排序方式: 共有33条查询结果,搜索用时 241 毫秒
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Weber F Rosenkranz S Castellan JP Osborn R Hott R Heid R Bohnen KP Egami T Said AH Reznik D 《Physical review letters》2011,107(10):107403
We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound 2H-NbSe2. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wave vector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments, combined with ab initio calculations, show that it is the wave vector dependence of the electron-phonon coupling that drives the CDW formation in 2H-NbSe2 and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in 2H-NbSe2 and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important. 相似文献
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Jean-Pierre Desvergne Alain Castellan Henri Bouas-Laurent 《Tetrahedron letters》1981,22(36):3529-3532
New dissymetrical non conjugated bichromophores linked by an oxygenated chain: (9-anthrylmethyloxymethyl)arenes[arenes:benzene(Ia),2-naphthalene(Ib),9-phenanthrene (Ic) and 1-pyrene (Id) and (9-phenanthrylmethyloxymethyl)-1-pyrene (II)] have been prepared. Ic–d and II exhibit intramolecular exciplex fluorescence in methylcyclohexane at room temperature. Intramolecular photoadducts were not found but Ia–d yield a mixture of head-to-head and head-to-tail anthracenic photodimers. 相似文献
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C. Noutary P. Fornier de Violet J. Vercauteren A. Castellan 《Research on Chemical Intermediates》1995,21(3-5):223-245
The contribution of phenols, absorbing above 300 nm, to the photodegradation of lignin was approached by studying a phenolic phenylcoumarone lignin model molecule PCO. The compound was irradiated in solution and adsorbed on filter paper. The isolation and analysis of the photoproducts formed during the irradiation indicate the presence of a catechol structure (involving demethylation of the starting material) and two dimeric stilbene compounds: a ketone and a conjugated quinone methide giving high colouration to the irradiated solution. Those compounds have been detected as well in the solid state. The structure elucidation of the photoproducts was mainly based on mass spectrometry and 2D NMR experiments at 500 MHz. The major role played by the phenol group was supported by studying the O-methylated analogue PCOMe. 相似文献
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The fluorescence of acenaphthylene follows a Stern-Volmer relationship in air-saturated ether giving a self-quenching constant KF=0.12M?1, indicating that the syn photodimer originates from the singlet state of acenaphthylene. By comparison, 1-cyanoacenaphthylene undergoes a more efficient self-quenching (KF= 2.8 M?1) in air-saturated ether. No excimer fluorescence was detected for 1-cyanoacenaphthylene nor the parent compound in solution. The triplet state of 1-cyanoacenaphthylene, obtained by sensitization or induced by heavy atom solvent (EtI), was shown to generate exclusively the head-to-head anti photodimer in a high chemical yield. Regiospecificity and stereospecificity observed in this reaction indicates the influence of the acenaphthylenic ring and the cyano group in stabilizing the diradicaloïd transition state. 相似文献
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Castellan JP Rosenkranz S Goremychkin EA Chung DY Todorov IS Kanatzidis MG Eremin I Knolle J Chubukov AV Maiti S Norman MR Weber F Claus H Guidi T Bewley RI Osborn R 《Physical review letters》2011,107(17):177003
We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations. 相似文献