Determination of oscillator strength of confined excitons in a semiconductor microcavity

We have achieved a significant experimental Rabi-splitting (3.4 meV) for confined polaritons in a planar semiconductor $\lambda$ microcavity for only a single quantum well (SQW) of GaAs (10 nm) placed at the antinode. The Rabi-splitting phenomena are discussed in detail based on the semiclassical theory, where two coupled harmonic oscillators (excitons and photons) are used to describe the system. In this way, we can obtain the dispersion curve of polaritons, the minimum value for the cavity reflectance and the oscillator strength to reach the strong coupling regime. This approach describes an ensemble of excitons confined in a SQW and includes a dissipation component. The results present a weak coupling regime, where an enhanced spontaneous emission takes place, and a strong coupling regime, where Rabi-splitting in the dispersion curve can be observed. The theoretical results are confronted with experimental data for the reflectance behavior in resonant and off-resonant conditions and present a great accuracy. This allows us to determine the oscillator strength of the confined excitons in the SQW with great precision.Comment: 11 pages, 7 figure

Loop and Path Spaces and Four-Dimensional BF Theories: Connections, Holonomies and Observables

We study the differential geometry of principal G-bundles whose base space is the space of free paths (loops) on a manifold M. In particular we consider connections defined in terms of pairs (A,B), where A is a connection for a fixed principal bundle P(M,G) and B is a 2-form on M. The relevant curvatures, parallel transports and holonomies are computed and their expressions in local coordinates are exhibited. When the 2-form B is given by the curvature of A, then the so-called non-abelian Stokes formula follows. For a generic 2-form B, we distinguish the cases when the parallel transport depends on the whole path of paths and when it depends only on the spanned surface. In particular we discuss generalizations of the non-abelian Stokes formula. We study also the invariance properties of the (trace of the) holonomy under suitable transformation groups acting on the pairs (A,B). In this way we are able to define observables for both topological and non-topological quantum field theories of the BF type. In the non topological case, the surface terms may be relevant for the understanding of the quark-confinement problem. In the topological case the (perturbative) four-dimensional quantum BF-theory is expected to yield invariants of imbedded (or immersed) surfaces in a 4-manifold M.Comment: TeX, 39 page

Loop observables for BF theories in any dimension and the cohomology of knots

A generalization of Wilson loop observables for BF theories in any dimension is introduced in the Batalin-Vilkovisky framework. The expectation values of these observables are cohomology classes of the space of imbeddings of a circle. One of the resulting theories discussed in the paper has only trivalent interactions and, irrespective of the actual dimension, looks like a 3-dimensional Chern-Simons theory.Comment: 13 page

Conversion of Lower Lignin Mutants of Sorghum bicolor to Ethanol

The United States Government has targeted aggressive development of ethanol as one route for decreasing oil dependence and lowering greenhouse gas emissions. Achieving future production targets depends on expanding feedstock sources beyond corn and towards lignocellulose. This is expected to include new low-input and high yielding energy crops. Utilizing dedicated energy crops allows for the possibility of breeding them for increased conversion efficiency to ethanol. In this study, the effect of reducing lignin content was evaluated for sugar and ethanol production in forage sorghum (Sorghum bicolor (L.)). The sample set included wild-type and brown midrib mutants for reduced lignin contents. The mutants had significantly reduced lignin contents compared to the wild-type, but did not significantly vary in carbohydrates. The set was treated with dilute-acid, washed, and the residual cellulose hydrolyzed with commercial cellulase. Extraction of glucose was negatively correlated with lignin content. Samples were again treated with dilute-acid, but this time the whole hydrolysate was neutralized and converted to ethanol using cellulases in combination with Saccharomyces yeast. The ethanol yield was also determined to be negatively correlated with lignin content. These results suggest that breeding for reduced lignin can lead to improved ethanol yields. In addition, selected samples were also evaluated using a more severe alkaline pretreatment. Once again higher ethanol yields were associated with lower lignin contents

Higgs Properties in the Fourth Generation MSSM: Boosted Signals Over the 3G Plan

The generalization of the MSSM to the case of four chiral fermion generations (4GMSSM) can lead to significant changes in the phenomenology of the otherwise familiar Higgs sector. In most of the 3GMSSM parameter space, the lighter CP-even $h$ is $\sim 115-125$ GeV and mostly Standard Model-like while $H,A,H^\pm$ are all relatively heavy. Furthermore, the ratio of Higgs vevs, $\tan \beta$, is relatively unconstrained. In contrast to this, in the 4GMSSM, heavy fourth generation fermion loops drive the masses of $h,H,H^\pm$ to large values while the CP-odd boson, $A$, can remain relatively light and $\tan \beta$ is restricted to the range 1/2 \lsim \tan \beta \lsim 2 due to perturbativity requirements on Yukawa couplings. We explore this scenario in some detail, concentrating on the collider signatures of the light CP-odd Higgs at both the Tevatron and LHC. We find that while $gg \to A$ may lead to a potential signal in the $\tau^+\tau^-$ channel at the LHC, $A$ may first be observed in the $\gamma \gamma$ channel due to a highly loop-enhanced cross section that can be more than an order of magnitude greater than that of a SM Higgs for $A$ masses of $\sim 115-120$ and $\tan\beta<1$. We find that the CP-even states $h,H$ are highly mixed and can have atypical branching fractions. Precision electroweak constraints, particularly for the light $A$ parameter space region, are examined in detail.Comment: 20 pages, 7 figures; typos fixed, refs adde

Localized Charge Transfer Process and Surface Band Bending in Methane Sensing by GaN Nanowires

The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A global resistive response for sensor devices of ensemble nanowires and a localized charge transfer process in single GaN nanowires are studied in situ scanning by Kelvin probe microscopy (SKPM). A localized charge transfer process, involving the VGa-3ON defect complex on nanowire surface is attributed in controlling the global gas sensing behavior of the oxygen rich ensemble GaN nanowires.Comment: 42 pages, 6 figures, Journa

Four-Dimensional Yang-Mills Theory as a Deformation of Topological BF Theory

The classical action for pure Yang--Mills gauge theory can be formulated as a deformation of the topological $BF$ theory where, beside the two-form field $B$, one has to add one extra-field $\eta$ given by a one-form which transforms as the difference of two connections. The ensuing action functional gives a theory that is both classically and quantistically equivalent to the original Yang--Mills theory. In order to prove such an equivalence, it is shown that the dependency on the field $\eta$ can be gauged away completely. This gives rise to a field theory that, for this reason, can be considered as semi-topological or topological in some but not all the fields of the theory. The symmetry group involved in this theory is an affine extension of the tangent gauge group acting on the tangent bundle of the space of connections. A mathematical analysis of this group action and of the relevant BRST complex is discussed in details.Comment: 74 pages, LaTeX, minor corrections; to be published in Commun. Math. Phy

Hybrid Approach in Microscale Transport Phenomena: Application to Biodiesel Synthesis in Micro-reactors

A hybrid engineering approach to the study of transport phenomena, based on the synergy among computational, analytical, and experimental methodologies is reviewed. The focus of the chapter is on fundamental analysis and proof of concept developments in the use of nano- and micro-technologies for energy efficiency and heat and mass transfer enhancement applications. The hybrid approach described herein combines improved lumped-differential modeling, hybrid numericalanalytical solution methods, mixed symbolic-numerical computations, and advanced experimental techniques for micro-scale transport phenomena. An application dealing with micro-reactors for continuous synthesis of biodiesel is selected to demonstrate the instrumental role of the hybrid approach in achieving improved design and enhanced performance

Enhanced Eshelby twist on thin wurtzite InP nanowires and measurement of local crystal rotation

We have performed a detailed study of the lattice distortions of InP wurtzite nanowires containing an axial screw dislocation. Eshelby predicted that this kind of system should show a crystal rotation due to the dislocation induced torque. We have measured the twisting rate and the dislocation Burgers vector on individual wires, revealing that nanowires with a 10-nm radius have a twist up to 100% larger than estimated from elasticity theory. The strain induced by the deformation has a Mexican-hat-like geometry, which may create a tube-like potential well for carriers