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61.
We propose a new method of resonant enhancement of optical Kerr nonlinearity that uses multilevel atomic coherence. The enhancement is accompanied by suppression of the other linear and nonlinear susceptibility terms of the medium. We show that the effect results in a modification of the nonlinear Faraday rotation of light propagating in an 87Rb vapor cell by changing the ellipticity of the light. 相似文献
62.
63.
Haskins RW Maier RS Ebeling RM Marsh CP Majure DL Bednar AJ Welch CR Barker BC Wu DT 《The Journal of chemical physics》2007,127(7):074708
We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Young's modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Young's modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Young's modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Young's modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Young's modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains. 相似文献
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Xu J Sun G Rossin R Hagooly A Li Z Fukukawa KI Messmore BW Moore DA Welch MJ Hawker CJ Wooley KL 《Macromolecules》2007,40(9):2971-2973
Radiolabeling studies were employed to investigate the influence of structure on the efficiency of surface functionalization for poly(acrylic acid)-coated shell crosslinked nanoparticles (SCKs) with two types of amine-terminated DOTA chelators. An intricate interplay between the chemical and physical properties of both the DOTA derivative and the SCK nanostructures was revealed, demonstrating the importance of structural control. 相似文献
66.
Dent ML McClaine EM Welch TE 《The Journal of the Acoustical Society of America》2007,122(6):3609-3614
The properties of the Franssen effect (FE) were measured in budgerigars and zebra finches. To elicit the FE, listeners are presented with a signal which has been split into a transient component, carrying an abrupt onset and ramped offset and separated in space from the sustained component which has a slowly rising onset and longer duration. When these two signals are played under certain conditions, the perception is that of a long-duration steady state tone being played at the location of the transient. The birds were trained using operant conditioning methods on a categorization task to peck a left key when presented with a stimulus from a left speaker and to peck a right key when presented with a stimulus from a right speaker. Once training was completed, FE stimuli were presented during a small proportion of trials. The FE was measured at speaker separations of 60 degrees and 180 degrees in both echoic and echoic-reduced conditions. Both species of birds exhibited the FE, although to varying degrees, across conditions. These results show that nonmammals also experience the FE illusion in confusing listening situations in a manner similar to mammals, suggestive of similar auditory processing mechanisms. 相似文献
67.
Chromatographic enantioseparations on the order of a few seconds can be achieved by supercritical fluid chromatography using short columns packed with chiral stationary phases. The evolution of ‘world record’ speeds for the chromatographic separation of enantiomers has steadily dropped from an industry standard of 20–40 min just two decades ago, to a current ability to perform many enantioseparations in well under a minute. Improvements in instrument and column technologies enabled this revolution, but the ability to predict optimal separation time from an initial method development screening assay using the tmin cc predictor greatly simplifies the development and optimization of high‐speed chiral chromatographic separations. In this study, we illustrate how the use of this simple tool in combination with the workhorse technique of supercritical fluid chromatography on customized short chiral columns (1–2 cm length) allows us to achieve ultrafast enantioseparations of pharmaceutically relevant compounds on the 5–20 s scale, bringing the technique of high‐throughput enantiopurity analysis out of the specialist realm and into the laboratories of most researchers. 相似文献
68.
Kenwright JL Galloway WR Blackwell DT Isidro-Llobet A Hodgkinson J Wortmann L Bowden SD Welch M Spring DR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(10):2981-2986
Herein, a new copper-catalysed strategy for the synthesis of rare nitrogen-linked seven-, eight- and nine-membered biaryl ring systems is described. It is proposed that the reaction proceeds through a highly activated intramolecularly co-ordinated copper catalyst. The process is technically simple, proceeds under relatively mild conditions, displays a broad substrate scope and forms biologically valuable products that are difficult to synthesise by other methods. We envisage that this methodology will prove useful in a wide synthetic context, with possible applications in both target-oriented and diversity-oriented synthesis. 相似文献
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70.
Welch Nathan J. Carey J. William Frash Luke P. Hyman Jeffrey D. Hicks Wes Meng Meng Li Wenfeng Menefee Anne H. 《Transport in Porous Media》2022,141(1):17-47
Transport in Porous Media - Fluid flow in fractures has the potential to drastically change the economic, environmental, and safety risks associated with a subsurface operation. This work focuses... 相似文献