\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

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

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

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

Semiclassical dynamics and long time asymptotics of the central-spin problem in a quantum dot

The spin of an electron trapped in a quantum dot is a promising candidate implementation of a qubit for quantum information processing. We study the central spin problem of the effect of the hyperfine interaction between such an electron and a large number of nuclear moments. Using a spin coherent path integral, we show that in this limit the electron spin evolution is well described by classical dynamics of both the nuclear and electron spins. We then introduce approximate yet systematic methods to analyze aspects of the classical dynamics, and discuss the importance of the exact integrability of the central spin Hamiltonian. This is compared with numerical simulation. Finally, we obtain the asymptotic long time decay of the electron spin polarization. We show that this is insensitive to integrability, and determined instead by the transfer of angular momentum to very weakly coupled spins far from the center of the quantum dot. The specific form of the decay is shown to depend sensitively on the form of the electronic wavefunction.Comment: 13 pages, 4 figures, accepted by PR

Quantum effects in a half-polarized pyrochlore antiferromagnet

We study quantum effects in a spin-3/2 antiferromagnet on the pyrochlore lattice in an external magnetic field, focusing on the vicinity of a plateau in the magnetization at half the saturation value, observed in CdCr$_2$O$_4$, and HgCr$_2$O$_4$. Our theory, based on quantum fluctuations, predicts the existence of a symmetry-broken state on the plateau, even with only nearest-neighbor microscopic exchange. This symmetry broken state consists of a particular arrangement of spins polarized parallel and antiparallel to the field in a 3:1 ratio on each tetrahedron. It quadruples the lattice unit cell, and reduces the space group from $Fd\bar{3}m$ to $P4_332$. We also predict that for fields just above the plateau, the low temperature phase has transverse spin order, describable as a Bose-Einstein condensate of magnons. Other comparisons to and suggestions for experiments are discussed

Degenerate perturbation theory of quantum fluctuations in a pyrochlore antiferromagnet

We study the effect of quantum fluctuations on the half-polarized magnetization plateau of a pyrochlore antiferromagnet. We argue that an expansion around the easy axis limit is appropriate for discussing the ground state selection amongst the classically degenerate manifold of collinear states with a 3:1 ratio of spins parallel/anti-parallel to the magnetization axis. A general approach to the necessary degenerate perturbation theory is presented, and an effective quantum dimer model within this degenerate manifold is derived for arbitrary spin $s$. We also generalize the existing semiclassical analysis of Hizi and Henley [Phys. Rev. B {\bf 73}, 054403 (2006)] to the easy axis limit, and show that both approaches agree at large $s$. We show that under rather general conditions, the first non-constant terms in the effective Hamiltonian for $s\geq 1$ occur only at {\sl sixth} order in the transverse exchange coupling. For $s\geq 3/2$, the effective Hamiltonian predicts a magnetically ordered state. For $s\leq 1$ more exotic possibilities may be realized, though an analytical solution of the resulting quantum dimer model is not possible

Structure of human transthyretin complexed with bromophenols: a new mode of binding

The binding of two organohalogen substances, pentabromophenol (PBP) and 2,4,6-tribromophenol (TBP), to human transthyretin (TTR), a thyroid hormone transport protein, has been studied by in vitro competitive binding assays and by X-ray crystallography. Both compounds bind to TTR with high affinity, in competition with the natural ligand thyroxine (