By calculation and analysis of the bare conduction bands in a large number of hole-doped high-temperature superconductors, we have identified the energy of the so-called axial-orbital as the essential, material-dependent parameter. It is uniquely related to the range of the intra-layer hopping. It controls the Cu 4s-character, influences the perpendicular hopping, and correlates with the observed Tc at optimal doping. We explain its dependence on chemical composition and structure, and present a generic tight-binding model. PACS numbers: 74.25.Jb, 74.62.Bf, 74.62.Fj, The mechanism of high-temperature superconductivity (HTSC) in the hole-doped cuprates remains a puzzle [1]. Many families with CuO 2 -layers have been synthesized and all exhibit a phase diagram with T c going through a maximum as a function of doping. The prevailing explanation is that at low doping, superconductivity is destroyed with rising temperature by the loss of phase coherence, and at high doping by pair-breaking [2]. For the materials-dependence of T c at optimal doping, T c max , the only known, but not understood, systematics is that for materials with multiple CuO 2 -layers, such as HgBa 2 Ca n−1 Cu n O 2n+2 , T c max increases with the number of layers, n, until n ∼3. There is little clue as to why for n fixed, T c max depends strongly on the family, e.g. why for n=1, T c max is 40 K for La 2 CuO 4 and 85 K for Tl 2 Ba 2 CuO 6 , although the Neel temperatures are fairly similar. A wealth of structural data has been obtained, and correlations between structure and T c have often been looked for as functions of doping, pressure, uniaxial strain, and family. However, the large number of structural and compositional parameters makes it difficult to find what besides doping controls the superconductivity. Insight was recently provided by Seo et al. [3] who grew ultrathin epitaxial La 1.9 Sr 0.1 CuO 4 films with varying degrees of strain and measured all relevant structural parameters and physical properties. For this single-layer material it was concluded that the distance between the charge reservoir and the CuO 2 -plane is the key structural parameter determining the normal state and superconducting properties.Most theories of HTSC are based on a Hubbard model with one Cu d x 2 −y 2 -like orbital per CuO 2 unit. The oneelectron part of this model is, in the k-representation:with t, t ′ , t ′′ , ... denoting the hopping integrals (≥ 0) on the square lattice (Fig. 1) Relation between the one-orbital model (t, t ′ , t ′′ , ...) and the nearest-neighbor four-orbital model [4] (ε d − εp ∼ 1 eV, t pd ∼ 1.5 eV, εs − εp ∼ 16 − 4 eV, tsp ∼ 2 eV) .The LDA band structure of the best known, and only stoichiometric optimally doped HTSC, YBa 2 Cu 3 O 7 , is more complicated than what can be described with the t-t ′ model. Nevertheless, careful analysis has shown [4] that the low-energy, layer-related features, which are the only generic ones, can be described by a nearest-neighbor, tight-binding model with four orbitals per layer (Fig. 1), Cu d x 2 −...

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We have analyzed the unusual electronic structure of Sr2FeMoO6 combining ab initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength ( U(eff)) at the Mo site due to these renormalized interaction strengths.

We present a study of the paramagnetic metallic and insulating phases of vanadium sesquioxide by means of the Nth order muffin-tin orbital implementation of density functional theory combined with dynamical meanfield theory. The transition is shown to be driven by a correlation-induced enhancement of the crystal-field splitting within the t 2g manifold, which results in a suppression of the hybridization between the a 1g and e g bands. We discuss the changes in the effective quasiparticle band structure caused by the correlations and the corresponding self-energies. At temperatures of about 400 K, we find the a 1g orbital displays coherent quasiparticle behavior, while a large imaginary part of the self-energy and broad features in the spectral function indicate that the e g orbitals are still far above their coherence temperature. The local spectral functions are in excellent agreement with recent bulk sensitive photoemission data. Finally, we also make a prediction for angle-resolved photoemission experiments by calculating momentum-resolved spectral functions.

The natural mineral azurite Cu(3)(CO(3))(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

We describe the screened Korringa-Kohn-Rostoker (KKR) method and the thirdgeneration linear muffin-tin orbital (LMTO) method for solving the single-particle Schrödinger equation for a MT potential. In the screened KKR method, the eigenvectors c RL,i are given as the non-zero solutions, and the energies ε i as those for which such solutions can be found, of the linear homogeneous equations: RL K a R ′ L ′ ,RL (ε i ) c RL,i = 0, where K a (ε) is the screened KKR matrix. The screening is specified by the boundary condition that, when a screened spherical wave ψ a RL (ε, r R ) is expanded in spherical harmonics Y R ′ L ′ (r R ′ ) about its neighboring sites R ′ , then each component either vanishes at a radius, r R ′ =a R ′ L ′ , or is a regular solution at that site. When the corresponding "hard" spheres are chosen to be nearly touching, then the KKR matrix is usually short ranged and its energy dependence smooth over a range of order 1 Ry around the centre of the valence band. The KKR matrix, K (ε ν ) , at a fixed, arbitrary energy turns out to be the negative of the Hamiltonian, and its first energy derivative,K (ε ν ) , to be the overlap matrix in a basis of kinked partial waves, Φ RL (ε ν , r R ) , each of which is a partial wave inside the MT-sphere, tailed with a screened spherical wave in the interstitial, or taking the other point of view, a screened spherical wave in the interstitial, augmented by a partial wave inside the sphere. When of short range, K (ε) has the two-centre tight-binding (TB) form and can be generated in real space, simply by inversion of a positive definite matrix for a cluster. The LMTOs, χ RL (ε ν ) , are smooth orbitals constructed from Φ RL (ε ν , r R ) andΦ RL (ε ν , r R ) , and the Hamiltonian and overlap matrices in the basis of LMTOs are expressed solely in terms of K (ε ν ) and its first three energy derivatives. The errors of the single-particle energies ε i obtained from the Hamiltonian and overlap matrices in the Φ (ε ν )-and χ (ε ν ) bases are respectively of second and fourth order in ε i − ε ν . Third-generation LMTO sets give wave functions which are correct to order ε i − ε ν , not only inside the MT spheres, but also in the interstitial region. As a consequence, the simple and popular formalism which previously resulted from the atomic-spheres approximation (ASA) now holds in general, that is, it includes downfolding and the combined correction. Downfolding to few-orbital, possibly short-ranged, low-energy, and possibly orthonormal Hamiltonians now works exceedingly well, as is demonstrated for a high-temperature superconductor. First-principles sp 3 and sp 3 d 5 TB Hamiltonians for the valence and lowest conduction bands of silicon are derived. Finally, we prove that the new method treats overlap of the potential wells correctly to leading order and we demonstrate how this can be exploited to get rid of the empty spheres in the diamond structure.

Using first-principles density functional calculations, we study the electronic and magnetic properties of the ferromagnetic insulating double perovskite compound La2NiMnO6, which has been reported to exhibit an interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals the existence of very soft infrared active phonons that couple strongly with spins at the Ni and Mn sites through modification of the superexchange interaction. We suggest that these modes are the origin for the observed dielectric anomaly in La2NiMnO6.

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45• within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation (LSDA)+U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favours a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.PACS numbers: 71.15.Mb, 71.70.Ej, The spinel compounds with a chemical formula of AB 2 X 4 where B sites are usually transition metal ions, form a frustrated pyrochlore lattice with corner-sharing tetrahedra. These compounds show a complex behavior including structural transitions from cubic to tetragonal symmetries which are often accompanied by an orbital order-disorder transition as well as complicated magnetic orderings at low temperatures 1 .The spinel MnV 2 O 4 has experienced a recent surge in activities due to new experimental observations in single crystals 2 revealing a lower symmetry structure than previously suggested 3 . This has important implications for the related orbital order at low temperatures which is still unclear. The presence of two magnetic ions in MnV 2 O 4 (Mn with spin 5/2 and V with spin 1) translates into more complex magnetic phase transitions in this system than in other vanadium spinel oxides such as ZnV 2 O 4 , MgV 2 O 4 or CdV 2 O 4 with nonmagnetic Asite ions. Recent experimental findings 2,4 indicated that MnV 2 O 4 undergoes a phase transition from paramagnetic to a collinear ferrimagnetic phase at 56K where the Mn and V spin moments point in opposite directions. At T = 53K a second magnetic phase transition to noncollinear ferrimagnetism follows accompanied by a structural transition from cubic to tetragonal phase.The cubic to tetragonal structural transition in MnV 2 O 4 is, similar to other vanadium spinels, associated with a compression of the VO 6 octahedron (c T /a T = 0.98). The octahedral environment of V (VO 6 ) splits the d states into lower t 2g and higher e g . Since V +3 is in a 3d 2 configuration, the t 2g orbitals are partially filled and possible orbital orderings may occur. Earlier experimental observations 3 indicated the tetragonal space group to be I4 1 /amd. However, recent precise measurements on a single crystal 2,4 showed that the tetragonal space group is I4 1 /a. Since the orbital order and, accordingly, the magnetic order are closely related to the underlying space group symmetry, it is very important to establish the space group symmetry unambiguously.The I4 1 /a space group breaks the mirror and glide symmetr...