Fractionalization in a square-lattice model with time-reversal symmetry

We propose a two-dimensional time-reversal invariant system of essentially non-interacting electrons on a square lattice that exhibits configurations with fractional charges e/2. These are vortex-like topological defects in the dimerization order parameter describing spatial modulation in the electron hopping amplitudes. Charge fractionalization is established by a simple counting argument, analytical calculation within the effective low-energy theory, and by an exact numerical diagonalization of the lattice Hamiltonian. We comment on the exchange statistics of fractional charges and possible realizations of the system.Comment: 4 pages, 3 figures, RevTex 4. (v2) improved discussion of lattice effects and confinement; clearer figure

Dodecahedral topology fails to explain quadrupole-octupole alignment

The CMB quadrupole and octupole, as well as being weaker than expected, align suspiciously well with each other. Non-trivial spatial topology can explain the weakness. Might it also explain the alignment? The answer, at least in the case of the Poincare dodecahedral space, is a resounding no.Comment: 5 pages, 1 figur

Invariance of Structure in an Aging Colloidal Glass

We study concentrated colloidal suspensions, a model system which has a glass transition. The non-equilibrium nature of the glassy state is most clearly highlighted by aging -- the dependence of the system's properties on the time elapsed since vitrification. Fast laser scanning confocal microscopy allows us to image a colloidal glass and track the particles in three dimensions. We analyze the static structure in terms of tetrahedral packing. We find that while the aging of the suspension clearly affects its dynamics, none of the geometrical quantities associated with tetrahedra change with age.Comment: Submitted to the proceedings of "The 3rd International Workshop on Complex Systems" in Sendai, Japa

Bottom-up Herbivore-Plant Feedbacks Trump Trophic Cascades in a Wolf-Elk-Grassland System

Top-down predator-prey effects that alter the abundance, biomass, or productivity of a population community across more than one link in a food web are referred to as trophic cascades. While these effects have been extensively studied in aquatic environments, fewer studies have examined trophic cascades in terrestrial ecosystems. And fewer still terrestrial studies have tested for trophic cascades between vertebrates and grassland vegetation. Across the globe, grassland plant biomass is driven by both precipitation and non-linear positive feedbacks between grazing and plant productivity, as predicted by the Intermediate Grazing Hypothesis. Yet little is known about the role that apex carnivores play in regard to trophic impacts on grassland biomass. We utilized a long-term dataset collected over the last two decades on a montane rough-fescue grassland adjacent to Banff National Park, Alberta, to test whether top-down effects regulate grassland biomass in a wolf-elk system. First, we measured annual growing season plant biomass from 2006 – 2018 at 61 repeat sampled plots in the grassland. Next, we measured wolf predation risk using a previously developed wolf resource selection function created from GPS radiocollar data from 5 wolf packs. Finally, we measured grazing intensity using Brownian Bridge Movement Models derived from GPS radiocollar data from 131 unique elk. We then tested top-down, bottom-up and abiotic hypotheses for grassland biomass over time in program R. The top model incorporated precipitation and positive non-linear effects of elk use, excluding predator effects and thus failing to support the trophic cascade hypothesis. This may be due to the observational nature of this study, or predation effects in this system may be obscured by human use. Alternatively, our results also support the hypothesis that intermediate grazing may outweigh the benefits of predation in grassland systems. Our study serves to help fill a gap in trophic cascade literature, and emphasizes that positive feedback between grazers and grasslands may trump top-down effects. Understanding when trophic cascade theory is or is not applicable is vital for carnivore management, conservation, and reintroduction efforts across North America