Synthesis and Ring-Opening Metathesis Polymerization of Second-Generation Dendronized Poly(ether) Monomers Initiated by Ruthenium Carbenes

The Ring-Opening Metathesis Polymerization (ROMP) of second-generation dendronized monomers is described. Using the highly active and fast-initiating third-generation ruthenium complex [(H_2IMes)(pyr)_2Cl_2RuCHPh], moderate to high molecular weight polymers (430-2230 kDa) are efficiently synthesized with low dispersities (Ð = 1.01-1.17). This study highlights the power of the metathesis approach toward polymer synthesis in a context where monomer structure can significantly impede polymerization

Hole dynamics and photoemission in a t-J model for SrCu_2(BO_3)_2

The motion of a single hole in a t-J model for the two-dimensional spin-gap compound SrCu_2(BO_3)_2 is investigated. The undoped Heisenberg model for this system has an exact dimer eigenstate and shows a phase transition between a dimerized and a Neel phase at a certain ratio of the magnetic couplings. We calculate the photoemission spectrum in the disordered phase using a generalized spin-polaron picture. By varying the inter-dimer hopping parameters we find a cross-over between a narrow quasiparticle band regime known from other strongly correlated systems and free-fermion behavior. The hole motion in the Neel-ordered phase is also briefly considered.Comment: 4 pages, 3 fig

Anharmonic double-phonon excitations in the interacting boson model

Double-$\gamma$ vibrations in deformed nuclei are analyzed in the context of the interacting boson model. A simple extension of the original version of the model towards higher-order interactions is required to explain the observed anharmonicities of nuclear vibrations. The influence of three- and four-body interactions on the moments of inertia of ground- and $\gamma$-bands, and on the relative position of single-$\gamma$ and double-$\gamma$ bands is studied in detail. As an example of a realistic calculation, spectra and transitions of the highly $\gamma$-anharmonic nuclei $^{164}$Dy, $^{166}$Er, and $^{168}$Er are interpreted in this approach.Comment: 38 pages, TeX (ReVTeX). 15 ps figures. Submitted to Phys. Rev.

Effects of Neuromuscular Electrical Stimulation Training on Skeletal Muscle Anabolic Signaling in Older Adults

Neuromuscular electrical stimulation (NMES) generates involuntary muscle contraction and may be a safe and effective alternative to voluntary resistance training, which is important for those who cannot perform voluntary exercise due to age-related conditions. However, further research is needed to better understand the skeletal muscle anabolic signaling response of the mTORC1 (mammalian target of rapamycin complex 1) pathway with repeated bouts of NMES. PURPOSE: To determine changes in skeletal muscle anabolic signaling in response to a 4-week NMES intervention in older adults. METHODS: Participants (n = 7) in this clinical trial were healthy, older adults (70.4 ± 2.9 years). NMES was applied to the quadriceps muscles for 40 min/treatment, 3x/week for 4 weeks (12 sessions). On Day 1 and Day 12 of the NMES intervention, skeletal muscle biopsies were obtained from the vastus lateralis Pre-NMES and 30 minutes Post-NMES and were analyzed for phosphorylation of mammalian target of rapamycin (mTOR) and p70-S6 Kinase 1 (S6K1) anabolic signaling proteins using the SDS-PAGE Western blot technique. Phosphorylation is expressed as the ratio of phosphorylated to total protein content. Data were analyzed using paired t-tests and data are reported as mean ± SE with statistical significance set at p ≤ 0.05. RESULTS: On Day 1, phosphorylation of S6K1 increased (Pre-NMES: 0.652 ± 0.145 AU vs. Post-NMES: 0.979 ± 0.151 AU, p = 0.037) and phosphorylation of mTOR increased (Pre-NMES: 0.464 ± 0.077 AU vs. Post-NMES: 1.046 ± 0.128 AU, p = 0.006) from Pre-NMES to Post-NMES. On Day 12, phosphorylation of S6K1 increased (Pre-NMES: 0.628 ± 0.108 AU vs. Post-NMES: 1.253 ± 0.288 AU, p = 0.048) with an increasing trend for mTOR (Pre-NMES: 0.485 ± 0.044 AU vs. Post-NMES: 0.700 ± 0.154 AU, p = 0.053) from Pre-NMES to Post-NMES. Phosphorylated S6K1 and mTOR protein content were not different between Day 1 and Day 12 at Pre-NMES (p \u3e 0.05) or at Post-NMES (p \u3e 0.05). CONCLUSION: The findings of this study suggest that the anabolic signaling response to a bout of NMES remains upregulated after 4-weeks of treatment; thus, the response is not attenuated with short-term repeated bouts of NMES. Funding: Research Enhancement Program Grant to J Mettler and L Kipp; Research Accelerator Grant, Texas State University, to J Mettler

Magnetic Phase Transition of the Perovskite-type Ti Oxides

Properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state is hardly changed so that the spin-exchange coupling along the c-axis changes nearly continuously from positive to negative and takes approximately zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes a rapid suppression of the critical temperature as is observed experimentally.Comment: 9 pages, 5 figure

Synthesis and Ring-Opening Metathesis Polymerization of Second-Generation Dendronized Poly(ether) Monomers Initiated by Ruthenium Carbenes

The Ring-Opening Metathesis Polymerization (ROMP) of second-generation dendronized monomers is described. Using the highly active and fast-initiating third-generation ruthenium complex [(H_2IMes)(pyr)_2Cl_2RuCHPh], moderate to high molecular weight polymers (430-2230 kDa) are efficiently synthesized with low dispersities (Ð = 1.01-1.17). This study highlights the power of the metathesis approach toward polymer synthesis in a context where monomer structure can significantly impede polymerization

Astronomy outreach in Namibia: H.E.S.S. and beyond

Astronomy plays a major role in the scientific landscape of Namibia. Because of its excellent sky conditions, Namibia is home to ground-based observatories like the High Energy Spectroscopic System (H.E.S.S.), in operation since 2002. Located near the Gamsberg mountain, H.E.S.S. performs groundbreaking science by detecting very-high-energy gamma rays from astronomical objects. The fascinating stories behind many of them are featured regularly in the ``Source of the Month'', a blog-like format intended for the general public with more than 170 features to date. In addition to other online communication via social media, H.E.S.S. outreach activities have been covered locally, e.g. through `open days' and guided tours on the H.E.S.S. site itself. An overview of the H.E.S.S. outreach activities are presented in this contribution, along with discussions relating to the current landscape of astronomy outreach and education in Namibia. There has also been significant activity in the country in recent months, whereby astronomy is being used to further sustainable development via human capacity-building. Finally, as we take into account the future prospects of radio astronomy in the country, momentum for a wider range of astrophysics research is clearly building -- this presents a great opportunity for the astronomy community to come together to capitalise on this movement and support astronomy outreach, with the overarching aim to advance sustainable development in Namibia.Comment: ICRC 2021 conference proceedings, 9 pages, 1 figure, 1 tabl

Field dependent thermodynamics and Quantum Critical Phenomena in the dimerized spin system Cu2(C5H12N2)2Cl4

Experimental data for the uniform susceptibility, magnetization and specific heat for the material Cu2(C5H12N2)2Cl4 (abbreviated CuHpCl) as a function of temperature and external field are compared with those of three different dimerized spin models: alternating spin-chains, spin-ladders and the bilayer Heisenberg model. It is shown that because this material consists of weakly coupled spin-dimers, much of the data is insensitive to how the dimers are coupled together and what the effective dimensionality of the system is. When such a system is tuned to the quantum critical point by application of a field, the dimensionality shows up in the power-law dependences of thermodynamic quantities on temperature. We discuss the temperature window for such a quantum critical behavior in CuHpCl.Comment: Revtex, 5 pages, 4 figures (postscript

Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach

The properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state changes only little whereas the spin-exchange coupling along the c-axis is expected to change from positive to negative nearly continuously and approaches zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes rapid suppression of the critical temperature, as observed experimentally. It may induce large quantum fluctuations in this region.Comment: 13 pages, 15 figure

Chaotic Waveguide-Based Resonators for Microlasers

We propose the construction of highly directional emission microlasers using two-dimensional high-index semiconductor waveguides as {\it open} resonators. The prototype waveguide is formed by two collinear leads connected to a cavity of certain shape. The proposed lasing mechanism requires that the shape of the cavity yield mixed chaotic ray dynamics so as to have the appropiate (phase space) resonance islands. These islands allow, via Heisenberg's uncertainty principle, the appearance of quasi bound states (QBS) which, in turn, propitiate the lasing mechanism. The energy values of the QBS are found through the solution of the Helmholtz equation. We use classical ray dynamics to predict the direction and intensity of the lasing produced by such open resonators for typical values of the index of refraction.Comment: 5 pages, 5 figure