Microstructure of GaN deposited by lateral confined epitaxy on patterned Si (111)

Lateral confined epitaxy (LCE) is an epitaxial growth method on substrates patterned to form uniform mesas separated by trenches for laterally restricting growth area. In this work, plan view and cross-sectional transmission electron microscopy (TEM) were used in order to characterize the microstructure of GaN films grown by metal-organic chemical vapor deposition on patterned Si (111) using the LCE method. Two kinds of propagation modes of the dislocations were observed. The dislocations in the center of the mesa mainly propagate vertically to the surface. On the other hand, dislocations close (1–2 μm) to the mesa edges tend to bend laterally, allowing dislocation reactions that result in a lower dislocation density. This suggests that the overall material quality improves with decreasing mesa size, which is consistent with the observed increase in photoluminescence band edge peak intensity.     

Geometrization in the Yang-Mills,extended supergravity,and Klein-Kaluza versions

We relate personal encounters of three kinds with geometrical approaches in the development of a relativistic quantum field theory of the fundamental interactions—including interactions with Nathan Rosen. We characterize the geometrical structures involved and discuss the more recent attempts to develop a unified theory based on a Klein-Kaluza contraction of the eightfold extended supergravity.Publisher's note: The full paper will appear inFoundations of Physics, Volume 15, Number 3, March 1985.     

Transition functions for high-frequency diffraction by a curvedperfectly conducting wedge. II. A partially uniform solution for ageneral wedge angle

For pt.I see ibid., vol.37, no.9, p.1073-9(1989). In pt.I, transition function solutions for the combined surface-edge diffraction were derived from the rigorous, canonical solution for a thin cylindrically curved sheet. Here, similar solutions are derived for the more general case of diffraction by a perfectly conducting curved wedge. In the absence of a canonical solution for this case, the theory developed here is a physical one. It is an extension of the spectral theory of diffraction to the Fock solution for the penumbra region field near a convex surface. For certain domains of illumination aspects and field points, this procedure recovers the results obtained by other authors, starting, however, from more plausible assumptions and providing a ne insight. For other domains, it yields asymptotic solutions for the first time, thus demonstrating greater generality than in the previous approaches. The results are checked in two ways: first, they reduce to the rigorous results of pt.I when specialized to a curved sheet; second, they are shown to agree with a moment-method solution for a structure involving a curved wedge     

CP and CPT symmetry violations,entropy and the expanding universe

The conjecture linking together statistical and cosmological time arrows implies interpreting the contracting phase in an oscillating cosmological model as a time-inverted expansion. The Fitch-Cronin effect now requires a redefinition of which is matter and which is antimatter in this time-inverted picture. However, ifCPT is also broken, then theT-violating effects yield a different set of laws altogether for such reactions.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR grant number EOOAR-68-0010, through the European Office of Aerospace Research.     

Magnetization transfer using inversion recovery during off-resonance irradiation

Estimation of magnetization transfer (MT) parameters in vivo can be compromised by an inability to drive the magnetization to a steady state using allowable levels of radiofrequency (RF) irradiation, due to safety concerns (tissue heating and specific absorption rate (SAR)). Rather than increasing the RF duration or amplitude, here we propose to circumvent the SAR limitation by sampling the formation of the steady state in separate measurements made with the magnetization initially along the -z and +z axis of the laboratory frame, i.e. with or without an on-resonance inversion pulse prior to the off-resonance irradiation. Results from human brain imaging demonstrate that this choice provides a tremendous benefit in the fitting procedure used to estimate MT parameters. The resulting parametric maps are characterized by notably increased tissue specificity as compared to those obtained with the standard MT acquisition in which magnetization is initially along the +z axis only.     

Novel technologies for the formation of 2-D and 3-D droplet interface bilayer networks

Droplet interface bilayer (DIB) networks have vast potential in the field of membrane biophysics, synthetic biology, and functional bio-electronics. However a technological bottleneck exists in network fabrication: existing methods are limited in terms of their automation, throughput, versatility, and ability to form well-defined 3-D networks. We have developed a series of novel and low-cost methodologies which address these limitations. The first involves building DIB networks around the contours of a microfluidic chip. The second uses flow rate and droplet size control to influence droplet packing geometries within a microfluidic chamber. The latter method enables the controlled formation of various 3-D network arrays consisting of thousands of interconnected symmetric and asymmetric lipid bilayers for the first time. Both approaches allow individual droplet position and composition to be controlled, paving the way for complex on-chip functional network synthesis.     

The impact of solvent polarity on intramolecular proton and electron transfer in 2-alkylamino-4-nitro-5-methyl pyridine N-oxides

The crystal structure of 2-butylamino-4-nitro-5-methyl pyridine N-oxide (2B5M) and solution studies of both 2B5M and 2-methylamino-4-nitro-5-methyl pyridine (2M5M) N-oxide are presented. Steady-state absorption and emission measurements were employed for both molecules while a picosecond fluorescence up-conversion technique was used to follow the dynamic behavior of the 2M5M system. The experimental methods were complemented by DFT and TD DFT B3LYP/6-31G(d,p) calculations involving ground and excited-state optimization which in the case of the smaller 2M5M molecule were extended to the CAM-B3LYP/6-31G(d,p) method. The solvent effect is incorporated by applying the polarizable continuum (PCM) model. The data reveal that the 2B5M molecule crystallizes in the monoclinic space group P2(1)/c and its crystal lattice is composed of monomers with intramolecular N-H···O [2.572(3) ?] hydrogen bonds, connected into a polymer network by weak intermolecular C-H…O [3.2-3.4 ?]-type interactions. Quantum-chemical calculations show that the aminoalkyl substitutent in aminoalkyl-pyridine N-oxides is a specific determinant of the CT nature of the lowest-lying excited electronic ππ* state, distinguishing them from other nitroaromatic compounds. The results of both picosecond fluorescence up-conversion experiments in different solvents and quantum-chemical calculations suggest that in nonpolar media the ESIPT process in 2M5M is favored, while in polar acetonitrile, the N* → PT* transition demands barrier-crossing and thus unfavorable thermodynamic conditions do not allow the ESIPT to occur. The signals of picosecond fluorescence up-conversion of 2M5M are solvent- and emission-wavelength dependent. The three time components found in a weakly polar isooctane-dioxane mixture have been attributed to solvation dynamics (～500 fs), and to relaxation of N* and PT* forms while in acetonitrile, a very rapid fluorescence decay with a time constant (2.3-4.0 ps) indicative of the presence of the normal (N*) form was observed. Much shorter fluorescence lifetimes in alcohols (a few picoseconds) and in D(2)O (less than 200 fs) than in aprotic solvents suggest that in protic media, the solvent molecules participate in the ESIPT, bridging between the methylamine group and the N-oxide group of 2M5M.     

Arc permutations

Arc permutations and unimodal permutations were introduced in the study of triangulations and characters. This paper studies combinatorial properties and structures on these permutations. First, both sets are characterized by pattern avoidance. It is also shown that arc permutations carry a natural affine Weyl group action, and that the number of geodesics between a distinguished pair of antipodes in the associated Schreier graph, and the number of maximal chains in the weak order on unimodal permutations, are both equal to twice the number of standard Young tableaux of shifted staircase shape. Finally, a bijection from non-unimodal arc permutations to Young tableaux of certain shapes, which preserves the descent set, is described and applied to deduce a conjectured character formula of Regev.     

Harmonic analysis on the Iwahori–Hecke algebra

These are purely expository notes of Opdam’s analysis [O1] of the trace form τ(f) = f(e) on the Hecke algebra H = C _c (I\G/I) of compactly supported functions f on a connected reductive split p-adic group G which are biinvariant under an Iwahori subgroup I, extending Macdonald’s work. We attempt to give details of the proofs, and choose notations which seem to us more standard. Many objects of harmonic analysis are met: principal series, Macdonald’s spherical forms, trace forms, Bernstein forms. The latter were introduced by Opdam under the name Eisenstein series for H. The idea of the proof is that the last two linear forms are proportional, and the proportionality constant is computed by projection to Macdonald’s spherical forms. Crucial use is made of Bernstein’s presentation of the Iwahori–Hecke algebra by means of generators and relations, as an extension of a finite dimensional algebra by a large commutative subalgebra. We give a complete proof of this using the universal unramified principal series right H-module M = C _c (A(O)N\G/I) to develop a theory of intertwining operators algebraically.