Electronic structure and magnetism in X_xW_{1-x}O_3 (X=Nb,V,Re) from supercell calculations

Some doped semiconductors have recently been shown to display superconductivity or weak ferromagnetism. Here we investigate the electronic structure and conditions for magnetism in a supercells of cubic XW_{26}O_{81}, where X=Nb,V and Re. The undoped material is an insulator, and although the slightly doped material is a metal, it is far from the Stoner criterion of magnetism. The conditions of a localized density-of-states which varies rapidly with the energy, resemble those of doped hexaborides. The virtual crystal approximation is used to vary the doping level. A small moment appears if the Fermi energy coincides with a large derivative of the DOS.Comment: 5 pages, 5 figures, to appear in JMM

Anti-ferromagnetism, spin-phonon interaction and the local-density approximation in high-TC_C superconductors

Results from different sets of band calculations for undoped and doped HgBa$_2$CuO$_4$ show that small changes in localization can lead to very different ground states. The normal LDA results are compared with 'modified' LDA results, in which different linearization energies make the O-p band more localized. The ground states in the normal calculations are far from the anti-ferromagnetic ones, while nearly AFM states are found in the modified calculations. The proximity of an AFM state in the doped system leads to increased $\lambda_{sf}$, and the modified band structure has favorable conditions for spin-phonon coupling and superconductivity mediated by spin fluctuations.Comment: 4 pages, 2 figs., Accepted in J. Physics: Condensed Matter as a lette

Temperature dependence of magnetism near defects in SrB_6

The T-dependence of magnetic moments in SrB_6 is studied through spin-polarized band calculations for a supercell of Sr_{27}B_{156} containing a B_6 vacancy. The magnetic moment decays rather quickly with T despite the fact that only electronic Fermi-Dirac effects are included. This result and the T-dependence of moments near a La impurity can hardly explain the reports of a very high Curie temperature in hexaborides, but suggest that the magnetism is caused by some other type of impurity.Comment: 3 pages, 2 figure

Spin-Phonon Coupling in High-T_c Copper Oxides

Band calculations on HgBa$_2$CuO$_4$ and La$_{(2-x)}$Sr$_x$CuO$_4$ with phonon and spin-waves within the CuO planes show that partial gaps are created at various energies depending on wavelengths. Spin and phonon gaps appear at different energies when the modulations are along [1,1,0], while they are at the same energy for modulations along [1,0,0]. It is shown that the ability to form gaps and antiferromagnetic waves is correlated with the strength of the interaction parameter $\lambda_{sf}$ for spin fluctuations. Many unusual properties of the high-T$_C$ oxides can be understood from spin-phonon coupling.Comment: 2 pages, proceedings of the LT24 conferenc

A model of the T-dependent pseudogap and its competition with superconductivity in copp er oxides

Results for pseudogaps are obtained from a band model, where the stability of the gap depends on the amplitudes of vibrational displacements, or magnetic moments, and their coupling to electrons. A one-particle gap is favored by normal thermal excitations of phonons or spin waves. Another gap can be generated by spontaneous waves at lower temperature, if the electronic energy gain overcomes the elastic/magnetic energy needed for increased amplitudes of the oscillations. This state is characterized by charge or spin density waves. The pseudogap has many features in common with the superconducting gap, and the model lends support to the interpretation that the pseudogap is a precursor of, and competes with, superconducting pairing.Comment: 4 pages, 2 figure

Properties of high-TC_C copper oxides from the nearly-free electron model

The generic band structure of high-T$_C$ copper oxides is simulated by the nearly free-electron model (NFE) in two dimensions (2-D) with parameters from band calculations. Interaction between phonons and spin waves will cause potential modulations and pseudogaps, and the strength of the modulations, the wave lengths and the doping, are all related. A Fermi-surface "arc" is found for dynamic spin/phonon waves. The confinement of superconductivity between two limiting dopings can be a result of competition with the pseudogap at low doping and weak coupling at high doping.Comment: 4 pages, 4 figures, 1 tabl

Electronic structure and magnetism for FeSi(1−x)_{(1-x)}Gex_x from supercell calculations

Recent studies of FeSi$_{(1-x)}$Ge$_x$, which found a transition from an insulating to a magnetic metallic state near $x$=0.25, have revived the discussion about the role of strong correlation in these systems. Here are spin polarized band calculations made for 64-atom supercells of FeSi$_{(1-x)}$Ge$_x$ for different $x$ and different volumes for large $x$. The results show that the small band gap in FeSi is closed for $x \geq 0.3$, because of both substitutional disorder and increased volume. Ferromagnetism appears near this composition and becomes enforced for increasing $x$. The $x$-dependence of the electronic specific heat can be understood from the exchange splitting of the density-of-states near the gap. Strong volume dependencies for the properties of FeGe suggest experiments using pressure instead of $x$ for investigations of the gap.Comment: (7 pages, 4 figures

Mechanisms for higher TCT_C in copper oxide superconductors; Ideas from band calculations

Band calculations for the hole doped La$_2$CuO$_4$ system show that artificial periodicities of Ba dopants can give the material different properties than from a uniform distribution of dopants. A periodicity within the planes make static pseudogaps which could be tuned to raise the density-of-states (DOS) at $E_F$ and the superconducting $T_C$. A periodic doping dependence perpendicular to the CuO planes can increase the matrix element for spin fluctuations.Comment: 1 Figur