A supramolecular assembly containing an unusually short n‐h…n hydrogen bond ‐ an x‐ray and neutron diffraction study

A supramolecular assembly formed between phthalimide and 2‐guanidinobenzimidazole, containing a short 2.692(4)AR N‐H…N hydrogen bond, is reported. The crystal structure of this species was determined by both X‐ray and neutron diffraction. The diffraction data reveal that the proton involved in the short hydrogen bond has been transferred from the phthalimide to the guanidinobenzimidazole to form an ion pair. There is also an interesting stacking interaction between the atoms involved in the short hydrogen bond and the π system of a phthalimide molecule that is approximately 3.3 Å away. The structure is compared with the structure of a similar assembly formed between 4‐nitrophthalimide and 2‐guanidinobenzimidazole.     

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Polaronic origin of the isotope effect on the London penetration depth in high-temperature superconducting oxides

Direct measurements of the in-plane London penetration depth λ _L have recently been performed on high-temperature superconducting copper oxides by a new low-energy muon spin rotation technique. The results show that λ _L is isotope dependent, evidencing unconventional electron–phonon interactions as its source. The data are interpreted here in terms of polaronic effects on the single-particle energies, which leads to level shifts and exponential band narrowing. Good agreement with the experimental data is obtained.     

Charged pion spectra and energy transfer following antiproton annihilation at rest in carbon and uranium

The momentum spectra of charged pions following antiproton annihilation at rest in carbon and uranium have been measured. This information complements our previous measurement of the neutral pion spectra. The total charged pion multiplicity is 2.84±0.10 and 2.47±0.09 for carbon and uranium, respectively, in good agreement with recent INC model predictions of 2.96 and 2.48 for the same quantities. However, structures predicted by the model near 200 MeV/c and 300 MeV/c related to delta-resonance production are not seen in the data. The total energy transfers to the nucleus are calculated to be 119±59 MeV(carbon) and 455±50 MeV(uranium). The possibility of exciting multifragmentation with a¯p beam impinging on heavy nuclei is discussed.     

Search space pruning and global optimisation of multiple gravity assist spacecraft trajectories

We introduce and describe the Multiple Gravity Assist problem, a global optimisation problem that is of great interest in the design of spacecraft and their trajectories. We discuss its formalization and we show, in one particular problem instance, the performance of selected state of the art heuristic global optimisation algorithms. A deterministic search space pruning algorithm is then developed and its polynomial time and space complexity derived. The algorithm is shown to achieve search space reductions of greater than six orders of magnitude, thus reducing significantly the complexity of the subsequent optimisation. This work was partially funded under the Ariadna scheme of the European Space Agency, contract number 18138/04/NL/MV.     

Harmonic measure,L 2-estimates and the Schwarzian derivative

We consider several results, each of which uses some type of “L ²” estimate to provide information about harmonic measure on planar domains. The first gives an a.e. characterization of tangent points of a curve in terms of a certain geometric square function. Our next result is anL ^p estimate relating the derivative of a conformal mapping to its Schwarzian derivative. One consequence of this is an estimate on harmonic measure generalizing Lavrentiev’s estimate for rectifiable domains. Finally, we considerL ² estimates for Schwarzian derivatives and the question of when a Riemann mapping ϕ has log ϕ′ in BMO. Supported in part by NSF Grant DMS-91-00671. Supported in part by NSF Grant DMS-86-025000.     

Spin-dependent resistivity and quantum Hall ferromagnetism in two-dimensional electrons confined to AlAs quantum wells

We observe a strong dependence of the amplitude and field position of longitudinal resistivity (ρ_xx) peaks in the spin-resolved integer quantum Hall regime on the spin orientation of the Landau level (LL) in which the Fermi energy resides. The amplitude of a given peak is maximal when the partially filled LL has the same spin as the lowest LL, and amplitude changes as large as an order of magnitude are observed as the sample is tilted in field. In addition, the field position of both the ρ_xx peaks and plateau–plateau transitions in the Hall resistance shift depending on the spin orientation of the LLs. The spin dependence of the resistivity points to a new explanation for resistivity spikes, associated with first-order quantum Hall ferromagnetic transitions, that occur at the edges of quantum Hall states.     

Cold tolerance and overwintering of an introduced New Zealand frog, the brown tree frog (Litoria ewingii)

The overwintering strategy of Litoria ewingii in Otago, New Zealand, was studied under laboratory and field conditions. Microhabitat temperature measurements showed that the frogs were often exposed to subzero temperatures. In the laboratory, Litoria ewingii tolerated freezing for up to 6 hrs at -1 degrees C, and after the completion of the freezing event (about 1 hr) at -2 degrees C. Frogs frozen with insulation survived freezing for 12 hrs at -1 degrees C. Frogs supercooled to -1.2 +/- 0.1 degrees C and -1.7 +/- 0.3 degrees C on wet and dry substrates respectively. L. Ewingii tolerated up to 47.5% of its body water frozen. Plasma glucose levels and osmolality were not increased during freezing. It is concluded that l. Ewingii cannot avoid freezing and is sufficiently freeze tolerant to survive the subzero temperatures encountered during winter in Otago.