Axion response in gapless systems

The strong topological insulator in 3D is expected to realize a quantized magneto-electric response, the so-called axion response. However, many of the materials predicted to be topological insulators have turned out to be metallic, with bulk Fermi surfaces. Following the result of Bergman et al. (Phys. Rev. B 82, 195417 (2010)) that the helical surface states of the topological insulator persist even when the band structure gap is closed, we explore the fate of the magneto-electric response in such systems. We find a non-quantized magneto-electric coupling remains once a bulk Fermi surface opens - a non-universal axion response. More generally we find that higher dimensional analogs of the intrinsic anomalous Hall effect appear for \emph{every} Chern form - non-quantized response coefficients for gapless systems, as opposed to quantized transport coefficients in gapped systems, both with a topological origin. In particular, the non-quantized magneto-electric response in 3D descends from the intrinsic anomalous Hall effect analog in 4D

Effective Hamiltonians for some highly frustrated magnets

In prior work, the authors developed a method of degenerate perturbation theory about the Ising limit to derive an effective Hamiltonian describing quantum fluctuations in a half-polarized magnetization plateau on the pyrochlore lattice. Here, we extend this formulation to an arbitrary lattice of corner sharing simplexes of $q$ sites, at a fraction $(q-2k)/q$ of the saturation magnetization, with $0<k<q$. We present explicit effective Hamiltonians for the examples of the checkerboard, kagome, and pyrochlore lattices. The consequent ground states in these cases for $k=1$ are also discussed.Comment: 10 pages, 2 figures,. Conference proceedings for Highly Frustrated Magnetism 200

\u3ci\u3eBaetis\u3c/i\u3e (Ephemeroptera: Baetidae) of Wisconsin

Data on life histories and environmental requirements for species in many mayfly genera remains sketchy at best This is certainly true of Baetis, which is one of the most common components of Wisconsin\u27s lotic fauna. Most Wisconsin streams that are not grossly pobted contain one or more species of the minnow-like nymphs, which are usually found clinging to surfaces of rocks or aquatic plants. Biological studies of Baetis in North America have been neglected primarily because of their enigmatic taxonomy. Even keys of Needham et al. (1935) and Burks (1953), which are considered standard referenas, are either incomplete or difficult to use when identifying Baetis

Grackles

Numbering in the tens of millions of birds, grackle populations in North America can cause a variety of conflicts with people. Grackles eat agricultural crops and livestock feed, damage property, spread pathogens, and collide with aircraft. Their large roosts can be a nuisance in urban and suburban areas. A combination of dispersal techniques, exclusion, and lethal removal may help to reduce grackle damage. Grackles adapt easily to human-dominated environments, and exploit human food and other features of human landscapes. Thus, an integrated damage management approach to grackle damage focuses on reducing and eliminating the damage, rather than simply controlling grackle populations. Three species of grackles are present in North America: the common grackle, the boat-tailed grackle, and the great-tailed grackle. A fourth species, the greater Antillean grackle (Q. niger) is present in Puerto Rico. All are part of the Family Icteridae that includes blackbirds, orioles, cowbirds, meadowlarks, and bobolinks. The population status of all three grackle species is considered common to overabundant

Description of the Nymph of \u3ci\u3eCentroptilum Walshi\u3c/i\u3e (Ephemeroptera: Baetidae), with Biological Notes

The nymph of Centroptilum walshi McDunnough is described. C. walshi appears to be bivoltine in Wisconsin, with emergences throughout June into early July and from late August to early November. Mature nymphs were smallest when stream temperatures were the warmest. The nymphs were closely associated with Ranunculus sp., and numbers increased when the Ranunculus beds became more dense

Origin of the Tc_c enhancement in heterostructure cuprate superconductors

Recent experiments on heterostructures composed of two or more films of cuprate superconductors of different oxygen doping levels\cite{Yuli,Gozar} have shown a remarkable T$_c$ enhancement (up to 50%) relative to single compound films. We provide here a simple explanation of the enhancement which arises naturally from a collection of experimental works. We show that the enhancement could be caused by a structural change in the lattice, namely an increase in the distance of the apical oxygen from the copper-oxygen plane. This increase modifies the effective off-site interaction in the plane which in turn enhances the d-wave superconductivity order parameter. To illustrate this point we study the extended Hubbard model using the fluctuation exchange approximation

Theory of dissipationless Nernst effects

We develop a theory of transverse thermoelectric (Peltier) conductivity, \alpha_{xy}, in finite magnetic field -- this particular conductivity is often the most important contribution to the Nernst thermopower. We demonstrate that \alpha_{xy} of a free electron gas can be expressed purely and exactly as the entropy per carrier irrespective of temperature (which agrees with seminal Hall bar result of Girvin and Jonson). In two dimensions we prove the universality of this result in the presence of disorder which allows explicit demonstration of a number features of interest to experiments on graphene and other two-dimensional materials. We also exploit this relationship in the low field regime and to analyze the rich singularity structure in \alpha_{xy}(B, T) in three dimensions; we discuss its possible experimental implications.Comment: 4.5 pages, 2 figure

Transient and chaotic low-energy transfers in a system with bistable nonlinearity

The low-energy dynamics of a two-dof system composed of a grounded linear oscillator coupled to a lightweight mass by means of a spring with both cubic nonlinear and negative linear components is investigated. The mechanisms leading to intense energy exchanges between the linear oscillator, excited by a low-energy impulse, and the nonlinear attachment are addressed. For lightly damped systems, it is shown that two main mechanisms arise: Aperiodic alternating in-well and cross-well oscillations of the nonlinear attachment, and secondary nonlinear beats occurring once the dynamics evolves solely in-well. The description of the former dissipative phenomenon is provided in a two-dimensional projection of the phase space, where transitions between in-well and cross-well oscillations are associated with sequences of crossings across a pseudo-separatrix. Whereas the second mechanism is described in terms of secondary limiting phase trajectories of the nonlinear attachment under certain resonance conditions. The analytical treatment of the two aformentioned low-energy transfer mechanisms relies on the reduction of the nonlinear dynamics and consequent analysis of the reduced dynamics by asymptotic techniques. Direct numerical simulations fully validate our analytical predictions

Bulk metals with helical surface states

In the flurry of experiments looking for topological insulator materials, it has been recently discovered that some bulk metals very close to topological insulator electronic states, support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present here a theoretical investigation of the nature of these "helical metals" - bulk metals with helical surface states. We explore how the surface and bulk states can mix, in both clean and disordered systems. Using the Fano model, we discover that in a clean system, the helical surface states are \emph{not} simply absorbed by hybridization with a non-topological parasitic metallic band. Instead, they are pushed away from overlapping in momentum and energy with the bulk states, leaving behind a finite-lifetime surface resonance in the bulk energy band. Furthermore, the hybridization may lead in some cases to multiplied surface state bands, in all cases retaining the helical characteristic. Weak disorder leads to very similar effects - surface states are pushed away from the energy bandwidth of the bulk, leaving behind a finite-lifetime surface resonance in place of the original surface states