G. Seibold

Dynamics of Metallic Stripes in Cuprates

We study the dynamics of metallic vertical stripes in cuprates within the three-band Hubbard model based on a recently developed time-dependent Gutzwiller approximation. As doping increases, the optical conductivity shows transfer of spectral weight from the charge-transfer band towards (i) an incoherent band centered at 1.3 eV, (ii) a Drude peak, due mainly to motion along the stripe, and (iii) a low-energy collective mode which softens with doping and merges with (ii) at optimum doping in good agreement with experiment. The softening is related to the quasidegeneracy between Cu-centered and O-centered mean-field stripe solutions close to optimal doping.

Sum rules and missing spectral weight in magnetic neutron scattering in the cuprates

We present estimates in the Hubbard and Heisenberg models for the spectral weight in magnetic neutron scattering experiments on the cuprates. With the aid of spin-wave theory and the time dependent Gutzwiller approximation we discuss how the spectral weight is distributed among the different channels and between high and low energies. In addition to the well known total moment sum rule we discuss sum rules for each component of the dynamical structure factor tensor which are peculiar for spin 1/2 systems. The various factors that reduce the spectral weight at the relevant energies are singled out and analyzed like: shielding factors, weight at electronic energies, multimagnon process etc. Although about 10% ~ 15% of the naively expected weight is detected in experiments after consideration of these factors the missing weight is within the experimental uncertainties. A large fraction of the spectral weight is hard to detect with present experimental conditions.

Time-Dependent Gutzwiller Approximation for the Hubbard Model

We develop a time-dependent Gutzwiller approximation (GA) for the Hubbard model analogous to the time-dependent Hartree-Fock (HF) method. This new formalism incorporates ground state correlations of the random phase approximation (RPA) type beyond the GA. Static quantities like ground state energy and double occupancy are in excellent agreement with exact results in one dimension up to moderate coupling and in two dimensions for all couplings. We find a substantial improvement over traditional GA and HF+RPA treatments. Dynamical correlation functions can be computed and are also substantially better than HF+RPA ones and obey well behaved sum rules.

Competing orders in FeAs layers.

Using the unrestricted Hartree-Fock approximation and Landau theory we identify possible phases competing with superconductivity in FeAs layers. We find that close to half-filling the transition from the paramagnet to the magnetically ordered phase is first order, making anharmonicities relevant and leading to a rich phase diagram. Between the already known one-dimensionally modulated magnetic stripe phase and the paramagnet we find a new phase which has the same structure factor as the former but in which magnetic moments at nearest-neighbor sites are at right angles making electrons acquire a nontrivial phase when circulating a plaquette at strong coupling. Another competing phase has magnetic and charge order and may be stabilized by charged impurities.

Role of the vortex-core energy on the Berezinskii-Kosterlitz-Thouless transition in thin films of NbN.

We analyze the occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below T(BKT) and of the resistivity above T(BKT). By direct comparison between the experimental data and the theory, we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor.

UNRESTRICTED SLAVE-BOSON MEAN-FIELD APPROXIMATION FOR THE TWO-DIMENSIONAL HUBBARD MODEL

The Kotliar-Ruckenstein slave-boson scheme is used to allow for an unrestricted variation of the bosonic and fermionic fields on the saddle-point level. Various inhomogeneous solutions, such as spin polarons and domain walls, are discussed within the two-dimensional Hubbard model and compared with results of unrestricted Hartree-Fock (HF) calculations. We find that the present approach drastically reduces the polarization of these states and leads to increased delocalized wave functions as compared to the HF model. The interaction between two spin polarons turns out to be attractive over a wide range of the on-site repulsion

Phase diagrams of voltage-gated oxide interfaces with strong Rashba coupling

U

Time-dependent Gutzwiller theory of magnetic excitations in the Hubbard model

. In addition we obtain the crossover from vertical to diagonal domain walls at a higher value of

Optical excitation of phase modes in strongly disordered superconductors

U

Gutzwiller magnetic phase diagram of the cuprates

than predicted by HF.