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 −...

Fe-doped ZnO nanocrystals are successfully synthesized and structurally characterized by using x-ray diffraction and transmission electron microscopy. Magnetization measurements on the same system reveal a ferromagnetic to paramagnetic transition temperature above 450 K with a low-temperature transition from the ferromagnetic to the spin-glass state due to canting of the disordered surface spins in the nanoparticle system. Local magnetic probes like electron paramagnetic resonance and Mössbauer spectroscopy indicate the presence of Fe in both valence states Fe 2+ and Fe 3+ . We argue that the presence of Fe 3+ is due to possible hole doping in the system by cation ͑Zn͒ vacancies. In a subsequent ab initio electronic structure calculation, the effects of defects ͑e.g., O and Zn vacancies͒ on the nature and origin of ferromagnetism are investigated for the Fe-doped ZnO system. Electronic structure calculations suggest hole doping ͑Zn vacancy͒ to be more effective to stabilize ferromagnetism in Fe-doped ZnO and our results are consistent with the experimental signature of hole doping in ferromagnetic Fe-doped ZnO samples.

In this paper we have applied the full-potential linearized muffin tin orbital method and the tight-binding linearized muffin tin orbital method to investigate in detail the electronic structure and magnetism of a series of half-Heusler compounds XMZ with X = Fe, Co, Ni, M = Ti, V, Nb, Zr, Cr, Mo, Mn and Z = Sb, Sn. Our detailed analysis of the electronic structure using various indicators of chemical bonding suggests that covalent hybridization of the higher-valent transition element X with the lower-valent transition element M is the key interaction responsible for the formation of the d-d gap in these systems. However, the presence of the sp-valent element is crucial to provide stability to these systems. The influence of the relative ordering of the atoms in the unit cell on the d-d gap is also investigated. We have also studied in detail some of these systems with more than 18 valence electrons which exhibit novel magnetic properties, namely half-metallic ferro-and ferrimagnetism. We show that the d-d gap in the paramagnetic state, the relatively large X-Sb hybridization and the large exchange splitting of the M atoms are responsible for the half-metallic property of some of these systems.

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.

Through first-principles calculations, we study the electronic structure of double-perovskite iridates with Ir in the d 4 valence state. Contrary to the expected strong spin-orbit driven J = 0 nonmagnetic state, we find finite moment at the Ir site, exhibiting breakdown of the J = 0 state. We further find the band structure effect rather than the crystal field effect to be responsible for this breakdown. The antiferromagnetic superexchange interaction between Ir moments, in general, makes these compounds insulating.