Anisotropic 2D metallicity: plasmons in Ge(1 0 0)-Au.

The low-energy plasmonic excitations of the Ge(0 0 1)-Au close to one monolayer coverage of Au were investigated by momentum-resolved high resolution electron energy loss spectroscopy. A very weak plasmonic loss was identified dispersing along the chain direction of the [Formula: see text] formed at these Au coverages. The measured dispersion was compared with the Tomonaga-Luttinger-liquid (TLL) model and with a model for an anisotropic Fermi liquid. Using the TLL model both for single and arrays of wires, no consistent picture turned up that could describe all available data. On the contrary, a quasi-one-dimensional model of a confined 2D electron gas gave a satisfactorily consistent description of the data. From these results for the collective low-energy excitations we conclude that the Ge(0 0 1)-Au system is reasonably well described by a strongly anisotropic 2D Fermi liquid, but is incompatible with a TLL.

Local density of states of the one-dimensional spinless fermion model.

We investigate the local density of states of the one-dimensional half-filled spinless fermion model with nearest-neighbour hopping t > 0 and interaction V in its Luttinger liquid phase -2t < V ≤ 2t. The bulk density of states and the local density of states in open chains are calculated over the full band width ∼4t with an energy resolution ≤0.08t using the dynamical density-matrix renormalization group (DDMRG) method. We also perform DDMRG simulations with a resolution of 0.01t around the Fermi energy to reveal the power-law behaviour D(ϵ) ∼ |ϵ - ϵ(F)|(α) predicted by the Luttinger liquid theory for bulk and boundary density of states. The exponents α are determined using a finite-size scaling analysis of DDMRG data for lattices with up to 3200 sites. The results agree with the exact exponents given by the Luttinger liquid theory combined with the Bethe Ansatz solution. The crossover from boundary to bulk density of states is analysed. We have found that boundary effects can be seen in the local density of states at all energies, even far away from the chain edges.

Spectral function of the one-dimensional Hubbard model away from half filling.

We calculate the photoemission spectral function of the one-dimensional Hubbard model away from half filling using the dynamical density-matrix renormalization group method. An approach for calculating momentum-dependent quantities in finite open chains is presented. Comparison with exact Bethe ansatz results demonstrates the unprecedented accuracy of our method. Our results show that the photoemission spectrum of the quasi-one-dimensional conductor TTF-TCNQ provides evidence for spin-charge separation on the scale of the conduction bandwidth.

Resonant inelastic X-ray scattering of the holon-antiholon continuum in SrCuO2.

We report a resonant inelastic x-ray scattering study of charge excitations in the quasi-one-dimensional Mott insulator SrCuO2. We observe a continuum of low-energy excitations, the onset of which exhibits a small dispersion of approximately 0.4 eV. Within this continuum, a highly dispersive feature with a large sinusoidal dispersion (approximately 1.1 eV) is observed. We have also measured the optical conductivity, and studied the dynamic response of the extended Hubbard model with realistic parameters, using a dynamical density-matrix renormalization group method. In contrast to earlier work, we do not find a long-lived exciton, but rather these results suggest that the excitation spectrum comprises a holon-antiholon continuum together with a broad resonance.

Optical conductivity of the half-filled hubbard chain

We combine well-controlled analytical and numerical methods to determine the optical conductivity of the one-dimensional Mott-Hubbard insulator at zero temperature. A dynamical density-matrix renormalization group method provides the entire absorption spectrum for all but very small coupling strengths. In this limit we calculate the conductivity analytically using exact field-theoretical methods. Above the Lieb-Wu gap the conductivity exhibits a characteristic square-root increase. For small to moderate interactions, a sharp maximum occurs just above the gap. For larger interactions, another weak feature becomes visible around the middle of the absorption band.