In the framework of the local spin-density approximation LSDA+ U method electronic-structure and magnetic properties of the intermetallic compound Gd 2 Fe 17 for both rhombohedral and hexagonal phases have been calculated. On top of that, ab initio exchange-interaction parameters within the Fe sublattice for all present nearest and next-nearest Fe ions have been obtained. It was found that for the first coordination sphereexchange interaction is ferromagnetic. For the second coordination sphere-exchange interaction is observed to be weaker and of antiferromagnetic type. Employing the theoretical values of exchange parameters Curie temperatures T C for the first as well as for the first and second coordination spheres of both hexagonal and rhombohedral phases of Gd 2 Fe 17 within Weiss mean-field theory were estimated. Obtained values of T C and its increase going from the hexagonal to rhombohedral crystal structure of Gd 2 Fe 17 agree well with experiment. Also for both structures LSDA+ U computed values of total magnetic moment agree well with experimental ones.

Material specific electronic band structure of the electron-doped high-Tc cuprate Nd1.85Ce0.15CuO4 (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Σ k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Σ k , which describes interaction of correlated conducting electrons with non-local Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for pseudo gap formation. Within this LDA+DMFT+Σ k approach we demonstrate the formation of pronounced "hot-spots" on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi2Sr2CaCu2O 8−δ (Bi2212), which have produced rather extended region of FS "destruction". There are several physical reasons for this fact: (i) the "hot-spots" in NCCO are located closer to Brillouin zone center; (ii) correlation length of AFM fluctuations ξ is larger for NCCO; (iii) pseudogap potential ∆ is stronger, than in Bi2212. Comparison of our theoretical data with recent bulk sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison alternative explanation of the van-Hove singularity at -0.3 eV is proposed. Optical conductivity both for Bi2212 and NCCO is also calculated within LDA+DMFT+Σ k and compared with experimental results, demonstrating satisfactory agreement.

Pseudogap regime for the prototype high-T c compounds hole doped Bi 2 Sr 2 CaCu 2 O 8-x (Bi2212) and electron doped Nd 2-x Ce x CuO 4 (NCCO) is described by means of novel generalized LDA+DMFT+Σ k approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Σ k . In the present case Σ k describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO 2 layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible "hot-spots" for NCCO while in Bi2212 there is just rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.

a b s t r a c tThe behavior of a three-dimensional isotropic Heisenberg ferromagnet in the presence of a magnetic field H is investigated in the random phase approximation (RPA) near the Curie temperature T c . It is shown that the magnetization M at the Curie temperature T c is described by the law MðT ¼ T c Þ $ H 1=5 and the initial magnetic susceptibility w 0 at temperatures T Z T c is given by w 0 ðT Z T c Þ $ ðTÀT c Þ À2 . It means that in the RPA the critical exponents for a three-dimensional Heisenberg ferromagnet coincide with the critical exponents for the Berlin-Kac spherical model of a ferromagnet rather than with the critical exponents of the mean field approximation (MFA). Hence it follows as well that, when a magnetic field H is risen from H ¼ 0 to H ¼ H a , the magnetic entropy S M will be decreased as

Slightly underdoped high-Tc system La1.86Sr0.14CuO4 (LSCO) is studied by means of high energy high resolution angular resolved photoemission spectroscopy (ARPES) and combined computational scheme LDA+DMFT+Σ k . Corresponding one band Hubbard model is solved via dynamical meanfield theory (DMFT), while model parameters needed are obtained from first principles within local density approximation (LDA). An "external" k-dependent self-energy Σ k describes interaction of correlated electrons with antiferromagnetic (AFM) pseudogap fluctuations. Experimental and theoretical data clearly show "destruction" of the LSCO Fermi surface in the vicinity of the (π,0) point and formation of "Fermi arcs" in the nodal directions. ARPES energy distribution curves (EDC) as well as momentum distribution curves (MDC) demonstrate deviation of the quasiparticle band from the Fermi level around (π,0) point. The same behavior of spectral functions follows from theoretical calculations suggesting AFM origin of the pseudogap state.