Coherent transport in a two-electron quantum dot molecule

We investigate the dynamics of two interacting electrons confined to a pair of coupled quantum dots driven by an external AC field. By numerically integrating the two-electron Schroedinger equation in time, we find that for certain values of the strength and frequency of the AC field we can cause the electrons to be localised within the same dot, in spite of the Coulomb repulsion between them. Reducing the system to an effective two-site model of Hubbard type and applying Floquet theory leads to a detailed understanding of this effect. This demonstrates the possibility of using appropriate AC fields to manipulate entangled states in mesoscopic devices on extremely short timescales, which is an essential component of practical schemes for quantum information processing.Comment: 4 pages, 3 figures; the section dealing with the perturbative treatment of the Floquet states has been substantially expanded to make it easier to follo

Mott Transition in Degenerate Hubbard Models: Application to Doped Fullerenes

The Mott-Hubbard transition is studied for a Hubbard model with orbital degeneracy N, using a diffusion Monte-Carlo method. Based on general arguments, we conjecture that the Mott-Hubbard transition takes place for U/W \propto \sqrt{N}, where U is the Coulomb interaction and W is the band width. This is supported by exact diagonalization and Monte-Carlo calculations. Realistic parameters for the doped fullerenes lead to the conclusion that stoichiometric A_3 C_60 (A=K, Rb) are near the Mott-Hubbard transition, in a correlated metallic state.Comment: 4 pages, revtex, 1 eps figure included, to be published in Phys.Rev.B Rapid Com

Metabolic changes during he transition period

We used four ruminally fistulated, multiparous, pregnant Holstein cows to measure changes in concentrations of plasma metabolite as the dairy cow transitions from one lactation to the next. Diets consisted of typical far-off and close-up diets, a late lactation diet containing wet corn gluten feed (20% DM), and an alfalfa hay-corn silage based early lactation diet. Calculated NEL (Mcal/lb), measured crude protein (%), and diet digestibilities (%; based on steers fed at 2% of BW) were 0.78, 18.7, 74.1; 0.70, 11.5, 66.2; 0.74, 15.6, 71.0; 0.73, 18.4, 70.7 for late lactation, far-off dry, close-up dry, and early lactation diets, respectively. Blood samples were obtained on day 79 prior to calving and weekly thereafter until calving and on days 1, 3, 5, 7, 15, 20, 25, 30, 60, and 90 after calving. Cows gained body weight and condition during the dry period, peaked just prior to calving, and lost weight and condition steadily through the first 11 weeks of lactation. Calculated energy balance was negative during the first 3 weeks of lactation. Plasma concentrations of non-esterified fatty acids (NEFA), glucose, and insulin to glucagon ratio remained fairly stable during the dry period. Plasma glucose increased just before calving, decreased markedly during early lactation, then increased and stabilized by day 30 of lactation. Plasma NEFA concentrations increased at calving and were elevated during early lactation, then returned to prepartum concentrations by day 30 of lactation. The insulin to glucagon ratio decreased just prior to calving, continued to decrease until day 7 of lactation, and then remained stable until the end of the trial. Changes in diet and intake affected plasma urea nitrogen, which decreased as dietary protein decreased during the far-off period, decreased with intake during the close-up period, and increased after calving consistent with the higher dietary protein and increase in dry matter intake. Most of the observed metabolic adaptations reflected the energy status of the cow with large shifts occurring around parturition. Certainly, some of the hormones associated with calving can initiate metabolic events favorable to lactation, but the changes in energy balance and nutrient supply support the continued diversion of nutrients to the mammary gland. These data support the concept that dairy cows experience a period of increased tissue mobilization from approximately 2 days prior to calving until 30 days after calving. In conclusion, a number of metabolic adaptations occur in transition dairy cows that provide clues to improve feeding and management guidelines

Characteristic changes of ruminal fermentation in transition dairy cows

Four-ruminally fistulated, multiparous, pregnant Holstein cows were used to delineate changes in ruminal fermentation in dairy cows as they experienced the transition from one lactation to the next. Diets consisted of typical far-off and close-up diets, a late lactation diet containing wet corn gluten feed (20% DM) and an alfalfa hay-corn silage based early lactation diet. Calculated NEL (Mcal/lb), measured crude protein (%), and diet digestibilities (%; based on steers fed at 2% of BW) were: 0.78, 18.7, 74.1; 0.70, 11.5, 66.2; 0.74, 15.6, 71.0; 0.73, 18.4, 70.7 for late lactation, far-off dry, close-up dry, and early lactation diets, respectively. Ruminal measurements were taken on days 72 (late lactation), 51 (faroff), 23, and 9 (close-up dry) before calving and on days 6, 20, 34, 48, 62, 76, and 90 days after calving. Ruminal samples were collected at hours 0, 3, 6, 9, and 12 after feeding on each sampling date. Major shifts in ruminal fermentations occurred when the close-up diet was consumed before calving and in concert with an increase in DM intake during the first 48 days of lactation. Dry matter digestibility increased after cows were switched to the close-up diet and continued this trend through day 6 postpartum. Ruminal pH decreased and total volatile fatty acids, peptides, and free amino acids increased after cows were switched to the early lactation diet. These data support the concept that alterations in ruminal fermentation reflect changes in both diet and intake

Changes in ruminal microbial populations in transition dairy cows

We used four ruminally fistulated, multiparous, pregnant Holstein cows to delineate microbial adaptations in dairy cows as they experienced the transition from one lactation to the next. Diets consisted of typical far-off and close-up diets, a late lactation diet containing wet corn gluten feed (20% DM) and an alfalfa hay-corn silage based early lactation diet. Calculated NEL (Mcal/lb), measured crude protein (%), and diet digestibilities (%; based on steers fed at 2% of BW) were: 0.78, 18.7, 74.1; 0.70, 11.5, 66.2; 0.74, 15.6, 71.0; 0.73, 18.4, 70.7 for late lactation, far-off dry, close-up dry, and early lactation, respectively. Microbial samples were obtained on days 72 (late lactation), 51 (far-off dry), 23, and 9 (close-up dry) prepartum and days 6, 20, 34, 48, 62, 76, and 90 postpartum. We analyzed ruminal samples for ciliated protozoa and viable counts of bacteria and fungi. Changing from a high forage to a high concentrate diet impacted bacterial counts less than ciliated protozoal and fungal counts. Switching diets from high concentrate to high forage increased ciliated protozoa and fungal counts, and counts decreased when diets were switched from high forage to high concentrate. Bacterial and ciliated protozoa counts increased in early lactation and decreased as cows approached peak dry matter intake. Dietary changes with the onset of lactation led to virtual disappearance of fungi from the rumen. In general, ruminal microbial populations of dairy cows respond to changes in diet and intake. Changes in diet affected populations of protozoa and fungi, whereas changes in intake affected populations of bacteria, protozoa, and fungi

Changes in rumen capacity of dairy cows during the periparturient period

Four-ruminally fistulated, multiparous, pregnant Holstein cows were studied to characterize ruminal adaptations during the transition from gestation to lactation. Cows were fed typical far-off and close-up diets, a late lactation diet containing wet corn gluten feed (20% DM), and an alfalfa hay, corn silage based early lactation diet. Ruminal measurements were obtained 72 (late lactation), 51 (far-off dry), 23 and 9 (close-up dry) days before expected parturition and 6, 20, and 34 days postpartum. Measurements included total fill, dry matter fill, fluid fill, and water-holding capacity of the rumen. Dry matter intake and milk production data were collected daily and body weight and body condition were determined weekly. Body weights and condition increased during the dry period, whereas intake as a percentage of body weight decreased. Ruminal water holding capacity, an indicator of rumen capacity, increased linearly from late lactation to 34 days postpartum. These data suggest rumen capacity is not the causative factor of intake depression in dairy cows during the final 3 wk of gestation