We present a simple, material-specific model Hamiltonian for stripes and use it to explain the observed differences between the angle-resolved photoemis-sion spectra of La 2−x Sr x CuO 4 and Bi 2 Sr 2 CaCu 2 O 8+x , e.g, along the nodal line and around (3π/4, 0). Next we compute the string tension for stripes and find it to be smallest in the materials with the highest observed T c max .

We present a simple, material-specific model Hamiltonian for stripes and use it to explain the observed differences between the angle-resolved photoemission spectra of La(2-x)Sr(x)CuO4 and Bi(2)Sr(2)CaCu(2)O(8+x), e.g, along the nodal line and around (3pi/4,0). Next we compute the string tension for stripes and find it to be smallest in the materials with the highest observed Tc,max.

Using an exact diagonalization method within the dynamical mean-field theory we analyze the stable stripe structures found in the two-dimensional Hubbard model doped by 0.03<<0.2 holes, and discuss a scenario for stripe melting. Our results demonstrate the importance of dynamical correlations which lead to the metallic stripes, in contrast to the Hartree-Fock picture. The spectral functions show a coexistence of the coherent quasiparticles (polaron band) close to the Fermi energy , and incoherent states at lower energies. The quasiparticles in the polaron band depend on hole doping, and hybridize strongly with the partly filled mid-gap band within the Mott-Hubbard gap, induced by stripe order. This explains the origin of nondispersive quasiparticles close to the Fermi energy mu, observed near the X=(pi ,o) and Y (0,pi) points for the samples with coexisting (10) and (01) stripes. We reproduce the gap which opens for charge excitations at the S =(pi /2,pi /2) point. observed in the angle-resolved photoemission experiments for La2-xSrxCuO4, and a pseudogap in the integrated spectral density pinned to mu, Finally, we show that large spectral weight close to mu moves from the X to the S point when the second neighbor hopping element increases, and the (01) stripe phase is destabilized by kink fluctuations.

Using an exact diagonalization method within the dynamical mean-field theory we found stable stripe phases in the two-dimensional Hubbard model doped by 0.03

Using an exact diagonalization method within the dynamical mean-field theory we found stable stripe phases in the two-dimensional Hubbard model doped by 0.03<delta<0.2 holes, with a crossover from diagonal to vertical site-centered stripes at doping delta approximately 0.05. The doping dependence of the size of magnetic domains and chemical potential shift Delta&mgr; approximately -delta(2) are in quantitative agreement with the experimental results for La2-xSrxCuO4. The one-dimensional metallic behavior along the domain walls explains the observed suppression of spectral weight along the Brillouin zone diagonal.

We present spectral and optical properties of the Hubbard model on a two-dimensional square lattice using a generalization of dynamical mean-field theory to magnetic states in a finite dimension. The self-energy includes the effect of spin fluctuations and screening of the Coulomb interaction due to particle-particle scattering. At half-filling the quasiparticles reduce the width of the Mott-Hubbard ``gap'' and have dispersions and spectral weights that agree remarkably well with quantum Monte Carlo and exact diagonalization calculations. Away from half-filling we consider incommensurate magnetic order with a varying local spin direction, and derive the photoemission and optical spectra. The incommensurate magnetic order leads to a pseudogap which opens at the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle states survive in the doped systems, but their dispersion is modified by the doping, and a rigid-band picture does not apply. Spectral weight in the optical conductivity is transferred to lower energies, and the Drude weight increases linearly with increasing doping. We show that incommensurate magnetic order also leads to midgap states in the optical spectra and to decreased scattering rates in the transport processes, in qualitative agreement with the experimental observations in doped systems. The gradual disappearence of the spiral magnetic order and the vanishing pseudogap with increasing temperature is found to be responsible for the linear resistivity. We discuss the possible reasons why these results may only partially explain the features observed in the optical spectra of high-temperature superconductors.

We investigate the consequences of an incommensurate magnetic order in doped La 2− x Sr x CuO 4 using dynamical mean-field theory for the effective single-band model. The high-energy optical transitions are due to high-energy excitations across large Mott–Hubbard “gap,” while low-energy excitations involve a pseudogap induced by the local spin order. The latter lead to a strong drop in the scattering rate at low ω, in qualitative agreement with the experimental data.

We have generalized the dynamical mean-field theory to study the doping dependence of the crossover from antiferromagnetic to short-range order modeled by an incommensurate spin density wave in the Hubbard model. The local self-energy which includes spin fluctuations gives quasiparticle weights and spectral properties in good agreement with quantum Monte Carlo and exact diagonalization data in two dimensions. The spectra at finite doping are characterized by a Mott-Hubbard "gap" accompanied by a pseudogap induced by the local spin order.

We investigate the magnetic instabilities of the nondegenerate (s-band) and a degenerate (d-band) Hubbard model in two dimensions using many-body effects due to the particle-particle diagrams and Hund's rule local correlations. The density of states and the position of Van Hove singularity change depending on the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly reduced in the s-band case, and ferromagnetism survives only if electron density is small, and the band is almost flat at small momenta due to next-nearest neighbor hopping. In contrast, for the d-band case the reduction of the Stoner parameter which follows from particle-particle correlations is much smaller and ferromagnetism survives to a large extent. Inclusion of local spin-spin correlations has a limited destabilizing effect on the magnetic states.