Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)

Time-resolved vibrational spectroscopy constitutes an invaluable experimental tool for monitoring hot-carrier induced surface reactions. However, the absence of a full understanding on the precise microscopic mechanisms causing the transient spectral changes has been limiting its applicability. Here we introduce a robust first-principles theoretical framework that successfully explains both the nonthermal frequency and linewidth changes of the CO internal stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct processes engender the changes: electron-hole pair excitations underlie the nonthermal frequency shifts, while electron-mediated vibrational mode coupling gives rise to linewidth changes. Furthermore, the origin and precise sequence of coupling events are finally identified.Comment: Article as accepted for publication in Physical Review Letters; 5 pages, 2 figures, 1 tabl

Pairing dynamics in strongly correlated superconductivity

Confirmation of the phononic origin of Cooper pair formation in superconductors came with the demonstration that the interaction was retarded and that the corresponding energy scales were associated with phonons. Using cellular dynamical mean-field theory for the two-dimensional Hubbard model, we identify such retardation effects in d-wave pairing and associate the corresponding energy scales with short-range spin fluctuations. We find which frequencies are relevant for pairing as a function of interaction strength and doping and show that the disappearance of superconductivity on the overdoped side coincides with the disappearance of the low energy feature in the antiferromagnetic fluctuations, as observed in neutron scattering experiments.Comment: LaTeX, 8 pages, 8 figure

Recent Migration Patterns in Rural and Small Town Canada

Community/Rural/Urban Development, Labor and Human Capital,

Effet de la dose de semis et de l’écartement entre les rangs sur l’interférence entre le Chenopodium album et le Triticum aestivum

Un essai répété 4 années a permis d'évaluer l'effet de la présence du chénopode (Chenopodium album) sur la productivité du blé (Triticum aestivum 'Max'). La céréale a été ensemencée à cinq doses de semis (100, 200,300, 400 et 500 grains viables m-2) et à deux écartements entre les rangs (10 et 18 cm). Une réduction de l'écartement entre les rangs a permis d'accroître les rendements en blé de 16 % en présence ou non du chénopode. La dose de semis est un facteur plus important que l'écartement entre les rangs. À une dose de semis de 100 grains m-2, la présence du chénopode a causé une réduction du rendement en grain de 23 % tandis que cette réduction était de 11 % avec une dose de semis de 500 grains m-2. La baisse de rendement en blé provoquée par la présence de chénopode est surtout attribuable à une réduction du nombre d'épis par unité de surface. Le chénopode a également réduit le nombre d'épis par plant et le nombre de grains par épi tandis que le poids de 1000 grains et l'indice de récolte ont été peu affectés. Les contenus en azote et en phosphore du grain de blé ont été réduits par la présence du chénopode. Une réduction de l'écartement entre les rangs ou une augmentation de la dose de semis du blé n'a pas permis de réduire le nombre de plants mais a réduit la biomasse du chénopode.A 4-yr study was conducted to evaluate the effect of two row spacings (10 and 18 cm) and five seeding rates (100, 200, 300, 400, and 500 viable seeds m-2) of wheat (Triticum aestivum 'Max') on the interference between lamb's-quarters (Chenopodium album) and the crop. Narrow row spacing increased wheat yield by 16% regardless of the presence or absence of lamb's-quarters. Seeding rate was a more important factor than row spacing. At a seeding rate of 100 seeds m-2, the yield reduction caused by lamb's-quarters was 23% whereas it was 11 % when wheat was seeded at 500 seeds m-2. Yield reductions were attributed to a decrease in the number of spikes m-2. Lamb's-quarters also reduced the number of spikes per plant and the number of kernels per spike, and, to a lesser extent, the 1000 kernel weight and the harvest index. Nitrogen and phosphorus content of wheat grain were reduced by the presence of Chenopodium. High seeding rates or narrow row spacing did not reduce the density of lamb's-quarters but did reduce the weed biomass