Fundamentals of Large Sensor Networks: Connectivity, Capacity, Clocks and Computation

Sensor networks potentially feature large numbers of nodes that can sense their environment over time, communicate with each other over a wireless network, and process information. They differ from data networks in that the network as a whole may be designed for a specific application. We study the theoretical foundations of such large scale sensor networks, addressing four fundamental issues- connectivity, capacity, clocks and function computation. To begin with, a sensor network must be connected so that information can indeed be exchanged between nodes. The connectivity graph of an ad-hoc network is modeled as a random graph and the critical range for asymptotic connectivity is determined, as well as the critical number of neighbors that a node needs to connect to. Next, given connectivity, we address the issue of how much data can be transported over the sensor network. We present fundamental bounds on capacity under several models, as well as architectural implications for how wireless communication should be organized. Temporal information is important both for the applications of sensor networks as well as their operation.We present fundamental bounds on the synchronizability of clocks in networks, and also present and analyze algorithms for clock synchronization. Finally we turn to the issue of gathering relevant information, that sensor networks are designed to do. One needs to study optimal strategies for in-network aggregation of data, in order to reliably compute a composite function of sensor measurements, as well as the complexity of doing so. We address the issue of how such computation can be performed efficiently in a sensor network and the algorithms for doing so, for some classes of functions.Comment: 10 pages, 3 figures, Submitted to the Proceedings of the IEE

The Link Overlap and Finite Size Effects for the 3D Ising Spin Glass

We study the link overlap between two replicas of an Ising spin glass in three dimensions using the Migdal-Kadanoff approximation and scaling arguments based on the droplet picture. For moderate system sizes, the distribution of the link overlap shows the asymmetric shape and large sample-to-sample variations found in Monte Carlo simulations and usually attributed to replica symmetry breaking. However, the scaling of the width of the distribution, and the link overlap in the presence of a weak coupling between the two replicas are in agreement with the droplet picture. We also discuss why it is impossible to see the asymptotic droplet-like behaviour for moderate system sizes and temperatures not too far below the critical temperature.Comment: 7 pages, 10 figure

Exciton generation and dissociation mechanisms in organic bulk heterojunction solar cell materials.

Characterization of the optical and electrical properties of organic solar cell materials is of prime importance to organic solar cell design. This thesis describes the use of capacitive photocurrent measurements to study the exciton generation and dissociation mechanisms of organic solar cell materials. The emphasis is on the study of the methanofullerene derivatives (e.g., PCBM) which act as the electron acceptor material. This is because much work has already been done studying electron donating polymers used in organic solar cells (in an effort to enhance their absorbance coefficient), but less information is available on the acceptor material. In the blend films of MDMO-PPV: PCBM, the charge generation rate in PCBM was discovered to be much higher than would be expected from the absorbance cross-section. This observation led to design of a photovoltage bleaching experiment to examine the charge generation mechanism. Here the effect of illumination by a tunable light source on the open circuit photovoltage of a MDMO-PPV: PCBM bulk heterojunction solar cell was measured. Illumination of light at the PCBM ground state singlet exciton causes a sharp decrease in the photovoltage, while illumination at the ground state MDMO-PPV exciton shows no change. A direct pathway of recombination of above gap generated charge carriers was identified. Photovoltage bleaching results suggests that excitation at the PCBM ground singlet state exciton increases the recombination rate of higher energy excitations, either by acting as a recombination center or by forcing higher energy carriers into short lived states that recombine before reaching the contacts. The fact that the photovoltage bleaching correlates with the ground state PCBM singlet exciton suggests that charge dissociation from PCBM preferentially generates long-lived localized states. Capacitive photocurrent measurements were then performed on isolated methanofullerene derivatives, with the polymer donor material absent. Several low energy transitions were resolved in the optical spectroscopy of methanofullerene derivatives. These low energy states lay below the optical energy band-gap of these materials, so that their presence was unexpected. It was determined that the low energy states overlapped with the plasmon state of the highly conducting substrates (Indium Tin Oxide) which were in close proximity with the PCBM. Plasmon states in ITO have been observed previously, but the results presented in this thesis are unique in that this is the first evidence of charge transfer from the plasmon state of ITO to a high electron affinity fullerene derivative. The results show the evidence of charge transfer from PCBM to ITO over a broad wavelength range of 400 - 2400 nm (3 eV - 0.5 eV). Few materials have been observed to have absorbance and charge transfer over such a large range of energies in the infra-red regime. These results open a new direction for development of organic solar cell design with higher power conversion efficiencies

Dispersion of narrow diameter carbon nanotubes for optical characterization.

Optical properties of carbon nanotubes have recently attracted considerable amount of attention. Due to there direct band gap material characteristic these and dimension of the order of nano meters they find potential applications in the field of nano photonics. Thus the optical study of carbon nanotubes is important for both fundamental research and for the next generation technical applications. In this thesis single walled carbon nanotubes were dispersed in various encapsulates such as surfactants, polymers, proteins etc to separate them individually and study there optical properties. The individually dispersed single walled carbon nanotubes displayed unique absorbance spectra. The optical absorption spectrum of a particular tube is expected to be dominated by a series of relatively sharp inter-band transitions, at energies associated with the van Hove singularities, the absorbance spectrum obtained on our samples were consistent with this expectation. Our samples also showed sharp photoluminescence peaks mostly from the semiconducting single walled carbon nanotubes. The detailed overlap of the absorbance spectra and photoluminescence spectra lead us to believe that our samples contain mainly individual tubes encapsulated in one of the surfactants or polymers. Thin film transistors (TFT) were made with a mesh of single walled carbon nanotubes as the active channel on top of a silicon/silicon oxide substrate, with silicon acting as a back gate and titanium/gold electrodes were evaporated on top of the nanotube film. Photocurrent properties of these thin films were investigated and they showed a huge change in the photocurrent in the presence and absence of the laser light. Thin films of carbon nanotubes were also deposited on rectangular silicon substrates and the influence of applied strain in the presence of laser light was investigated. These films showed a huge change in resistance on application of mechanical strain. Raman measurements were also performed on these thin films and the obtained radial breathing mode (RBM) data helped resolve the diameter of many single walled carbon nanotubes

Automated detection of naming conflicts in schema integration: Experiments with quiddities*

This paper discusses experiments involving a method for the automatic detection, prior to the integration of data base schemas, of conflicts in the naming of data elements within these schemas. The method relies on the representation of semantic information (called quiddity) about the data elements present in the various schemas. We develop several inference procedures which, utilizing this information, determine whether to distinctly named elements in fact represent the same object, or if elements with the same name actually represent different objects. The experiments are concerned with (a) examining the accuracy and consistency with which quiddities of data elements might be declared by different database designers, and (b) evaluating the accuracy and errors of these automated procedures. Our results indicate that the method has promise for use in detection of naming conflicts, and that certain inference procedures are superior to others in terms of their accuracy and error ratesNaval Postgraduate School, Monterey, CAhttp://archive.org/details/automateddetecti00bharO&MN Direct FundingNAApproved for public release; distribution is unlimited

Evolution of Quantum Discord and its Stability in Two-Qubit NMR Systems

We investigate evolution of quantum correlations in ensembles of two-qubit nuclear spin systems via nuclear magnetic resonance techniques. We use discord as a measure of quantum correlations and the Werner state as an explicit example. We first introduce different ways of measuring discord and geometric discord in two-qubit systems and then describe the following experimental studies: (a) We quantitatively measure discord for Werner-like states prepared using an entangling pulse sequence. An initial thermal state with zero discord is gradually and periodically transformed into a mixed state with maximum discord. The experimental and simulated behavior of rise and fall of discord agree fairly well. (b) We examine the efficiency of dynamical decoupling sequences in preserving quantum correlations. In our experimental setup, the dynamical decoupling sequences preserved the traceless parts of the density matrices at high fidelity. But they could not maintain the purity of the quantum states and so were unable to keep the discord from decaying. (c) We observe the evolution of discord for a singlet-triplet mixed state during a radio-frequency spin-lock. A simple relaxation model describes the evolution of discord, and the accompanying evolution of fidelity of the long-lived singlet state, reasonably well.Comment: 9 pages, 7 figures, Phys. Rev. A (in press

Electric field Induced Patterns in Soft Visco-elastic films: From Long Waves of Viscous Liquids to Short Waves of Elastic Solids

We show that the electric field driven surface instability of visco-elastic films has two distinct regimes: (1) The visco-elastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as ~ 4 x film thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus.Comment: Accepted for Publication in Physical Review Letter