A. I. Liechtenstein

Local spin density functional approach to the theory of exchange interactions in ferromagnetic metals and alloys

Abstract Rigorous expressions for the exchange parameters of classical Heisenberg model applied to crystals are obtained using a local spin density functional (LSDF) approach and KKR-Green functions formalism. The spin wave stiffness constant and Curie temperature ( T c ) of ferromagnetic metals are obtained without any model assumptions as to the character of exchange interactions. The concentration dependence of T c for binary ferromagnetic alloys is investigated in the framework of the single-site CPA-theory. The corresponding calculations are carried out for simple metals Fe, Ni and disordered NiPd alloys.

LDA energy bands, low-energy hamiltonians, t′, t″, t⊥ (k), and J⊥

Abstract We describe the LDA band structre of YBa2Cu3O7 in the ϵF ± 2 eV range using orbital projections and compare with YBa2Cu4O8. Then, the high-energy and chain-related degrees of freedom are integrated out and we arrive at two, nearest-neighbor, orthogonal, two-center, 8-band Hamiltonians, H8+ and H8−, for respectively the even and odd bands of the bi-layer. Of the 8 orbitals, Cux2 −y2, O2x, O3y, and Cus have σ character and Cuxz, Cuyz, O2z, and O3z have π character. The roles of the Cus orbital, which has some Cu3z2 − 1 character, and the four π orbitals are as follows: Cus provides 2nd- and 3rd-nearest-neighbor (t′ and t″) intra-plane hopping, as well as hopping between planes (t⊥). The π-orbitals are responsible for bifurcation of the saddle-points for dimpled planes. The 4-σ-band Hamiltonian is generic for flat CuO2 planes and we use it for analytical studies. The k∥-dependence is expressed as one on u ≡ (cos bk y + cos ak x ) 2 and one on v ≡ (cos bk y − cos ak x ) 2 . The latter arises solely through the influence of Cus. The reduction of the σ-Hamiltonian to 3- and 1-band Hamiltonians is explicitly discussed and we point out that, in addition to the hoppings commonly included in many-body calculations, the 3-band Hamiltonian should include hopping between all 2nd-nearest-neighbor oxygens and that the 1-band Hamiltonian should include 3rd-nearest-neighbor hoppings. We calculate the single-particle hopping between the planes of a bi-layer and show that it is generically: t⊥ (k ∥ ) ≈ 0.25 eV · v 2 (1 − 2ut′ t ) −2 . The hopping through insulating spacers such as (BaO)Hg(BaO) is an order of magnitude smaller, but seems to have the same k∥-dependence. We show that the inclusion of t′ is crucial for understanding ARPES for the anti-ferromagnetic insulator Sr2CuO2Cl2. Finally, we estimate the value of the inter-plane exchange constant J⊥ for an un-doped bi-layer in meanfield theory using different single-particle Hamiltonians, the LDA for YBa2Cu3O6, the eight- and four-band Hamiltonians, as well as an analytical calculation for the latter. We conclude that J⊥ ~ − 20 meV.

Exchange interactions and spin-wave stiffness in ferromagnetic metals

Rigorous expressions for the exchange parameters and the spin-wave stiffness constant of ferromagnetic metals are obtained using a local spin-density functional formalism and the muffin-tin potential model. The calculated results for ferromagnetic iron are given as an example of the approach.

Electrons, phonons, and their interaction in YBa2Cu3O7

Abstract We have performed LDA calculations of structure, phonon frequencies, and the electron-phonon interaction. We find good agreement with available data and estimate: λ=1. Saddlepoints 20 meV below the Fermi level are important.

Local spin excitations and Curie temperature of iron

Abstract The expression for the Curie temperature of ferromagnetic metals in the mean field approximations is obtained for arbitrary types of exchange interactions. In the framework of local spin density functional approach an exact formula for the effective exchange parameter J0 is derived. The numerical calculations for ferromagnetic iron illustrate the possibilities of the method.

S-WAVE SUPERCONDUCTIVITY FROM AN ANTIFERROMAGNETIC SPIN-FLUCTUATION MODEL FOR BILAYER MATERIALS

It is usually believed that the spin-fluctuation mechanism for high-temperature superconductivity results in d-wave pairing, and that it is destructive for the conventional phonon-mediated pairing. We show that in bilayer materials, due to nearly perfect antiferromagnetic spin correlations between the planes, the stronger instability is with respect to a superconducting state whose order parameters in the even and odd plane-bands have opposite signs, while having both two-dimensional

Out-of-plane instability and electron-phonon contribution tos- andd-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model

s

Dynamical Mean-Field Theory for Doped Antiferromagnets

-symmetry. The interaction of electrons with Raman- (infrared-) active phonons enhances (suppresses) the instability.

Plasmon damping and response function in doped compounds

AbstractThe equilibrium structure, energy bands, phonon dispersions, and s- and d-channel electron-phonon interactions (EPIs) are calculated for the infinite-layer superconductor CaCuO2 doped with 0.12 holes per CuO2. The LDA and the linear-response full-potential LMTO method were used. In the equilibrium structure, oxygen is found to buckle slightly out of the plane and, as a result, the characters of the energy bands near εF are found to be similar to those of other optimally doped HTSCs. For the EPI we find λs ~ 0.4, in accord with previous LDA calculations for YBa2 Cu3O7. This supports the common belief that the EPI mechanism alone is insufficient to explain HTSC.