Fluctuations of g factors in metal nanoparticles: effects of electron-electron interaction and spin-orbit scattering

We investigate the combined effect of spin-orbit scattering and electron-electron interactions on the probability distribution of g factors of metal nanoparticles. Using random matrix theory, we find that even a relatively small interaction strength significantly increases g-factor fluctuations for not-too-strong spin-orbit scattering (ratio of spin-orbit rate and single-electron level spacing 1/tau(so)delta < or near 1), and leads to the possibility to observe g factors larger than 2.     

An efficient peak assignment algorithm for two-dimensional NMR correlation spectra of framework structures

An algorithm is described for efficiently assigning the resonances in NMR spectra to the inequivalent atoms in the structure under study based on the information in two-dimensional NMR correlation experiments and the 'connectivities' known from the structure. The algorithm, which is based on basic graph theory concepts, finds all possible assignments sets which are consistent with the experimentally observed correlations and known connectivities in a very efficient manner. It is designed to deal with less than ideal experimental data in which there may be overlapping peaks and uncertainty about the presence or absence of correlation peaks. The algorithm was primarily developed for assigning the peaks in the high-resolution solid-state 29Si MAS NMR spectra of highly siliceous zeolites based on two-dimensional 29Si INADEQUATE spectra and is described using the zeolites ZSM-12 and ZSM-5 as working examples. Peak assignment for zeolite frameworks is particularly challenging since there is often little or no information to distinguish peaks from one another such as characteristic chemical shifts, relative intensities, or different relaxation times. The algorithm may be a useful tool for easily, reliably, and efficiently working out peak assignments from other types of correlation experiments on other types of systems and further examples are provided in the Supplementary material.     

Ehrenfest-time dependence of weak localization in open quantum dots

Semiclassical theory predicts that the weak localization correction to the conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys. Rev. B 54, 14423 (1996)]. We report numerical simulations of weak localization in the open quantum kicked rotator that confirm this prediction. Our results disagree with the "effective random matrix theory" of transport through ballistic chaotic cavities.     

Bound on anisotropy in itinerant ferromagnets from random impurities

We calculate the anisotropy energy of a single-domain ferromagnetic particle in which the only source of anisotropy is the presence of nonmagnetic impurities. Such anisotropy has easy-axis and easy-plane contributions, with random orientations of the axes. Typically the anisotropy energy is of order N1/2plankv/tau(so), where N is the number of electrons in the ferromagnetic particle and tau(so) is the spin-orbit time.     

Notes on binary codes related to the O(5, q) generalized quadrangle for odd q

In this note we determine the dimensions of the binary codes spanned by the lines or by the point neighborhoods in the generalized quadrangle Sp(4, q) and its dual O(5, q), where q is odd. Several more general results are given. As a side result we find that if a square generalized quadrangle of odd order has an antiregular point, then all of its points are antiregular.On leave from the Indian Statistical Institute, Calcutta; research supported by a grant from NWO.     

Chain length effects in the competition between intramolecular hydrogen migrations in the molecular ions of hydrogen migration in the molecular ions of α-benzylamino-ω-benzyloxyalkanes as studied by deuterium labelling

The unimolecular decompositions upon electron impact of the bifunctional compounds ØCH₂-NH? (CH₂)_n-O? CH₂Ø(n=2 to 7)have been studied by deuterium labelling. The molecular ions lose a benzyl radical and/or a benzaldehyde molecule, depending on the length of the aliphatic chain. Both reactions appear to be the result of the transfer of a hydrogen atom to the charge localized amino function in a cyclic transition state. The ring size of this transition state turns out to play a far more important role than the difference in reactivity of the migrating hydrogen atoms.     

Synthesis of cyclic peptidosulfonamides by ring-closing metathesis

N-Protected beta-aminoethanesulfonyl chlorides (2a-e) were used in the preparation of sulfonamides 4, 8, 11a-c, and 15. Ring-closing metathesis of sulfonamides 4 and 8 did not lead to the expected nine-membered cyclic peptidosulfonamides. In contrast, the allylated peptidosulfonamides 11a-c and 15 turned out to be suitable precursor systems for ring-closing metathesis using second-generation Grubbs catalyst and nine-membered cyclic peptidosulfonamides were obtained in 47-60% yields. The possibility for incorporation of these cyclic peptidosulfonamides into a peptide sequence was illustrated by the incorporation of an amino acid on the "S"- or "N"-terminus leading to 16 and 18-20, respectively. A model of cyclic peptidosulfonamide 16 hints at an extended-like structure.     

Fano resonances as a probe of phase coherence in quantum dots

In the presence of direct trajectories connecting source and drain contacts, the conductance of a quantum dot may exhibit resonances of the Fano type. Since Fano resonances result from the interference of two transmission pathways, their line shape (as described by the Fano parameter q) is sensitive to dephasing in the quantum dot. We show that under certain circumstances the dephasing time can be extracted from a measurement of q for a single resonance. We also show that q fluctuates from level to level, and we calculate its probability distribution for a chaotic quantum dot. Our results are relevant to recent experiments by G?res et al. [Phys. Rev. B 62, 2188 (2000)].