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排序方式: 共有184条查询结果,搜索用时 15 毫秒
91.
92.
Keith Andrew Brett Bolen Chad A. Middleton 《General Relativity and Gravitation》2007,39(12):2061-2071
We examine the effect on cosmological evolution of adding a Gauss–Bonnet term to the standard Einstein–Hilbert action for
a (1 + 3) + d dimensional Friedman–Robertson–Walker (FRW) metric. By assuming that the additional dimensions compactify as a power law
as the usual 3 spatial dimensions expand, we solve the resulting dynamical equations and find that the solution may be of
either de Sitter or Kasner form depending upon whether the Gauss–Bonnet term or the Einstein term dominates. 相似文献
93.
Middleton DA Ahmed Z Glaubitz C Watts A 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,147(2):366-370
A method is presented for the calculation of REDOR dephasing for specifically labeled membrane-spanning peptides in uniformly aligned lipid bilayers under magic angle oriented sample spinning (MAOSS) conditions. Numerical simulations are performed for dephasing of (13)C signal by (15)N when the labels are placed in an alpha-helical peptide at the carbonyl of residue (i) and amide nitrogen of residue (i + 2) to show the dependency of REDOR echo intensity on the peptide tilt angle relative to the membrane normal. The approach was applied to the labeled transmembrane domain of phospholamban ([(15)N-Leu(37), (13)C-Leu(39)]PLBTM) incorporated into dimyristoylphosphatidylcholine bilayers. The dephasing observed for a random membrane dispersion showed that the peptide was alpha-helical in the region including the two labels, and dephasing in oriented membranes showed that the peptide helix was tilted by 25 degrees +/- 7 degrees relative to the bilayer normal. These results agree with those obtained by other spectroscopic methods. 相似文献
94.
A model for ac-driven systems, based on the Tang-Wiesenfeld-Bak-Coppersmith-Littlewood automaton for an elastic medium, exhibits mode-locked steps with frequencies that are irrational multiples of the drive frequency, when the pinning is spatially quasiperiodic. Detailed numerical evidence is presented for the large-system-size convergence of such a mode-locked step. The irrational mode locking is stable to small thermal noise and weak disorder. Continuous-time models with irrational mode locking and possible experimental realizations are discussed. 相似文献
95.
Exact numerical minimization of interface energies is used to test the functional renormalization group analysis for interfaces pinned by quenched disorder. The fixed-point function R(u) (the correlator of the coarse-grained disorder) is computed. In dimensions D=d+1, a linear cusp in R'(u) is confirmed for random bond (d=1, 2, 3), random field (d=0, 2, 3), and periodic (d=2, 3) disorders. The functional shocks that lead to this cusp are seen. Small, but significant, deviations from the 1-loop calculation are compared to 2-loop corrections and chaos is measured. 相似文献
96.
Solvent and solvent isotope effects on the vibrational cooling dynamics of a DNA base derivative 总被引:1,自引:0,他引:1
Vibrational cooling by 9-methyladenine was studied in a series of solvents by femtosecond transient absorption spectroscopy. Signals at UV and near-UV probe wavelengths were assigned to hot ground state population created by ultrafast internal conversion following electronic excitation by a 267 nm pump pulse. A characteristic time for vibrational cooling was determined from bleach recovery signals at 250 nm. This time increases progressively in H2O (2.4 ps), D2O (4.2 ps), methanol (4.5 ps), and acetonitrile (13.1 ps), revealing a pronounced solvent effect on the dissipation of excess vibrational energy. The trend also indicates that the rate of cooling is enhanced in solvents with a dense network of hydrogen bonds. The faster rate of cooling seen in H2O vs D2O is noteworthy in view of the similar hydrogen bonding and macroscopic thermal properties of both liquids. We propose that the solvent isotope effect arises from differences in the rates of solute-solvent vibrational energy transfer. Given the similarities of the vibrational friction spectra of H2O and D2O at low frequencies, the solvent isotope effect may indicate that a considerable portion of the excess energy decays by exciting relatively high frequency (>/=700 cm-1) solvent modes. 相似文献
97.
M.B. Smith G.J. Campbell R. Chapman P.O. Tjøm R.A. Bark G.B. Hagemann N. Keeley D.J. Middleton H. Ryde K.-M. Spohr 《The European Physical Journal A - Hadrons and Nuclei》1999,5(1):37-42
High-spin states in 167Hf, populated in the 141Pr(30Si, p3n)167Hf reaction, have been studied using the nordball Ge detector array. Three rotational cascades have been observed for the first time and the previously-known level scheme
has been extended to significantly higher spin. Band-crossing effects are discussed within the framework of Woods-Saxon cranking
calculations and are found to be in good agreement.
Received: 9 April 1999 / Revised version: 13 May 1999 相似文献
98.
M.B. Smith R. Chapman J.F.C. Cocks O. Dorvaux K. Helariutta P.M. Jones R. Julin S. Juutinen H. Kankaanpää H. Kettunen P. Kuusiniemi Y. Le Coz M. Leino D.J. Middleton M. Muikku P. Nieminen P. Rahkila A. Savelius K.-M. Spohr 《The European Physical Journal A - Hadrons and Nuclei》1999,5(1):43-47
Excited states in the Z= 85 nucleus 197At have been identified for the first time using the recoil-decay-tagging (RDT) technique. The excitation energy of these
states is found to be consistent with the systematics of neutron-deficient astatine nuclei and with theoretical calculations
indicating that the nucleus may be deformed in its ground state.
Received: 8 January 1999 相似文献
99.
T. R. Faulkner D. Middleton 《International Journal of Mathematical Education in Science & Technology》2013,44(3):417-424
In contrast to the majority of mathematical investigations into the dynamics of the spread of the human immunodeficiency virus (HIV), which have been concerned with sexual transmission, this investigation deals with its spread by intravenus drug use (IVDU). An ‘addict based’ approach is presented in which the proportion of the susceptible population to be infected is determined by Monte Carlo means. This is in contrast to a 1989 ‘shooting gallery’ approach by Kaplan. 相似文献
100.
A Yoshimura KR Middleton C Zhu VN Nemykin VV Zhdankin 《Angewandte Chemie (International ed. in English)》2012,51(32):8059-8062
Look, no metal: A metal-free catalytic procedure for aziridination of alkenes using tetrabutylammonium iodide as the catalyst, m-chloroperoxybenzoic acid (mCPBA) as the terminal oxidant, and N-aminophthalimide as the nitrenium precursor has been developed (see scheme; right: X-ray structure of one of the products). Control experiments suggests that the active oxidant is in?situ generated hypoiodous acid (HIO). 相似文献