Stress effects on the Raman spectrum of an amorphous material: theory and experiment on a-Si:H

Strain in a material induces shifts in vibrational frequencies, which is a probe of the nature of the vibrations and interatomic potentials, and can be used to map local stress/strain distributions via Raman microscopy. This method is standard for crystalline silicon devices, but due to lack of calibration relations, it has not been applied to amorphous materials such as hydrogenated amorphous silicon (a-Si:H), a widely studied material for thin-film photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H \ab initio under different strains $\epsilon$ and found peak shifts $\Delta \omega = \left( -460 \pm 10\ \mathrm{cm}^{-1} \right) {\rm Tr}\ \epsilon$. This proportionality to the trace of the strain is the general form for isotropic amorphous vibrational modes, as we show by symmetry analysis and explicit computation. We also performed Raman measurements under strain and found a consistent coefficient of $-510 \pm 120\ \mathrm{cm}^{-1}$. These results demonstrate that a reliable calibration for the Raman/strain relation can be achieved even for the broad peaks of an amorphous material, with similar accuracy and precision as for crystalline materials.Comment: 12 pages, 3 figures + supplementary 8 pages, 4 figure

Thermodynamic limits to energy conversion in solar thermal fuels

Solar thermal fuels (STFs) are an unconventional paradigm for solar energy conversion and storage which is attracting renewed attention. In this concept, a material absorbs sunlight and stores the energy chemically via an induced structural change, which can later be reversed to release the energy as heat. An example is the azobenzene molecule which has a cis-trans photoisomerization with these properties, and can be tuned by chemical substitution and attachment to templates such as carbon nanotubes, small molecules, or polymers. By analogy to the Shockley-Queisser limit for photovoltaics, we analyze the maximum attainable efficiency for STFs from fundamental thermodynamic considerations. Microscopic reversibility provides a bound on the quantum yield of photoisomerization due to fluorescence, regardless of details of photochemistry. We emphasize the importance of analyzing the free energy, not just enthalpy, of the metastable molecules, and find an efficiency limit for conversion to stored chemical energy equal to the Shockley-Queisser limit. STF candidates from a recent high-throughput search are analyzed in light of the efficiency limit.Comment: 16 pages, 4 figure

In Memoriam Professor Emeritus Egon Guttmon

When I think about Egon, the first thing that comes to mind are the memories when we met in 1982 at the Washington College of Law (WCL), where he was working as a full-time faculty member. I was coming at that time from my sabbatical in the Netherlands and as a Fulbright Scholar. From the beginning, Egon sought to provide me with a welcoming environment. He approached me, finding shared backgrounds and interests, which is always greatly appreciated, particularly when you are in a new institution. Egon noted that we both had an intellectual interest in international law. In addition to his teaching and research, Egon has had an important educational experience as a faculty member in Sudan and would often talk to me about the life-changing nature of that academic endeavor

Rural water districts in Illinois

Research was conducted on the legal-organization, the economic and the spatial aspects of rural water systems. With encouragement from subsidized federal government credit, rural water systems serving farmers, nonfarm residents in the open country and residents of towns under 10,000 people have been developed to meet the demands for a dependable quality domestic water supply. The 59 districts financed in part by the Farmers Home Administration, USDA, and serving only farmers and nonfarm rural residence customers in Illinois were the objects of the research. In general, these systems serve 24,000 customers and maintain 4,200 miles of line. They are located in the southern, the west-central, and the east-central regions of Illinois. The systems received financial assistance from the federal government in the form of construction grants and/or low interest loans authorized initially in 1954. The systems are generally owned and operated by "water supply districts," a special unit of local government and have to comply with all operating procedures and regulations required of public water supplies under Illinois Environmental Protection Agency authority. In analyzing the costs of rural water service, districts with greater volume and/or higher user density generally had lower operating costs. The median number of users reported was 278. In 1980 dollars, the per user average outlay for operating costs and debt retirement was approximately $16.00 per month. Only 22 percent of rural water district users were farmers. Consumption levels averaged 4.64 thousand gallons per month at an average price of$5.77. The demand for rural water service was inelastic over much of the relevant range of observations. Little evidence was discovered supporting the contention that rural water service is a major force in the shift of agricultural land to nonfarm residential use.U.S. Department of the InteriorU.S. Geological SurveyOpe

A theory of urban structure : including an application of certain aspects of the theory to the relationship between land use and transportation in Providence, Rhode Island

Thesis (M.C.P.)--Massachusetts Institute of Technology, Dept. of City and Regional Planning, 1953.Includes bibliographical references (leaf 30).by David Abraham Grossman.M.C.P

CP Violation in Three-Body Chargino Decays

CP violation in supersymmetry can give rise to rate asymmetries in the decays of supersymmetric particles. In this work we compute the rate asymmetries for the three-body chargino decays \tilde\chi^\pm_2 \to \tilde\chi^\pm_1 HH, \tilde\chi^\pm_2 \to \tilde\chi^\pm_1 ZZ, \tilde\chi^\pm_2 \to \tilde\chi^\pm_1 W^+ W^- and \tilde\chi^\pm_2 \to tilde\chi^\pm_1 ZH. Each of the decays contains contributions mediated by neutral Higgs bosons that can possibly go on shell. Such contributions receive a resonant enhancement; furthermore, the strong phases required for the CP asymmetries come from the widths of the exchanged Higgs bosons. Our results indicate that the rate asymmetries can be relatively large in some cases, while still respecting a number of important low-energy bounds such as those coming from B meson observables and electric dipole moments. For the parameters that we consider, rate asymmetries of order 10% are possible in some cases.Comment: 17 pages, 4 figures, published versio

Fractal dimensions of the Q-state Potts model for the complete and external hulls

Fortuin-Kastelyn clusters in the critical $Q$-state Potts model are conformally invariant fractals. We obtain simulation results for the fractal dimension of the complete and external (accessible) hulls for Q=1, 2, 3, and 4, on clusters that wrap around a cylindrical system. We find excellent agreement between these results and theoretical predictions. We also obtain the probability distributions of the hull lengths and maximal heights of the clusters in this geometry and provide a conjecture for their form.Comment: 9 pages 4 figure

B-Decay CP Asymmetries, Discrete Ambiguities and New Physics

The first measurements of CP violation in the $B$ system will likely probe $\sin 2\alpha$, $\sin 2\beta$ and $\cos 2\gamma$. Assuming that the CP angles $\alpha$, $\beta$ and $\gamma$ are the interior angles of the unitarity triangle, these measurements determine the angle set $(\alpha,\beta,\gamma)$ except for a twofold discrete ambiguity. If one allows for the possibility of new physics, the presence of this discrete ambiguity can make its discovery difficult: if only one of the two candidate solutions is consistent with constraints from other measurements in the $B$ and $K$ systems, one is not sure whether new physics is present or not. We review the methods used to resolve the discrete ambiguity and show that, even in the presence of new physics, they can usually be used to uncover this new physics. There are some exceptions, which we describe in detail. We systematically scan the parameter space and present examples of values of $(\alpha,\beta,\gamma)$ and the new-physics parameters which correspond to all possibilities. Finally, we show that if one relaxes the assumption that the bag parameters \BBd and \BK are positive, one can no longer definitively establish the presence of new physics.Comment: 29 pages, LaTeX, 1 figures, presentation substantially reworked, physics conclusions unchanged. This version will be published in Phys. Rev.

New-Physics Effects on Triple-Product Correlations in Lambda_b Decays

We adopt an effective-lagrangian approach to compute the new-physics contributions to T-violating triple-product correlations in charmless Lambda_b decays. We use factorization and work to leading order in the heavy-quark expansion. We find that the standard-model (SM) predictions for such correlations can be significantly modified. For example, triple products which are expected to vanish in the SM can be enormous (~50%) in the presence of new physics. By measuring triple products in a variety of Lambda_b decays, one can diagnose which new-physics operators are or are not present. Our general results can be applied to any specific model of new physics by simply calculating which operators appear in that model.Comment: 20 pages, LaTeX, no figures. Added a paragraph (+ references) discussing nonfactorizable effects. Conclusions unchange

Triple-Product Correlations in B -> V1 V2$ Decays and New Physics

In this paper we examine T-violating triple-product correlations (TP's) in B -> V1 V2 decays. TP's are excellent probes of physics beyond the standard model (SM) for two reasons: (i) within the SM, most TP's are expected to be tiny, and (ii) unlike direct CP asymmetries, TP's are not suppressed by the small strong phases which are expected in B decays. TP's are obtained via the angular analysis of B -> V1 V2. In a general analysis based on factorization, we demonstrate that the most promising decays for measuring TP's in the SM involve excited final-state vector mesons, and we provide estimates of such TP's. We find that there are only a handful of decays in which large TP's are possible, and the size of these TP's depends strongly on the size of nonfactorizable effects. We show that TP's which vanish in the SM can be very large in models with new physics. The measurement of a nonzero TP asymmetry in a decay where none is expected would specifically point to new physics involving large couplings to the right-handed b-quark.Comment: 42 pages, LaTeX, no figures. Title changed, several explanatory paragraphs added, references added, analysis and conclusions unchange