Coherent phonons, nanoseismology and THz radiation in InGaN/GaN heterostructures

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. We discuss the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also discuss the THz radiation emitted from these phonons.     

Low order adaptive optics on Z-Beamlet using a single actuator deformable mirror

The Z-Beamlet laser at Sandia National Laboratories can perform a full system shot every 3-4 h. This shot rate is limited by thermal aberrations that result from the flashlamp pumped Nd:phosphate amplifier slabs. The lowest order as well as the strongest aberration is of cylindrical shape. Therefore, a single actuator deformable mirror assembly for correction of cylindrical aberration was developed. Mirror performance was modeled using finite element analysis and showed good agreement with derived analytical expressions. Quantitative measurements were performed with an interferometer and thermal lens compensation was achieved in the Z-Beamlet laser system leading to an increased shot rate of one in every 2 h.     

Search for cluster emission and extremely deformed shapes using charged-particle spectroscopy

The possible occurrence of highly deformed configurations is investigated in the N=Z nuclei ⁴⁰Ca, ⁴⁴Ti, and ⁵⁶Ni as formed in the ²⁸Si+¹²C, ¹⁶O+²⁸Si, and ²⁸Si+²⁸Si reactions by using the properties of the emitted light charged particles (LCP). The energy spectra, in-plane and out-of-plane angular correlations of LCP’s are analysed for each of the 3 studied reactions within the framework of the statistical model. Strong deformation effects are deduced from Hauser-Feshbach calculations performed with the Monte Carlo code CACARIZO by using a consistent set of parameters with spin-dependent level densities. The analysis of α particles in coincidence with ³²S fragments emitted in ²⁸Si+¹²C shows a strong contribution from a ⁸Be cluster emission. The binary nature of this cluster emission has been verified in three other reactions involving a ¹²C target: ²⁷A1+¹²C, ³¹P+¹²C, and ³²S+¹²C.     

Disjoint spine phenomena in certain contractible n-manifolds (n5)

If M is a compact PL manifold with boundary containing a subpolyhedron K in its interior, then K is said to be a PL spine of M provided M collapses to K (MK). Guilbault [Topology 34 (1) (1995) 99–108] has shown that certain nontrivial contractible manifolds possess disjoint spines. His results stem from a standing conjecture regarding disjoint spines in contractible 4-manifolds constructed by Mazur. More to the point, there is a dimensional requirement introduced by his techniques; Guilbault produces such manifolds in dimensions n9. We shall provide techniques which allow the construction of examples in dimensions n5 following the path laid out by Guilbault. The new techniques will provide a slight strengthening of some other Guilbault results as well.     

Reactions of Organic Peroxides with Alcohols in Atmospheric Pressure Chemical Ionization—the Pitfalls of Quantifying Triacetone Triperoxide (TATP)

Over the last several decades, mass spectrometry has become one of the principle methods for compound identification and quantification. While for analytical purposes, fragments which are not fully characterized in terms of origin and intensity as a function of experimental conditions have been used, understanding the nature of those species is very important. Herein we discuss such issues relative to triacetone triperoxide (TATP) and its frequently observed fragment at m/z 89. This “fragment” has been identified as the gas-phase reaction product of TATP with one or two methanol molecules/ions. Additionally, the origin and conditions of other fragments at m/z 91, 75, and 74 associated with TATP will be addressed. Similar analytical issues associated with other multi-peroxide organic compounds [hexamethylene triperoxide diamine (HMTD), methyl ethyl ketone peroxides (MEKP)] will also be discussed. Solution storage conditions for TATP, HMTD, and tetramethylene diperoxide diamine dialdehyde have been determined.

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Direction-Projection-Permutation for High-Dimensional Hypothesis Tests

High-dimensional low sample size (HDLSS) data are becoming increasingly common in statistical applications. When the data can be partitioned into two classes, a basic task is to construct a classifier that can assign objects to the correct class. Binary linear classifiers have been shown to be especially useful in HDLSS settings and preferable to more complicated classifiers because of their ease of interpretability. We propose a computational tool called direction-projection-permutation (DiProPerm), which rigorously assesses whether a binary linear classifier is detecting statistically significant differences between two high-dimensional distributions. The basic idea behind DiProPerm involves working directly with the one-dimensional projections of the data induced by binary linear classifier. Theoretical properties of DiProPerm are studied under the HDLSS asymptotic regime whereby dimension diverges to infinity while sample size remains fixed. We show that certain variations of DiProPerm are consistent and that consistency is a nontrivial property of tests in the HDLSS asymptotic regime. The practical utility of DiProPerm is demonstrated on HDLSS gene expression microarray datasets. Finally, an empirical power study is conducted comparing DiProPerm to several alternative two-sample HDLSS tests to understand the advantages and disadvantages of each method.