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961.
The study of ecological systems has generated deep interest in exploring the complexity of chaotic food chains. The role of chaos in ecosystems is not entirely understood. One approach to have a better comprehension of ecological chaos is by analyzing it in mathematical models of basic food chains. In this article it is considered a classical chaotic food chain model from the literature. We use the theory of symbolic dynamics to study the topological entropy and the parameter space ordering of kneading sequences associated with one-dimensional maps that reproduce significant aspects of the model dynamics. The topological entropy allows us to distinguish different chaotic states in some realistic system parameter region. Another numerical invariant is introduced in order to characterize isentropic dynamics. Studying a set of maps with the same topological entropy, we exhibit numerical results about the relation between the second topological invariant and each of the control parameters in consideration. This work provides an illustration of how our understanding of ecological models can be enhanced by the theory of symbolic dynamics. 相似文献
962.
We present a linear-optical implementation of a class of two-qubit partial SWAP gates for polarization states of photons. Different gate operations, including the SWAP and entangling sqrt[SWAP], can be obtained by changing a classical control parameter, namely, the path difference in the interferometer. Reconstruction of output states, full quantum process tomography, and an evaluation of entanglement of formation prove very good performance of the gates. 相似文献
963.
S. González-Pérez I. R. Martín F. Lahoz P. Haro-González J. Herreros 《Applied Physics A: Materials Science & Processing》2008,93(4):983-986
Local crystalline formation in erbium doped oxyfluoride glass has been obtained under a cw Argon laser irradiation up to 1.8
W pumping power. By exciting at 514 nm, the emission from 800 nm and 850 nm corresponding to the 4S3/2(2H11/2)→4I13/2 electronic transitions have been analyzed both inside and outside the irradiated area. The changes in the emission spectra
indicate that the high power Ar laser irradiation has resulted in a localized desvitrification process. The temperature dependence
of the fluorescence intensity ratio of the 800 nm and 850 nm emission bands has been used to determine the temperature of
the irradiated zone. Moreover, the average lifetime of the 4S3/2(2H11/2) thermalized levels have been measured as a function of the excitation spot position. An important decrease is observed at
the irradiated area. These results confirm that a localized cristalline phase has been created by the laser action. 相似文献
964.
965.
Gábor Kónya 《Journal of Russian Laser Research》2018,39(5):473-483
We consider a harmonic oscillator coupled to a Markovian environment, i.e. a thermal reservoir. We derive the Keldysh path integral and the corresponding lesser, greater, time-ordered, antitime-ordered, retarded, advanced, and Keldysh Green’s functions for this system. In order to give a conceptually clear derivation, we use the superoperator formalism and also define supercoherent states. The supercoherent states and the creation and annihilation superoperators provide a natural way to rewrite the master equation into a path integral. 相似文献
966.
967.
968.
Arthur H. G. David Pablo García-Cerezo Dr. Araceli G. Campaña Prof. Francisco Santoyo-González Dr. Victor Blanco 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(24):6170-6179
We report the application of the click Michael-type addition reaction to vinyl sulfone or vinyl sulfonate groups in the synthesis of rotaxanes through the threading-and-capping method. This methodology has proven to be efficient and versatile as it allowed the preparation of rotaxanes using template approaches based on different noncovalent interactions (i.e., donor-acceptor π–π interactions or hydrogen bonding) in yields of generally 60–80 % and up to 91 % aided by the mild conditions required (room temperature or 0 °C and a mild base such as Et3N or 4-(N,N-dimethylamino)pyridine (DMAP)). Furthermore, the use of vinyl sulfonate moieties, which are suitable motifs for coupling-and-decoupling (CAD) chemistry, implies another advantage because it allows the controlled chemical disassembly of the rotaxanes into their components through nucleophilic substitution of the sulfonates resulting from the capping step with a thiol under mild conditions (Cs2CO3 and room temperature). 相似文献
969.
Dr. Carlos Sánchez-Sánchez Dr. Thomas Dienel Dr. Adrien Nicolaï Dr. Neerav Kharche Dr. Liangbo Liang Colin Daniels Prof. Vincent Meunier Dr. Junzhi Liu Prof. Xinliang Feng Prof. Klaus Müllen Dr. Juan Ramón Sánchez-Valencia Dr. Oliver Gröning Dr. Pascal Ruffieux Prof. Roman Fasel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(52):12074-12082
A bottom up method for the synthesis of unique tetracene-based nanoribbons, which incorporate cyclobutadiene moieties as linkers between the acene segments, is reported. These structures were achieved through the formal [2+2] cycloaddition reaction of ortho-functionalized tetracene precursor monomers. The formation mechanism and the electronic and magnetic properties of these nanoribbons were comprehensively studied by means of a multitechnique approach. Ultra-high vacuum scanning tunneling microscopy showed the occurrence of metal-coordinated nanostructures at room temperature and their evolution into nanoribbons through formal [2+2] cycloaddition at 475 K. Frequency-shift non-contact atomic force microscopy images clearly proved the presence of bridging cyclobutadiene moieties upon covalent coupling of activated tetracene molecules. Insight into the electronic and vibrational properties of the so-formed ribbons was obtained by scanning tunneling microscopy, Raman spectroscopy, and theoretical calculations. Magnetic properties were addressed from a computational point of view, allowing us to propose promising candidates to magnetic acene-based ribbons incorporating four-membered rings. The reported findings will increase the understanding and availability of new graphene-based nanoribbons with high potential in future spintronics. 相似文献
970.
Katharina Zielke Ondřej Kováč Michael Winter Dr. Jiří Pospíšil Prof. Mario Waser 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(34):8163-8168
The first highly asymmetric catalytic synthesis of densely functionalized dihydrobenzofurans is reported, which starts from ortho-hydroxy-containing para-quinone methides. The reaction relies on an unprecedented formal [4+1]-annulation of these quinone methides with allenoates in the presence of a commercially available chiral phosphine catalyst. The chiral dihydrobenzofurans were obtained as single diastereomers in yields up to 90 % and with enantiomeric ratios up to 95:5. 相似文献