Marcus Fleck

One-dimensional metallic behavior of the stripe phase in La2-xSrxCuO4

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.

MAGNETIC PHASES NEAR THE VAN HOVE SINGULARITY IN S- AND D-BAND HUBBARD MODELS

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 particle-particle diagrams and Hund’s rule local correlations. The density of states and the position of the 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 the electron density is small and the band has flat regions. Due to next-nearest-neighbor hopping there are flat regions in Γ−X and Γ−Y directions. 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. (Less)

Dynamical Mean-Field Theory for Doped Antiferromagnets

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.

Dynamical Mean-Field Theory of Stripe Ordering

Applying the dynamical mean-field theory to the two-dimensional Hubbard model, we calculate self-consistent solutions of doped antiferromagnets with spatially varying spin density ‹n iσ › using realistic tight-binding parameters. The local self-energy of the supercell includes transverse and longitudinal spin fluctuations with an effective local potential due to short-range electron-electron correlations. It is found that metallic stripes are stabilized by a pseudogap. The stripes along (1,0) direction filled by one hole per two-domain wall unit cells change with increasing Coulomb interaction U to the more extended stripes along (1,1) direction consisting of four atoms filled by 1/4 doped hole each. These findings agree qualitatively with the experimental observations in the superconducting cuprates, and predict a qualitative difference between various compounds due to differences in the extended hopping parameters.

Quasiparticle broadening in insulating copper oxides

We calculate the spectral function of the half-filled two-dimensional Hubbard model with realistic hopping integrals for Sr2CuO2Cl2 within the dynamical mean-field theory at finite temperatures. It is shown that precursor effects of incommensurate spin dynamics lead to an incoherent broadening of the quasiparticle peaks on an energy scale that depends on local spin order. This might serve as an indirect method of detecting changes of local spin order under doping.

Optical Properties of Doped Antiferromagnets

We investigate the consequences of an incommensurate magnetic order in doped La2−xSrxCuO4 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.

Metallic stripes in doped Hubbard model

We report new types of metallic stripes with ferromagnetic order along the domain walls for the two-dimensional Hubbard model using the dynamical mean-field theory. The (1,0) stripes have the observed filling close to one doped hole per two atoms, while the (1,1) stripes are more extended. The stripe structures are stable at low temperatures T<70 K, and change into spin spirals at higher temeratures.

Magnetism near the Van Hove singularity

We study the phase diagrams of the s-band and d-band Hubbard model in two dimensions including electron correlations. While the ferromagnetic instability in the s-band case occurs only near the Van Hove singularity, ferromagnetism is more stable in the d-band model, and the values of the Stoner parameters in transition metals are reduced below their LDA values.