Showing 3431 - 3440 of 5713 Items
Convexity Properties of the Diestel-Leader Group Γ_3(2)
Date: 2014-05-01
Creator: Peter J Davids
Access: Open access
- The Diestel-Leader groups are a family of groups first introduced in 2001 by Diestel and Leader in [7]. In this paper, we demonstrate that the Diestel-Leader group Γ3(2) is not almost convex with respect to a particular generating set S. Almost convexity is a geometric property that has been shown by Cannon [3] to guarantee a solvable word problem (that is, in any almost convex group there is a finite-step algorithm to determine if two strings of generators, or “words”, represent the same group element). Our proof relies on the word length formula given by Stein and Taback in [10], and we construct a family of group elements X that contradicts the almost convexity condition. We then go on to show that Γ3(2) is minimally almost convex with respect to S.
Thermal fractionation of air in polar firn by seasonal temperature gradients
Date: 2001-07-01
Creator: Jeffrey P. Severinghaus, Alexi Grachev, Mark Battle
Access: Open access
- Air withdrawn from the top 5-15 m of the polar snowpack (firn) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we explore and quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2-m-depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter sampling at the pole. We analyzed 15N/14N, 40Ar/36Ar, 40Ar/38Ar, 18O/16O of O2, O2/N2, 84Kr/36Ar, and 132Xe/36Ar. The results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative values of different gas pairs may. At Siple Dome, δ40Ar/4 is 66 ± 2% as sensitive to thermal diffusion as δ15N, in agreement with laboratory calibration; δ18O/2 is 83 ± 3%, and δ84Kr/48 is 33 ± 3% as sensitive as δ15N. The corresponding figures for summer South Pole are 64 ± 2%, 81 ± 3%, and 34 ± 3%. Accounting for atmospheric change, the figure for δO2/N2/4 is 90 ± 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 ± 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or a rectifier effect is seen.
Egg Size, Breeding Phenology, and Parental Investment in Leach’s Storm Petrels Access to this record is restricted to members of the Bowdoin community. Log in here to view.
Date: 2020-01-01
Creator: James L. O'Shea
Access: Access restricted to the Bowdoin Community
Bowdoin College Catalogue (1921-1922)
Date: 1922-01-01
Access: Open access
- Bowdoin College Bulletin no. 114
Quantum lifetime of two-dimensional holes
Date: 2007-08-01
Creator: J. P. Eisenstein, D. Syphers, L. N. Pfeiffer, K. W. West
Access: Open access
- The quantum lifetime of two-dimensional holes in a GaAs/AlGaAs double quantum well is determined via tunneling spectroscopy. At low temperatures the lifetime is limited by impurity scattering but at higher temperatures hole-hole Coulomb scattering dominates. Our results are consistent with the Fermi liquid theory, at least up to rs = 11. At the highest temperatures the measured width of the hole spectral function becomes comparable to the Fermi energy. A new, tunneling-spectroscopic method for determining the in-plane effective mass of the holes is also demonstrated. © 2007 Elsevier Ltd. All rights reserved.
Superconformal coset equivalence from level-rank duality
Date: 1997-11-17
Creator: Stephen G. Naculich, Howard J. Schnitzer
Access: Open access
- We construct a one-to-one map between the primary fields of the N = 2 superconformal Kazama-Suzuki models G(m, n, k) and G(k, n, m) based on complex Grassmannian cosets, using level-rank duality of Wess-Zumino-Witten models. We then show that conformal weights, superconformal U(1) charges, modular transformation matrices, and fusion rules are preserved under this map, providing strong evidence for the equivalence of these coset models. © 1997 Elsevier Science B.V.
The guanine nucleotide exchange factor trio mediates axonal development in the Drosophila embryo
Date: 2000-01-01
Creator: Jack Bateman, Huidy Shu, David Van Vactor
Access: Open access
- Recent analysis of Rho subfamily GTPases in Drosophila revealed roles for Rac and Cdc42 during axonogenesis. Here, we describe the identification and characterization of the Drosophila counterpart of Trio, a guanine nucleotide exchange factor (GEF) that associates with the receptor phosphatase LAR and regulates GTPase activation in vertebrate cells. Mutants deficient in trio activity display defects in both central and peripheral axon pathways reminiscent of pheno-types observed in embryos deficient in small GTPase function. Double mutant analysis shows that trio interacts with Rac in a dose-sensitive manner but not with Rho. Moreover, reduction of trio activity potentiates the phenotype of mutations in the LAR homolog Dlar, suggesting that these proteins collaborate in orchestrating the cytoskeletal events that underlie normal axonogenesis.
Measurement of the decay asymmetry parameters in Λc+ → Λπ+ and Λc+ → Σ+π0
Date: 1995-05-11
Creator: M. Bishai, J. Fast, E. Gerndt, J. W. Hinson, R. L., McIlwain, T. Miao, D. H. Miller, M. Modesitt, D. Payne, E. I. Shibata, I. P.J. Shipsey, P. N. Wang, M. Battle, J. Ernst, L. Gibbons, Y. Kwon, S. Roberts, E. H. Thorndike, C. H. Wang, J. Dominick, M. Lambrecht, S. Sanghera, V. Shelkov, T. Skwarnicki, R. Stroynowski, I. Volobouev, G. Wei, M. Artuso, M. Gao, M. Goldberg, D. He
Access: Open access
- We have measured the weak decay asymmetry parameters (αΛc) for two Λc+ decay modes. Our measurements are αΛc = -0.94-0.06-0.06+0.21+0.12 for the decay mode Λc+ → Λπ+ and αΛc = -0.45 ±0.31 ±0.06 for the decay mode Λc → Σ+ π0. By combining these measurements with the previously measured decay rates, we have extracted the parity-violating and parity-conserving amplitudes. These amplitudes are used to test models of nonleptonic charmed baryon decay. © 1995.