Frustrated spin systems can show phases with spontaneous breaking of spin-rotational symmetry without the formation of local magnetic order. We study the dynamic response of the spin-nematic phase of one-dimensional spin-1/2 systems, characterized by
slow large-distance decay of quadrupolar correlations, by numerically computing one-spin and two-spin dynamical structure factors at zero temperature using time-dependent density matrix renormalization group methods. We interpret the results in terms of an effective theory of gapped magnon excitations interacting with a quasi-condensate of bound magnon pairs. This employs an extension of the well-known Tomonaga-Luttinger liquid theory which includes the magnon states as a mobile impurity. A good qualitative understanding of the characteristic thresholds and their intensity in the structure factors is obtained this way. Our results are useful in the interpretation of inelastic neutron scattering and resonant inelastic x-ray scattering experiments.
Atomic nanowires on semiconductor surfaces induced by the adsorption of metallic atoms have attracted a lot of attention as possible hosts of the elusive, Tomonaga-Luttinger liquid. The Au/Ge(100) system in particular is the subject of controversy as
to whether the Au-induced nanowires do indeed host exotic, 1D metallic states. We report on a thorough study of the electronic properties of high quality nanowires formed at the Au/Ge(100) surface. High resolution ARPES data show the low-lying Au-induced electronic states to possess a dispersion relation that depends on two orthogonal directions in k-space. Comparison of the E(k$_x$,k$_y$) surface measured using ARPES to tight-binding calculations yields hopping parameters in the two different directions that differ by a factor of two. We find that the larger of the two hopping parameters corresponds, in fact, to the direction perpendicular to the nanowires (t$_{perp}$). This, the topology of the $E$=$E_F$ contour in k$_{||}$, and the fact that $t_{||}$/$t_{perp}sim 0.5$ proves that the Au-induced electron pockets possess a 2D, closed Fermi surface, this firmly places the Au/Ge(100) nanowire system outside being a potential hosts of a Tomonaga-Luttinger liquid. We combine these ARPES data with STS measurements of the spatially-resolved electronic structure and find that the spatially straight conduction channels observed up to energies of order one electron volt below the Fermi level do not originate from the Au-induced states seen in the ARPES data. The former are more likely to be associated with bulk Ge states that are localized to the subsurface region. Despite our proof of the 2D nature of the Au-induced nanowire and sub-surface Ge-related states, an anomalous suppression of the density of states at the Fermi level is observed in both the STS and ARPES data, this phenomenon is discussed in the light of the effects of disorder.
