A reading workbook of stories and exercises with first grade vocabulary and fourth grade interests

Thesis (Ed.M.)--Boston Universit

Semimetallic features in quantum transport through a gate-defined point contact in bilayer graphene

We demonstrate that, at the onset of conduction, an electrostatically defined quantum wire in bilayer graphene (BLG) with an interlayer asymmetry gap may act as a 1D semimetal, due to the multiple minivalley dispersion of its lowest subband. Formation of a non-monotonic subband coincides with a near-degeneracy between the bottom edges of the lowest two subbands in the wire spectrum, suggesting an $8e^2/h$ step at the conduction threshold, and the semimetallic behaviour of the lowest subband in the wire would be manifest as resonance transmission peaks on an $8e^2/h$ conductance plateau.Comment: 9 pages, 8 figures (including appendices

Twist-controlled Resonant Tunnelling between Monolayer and Bilayer Graphene

We investigate the current-voltage characteristics of a field-effect tunnelling transistor comprised of both monolayer and bilayer graphene with well-aligned crystallographic axes, separated by three layers of hexagonal boron nitride. Using a self-consistent description of the device's electrostatic configuration we relate the current to three distinct tunable voltages across the system and hence produce a two-dimensional map of the I-V characteristics in the low energy regime. We show that the use of gates either side of the heterostructure offers a fine degree of control over the device's rich array of characteristics, as does varying the twist between the graphene electrodes.Comment: 5 pages including references and 3 figure

Direct radiative capture of p-wave neutrons

The neutron direct radiative capture (DRC) process is investigated, highlighting the role of incident p-wave neutrons. A set of calculations is shown for the 12-C(n,gamma) process at incoming neutron energies up to 500 keV, a crucial region for astrophysics. The cross section for neutron capture leading to loosely bound s, p and d orbits of 13-C is well reproduced by the DRC model demonstrating the feasibility of using this reaction channel to study the properties of nuclear wave functions on and outside the nuclear surface. A sensitivity analysis of the results on the neutron-nucleus interaction is performed for incident s- as well as p-waves. It turned out that the DRC cross section for p-wave neutrons is insensitive to this interaction, contrary to the case of incident s-wave neutrons. PACS number(s): 25.40Lw,21.10Gv,23.40.HcComment: 16 pages, REVTeX file, PostScript file, .dvi fil

Ultracold, radiative charge transfer in hybrid Yb ion - Rb atom traps

Ultracold hybrid ion-atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the Yb$^{+}$ ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes. Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion-atom collisions dominates cold ion-atom collisions. For spin dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole interaction and the second-order spin-orbit coupling can play important roles, inducing couplingbetween the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately 5 orders of magnitude higher than the charge-exchange collision rate \cite{kohl13}. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question.Comment: 7 figures, 1 table accepted for publication in J. Phys. B: At. Mol. Opt. Phys. arXiv admin note: text overlap with arXiv:1107.114

Use of insulin to increase epiblast cell number: towards a new approach for improving ESC isolation from human embryos

Human embryos donated for embryonic stem cell (ESC) derivation have often been cryopreserved for 5–10 years. As a consequence, many of these embryos have been cultured in media now known to affect embryo viability and the number of ESC progenitor epiblast cells. Historically, these conditions supported only low levels of blastocyst development necessitating their transfer or cryopreservation at the 4–8-cell stage. As such, these embryos are donated at the cleavage stage and require further culture to the blastocyst stage before hESC derivation can be attempted. These are generally of poor quality, and, consequently, the efficiency of hESC derivation is low. Recent work using a mouse model has shown that the culture of embryos from the cleavage stage with insulin to day 6 increases the blastocyst epiblast cell number, which in turn increases the number of pluripotent cells in outgrowths following plating, and results in an increased capacity to give rise to ESCs. These findings suggest that culture with insulin may provide a strategy to improve the efficiency with which hESCs are derived from embryos donated at the cleavage stage.Jared M. Campbell, Michelle Lane, Ivan Vassiliev, and Mark B. Nottl

Carotenoids are the likely precursor of a significant fraction of marine dissolved organic matter.

The ocean's biota sequester atmospheric carbon dioxide (CO2) in part by producing dissolved organic matter (DOM) that persists in the ocean for millennia. This long-term accumulation of carbon may be facilitated by abiotic and biotic production of chemical structures that resist degradation, consequently contributing disproportionately to refractory DOM. Compounds that are selectively preserved in seawater were identified in solid-phase extracted DOM (PPL-DOM) using comprehensive gas chromatography (GC) coupled to mass spectrometry (MS). These molecules contained cyclic head groups that were linked to isoprenoid tails, and their overall structures closely resembled carotenoid degradation products (CDP). The origin of these compounds in PPL-DOM was further confirmed with an in vitro β-carotene photooxidation experiment that generated water-soluble CDP with similar structural characteristics. The molecular-level identification linked at least 10% of PPL-DOM carbon, and thus 4% of total DOM carbon, to CDP. Nuclear magnetic resonance spectra of experimental CDP and environmental PPL-DOM overlapped considerably, which indicated that even a greater proportion of PPL-DOM was likely composed of CDP. The CDP-rich DOM fraction was depleted in radiocarbon (14C age > 1500 years), a finding that supports the possible long-term accumulation of CDP in seawater. By linking a specific class of widespread biochemicals to refractory DOM, this work provides a foundation for future studies that aim to examine how persistent DOM forms in the ocean

Dynamical symmetry of isobaric analog 0+ states in medium mass nuclei

An algebraic sp(4) shell model is introduced to achieve a deeper understanding and interpretation of the properties of pairing-governed 0+ states in medium mass atomic nuclei. The theory, which embodies the simplicity of a dynamical symmetry approach to nuclear structure, is shown to reproduce the excitation spectra and fine structure effects driven by proton-neutron interactions and isovector pairing correlations across a broad range of nuclei.Comment: 7 pages, 5 figure

The Phases Differential Astrometry Data Archive. IV. The Triple Star Systems 63 Gem A and HR 2896

Differential astrometry measurements from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) are used to constrain the astrometric orbit of the previously known ≾2 day subsystem in the triple system 63 Gem A and have detected a previously unknown two-year Keplerian wobble superimposed on the visual orbit of the much longer period (213 years) binary system HR 2896. 63 Gem A was already known to be triple from spectroscopic work, and absorption lines from all three stars can be identified and their individual Doppler shifts measured; new velocities for all three components are presented to aid in constraining the orbit and measuring the stellar masses. In fact, 63 Gem itself is a sextuple system: the hierarchical triple (Aa1-Aa2)-Ab (in which Aa1 and Aa2 orbit each other with a rapid period just under 2 days, and Ab orbits these every two years), plus three distant common proper motion companions. The very small astrometric perturbation caused by the inner pair in 63 Gem A stretches the limits of current astrometric capabilities, but PHASES observations are able to constrain the orientation of the orbit. The two bright stars comprising the HR 2896 long-period (213 year) system have a combined spectral type of K0III and the newly detected object's mass estimate places it in the regime of being an M dwarf. The motion of the stars are slow enough that their spectral features are always blended, preventing Doppler studies. The PHASES measurements and radial velocities (when available) have been combined with lower precision single-aperture measurements covering a much longer time frame (from eyepiece measurements, speckle interferometry, and adaptive optics) to improve the characterization of the long-period orbits in both binaries. The visual orbits of the short- and long-period systems are presented for both systems and used to calculate two possible values of the mutual inclinations between inner and outer orbits of 152° ± 12° or a less likely value of 31° ± 11° for 63 Gem A and 10.°2 ± 2.°4 or 171.°2 ± 2.°8 for HR 2896. The first is not coplanar, whereas the second is either nearly coplanar or anti-coplanar