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Missouri University of Science and Technology

2008

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Articles 1 - 30 of 137

Full-Text Articles in Physical Sciences and Mathematics

A Uniformly Dissipative Scheme For Stationary Statistical Properties Of The Infinite Prandtl Number Model, Wenfang (Wendy) Cheng, Xiaoming Wang Dec 2008

A Uniformly Dissipative Scheme For Stationary Statistical Properties Of The Infinite Prandtl Number Model, Wenfang (Wendy) Cheng, Xiaoming Wang

Mathematics and Statistics Faculty Research & Creative Works

The purpose of this short communication is to announce that a class of numerical schemes, uniformly dissipative approximations, which uniformly preserve the dissipativity of the continuous infinite dimensional dissipative complex (chaotic) systems possess desirable properties in terms of approximating stationary statistics properties. in particular, the stationary statistical properties of these uniformly dissipative schemes converge to those of the continuous system at vanishing mesh size. the idea is illustrated on the infinite Prandtl number model for convection and semi-discretization in time, although the general strategy works for a broad class of dissipative complex systems and fully discretized approximations. as far as …

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A Semi-Implicit Scheme For Stationary Statistical Properties Of The Infinite Prandtl Number Model, Wenfang Cheng, Xiaoming Wang Dec 2008

A Semi-Implicit Scheme For Stationary Statistical Properties Of The Infinite Prandtl Number Model, Wenfang Cheng, Xiaoming Wang

Mathematics and Statistics Faculty Research & Creative Works

We propose a semisecret in time semi-implicit numerical scheme for the infinite Prandtl model for convection. Besides the usual finite time convergence, this scheme enjoys the additional highly desirable feature that the stationary statistical properties of the scheme converge to those of the infinite Prandtl number model at vanishing time stop. One of the key characteristics of the scheme is that it preserves the dissipativity of the infinite Prandtl number model uniformly in terms of the time stop. So far as wo know, this is the first rigorous result on convergence of stationary statistical properties of numerical schemes for infinite …

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Using Clinical Decision Support To Maintain Medication And Problem Lists: A Pilot Study To Yield Higher Patient Safety, Chiang S. Jao, Daniel B. Hier, William L. Galanter Dec 2008

Using Clinical Decision Support To Maintain Medication And Problem Lists: A Pilot Study To Yield Higher Patient Safety, Chiang S. Jao, Daniel B. Hier, William L. Galanter

Chemistry Faculty Research & Creative Works

To Investigate Whether Clinical Decision Support that Automates the Matching of Ordered Drugs to Problems (Clinical Diagnoses) on the Problem List Can Enhance the Maintenance of Both Medication and Problem Lists in the Electronic Medical Record, We Designed a Clinical Decision Support System to Match Ordered Drugs on the Medication List and Ongoing Problems on the Problem List. We Evaluated the Capability and Performance of This Clinical Decision Support System in Medication-Problem Matching using Physician Expert Chart Audits to Match Ordered Drugs to Ongoing Clinical Problems. a Clinical Decision Support System Was Shown to Be Useful in Improving Medication-Problem Matches …

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A Model Based Fault Detection And Prognostic Scheme For Uncertain Nonlinear Discrete-Time Systems, Balaje T. Thumati, Jagannathan Sarangapani Dec 2008

A Model Based Fault Detection And Prognostic Scheme For Uncertain Nonlinear Discrete-Time Systems, Balaje T. Thumati, Jagannathan Sarangapani

Electrical and Computer Engineering Faculty Research & Creative Works

A new fault detection and prognostics (FDP) framework is introduced for uncertain nonlinear discrete time system by using a discrete-time nonlinear estimator which consists of an online approximator. A fault is detected by monitoring the deviation of the system output with that of the estimator output. Prior to the occurrence of the fault, this online approximator learns the system uncertainty. In the event of a fault, the online approximator learns both the system uncertainty and the fault dynamics. A stable parameter update law in discrete-time is developed to tune the parameters of the online approximator. This update law is also …

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Neural Network Output Feedback Control Of A Quadrotor Uav, Jagannathan Sarangapani, Travis Alan Dierks Dec 2008

Neural Network Output Feedback Control Of A Quadrotor Uav, Jagannathan Sarangapani, Travis Alan Dierks

Electrical and Computer Engineering Faculty Research & Creative Works

A neural network (NN) based output feedback controller for a quadrotor unmanned aerial vehicle (UAV) is proposed. The NNs are utilized in the observer and for generating virtual and actual control inputs, respectively, where the NNs learn the nonlinear dynamics of the UAV online including uncertain nonlinear terms like aerodynamic friction and blade flapping. It is shown using Lyapunov theory that the position, orientation, and velocity tracking errors, the virtual control and observer estimation errors, and the NN weight estimation errors for each NN are all semi-globally uniformly ultimately bounded (SGUUB) in the presence of bounded disturbances and NN functional …

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Built-In Electric Field Assisted Spin Injection In Cr And Mn Δ-Layer Doped Aln/Gan(0001) Heterostructures From First Principles, X. Y. Cui, Julia E. Medvedeva, B. Delley, C. Stampfl, Arthur J. Freeman Dec 2008

Built-In Electric Field Assisted Spin Injection In Cr And Mn Δ-Layer Doped Aln/Gan(0001) Heterostructures From First Principles, X. Y. Cui, Julia E. Medvedeva, B. Delley, C. Stampfl, Arthur J. Freeman

Physics Faculty Research & Creative Works

Highly spin-polarized diluted ferromagnetic semiconductors are expected to be widely used as ideal spin injectors. Here, extensive first-principles density-functional theory calculations have been performed to investigate the feasibility of using Cr- and Mn-doped wurtzite polar AlN/GaN(0001) heterostructures, with the aim to realize the appealing half-metallic character and, hence, efficient electrical spin injection. To overcome the formation of detrimental embedded clusters, we propose digital delta-layer doping perpendicular to the growth direction so as to realize enhanced performance at room temperature. The formation energy, electronic and magnetic properties, and the degree of spin polarization for both neutral and charged valence states for …

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Retrieving Photorecombination Cross Sections Of Atoms From High-Order Harmonic Spectra, Shinichiro Minemoto, Toshihito Umegaki, Yuichiro Oguchi, Toru Morishita, Anh-Thu Le, Shinichi Watanabe, Hirofumi Sakai Dec 2008

Retrieving Photorecombination Cross Sections Of Atoms From High-Order Harmonic Spectra, Shinichiro Minemoto, Toshihito Umegaki, Yuichiro Oguchi, Toru Morishita, Anh-Thu Le, Shinichi Watanabe, Hirofumi Sakai

Physics Faculty Research & Creative Works

We observe high-order harmonic spectra generated from a thin atomic medium, Ar, Kr, and Xe, by intense 800-nm and 1300-nm femtosecond pulses. A clear signature of a single-atom response is observed in the harmonic spectra. Especially in the case of Ar, a Cooper minimum, reflecting the electronic structure of the atom, is observed in the harmonic spectra. We successfully extract the photorecombination cross sections of the atoms in the field-free condition with the help of an accurate recolliding electron wave packet. The present protocol paves the way for exploring ultrafast imaging of molecular dynamics with attosecond resolution.

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Interference Effects Due To Projectile Target Nucleus Scattering In Single Ionization Of H₂ By 75-Kev Proton Impact, Jason S. Alexander, Aaron C. Laforge, Ahmad Hasan, Z. S. Machavariani, M. F. Ciappina, R. D. Rivarola, Don H. Madison, Michael Schulz Dec 2008

Interference Effects Due To Projectile Target Nucleus Scattering In Single Ionization Of H₂ By 75-Kev Proton Impact, Jason S. Alexander, Aaron C. Laforge, Ahmad Hasan, Z. S. Machavariani, M. F. Ciappina, R. D. Rivarola, Don H. Madison, Michael Schulz

Physics Faculty Research & Creative Works

Doubly differential cross sections (DDCSs) for single ionization of molecular hydrogen by 75-keV proton impact have been measured and calculated as a function of the projectile scattering angle and energy loss. Interference structures are observed in the scattering angular dependence of the DDCSs, which disappear, however, at electron speeds near the projectile speed. The comparison to our calculations shows that the projectile-target nucleus interaction plays a central role. Furthermore, our data suggest that for a given scattering angle, ionization favors well-defined molecular orientations.

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Spin- And Fine-Structure-Resolved Ionization Of Krypton, S. Bellm, J. Lower, R. P. Mceachran, E. Weigold, Ciarán Ryan-Anderson, Don H. Madison Dec 2008

Spin- And Fine-Structure-Resolved Ionization Of Krypton, S. Bellm, J. Lower, R. P. Mceachran, E. Weigold, Ciarán Ryan-Anderson, Don H. Madison

Physics Faculty Research & Creative Works

The influence of exchange, correlation, and relativistic effects in the ionization of heavy-target atoms can be sensitively probed by kinematically complete studies involving spin-polarized electrons, in particular when the fine structure of the residual ion is resolved. We present spin asymmetries, triple differential cross sections, and branching ratios for the ionization, by 127.5-eV and 114.3-eV electrons, of ground-state krypton atoms leading to the Kr+ 4s 2S1/2, 4p5 2P1/2, and 4p5 2P3/2 states. In order to untangle contributions from different physical effects, the experimental results are compared to those from distorted-wave Born approximation calculations (non- and semirelativistic) in which bound-state and …

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Neural-Network-Based State Feedback Control Of A Nonlinear Discrete-Time System In Nonstrict Feedback Form, Pingan He, Jagannathan Sarangapani Dec 2008

Neural-Network-Based State Feedback Control Of A Nonlinear Discrete-Time System In Nonstrict Feedback Form, Pingan He, Jagannathan Sarangapani

Electrical and Computer Engineering Faculty Research & Creative Works

In this paper, a suite of adaptive neural network (NN) controllers is designed to deliver a desired tracking performance for the control of an unknown, second-order, nonlinear discrete-time system expressed in nonstrict feedback form. In the first approach, two feedforward NNs are employed in the controller with tracking error as the feedback variable whereas in the adaptive critic NN architecture, three feedforward NNs are used. In the adaptive critic architecture, two action NNs produce virtual and actual control inputs, respectively, whereas the third critic NN approximates certain strategic utility function and its output is employed for tuning action NN weights …

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Pas: Predicate-Based Authentication Services Against Powerful Passive Adversaries, Sriram Chellappan, Xiaole Bai, Wenjun Gu, Xun Wang, Dong Xuan, Bin Ma Dec 2008

Pas: Predicate-Based Authentication Services Against Powerful Passive Adversaries, Sriram Chellappan, Xiaole Bai, Wenjun Gu, Xun Wang, Dong Xuan, Bin Ma

Computer Science Faculty Research & Creative Works

Securely authenticating a human user without assistance from any auxiliary device in the presence of powerful passive adversaries is an important and challenging problem. Passive adversaries are those that can passively monitor, intercept, and analyze every part of the authentication procedure, except for an initial secret shared between the user and the server. In this paper, we propose a new secure authentication scheme called predicate-based authentication service (PAS). In this scheme, for the first time, the concept of a predicate is introduced for authentication. We conduct analysis on the proposed scheme and implement its prototype system. Our analytical data and …

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Differential Cross Sections For The Ionization Of Oriented H₂ Molecules By Electron Impact, James Colgan, Michael S. Pindzola, Francis J. Robicheaux, Christian V. Kaiser, Andrew James Murray, Don H. Madison Dec 2008

Differential Cross Sections For The Ionization Of Oriented H₂ Molecules By Electron Impact, James Colgan, Michael S. Pindzola, Francis J. Robicheaux, Christian V. Kaiser, Andrew James Murray, Don H. Madison

Physics Faculty Research & Creative Works

A nonperturbative close-coupling technique is used to calculate differential cross sections for the electron-impact ionization of H2 at an energy of 35.4 eV. Our approach allows cross sections for any orientation of the molecule with respect to the incident electron beam to be analyzed. New features in the resulting cross sections are found compared with the case where the molecular orientation is averaged, and also with cross sections for He at equivalent electron kinematics. When averaged over all possible molecular orientations, good agreement is found with recent experimental results.

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A Systematic Framework For Structured Object-Oriented Security Requirements Analysis In Embedded Systems, Sojan Markose, Xiaoqing Frank Liu, Bruce M. Mcmillin Dec 2008

A Systematic Framework For Structured Object-Oriented Security Requirements Analysis In Embedded Systems, Sojan Markose, Xiaoqing Frank Liu, Bruce M. Mcmillin

Computer Science Faculty Research & Creative Works

The primary goal of this paper is to develop a structured objectoriented security requirements analysis methodology for the elicitation and analysis of security requirements in embedded systems. There are several approaches to elicit, analyze and specify security requirements in embedded systems ranging from formal mathematical models for proof of certain security properties to informal methods that are easily understood. Applicability of formal security models is limited because they are complex and it is time consuming to develop. On the other hand, informal security requirements analysis methods are not integrated with conceptual models in requirements analysis, and although both external and …

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Implementation And Analysis Of Practical Algorithm For Data Security, Willi Ballenthin, F. Kacani, Julia Albath, Sanjay Kumar Madria Dec 2008

Implementation And Analysis Of Practical Algorithm For Data Security, Willi Ballenthin, F. Kacani, Julia Albath, Sanjay Kumar Madria

Computer Science Faculty Research & Creative Works

In this paper, we present a complete implementation of the Practical Algorithm for Data Security (PADS) proposed by Albath et al., an end-to-end security scheme employing symmetric key encryption. The implementation takes full advantage of the modular design of the TinyOS environment. The simplicity of the algorithm allows for efficient implementation in hardware, a requirement for resource constrained devices. The protocol adds only four bytes of data per packet, on par with industry standards. Simulation and empirical results of the scheme are also provided. The analysis shows that the Practical Algorithm for Data Security is superior to standard security schemes.

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Secure Communications Over Hybrid Military Networks, Vamsi Paruchuri, Arjan Durresi, Sriram Chellappan Nov 2008

Secure Communications Over Hybrid Military Networks, Vamsi Paruchuri, Arjan Durresi, Sriram Chellappan

Computer Science Faculty Research & Creative Works

Stealthiness can be described as a disposition to be sly and to do things surreptitiously. This paper presents a new architecture for flexible and secure networking in battlefields that enables stealthy and covert communication in the presence of node mobility. Our architecture is based on the combination of optical (fiber) and wireless links. Our objective is to be able to carry on undeterred communication without the attack/eavesdropping nodes being able to detect the presence of any communication. This objective is not only crucial for successful completion of the operation, but also for the safety of our mobile nodes, by not …

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Three-Dimensional Modeling Of The Plasma Arc In Arc Welding, Gu Xu, J. Hu, Hai-Lung Tsai Nov 2008

Three-Dimensional Modeling Of The Plasma Arc In Arc Welding, Gu Xu, J. Hu, Hai-Lung Tsai

Mechanical and Aerospace Engineering Faculty Research & Creative Works

Most previous three-dimensional modeling on gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) focuses on the weld pool dynamics and assumes the two-dimensional axisymmetric Gaussian distributions for plasma arc pressure and heat flux. In this article, a three-dimensional plasma arc model is developed, and the distributions of velocity, pressure, temperature, current density, and magnetic field of the plasma arc are calculated by solving the conservation equations of mass, momentum, and energy, as well as part of the Maxwell's equations. This three-dimensional model can be used to study the nonaxisymmetric plasma arc caused by external perturbations such as …

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Revisiting Unitarity Corrections For Electromagnetic Processes In Collisions Of Relativistic Nuclei, Ulrich D. Jentschura, Kai Hencken, Valery G. Serbo Nov 2008

Revisiting Unitarity Corrections For Electromagnetic Processes In Collisions Of Relativistic Nuclei, Ulrich D. Jentschura, Kai Hencken, Valery G. Serbo

Physics Faculty Research & Creative Works

Unitarity corrections to several electromagnetic processes in collisions of relativistic heavy nuclei are considered. They are due to the unitarity requirement for the S-matrix and correspond to the exchange of a light-by-light scattering block between colliding nuclei. We obtain improved results for the corrections to e+ e- and μ+ μ- pair production as well as new results for unitarity corrections to the production of photons via virtual Compton and virtual Delbrück scattering. These corrections can be numerically large; e.g., the μ+ μ- pair production cross section is reduced by about 50% and nuclear bremsstrahlung …

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New (E,2e) Studies Of Atomic And Molecular Targets, E. M. Staicu-Casagrande, A. Naja, A. Lahmam-Bennani, A. S. Kheifets, Don H. Madison, B. Joulakian Nov 2008

New (E,2e) Studies Of Atomic And Molecular Targets, E. M. Staicu-Casagrande, A. Naja, A. Lahmam-Bennani, A. S. Kheifets, Don H. Madison, B. Joulakian

Physics Faculty Research & Creative Works

We report new coplanar (e,2e) measurements characterised by large energy transfer and close to minimum momentum transfer from the projectile to the target. Ionisation of the two-electron targets He and H2 is investigated under these particular kinematics. The experimental data are compared with the predictions of the most elaborate theoretical models. The obtained good agreement motivated us to extend our research to the case of more complex targets such as Ar. Comparison with the most elaborate models in the case of multi-electron targets is excellent. Destructive and constructive interference effects in the case of H2 are observed and …

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Localizing Sensor Networks In Un-Friendly Environments, Sriram Chellappan, Vamsi Paruchuri, Dylan Mcdonald, Arjan Durresi Nov 2008

Localizing Sensor Networks In Un-Friendly Environments, Sriram Chellappan, Vamsi Paruchuri, Dylan Mcdonald, Arjan Durresi

Computer Science Faculty Research & Creative Works

In this paper, we study the issue of defending against a wireless sensor network (WSN) that has been deployed by a malicious enemy agent in an area of interest to us. While there can be many approaches to defend against maliciously deployed WSNs, we propose the design of a localization centric approach. Specifically, the problem we address is: given an enemy deployed WSN in an area of interest to us, how can we determine locations of the sensors without co-operating with the sensors themselves during localization. In our approach, we employ a physically mobile agent called the localizer (e.g., a …

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Laser Channeling Of Bethe-Heitler Pairs, Erik Lotstedt, Ulrich D. Jentschura, Christoph H. Keitel Nov 2008

Laser Channeling Of Bethe-Heitler Pairs, Erik Lotstedt, Ulrich D. Jentschura, Christoph H. Keitel

Physics Faculty Research & Creative Works

Electron-positron pair creation is analyzed for an arrangement involving three external fields: a high-frequency gamma photon, the Coulomb field of a nucleus, and a strong laser wave. The frequency of the incoming gamma photon is assumed to be larger than the threshold for pair production in the absence of a laser, and the peak electric field of the laser is assumed to be much weaker than Schwinger's critical field. The total number of pairs produced is found to be essentially unchanged by the laser field, while the differential cross section is drastically modified. We show that the laser can channel …

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Simulations Of Baryon Acoustic Oscillations - I. Growth Of Large-Scale Density Fluctuations, Ryuichi Takahashi, Naoki Yoshida, Shun Saito, For Full List Of Authors, See Publisher's Website. Oct 2008

Simulations Of Baryon Acoustic Oscillations - I. Growth Of Large-Scale Density Fluctuations, Ryuichi Takahashi, Naoki Yoshida, Shun Saito, For Full List Of Authors, See Publisher's Website.

Physics Faculty Research & Creative Works

We critically examine how well the evolution of large-scale density perturbations is followed in cosmological N-body simulations. We first run a large volume simulation and perform a mode-by-mode analysis in three-dimensional Fourier space. We show that the growth of large-scale fluctuations significantly deviates from linear-theory predictions. The deviations are caused by non-linear coupling with a small number of modes at largest scales owing to finiteness of the simulation volume. We then develop an analytic model based on second-order perturbation theory to quantify the effect. Our model accurately reproduces the simulation results. For a single realization, the second-order effect appears typically …

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A Model Based Fault Detection Scheme For Nonlinear Multivariable Discrete-Time Systems, Balaje T. Thumati, Jagannathan Sarangapani Oct 2008

A Model Based Fault Detection Scheme For Nonlinear Multivariable Discrete-Time Systems, Balaje T. Thumati, Jagannathan Sarangapani

Electrical and Computer Engineering Faculty Research & Creative Works

In this paper, a novel robust scheme is developed for detecting faults in nonlinear discrete time multi-input and multi-output systems in contrast with the available schemes that are developed in continuous-time. Both state and output faults are addressed by considering separate time profiles. The faults, which could be incipient or abrupt, are modeled using input and output signals of the system. By using nonlinear estimation techniques, the discrete-time system is monitored online. Once a fault is detected, its dynamics are characterized using an online approximator. A stable parameter update law is developed for the online approximator scheme in discrete-time. The …

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Who Can Help Me With This Source Code Change, Huzefa Kagdi, Maen Hammad, Jonathan I. Maletic Oct 2008

Who Can Help Me With This Source Code Change, Huzefa Kagdi, Maen Hammad, Jonathan I. Maletic

Computer Science Faculty Research & Creative Works

An approach to recommend a ranked list of developers to assist in performing software changes to a particular file is presented. The ranking is based on change expertise, experience, and contributions of developers, as derived from the analysis of the previous commits involving the specific file in question. The commits are obtained from a software system's version control repositories (e.g., Subversion). The basic premise is that a developer who has substantially contributed changes to specific files in the past is likely to best assist for their current or future change. Evaluation of the approach on a number of open source …

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Joint Adaptive Distributed Rate And Power Control For Wireless Networks, James W. Fonda, Jagannathan Sarangapani, Steve Eugene Watkins Oct 2008

Joint Adaptive Distributed Rate And Power Control For Wireless Networks, James W. Fonda, Jagannathan Sarangapani, Steve Eugene Watkins

Electrical and Computer Engineering Faculty Research & Creative Works

A novel adaptive distributed rate and power control (ADRPC) protocol is introduced for wireless networks. The proposed controller contrasts from others by providing nonlinear compensation to the problem of transmission power and bit-rate adaptation. The protocol provides control of both signal-to-interference ratio (SIR) and quality-of-service (QoS) support to bit-rate adaptation. Bit-rate adaptation is performed by local estimation of congestion levels, rendering little packet overhead, using Lyapunov based adaptive control methods. Performance of the proposed control scheme is shown through analytical proof and simulation examples.

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Reexamining Blackbody Shifts For Hydrogenlike Ions, Ulrich D. Jentschura, Martin K. Haas Oct 2008

Reexamining Blackbody Shifts For Hydrogenlike Ions, Ulrich D. Jentschura, Martin K. Haas

Physics Faculty Research & Creative Works

We investigate blackbody-induced energy shifts for low-lying levels of atomic systems, with a special emphasis on transitions used in current and planned high-precision experiments on atomic hydrogen and ionized helium. Fine-structure- and Lamb-shift-induced blackbody shifts are found to increase with the square of the nuclear charge number, whereas blackbody shifts due to virtual transitions decrease with increasing nuclear charge as the fourth power of the nuclear charge. We also investigate the decay width acquired by the ground state of atomic hydrogen, due to interaction with blackbody photons. The corresponding width is due to an instability against excitation to higher excited …

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Novel Dynamic Representation And Control Of Power Networks Embedded With Facts Devices, Shahab Mehraeen, Jagannathan Sarangapani, Mariesa Crow Oct 2008

Novel Dynamic Representation And Control Of Power Networks Embedded With Facts Devices, Shahab Mehraeen, Jagannathan Sarangapani, Mariesa Crow

Electrical and Computer Engineering Faculty Research & Creative Works

FACTS devices have been shown to be powerful in damping power system oscillations caused by faults; however, in the multi machine control using FACTS, the control problem involves solving differential-algebraic equations of a power network which renders the available control schemes ineffective due to heuristic design and lack of know how to incorporate FACTS into the network. A method to generate nonlinear dynamic representation of a power system consisting of differential equations alone with universal power flow controller (UPFC) is introduced since differential equations are typically preferred for controller development. Subsequently, backstepping methodology is utilized to reduce the generator oscillations …

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Optimal Energy-Delay Routing Protocol With Trust Levels For Wireless Ad Hoc Networks, Eyad Taqieddin, Ann K. Miller, Jagannathan Sarangapani Sep 2008

Optimal Energy-Delay Routing Protocol With Trust Levels For Wireless Ad Hoc Networks, Eyad Taqieddin, Ann K. Miller, Jagannathan Sarangapani

Electrical and Computer Engineering Faculty Research & Creative Works

This paper presents the Trust Level Routing (TLR) pro- tocol, an extension of the optimized energy-delay rout- ing (OEDR) protocol, focusing on the integrity, reliability and survivability of the wireless network. TLR is similar to OEDR in that they both are link state routing proto- cols that run in a proactive mode and adopt the concept of multi-point relay (MPR) nodes. However, TLR aims at incorporating trust levels into routing by frequently changing the MPR nodes as well as authenticating the source node and contents of control packets. TLR calcu- lates the link costs based on a composite metric (delay …

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Phase Transitions In Lafeaso: Structural, Magnetic, Elastic, And Transport Properties, Heat Capacity And Mössbauer Spectra, Michael A. Mcguire, Andrew D. Christianson, Athena S. Sefat, Brian C. Sales, Mark D. Lumsden, Rongying Jin, Edward Andrew Payzant, David G. Mandrus, Yanbing Luan, Veerle M. Keppens, Vijayalaksmi Varadarajan, Joseph W. Brill, Raphäel P. Hermann, M. T. Sougrati, Fernande Grandjean, Gary J. Long Sep 2008

Phase Transitions In Lafeaso: Structural, Magnetic, Elastic, And Transport Properties, Heat Capacity And Mössbauer Spectra, Michael A. Mcguire, Andrew D. Christianson, Athena S. Sefat, Brian C. Sales, Mark D. Lumsden, Rongying Jin, Edward Andrew Payzant, David G. Mandrus, Yanbing Luan, Veerle M. Keppens, Vijayalaksmi Varadarajan, Joseph W. Brill, Raphäel P. Hermann, M. T. Sougrati, Fernande Grandjean, Gary J. Long

Chemistry Faculty Research & Creative Works

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of “FeAs” based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonal-orthorhombic crystallographic phase transition at ~160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutron-diffraction measurements is presented. A magnetic moment of ~0.35µB per iron is derived from Mössbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature (up to near 200 K) in the diffraction data as well …

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Iron Oxide Deposits Associated With The Ectosymbiotic Bacteria In The Hydrothermal Vent Shrimp Rimicaris Exoculata, Laure Corbari, Marie Anne Cambon-Bonavita, Gary J. Long, Fernande Grandjean, Magali Zbinden, Françoise Gaill, Philippe Compère Sep 2008

Iron Oxide Deposits Associated With The Ectosymbiotic Bacteria In The Hydrothermal Vent Shrimp Rimicaris Exoculata, Laure Corbari, Marie Anne Cambon-Bonavita, Gary J. Long, Fernande Grandjean, Magali Zbinden, Françoise Gaill, Philippe Compère

Chemistry Faculty Research & Creative Works

The Rimicaris exoculata shrimp is considered as a primary consumer that dominates the fauna of most Mid-Atlantic Ridge (MAR) hydrothermal ecosystems. These shrimps harbour in their gill chambers an important ectosymbiotic community of chemoautotrophic bacteria associated with iron oxide deposits. The structure and elemental composition of the mineral concretions associated with these bacteria have been investigated by using LM, ESEM, TEM STEM and EDX microanalyses. The nature of the iron oxides in shrimps obtained from the Rainbow vent field has also been determined by Mössbauer spectroscopy. This multidisciplinary approach has revealed that the three layers of mineral crust in the …

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Photorefractivity In A Polymeric Composite Photosensitized With Nis Nanocrystals, Tyler M. Fears, Charles Anderson, Jeffrey G. Winiarz Sep 2008

Photorefractivity In A Polymeric Composite Photosensitized With Nis Nanocrystals, Tyler M. Fears, Charles Anderson, Jeffrey G. Winiarz

Chemistry Faculty Research & Creative Works

The photorefractive performance of a polymeric composite photosensitized through the inclusion of NiS nanocrystals is described. The nanocrystals were characterized using visible-absorption spectroscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. We further demonstrate the ability to enhance various aspects of the composite's photorefractive performance by performing ligand exchange on the nanocrystals prior to their incorporation into the polymer composite. This procedure resulted in a lowering of the overmodulation voltage from ~70 to ~50 V/µm without affecting the maximum diffraction efficiency of ~40%. An increase in the two-beam-coupling gain coefficient was similarly observed, increasing from 38 to 79 cm−1. The photoconductivities …

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