Nanotube Piezoelectricity

We combine ab initio, tight-binding methods and analytical theory to study piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity of a heteropolar nanotube depends on its chirality and diameter and can be understood starting from the piezoelectric response of an isolated planar sheet, along with a structure specific mapping from the sheet onto the tube surface. We demonstrate that coupling between the uniaxial and shear deformation are only allowed in the nanotubes with lower chiral symmetry. Our study shows that piezoelectricity of nanotubes is fundamentally different from its counterpart in three dimensional (3D) bulk materials.Comment: 4 pages, with 3 postscript figures embedded. Uses REVTEX4 macros. Also available at http://www.physics.upenn.edu/~nsai/preprints/bn_piezo/index.htm

Using Markov Models and Statistics to Learn, Extract, Fuse, and Detect Patterns in Raw Data

Many systems are partially stochastic in nature. We have derived data driven approaches for extracting stochastic state machines (Markov models) directly from observed data. This chapter provides an overview of our approach with numerous practical applications. We have used this approach for inferring shipping patterns, exploiting computer system side-channel information, and detecting botnet activities. For contrast, we include a related data-driven statistical inferencing approach that detects and localizes radiation sources.Comment: Accepted by 2017 International Symposium on Sensor Networks, Systems and Securit

Overcoming the compensation of acceptors in GaN:Mg by defect complex formation

In GaN:Mg, the MgGa acceptor is compensated extensively by the formation of nitrogen vacancies (VN) and Mg interstitials (Mgi). However, we show that such compensation can be overcome by forming two kinds of Mg-rich complexes: one that contains VN and the other that contains only MgGa and Mgi. Such complexing not only neutralizes VN and Mgi but also forms better complex acceptors that have lower formation energies and smaller hole localization energies than isolated MgGa. Our results help explain the different doping behaviors in samples grown by different methods

Digital phase tightening for improved spatial resolution in millimeter-wave imaging systems

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.Includes bibliographical references (leaves 68-69).Imaging systems using millimeter-wave frequencies allow for the possibilities of vehicular radar and concealed weapons detection. By using silicon technology, the integration of millimeter-wave circuits can reach new levels that were previously impossible. This thesis discusses the challenge and design of a mm-wave imaging system using a technique called digital phase tightening for improved spatial resolution. Digital phase tightening uses feedback and oversampling to accurately measure the amplitude and phase of an incoming signal. Furthermore, it can be implemented using only a delay-lock loop, an analog-to-digital converter, and a counter. A proof of concept system utilizing a 2.4GHz delay-lock loop with supporting circuitry is designed in 90nm CMOS. Test results demonstrate a proof of concept system with a measured DLL resolution of 41.7ps that consumes 36mW of power. The goal of the system is to reduce the jitter of phase measurements to the order of femto-seconds. In the proto system, the quantization error is larger than the Gaussian noise; therefore, significant improvements in the accuracy of the phase measurements were not observed.by Ke Lu.S.M

Lessons for Chinese mega-mall development : a case study of the South China Mall

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Architecture, 2007.Includes bibliographical references (p. 90-93).China is embracing mega-mall development: Seven out of the ten largest shopping malls in the world will have been located in China by the year 2010. All the completed mega-malls are now suffering from high vacancy rates and therefore experiencing enormous economic losses. To avoid the failure of future mega-mall projects, it is important to establish rigorous guidelines for design, leasing, financing, and management of Chinese mega-malls. However, research in this field has not yet been conducted. This paper will analyze five problematic issues of the South China Mall, the largest shopping mall in the world, and describe a dynamic process involving governments, developers, and banks behind the failure of the Mall. The paper will also provide strategic suggestions on the development and management of the Mall. Given its dimensions and aspirations, the South China Mall is a mirror of contemporary Chinese mega-mall development. It is hoped that lessons from the South China Mall can be applied to other Chinese mega-malls currently experiencing economic stagnancy.by Lu Ai.S.M

A new approach in data visualization to integrate time and space variability of daylighting in the design process

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Architecture, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 124-126).Daylighting design has great impact on the performance and aesthetical quality of a work of architecture but requires many issues to be addressed during the design process. The way existing daylighting tools deliver data to designers is still inefficient. The output display has no quick switch between quantitative and qualitative data and simply considers single moments with fixed weather condition. Designers are interrupted in their design process, and they usually need to make a data synthesis themselves, with the risk of overlooking critical periods or aspects of the design. Therefore, this thesis proposed a new data visualization method to improve this situation and create a more efficient data transmission between the designer and the program to better inform and support the design process. It used some existing research work in progress and developed a functional data visualization platform to simultaneously present sufficient quantitative and qualitative data over the year while linking closely the performance to annual weather variations, sun positions, and surroundings. As a result, designers are able to focus on refining their design while still taking into account the environmental influence over time in a convenient way. The proposed platform will work as an analysis interface for the ongoing LightSolve project at MIT Daylighting Lab.by Lu Yi.S.M

An Inductorless Direct-injection Power Control Circuit for the Distribution Grid

This paper presents an inductorless direct-injection power flow and quality control topology. The circuit removes any bulky low-frequency transformer and includes a shunt active front end and high-frequency links that powers a series floating module per phase. These floating modules are connected in series with each phase and omit any ground connection, thereby only dealing with a small portion of the voltage difference. Unlike other low-voltage power controllers with full converters, the direct-injection circuit presented here does not need to handle full power, which allows for the utilization of low-voltage high-current GaN transistors. As a result, high-frequency switching can be adopted, and the conventional filtering inductor in series can be replaced by parasitic inductance naturally occurring between cables. Overall, the circuit is exceptionally small and compact by employing low-power components and omitting line transformers and inductors

Imaging of 3D tissue-engineered models of oral cancer using 890 and 1300 nm optical coherence tomography

© 2015, Nizhny Novgorod State Medical Academy. All rights reserved. Optical coherence tomography (OCT) generates its primary form of contrast from elastic backscatter. It is now the gold standard technique for retinal screening and is emerging rapidly in cardiovascular research however it remains a research goal to establish it to the same degree in epithelial cancer detection and diagnosis. In this report we compare two different OCT systems: an 890 nm spectrometer-based OCT system with 2.5 µm axial resolution and a 1300 nm swept-source OCT system with 7.5 µm axial resolution to determine the effect of these different OCT parameters on the endogenous backscatter contrast of dysplastic/malignant oral mucosa models relative to normal mucosa models. Tissueengineered oral mucosa models constructed with a dysplastic cell line (DOK), a malignant cell line (Cal27) and normal cell were imaged with both of these OCT platforms and comparisons made with regard to apparent epithelial thickness and the visibility of the epithelium relative to the underlying stroma. For the Cal27’s, hematoxylin and eosin staining confirmed the formation of a keratinized layer superficial to a thickened layer of viable cells on top of the stroma. The keratinized layer presented as a hyperreflective thickened layer superficial to a darker region on both OCT platforms. The keratinized layer caused a steep fall in signal at 890 nm, making it difficult to visualise underlying structures, whereas 1300 nm OCT clearly visualized both the epithelial cells and the stroma lying beneath. For the DOK cells, hematoxylin and eosin staining confirmed the formation of an epithelial layer frequently presenting an abnormal morphology especially at the epidermal/stromal junction, with features such as infiltrating, bulbous rete pegs. These were more clearly visualized under 890 nm OCT. These observations show that 890 nm OCT retains some of its known advantages of higher contrast between anatomical tissue layers when used to observe dysplastic and malignant 3D oral mucosa constructs. However 1300 nm OCT is confirmed to possess a greater ability to image the full thickness of the model epithelia and in particular it is more suited to imaging through the keratinized layer

Reconstruction of some cosmological models in f(R,T) gravity

In this paper, we reconstruct cosmological models in the framework of $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of the stress-energy tensor. We show that the dust fluid reproduces $\Lambda$CDM, phantom-non-phantom era and the phantom cosmology. Further, we reconstruct different cosmological models including, Chaplygin gas, scalar field with some specific forms of $f(R,T)$. Our numerical simulation for Hubble parameter shows good agreement with the BAO observational data for low redshifts $z<2$.Comment: 12 pages, 2 figure

InPBi Single Crystals Grown by Molecular Beam Epitaxy

InPBi was predicted to be the most robust infrared optoelectronic material but also the most difficult to synthesize within In-VBi (V = P, As and Sb) 25 years ago. We report the first successful growth of InPBi single crystals with Bi concentration far beyond the doping level by gas source molecular beam epitaxy. The InPBi thin films reveal excellent surface, structural and optical qualities making it a promising new III–V compound family member for heterostructures. The Bi concentration is found to be 2.4 ± 0.4% with 94 ± 5% Bi atoms at substitutional sites. Optical absorption indicates a band gap of 1.23 eV at room temperature while photoluminescence shows unexpectedly strong and broad light emission at 1.4–2.7 μm which can't be explained by the existing theory