On conformally flat circle bundles over surfaces

We study surface groups $\Gamma$ in $SO(4,1)$, which is the group of Mobius tranformations of $S^3$, and also the group of isometries of $\mathbb{H}^4$. We consider such $\Gamma$ so that its limit set $\Lambda_\Gamma$ is a quasi-circle in $S^3$, and so that the quotient $(S^3 - \Lambda_\Gamma) / \Gamma$ is a circle bundle over a surface. This circle bundle is said to be conformally flat, and our main goal is to discover how twisted such bundle may be by establishing a bound on its Euler number. By combinatorial approaches, we have two soft bounds in this direction on certain types of nice structures. In this article we also construct new examples, a "grafting" type path in the space of surface group representations into $SO(4,1)$: starting inside the quasi-Fuschsian locus, going through non-discrete territory and back.Comment: 28 pages, 7 figures. Updated from Thesis version: more correct bound of (3/2)n^2, updated exposition in section 3.

Commuting-projector Hamiltonians for chiral topological phases built from parafermions

We introduce a family of commuting-projector Hamiltonians whose degrees of freedom involve $\mathbb{Z}_{3}$ parafermion zero modes residing in a parent fractional-quantum-Hall fluid. The two simplest models in this family emerge from dressing Ising-paramagnet and toric-code spin models with parafermions; we study their edge properties, anyonic excitations, and ground-state degeneracy. We show that the first model realizes a symmetry-enriched topological phase (SET) for which $\mathbb{Z}_2$ spin-flip symmetry from the Ising paramagnet permutes the anyons. Interestingly, the interface between this SET and the parent quantum-Hall phase realizes symmetry-enforced $\mathbb{Z}_3$ parafermion criticality with no fine-tuning required. The second model exhibits a non-Abelian phase that is consistent with $\text{SU}(2)_{4}$ topological order, and can be accessed by gauging the $\mathbb{Z}_{2}$ symmetry in the SET. Employing Levin-Wen string-net models with $\mathbb{Z}_{2}$-graded structure, we generalize this picture to construct a large class of commuting-projector models for $\mathbb{Z}_{2}$ SETs and non-Abelian topological orders exhibiting the same relation. Our construction provides the first commuting-projector-Hamiltonian realization of chiral bosonic non-Abelian topological order.Comment: 29+18 pages, 25 figure

Formation of high-quality Ag-based ohmic contacts to p-type GaN

Low resistance and high reflectance ohmic contacts on p-type GaN were achieved using an Ag-based metallization scheme. Oxidation annealing was the key to achieve ohmic behavior of Ag-based contacts on p-type GaN. A low contact resistivity of similar to 5x10(-5) Omega cm(2) could be achieved from Me (=Ni, Ir, Pt, or Ru)/Ag (50/1200 angstrom) contacts after annealing at 500 degrees C for 1 min in O(2) ambient. Oxidation annealing promoted the out-diffusion of Ga atoms from the GaN layer, and Ga atoms dissolved in the in-diffused Ag layer with the formation of Ag-Ga solid solution, resulting in ohmic contact formation. Using Ru/Ni/Au (500/200/500 angstrom) overlayers on the Me/Ag contacts, the excessive incorporation of oxygen molecules into the contact interfacial region, and the out-diffusion and agglomeration of Ag, were effectively prevented during oxidation annealing. As a result, a high reflectance of 87.2% at the 460 nm wavelength and a smooth surface morphology could be obtained simultaneously. (C) 2008 The Electrochemical Society.open111618sciescopu

The Eco-School Project at the Daegu National University of Education

Asia Education Symposium 2006, Session 3: “Making the most of Resources of International Education”( Day Two; Sun., October 16

In vitro culture systems of hybrid aspen as tools for tree improvement programs and commercial applications

Tree improvement programs have two important components: (1) induction of desirable types of genetic variation by breeding, and (2) selection and maintenance of the traits by sexual or asexual propagation methods. By combining tissue culture systems and morden DNA technology with conventional breeding methods, both aspects of tree improvement will benfit. New tissue culture systems are also important to the development of commercial applications of improved trees;Therefore, several new approaches to the in vitro culture of hybrid aspen (Populus alba L. x P. grandidentata Michx. \u27Crandon\u27) were studied. To investigate multiple shoot regeneration capacity of different cells, tissues, and organs of this hybrid aspen, ex vitro and in vitro derived stem nodes, root, and callus derived from each tissues were examined. Although high levels of cytokinin produced greatly increased numbers of proliferated shoots, the survival (in vitro) of the shoots were relatively low compared with the shoots obtained by low levels of cytokinin. In a subsequent study, techniques were developed to rescue and use large number of shoots induced by high cytokinin treatments. After development of these rapid propagation systems, the variation of tissue-culture-derived plants was investigated at the phenotypic, chromosomal, and protein levels;To obtain large numbers of syncronized micropropagules for transplanting to nursery beds or as a tool for germplasm conservation, in vitro cold-storage systems were developed and evoluated. To achieve high survival rates of tissue culture plants in field conditions, different types of acclimatization methods were tested using thousands of tissue culture derived plants. An integrated system using polyterra peat plugs, intermittent misting, and 30% shade in greenhouse conditions gave almost 100% shoot survival

Spin force and intrinsic spin Hall effect in spintronics systems

We investigate the spin Hall effect (SHE) in a wide class of spin-orbit coupling systems by using spin force picture. We derive the general relation equation between spin force and spin current and show that the longitudinal force component can induce a spin Hall current, from which we reproduce the spin Hall conductivity obtained previously using Kubo's formula. This simple spin force picture gives a clear and intuitive explanation for SHE