but not to eliminate the staggered magnetic moment. In addition, we consider the renormalization of the square lattice phonon spectrum due to spin-phonon coupling in the adiabatic approximation. Towards low temperatures significant softening mainly of zone boundary phonons is found, especially around the (π,0) point of the Brillouin zone. This result is compatible with the tendency to plaquette formation in the static limit. We also point out the importance of a "magnetic pressure" on the lattice due to spin-phonon coupling. At low temperatures, this results in a tendency towards shear instabilities of the lattice.

Ferromagnetic Nickel is the most celebrated iron group metal with pronounced discrepancies between the experimental electronic properties and predictions of density functional theories. In this work, we show in detail that the recently developed multi-band Gutzwiller theory provides a very good description of the quasi-particle band structure of nickel. We obtain the correct exchange splittings and we reproduce the experimental Fermi-surface topology. The correct (111)-direction of the magnetic easy axis and the right order of magnitude of the magnetic anisotropy are found. Our theory also reproduces the experimentally observed change of the Fermi-surface topology when the magnetic moment is oriented along the (001)-axis. In addition to the numerical study, we give an analytical derivation for a much larger class of variational wave-functions than in previous investigations. In particular, we cover cases of superconductivity in multi-band lattice systems.

Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The exact evaluation of these variational ground states in the limit of large coordination number allows the identification of quasi-particle band structures, and the calculation of a variational spinwave dispersion. The study of a generic two-band model elucidates the co-operation of the Coulomb repulsion and the Hund's-rule exchange for itinerant ferromagnetism. We present results of calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis of $4s$, $4p$ and $3d$ valence electrons. The quasi-particle energy bands agree much better with the photo-emission and Fermi surface data than the band structure obtained from spin-density functional theory (SDFT).

Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The exact evaluation of these variational ground states in the limit of large coordination number allows the identification of quasi-particle band structures, and the calculation of a variational spinwave dispersion. The study of a generic two-band model elucidates the co-operation of the Coulomb repulsion and the Hund’s-rule exchange for itinerant ferromagnetism. We present results of calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis of 4s, 4p and 3d valence electrons. The quasiparticle energy bands agree much better with the photo-emission and Fermi surface data than the band structure obtained from spin-density functional theory (SDFT). 1 Exchange versus Correlations More than 50 years ago two basically different scenarios had emerged from early quantum-mechanical considerations on electrons in metals with partly filled d bands. Scenario I: As proposed by Slater [1] and Stoner [2], band theory alone was argued to account for itinerant ferromagnetism. Due to the Pauli principle, electrons with parallel spins cannot come arbitrarily close to each other (“Pauli” or “exchange hole”), and, thus, a ferromagnetic alignment of the electron spins reduces the total Coulomb energy with respect to the paramagnetic situation (“exchange field energy”). Scenario II: As emphasized by van Vleck [3], electronic correlations are important in narrow-band materials. Due to the strong electron-electron interaction, charge fluctuations in the atomic d shells are strongly suppressed (“minimum polarity model”). The atomic magnetic moments arise due to the local Coulomb interactions (in particular, Hund’s-rule couplings) and the electrons’ motion through the crystal may eventually align them at low enough temperatures. In principle, such a dispute can be resolved in natural sciences. The corresponding theories have to be worked out in detail, and their results and predictions have to be compared to experiments. This was indeed done for scenario I [4,5]. The (spin-)density functional theory is a refined band theory which describes some iron group metals with considerable success. Unfortunately, progress for scenario II was much slower. It calls 2 Werner Weber, Jörg Bünemann, and Florian Gebhard for a theory of correlated electrons, i.e., a genuine many-body problem has to be solved. It was only recently that reliable theoretical tools became available which allow to elucidate scenario II in more detail [6,7,8,9,10,11]. A first step in this direction was the formulation of appropriate model Hamiltonians which allowed to discuss matters concisely, e.g., the Hubbard model [12,13,14,15]. This model covers both aspects of d electrons on a lattice: they can move through the crystal, and they strongly interact when they sit on the same lattice site. The model is discussed in more detail in Sec. 2. Even nowadays, it is impossible to calculate exact ground-state properties of such a model in three dimensions. In 1963/1964 Gutzwiller introduced a trial state to examine variationally the possibility of ferromagnetism in such a model [12,13]. His wave function covers both limits of weak and strong correlations and should, therefore, be suitable to provide qualitative insights into the magnetic phase diagram of the Hubbard model. Gutzwiller-correlated wave functions for multi-band Hubbard models are defined and analyzed in Sec. 3. The evaluation of multi-band Gutzwiller wave functions itself poses a most difficult many-body problem. Perturbative treatments [16,17] are constrained to small to moderate interaction strengths. The region of strong correlations could only be addressed within the so-called “Gutzwiller approximation” [12,13,18] and its various extensions [19,20]. Some ten years ago, the Gutzwiller approximation was found to become exact for the one-band Gutzwiller wave function in the limit of infinite spatial dimensions, d → ∞ [21,22,23], and Gebhard [24] developed a compact formalism which allows the straightforward calculation of the variational ground-state energy in infinite dimensions. Recently, Gebhard’s approach was generalized by us to the case of multi-band Gutzwiller wave functions [10]. Thereby, earlier results by Bünemann and Weber [25], based on a generic extension of the Gutzwiller approximation [26], were found to become exact in infinite dimensions [27]. As shown in Sect. 4 for a two-band toy model, the Gutzwiller variational scheme approach also allows the calculation of spinwave spectra [28]. In this way, the dispersion relation of the fundamental low-energy excitations can be derived consistently. Albeit the description is based on itinerant electrons, the results for strong ferromagnets resemble those of a Heisenberg model for localized spins whereby a unified description of localized and itinerant aspects of electrons in transition metals is achieved. In Sect. 5 we discuss results from a full-scale calculation for nickel. The additional local correlations introduced in the Gutzwiller scheme lead to a much better description of the quasi-particle properties of nickel than in previous calculations based on spin-density functional theory.

Physikalische BlätterVolume 57, Issue 4 p. 21-22 Im BrennpunktOpen Access MgB2: Ein neuer „klassischer”︁ Supraleiter bei 39 K? Werner Weber, Werner Weber weber@fkt.physik.uni-dortmund.de Prof. Dr. Werner Weber, Institut für Physik, Universität Dortmund, 44221 DortmundSearch for more papers by this author Werner Weber, Werner Weber weber@fkt.physik.uni-dortmund.de Prof. Dr. Werner Weber, Institut für Physik, Universität Dortmund, 44221 DortmundSearch for more papers by this author First published: April 2001 https://doi.org/10.1002/phbl.20010570409Citations: 1 AboutPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Literatur 1 Jun Nagamatsu et al., Nature 410, 63 (2001). 2 D. K. Finnemore et al., de.arxiv.org, cond-mat/0102114. 3 J. Kortus et al., cond-mat/0101446. 4 Y. Kong et al., cond-mat/0102499. 5 R. K. Kremer et al., cond-mat/0102432. 6 S.L. Budko et al., Phys. Rev. Lett. 86, 1877 (2001), cond-mat/0101463. 7 K.-H. Müller et al., cond-mat/0102517. Citing Literature Volume57, Issue4April 2001Pages 21-22 ReferencesRelatedInformation

Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The approximate evaluation of these variational ground states becomes exact in the limit of large coordination number. The result allows the identification of quasi-particle band structures for correlated electron systems. As a first application, we summarize a study of itinerant ferromagnetism in a two-band model, thereby elucidating the co-operation of the Coulomb repulsion and the Hund's-rule exchange. Then, we present results of calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis of 4s, 4p and 3d valence electrons. Good agreement with the experimental ground-state properties of nickel is obtained. In particular, the quasi-particle energy bands agree much better with the photo-emission and Fermi surface data than the band structure obtained from spin-density functional theory. Finally, we present results for the variational spinwave dispersion for our two-band model.

We have studied the electronic structure and chemical bonding of the novel ternary hydride nitrides Sr2(H)N and Ba2(H)N by means of density functional theory. In these insulating materials the hydrogen atom is found to be close to an H− anion state, while there is considerable covalency in the metal-nitrogen bond, which reduces the effective N charge to ≈ − 1.8. To resolve a discrepancy with experimental data concerning the nature of the optical gap in Sr2(H)N we have also calculated the energy bands of the imide Sr(NH). In addition, we have simulated, in a rough model, the imide formation in these hydride nitrides, which occurs when the materials are illuminated with light.

Using a Rayleigh-Schrodinger perturbation expansion of multiband Hubbard models, we present analytic expressions for the superexchange coupling constants between magnetic transition-metal ions of arbitrary separation in Mott-Hubbard insulators. The only restrictions are (i) all ligand ions be closed shell anions and (ii) all contributing interaction paths be of equal length. For short paths, our results essentially confirm the Goodenough-Kanamori-Anderson rules, yet in general there does not exist any simple rule to predict the sign of the magnetic coupling constants. The most favorable situation for ferromagnetic coupling is found for ions with less than half-filled d shells; the (relative) tendency to ferromagnetic coupling increases with increasing path length. As an application, the magnetic interactions of the Cr compounds Rb2CrCl4, CrCl3, CrBr3, and CrI3 are investigated, all of which except CrCl3 are ferromagnets.

A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.

Using the generalized Gutzwiller method we present results on the ferromagnetic behavior of extended Hubbard models with two degenerate d(e(g)) orbitals. We find significant differences from the results obtained from Hartree-Fock theory.

We introduce Gutzwiller-correlated wave functions for the variational investigation of general multi-band Hubbard models. We set up a diagrammatic formalism which allows us to evaluate analytically ground-state properties in the limit of infinite spatial dimensions. In this limit recent results obtained within the Gutzwiller approximation are seen to become exact for these wave functions. We further show that the slave-boson mean-field theory for degenerate bands becomes variationally controlled at zero temperature in infinite dimensions. Lastly, we briefly comment on the variational approach to the Anderson transition in strongly correlated electron systems.

We consider the spin 12 Heisenberg antiferromagnet with nearest (J1) and next nearest neighbour (J2) interaction on the square lattice with 16, 18 and 20 sites. For strong frustration (J2/J1 around 0.5) the conventional Néel like order breaks down and a spin liquid arises. We find evidence of chiral order in this spin liquid which breaks parity but conserves time-reversal symmetry.