ВЛИЯНИЕ ПРЕДВАРИТЕЛ ЬНОГО ОСВЕЩЕНИЯ ha ХАРАКТЕРИСТИКИ МЕДН ОЗАКИСНЫХ ВЫПРЯМИТЕ ЛЕЙ

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ВЛИЯНИЕ ПРЕДВАРИТЕЛ ЬНОГО ОСВЕЩЕНИЯ НА ПР ОВОДИМОСТЬ И ФОТОПРОВОДИМОСТЬ З АКИСИ МЕДИ

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The effect of preliminary illumination on conductivity and photoconductivity of cuprous oxide

     

End-to-end diffusion coefficients and distance distributions from fluorescence energy transfer measurements: Enhanced resolution by using multiple donors with different lifetimes

We describe a method to improve the resolution of donor-to-acceptor distance distributions in molecules which are flexing on the timescale of the fluorescence lifetime. We measured the timedependent donor decays of two donor (D)-acceptor (A) pairs, where the donor lifetimes were substantially different. The donors were an indole residue (5.7 ns) and a naphthalene residue (24.4 ns). The same dansyl acceptor was used for both D-A pairs. The donor decays are complex due to both a distribution of D-A distances and D-A diffusion. Using the donor decay data for each D-A pair alone, it is difficult to resolve both the distance distribution and the D-to-A diffusion coefficient. However, these values are unambiguously recovered from global analysis of the data from both D-A pairs. The increased resolution from the global analysis is apparently the result of the complementary information content of the data for each D-A pair. The shorter-lived indole donor provides more information on the time-zero distance distribution because there is less time for D-A diffusion, and the longer-lived naphthyl donor is quenched to a greater extent than indole due to the longer time for diffusion-enhanced energy transfer. Simulations were also used to demonstrate the increased resolution of global analysis with different lifetime donors to obtain distance distribution parameters in the presence of D-A diffusion.     

On Abelian Branched Coverings of the Sphere

 We obtain an enumeration formula for the number of weak equivalence classes of the branched (?×ℬ)-covering of the sphere with m-branch points, when ? and ℬ are finite abelian groups with (|?|,|ℬ|)=1. From this, we can deduce an explicit formula for enumerating the weak equivalence classes of pseudofree spherical (ℤ _p ×ℤ _q )-actions on a given surface, when p and q are distinct primes. Received: August 10, 1999 Final version received: June 19, 2000     

Low-energy charge-density excitations in MgB2: Striking interplay between single-particle and collective behavior for large momenta

A sharp feature in the charge-density excitation spectra of single-crystal MgB2, displaying a remarkable cosinelike, periodic energy dispersion with momentum transfer (q) along the c* axis, has been observed for the first time by high-resolution nonresonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-q collective mode residing in the single-particle excitation gap of the B pi bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB2.     

Efficient algorithms for a constrained k-tree core problem in a tree network

Let T=(V,E) be a free tree in which each vertex has a weight and each edge has a length. Let n=|V|. Given T and parameters k and l, a (k,l)-tree core is a subtree X of T with diameter l, having k leaves, which minimizes the sum of the weighted distances from all vertices in T to X. In this paper, two efficient algorithms are presented for finding a (k,l)-tree core of T. The first algorithm has O(n²) time complexity for the case that each edge has an arbitrary length. The second algorithm has O(lkn) time complexity for the case that the lengths of all edges are 1. The (k,l)-tree core problem has an application in distributed database systems.     

Room-temperature ferromagnetism of Cu-doped ZnO films probed by soft X-ray magnetic circular dichroism

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu2+ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V(O)) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic "indirect double-exchange" model, in which alignments of localized large moments of Cu in the vicinity of the V(O) are mediated by the large-sized vacancy orbitals.