Ab initio determination of exchange integrals and Neel temperature in the chain cuprates

We report ab initio quantum chemical cluster calculations of the chain (J_a) and the largest interchain (J_b) Heisenberg exchange of the chain cuprates Ca_2CuO_3 and Sr_2CuO_3. We find that J_a is comparable to the in-plane J in layered cuprates and J_a/J_b ~250-400. Using recent theory we obtain close agreement with experiment for the staggered moments and the critical temperatures. This implies that T_N does not depend on the third parameter J_c << J_b, and cannot be calculated using spin-wave theory. We propose an explanation for this interms of a 1D->2D cross-over.Comment: ps, 19 pages. To appear in Chemical Physics Letter

Heisenberg exchange enhancement by orbital relaxation in cuprate compounds

We calculate the Heisenberg exchange J in the quasi-2D antiferromagnetic cuprates La2CuO4, YBa2Cu3O6, Nd2CuO4 and Sr2CuO2Cl2. We apply all-electron (MC)SCF and non-orthogonal CI calculations to [Cu2O11]18-, [Cu2O9]14-, [Cu2O7]10- and [Cu2O7Cl4]14- clusters in a model charge embedding. The (MC)SCF triplet and singlet ground states are well characterized by Cu2+ (dx2-y2) and O2-. The antiferromagnetic exchange is strongly enhanced by admixing relaxed (MC)SCF triplet and singlet excited states, in which a single electron is transferred from the central O ion to Cu. We ascribe this effect to orbital relaxation in the charge transfer component of the wave function. Close agreement with experiment is obtained.Comment: publishe

Heisenberg Exchange in La2CuO4

We study the exchange interaction in the quasi-2D antiferromagnetic high Tc superconductor parent compound La2CuO4 by all-electron, embedded cluster methods. Our material model is the cluster Cu2O11, embedded in a matrix of point charges. The SCF ground state configuration is characterized by Cu2+ and O2- with the d-hole oriented in the [CuO2]2- plane along the Cu-O bonds. We admix to this an excited SCF configuration that has one Cu-hole transferred to the central O into the ground state configuration by nonorthogonal CI, while paying special attention to the variational balance of the singlet and triplet states. This results in a value for the exchange parameter of J = -120 meV, in good agreement with the experimental value of J = -128 meV.

Cluster Calculations on Localized Holes in La2CuO4

The electronic structure of [CuO2]2- sheets, as occur in La2CuO4, is studied by ab initio non-orthogonal configuration interaction (NOCI) calculations on the small copper-oxide clusters CuO4 and Cu2O7. In the NOCI approach the wave function is obtained from CI calculations between relaxed Hartree-Fock states. With this approach important local relaxation effects, as well as charge transfer interactions can be taken into account. The cluster ground states can be characterized by Cu(d9) O(p6). States with one extrinsic hole have predominant Cu(d9) O(p5) character with admixture of Cu(d8) O(p6). This admixture is especially important for states with the extrinsic hole mainly in O(pσ).

Habitat selection of brood-rearing Northern Wheatears Oenanthe oenanthe and their invertebrate prey

Birds consider both variation in prey abundance and accessibility in their decision of where to forage. Acidification and nitrogen deposition affect both prey abundance and accessibility by stimulating growth of nitrophilic grasses at the expense of forbs. Management practises such as mowing or grazing primarily affect vegetation structure which also influences the abundance and accessibility of invertebrates. Hence, for effective management and conservation purposes It is paramount to understand the relationships between vegetation structure, densities of preferred prey and habitat-use of birds. In this study we explore such relationships for the nationally endangered Northern Wheatear Oenanthe oenanthe in dune grasslands along the Dutch coast. Our findings support the hypothesis that forager mobility and food accessibility are of greater importance during patch selection than food abundance per se in ground foraging birds. The abundance of all potential prey and three of the four most important actual prey groups was highest in tall grass, but Northern Wheatears foraged preferentially in short grass. Clearly, encroachment by tall grass will diminish habitat suitability for Northern Wheatears due to lowered prey accessibility. We propose that a mixture of short and tall vegetation and landscape management allows for dispersal of arthropods between different (micro)habitats. We provide densities of the important prey in a coastal area where Northern Wheatears still successfully breed. This enables site-managers to efficiently investigate presence and abundance of important prey In seemingly suitable areas but where Northern Wheatears do not breed. Eventually we may be able to discern whether food shortage poses a bottleneck for Northern Wheatears in these uninhabited areas

The Fine Arts in Architecture: Creation of the WKU College of Fine Arts

Over the years, the fine arts have served as the embodiment of a cultures beliefs and practices. From Art and Music to Language and Communications, the fine arts have shaped the way we interact and live our daily lives. Architecture is considered one of the many fine arts. Architecture has served as a physical monument of the representation of these ideals. Many artistic principles such as rhythm and repetition are prevalent in the detail orientation of architecture, and other principles such as performance and expression, showcase the beauty behind design that influences people’s thoughts and feelings. This research will establish how architecture is the full embodiment of all the fine arts and how the arts work to influence and provoke thoughts of creativity and individualism in a modern society. With the increasing emphasis placed on STEM career fields, this project will examine how the incorporation of the humanities can aid in the development and overall success of all individuals. The research will focus on how the established principles can be applied to WKU with the culmination of the research leading to the development of the WKU College of Fine Arts. This newly designed fine arts center will encourage and facilitate the growth of the fine arts and serve as the embodiment of the philosophy established in this project

Multicellular mathematical modelling of mesendoderm formation in amphibians

The earliest cell fate decisions in a developing embryo are those associated with establishing the germ layers. The specification of the mesoderm and endoderm is of particular interest as the mesoderm is induced from the endoderm, potentially from an underlying bipotential group of cells, the mesendoderm. Mesendoderm formation has been well studied in an amphibian model frog, Xenopus laevis, and its formation is driven by a gene regulatory network (GRN) induced by maternal factors deposited in the egg. We have recently demonstrated that the axolotl, a urodele amphibian, utilises a different topology in its GRN to specify the mesendoderm. In this paper, we develop spatially structured mathematical models of the GRNs governing mesendoderm formation in a line of cells. We explore several versions of the model of mesendoderm formation in both Xenopus and the axolotl, incorporating the key differences between these two systems. Model simulations are able to reproduce known experimental data, such as Nodal expression domains in Xenopus, and also make predictions about how the positional information derived from maternal factors may be interpreted to drive cell fate decisions. We find that whilst cell–cell signalling plays a minor role in Xenopus, it is crucial for correct patterning domains in axolotl