D. M. King

Graded allocation between vegetative and reproductive growth for annual plants in growing seasons of random length

Abstract Optimal allocation strategies are calculated for annual plants in fluctuating environments using a two-component model of plant growth, in which photosynthate is partitioned between a vegetative and a reproductive component. Previous studies have shown that final reproductive yield is maximized by a sequence of complete switches from purely vegetative to purely reproductive growth in an environment of fixed length. In most cases this final yield is maximized by a single switch. Here we assume that in temporally varying environments natural selection acts to maximize the geometric mean of final yield. We show that the geometric mean of final yield is maximized by a graded allocation strategy that prescribes a mix of vegetative and reproductive growth. Examples of graded optimal allocation strategies are provided.

Photoemission Studies of High-Tc Superconductors: The Superconducting Gap

Over the last several years there have been great improvements in the energy resolution and detection efficiency of angle-resolved photoemission spectroscopy. These improvements have made it possible to discover a number of fascinating features in the electronic structure of the high transition temperature (Tc) superconductors: apparently bandlike Fermi surfaces, flat-band saddle points, and nested Fermi surface sections. Recent work suggests that these features, previously thought explainable only by one-electron band theory, may be better understood with a many-body approach. Furthermore, other properties of the high-Tc superconductors, which are difficult to understand with band theory, are well described using a many-body picture. Angle-resolved photoemission spectroscopy has also been used to investigate the nature of the superconducting pairing state, revealing an anisotropic gap consistent with a d-wave order parameter and fueling the current debate over s-wave versus d-wave superconductivity.

Multiple switches between vegetative and reproductive growth in annual plants

Abstract A model of growth and reproduction in annual plants was developed by Cohen (1971, J. Theor. Biol. 33 , 299–307) to determine the allocation strategy which maximizes seed yield. The model divides the plant into vegetative and reproductive parts and predicts that yield is maximized by a strategy consisting of a switch from purely vegetative to strictly reproductive growth. We generalize Cohens model to include vegetative and reproductive loss terms. Both growth and loss rates are allowed to vary with time. Using optimal control theory we find that seed yield is maximized by a strategy consisting of multiple switches between vegetative and reproductive growth, for certain ranges of the model parameters. In natural systems a predictable vegetative loss burst may be necessary to promote multiple switches.

Energy Allocation Patterns of the California Grassland Annuals Plantago Erecta and Clarkia Rubicunda

Allocation models for annual plants predict that reproductive yield is maximized by an instantaneous switch from vegetative to reproductive growth. The switch is timed so that final reproductive yield is equal to the vegetative mass at the time of the switch, provided vegetative and reproductive growth efficiencies are equal. Measured allocation patterns for Plantago erecta and Clarkia rubicunda show a gradual shift from vegetative to reproductive growth in both species. This graded pattern results in a 5.5% decrease in reproductive yield for Plantago relative to the theoretical maximum. The timing of that graded pattern appears optimal.

Electronic structure evolution from Mott insulator to superconductor : an angle-resolved photoemission investigation

Abstract A critical analysis of new and earlier experimental data from the cuprate superconductors, in light of recent advances in many-body theory, shows that effects previously thought to be explained by band theory alone (such as the large Fermi surface) may also be accounted for by an idealized many-body approach. In addition, there are many elements of the data which can be better described using many-body approaches. Therefore, it appears that a comprehensive many-body description is needed to understand the electronic structure of the cuprate superconductors in the normal state. Angle-resolved photoemission has also revealed a large gap anisotropy, fueling the current debate on the symmetry of the pairing state

Recent results from Bi2Sr2CaCu2O8 and Nd2-xCexCuO4

Abstract We review recent angle-resolved photoemission data from Bi2212 and NCCO. We concentrate on the data revealing the charge excitations near the Fermi level, the shape of the Fermi surfaces and the anisotropic nature of the superconducting gap.

Band mapping of the model insulator Sr2CuO2Cl2—Dispersion of a single hole in an antiferromagnetic background

Abstract We report angle resolved photoemission measurements on Sr 2 CuO 2 Cl 2 , an insulating layered copper oxide. We map the energy-momentum dispersion of the highest energy band. We find that the width of this band is consistent with calculations based on variations of the Hubbard model. The dispersion in some directions and the position of the valence band maximum at ( π 2 , π 2 ) match predictions of the t − J model. However the overall shape of the band differs from these predictions. A comparison of our data with spectra previously reported on metallic samples leads to surprising new suggestions for the phenomenology of doping.

Clustering of Metals on Semiconductors Surfaces: Relation to Metallicity, Surface Diffusion, Growth Modes and Schottky Barriers

Photoelectron spectroscopy (PES) has been one of the primary techniques with which Schottky barrier formation on III-V semiconductor substrates has been measured and correlated with overlayer morphology and chemistry. One important class of overlayers is the unreactive materials such as In, Ga and Ag, which cluster on III-V substrates kept at room temperature. Partially reactive materials such as Al and Au have also been observed to cluster on these substrates. Clustered systems have a nonuniform distribution of pinning sites on the surface, and as a result, nonuniformities in the surface potential are also anticipated.1 Such nonuniform surface potentials complicate the interpretation of band bending measurements from these systems. It is the purpose of this paper to describe a model treating the clustering problem.

Photoemission Study of Single Crystal C 60

We report angle-resolved photoemission data from single crystals of C 60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C 60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C 60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C 60 . We assert that the width of the photoemission peak of C 60 does not reflectthe intrinsic band width.