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  • The Néel temperatures of both the hydrated and 96% deuterated single crystal specimen of antiferromagnetic manganese chloride, have been compared. The sample was placed in the tank circuit of a radio-frequency oscillator; the inductance of the coil and hence the frequency of oscillation thus depended on the susceptibility of the specimen. As the crystal in the liquid helium bath was warmed through the Neel temperature, the oscillation frequency was monitored by a frequency counter. By this method, the Neel temperature could be deduced to change by -2.3% when the crystal was 96% deuterated. This result is comparable to the measurements on CoCl₂.6H₂O and CuCl₂.2H₂O. A semi-quantitative explanation proposes an intimate relationship between the superexchange interaction and the hydrogen bond strength.
    Data Types:
    • Physical Object
  • A pressure transducer, sensitive to acoustic level pressures, was designed and used to measure amplitude, frequency and phase of fluctuating pressure on the surface of a three inch diameter circular cylinder at rest and exhibiting large-amplitude vortex-excited oscillation in a uniform incident wind flow. The phase of the fluctuating pressure relative to the cylinder motion and the cylinder amplitude and frequency were recorded. A disc probe connected to the pressure transducer was used in wake surveys for the stationary and oscillating cylinder. Measurements, made in the Reynolds number range 1.5(10⁴)
    Data Types:
    • Physical Object
  • The flow around a circular cylinder exhibiting vortex-induced oscillation is modelled by 2 potential vortices in a 2-dimensional, inviscid and irrotational flow. The lift on the cylinder is obtained from the general form of the Blasius equation. Pressure distribution is obtained from the pressure equation in a moving frame of reference. The lift expression is coupled to the dynamic equation of the cylinder. The phase and amplitude of oscillation are determined by the method of equivalent linearization. A relationship between amplitude of oscillation and strength of the vortices is proposed. Boot mean square pressure distribution at the Strouhal frequency on the surface of the oscillating cylinder is determined.
    Data Types:
    • Physical Object
  • A method based on phaselock techniques, for synchronizing the local oscillator signals in a proposed two-dish supersynthesis at 1420 MHz is presented. To demonstrate the feasibility of this method, the design and construction of a working system that provides phase-coherent, 1390 MHz signals at two sites, separated by a time-varying path length, is described. The phase accuracy of this system is ±5°. A provision for introducing a known phase difference between the two signals, in a manner that is suitable for interfacing with a digital computer, is included. Also, operation of the system over a frequency range greater than the expected range of doppler shift is possible, without the risk of locking to a wrong sideband. Transistor microwave oscillators at 1.4 GHz are used as voltage-controlled oscillators in this system. The performance of these devices is compared with that of the conventional voltage-controlled crystal oscillator/multiplier chain. Test results are given, which indicate that the system is suitable for use in an operational environment.
    Data Types:
    • Physical Object
  • Liquid crystal flexoelectric actuation uses an imposed electric field to create mem- brane bending and it is used by the Outer Hair Cells (OHC) located in the inner ear, whose role is to amplify sound through generation of mechanical power. Oscillations in the OHC membranes create periodic viscoelastic flows in the contacting fluid media. A key objective of this work on flexoelectric actuation relevant to OHC is to find the relations and impact of the electro-mechanical properties of the membrane, the rheolog- ical properties of the viscoelastic media, and the frequency response of the generated mechanical power output. The model developed and used in this work is based on the integration of: (i) the flexoelectric membrane shape equation applied to a circular mem- brane attached to the inner surface of a circular capillary, and (ii) the coupled capillary flow of contacting viscoelastic phases, such that the membrane flexoelectric oscillations drive periodic viscoelastic capillary flows, as in OHCs. By applying the Fourier transform formalism to the governing equation an analytical expression for the transfer function, associated to the curvature and electrical field, power dissipation elastic storage were found. The integrated flexoelectric/viscoelastic model and the novel findings contribute to the ongoing quest for a fundamental understanding of the functioning of outer hair cells (OHC), especially on the role of membrane deformation in delivering mechanical power through electromotility and its frequency-dependent power conversion efficiency.
    Data Types:
    • Other
  • Superpositions of indirectly coupled states are possible in quantum mechanics even when the intermediate states are far apart in energy. This is achieved via higher-order transitions in which the energetically forbidden intermediate states are only virtually occupied. Interest in such long-range transitions has increased recently within the context of quantum information processing with the possibility of low dissipation transfer of quantum states or coherent manipulation of two distant qubits .The recently achieved control and tunability of triple quantum dots allow to investigate phenomena relying on quantum superpositions of distant states mediated by tunneling. Recent experiments in these devices show clear evidence of charge and spin electron exchange between the outermost dots [1, 2, 3]. In the present talk I will discuss configurations of triple dots in series where long range transfer and quantum interferences determine the transport properties. I will show that the destructive interference between two virtual paths can lead to current cancelation, what we termed superexchange blockade[4]. Finally I will address long-range transport and quantum interferences in ac driven triple dots where transitions between distant and detuned dots are mediated by the exchange of photons[5]. We propose the phase difference between the two ac voltages as an external parameter, which can be easily tuned to manipulate the current characteristics. For gate voltages in phase opposition we find quantum destructive interferences among long-range and direct photon- assisted transitions, analogous to the interferences in closed-loop undriven triple dots. As the voltages oscillate in phase, interferences between multiple virtual paths give rise to dark states. Those totally cancel the current, and could be experimentally resolved. References [1] M.Busletal.,Bipolarspinblockadeandcoherentstatesuperpositionsinatriplequantumdot,NatureNanotech.8,261(2013). [2] F.Braakmanetal.,Long-distancecoherentcouplinginaquantumdotarray,NatureNanotech,8,432(2013). [3] R.Sanchezetal.,Long-RangeSpinTransferinTripleQuantumDots,Phys.Rev.Lett.,112,176803(2014). [4] R.Sanchez,F.Gallego-MarcosandG.Platero,Superexchangeblockadeintriplequantumdots,Phys.Rev.B,89,16140(R)(2014). [5] F. Gallego-Marcos,R. Sanchez and G. Platero, Coupled Landau-Zener-Stuckelberg quantum dot interferometers, Phys. Rev. B, 93, 075424 (2016).
    Data Types:
    • Other
  • In experimental particle physics, researchers must often construct a mathematical model of the experiment that can be used in fits to extract parameter values. With very large data sets, the statistical precision of measurements improves, and the required level of detail of the model increases. It can be extremely difficult or impossible to write a sufficiently precise analytical model for modern particle physics experiments. To avoid this problem, we have developed a new method for estimating parameter values from experimental data, using a Maximum Likelihood fit which compares the data distribution with a “Monte Carlo Template”, rather than an analytical model. In this technique, we keep a large number of simulated events in computer memory, and for each iteration of the fit, we use the stored true event and the current guess at the parameters to re-weight the event based on the probability functions of the underlying physical models. The re-weighted Monte-Carlo (MC) events are then used to recalculate the template histogram, and the process is repeated until convergence is achieved. We use simple probability functions for the underlying physical processes, and the complicated experimental resolution is modeled by a highly detailed MC simulation, instead of trying to capture all the details in an analytical form. We derive and explain in detail the “Monte-Carlo Re-Weighting” (MCRW) fit technique, and then apply it to the problem of measuring the neutral B meson mixing frequency. In this thesis, the method is applied to simulated data, to demonstrate the technique, and to indicate the results that could be expected when this analysis is performed on real data in the future.
    Data Types:
    • Physical Object
  • The chemical and physico - chemical properties of deoxyribonucleic acid preparations isolated from small amounts of liver and intestinal mucosa of rat (1-10 g.) by five different procedures, have been compared. The first method (29), used for preparation of deoxyribonucleic acid was based on the separation of nuclei from tissue homogenates, followed by extraction and deproteinization of deoxyribonucleic acid with strong salt solutions. The second method (20, 31) consisted of the extraction and deproteinization of nucleic acids by detergent solutions, and separation of ribonucleic acid and deoxyribonucleic acid by fractional precipitation with iso-propyl alcohol. In the third procedure crude deoxyribonucleic acid was isolated from nuclei according to the first method and the crude product was further purified according to the second procedure. The fourth method (32) was based on the disintegration of tissues by high frequency sonic oscillations, extraction of nucleoprotein from the nuclear fragments with strong salt solutions and deproteinization of deoxyribonucleic acid with chloroform -amyl alcohol mixtures. In the fifth method (36, 37) nucleic acids were extracted from tissues by hot, strong salt solutions, ribonucleic acid and deoxyribonucleic acid were separated by alkali treatment and deoxyribonucleic acid was precipitated with concentrated acid solutions. The advantages and shortcomings of the different procedures with respect to yield, purity and macromolecular state of the isolated material have been discussed. An improved technique has been described for the elution of purine and pyrimidine bases from paper chromatograms.
    Data Types:
    • Other
  • In mixed-conifer forests of western North America, fire ecologists and managers are increasingly recognizing the prevalence and importance of mixed-severity fire regimes. However, these fire regimes remain poorly understood compared to those of high- and low-severity. To enhance understanding of fire regimes in the montane forest of Jasper National Park (JNP), I reconstructed fire history and assessed forest composition, age and size structure at 29 sites (Chapter 2). Historic fires were of mixed severity through time at 18 sites, whereas the remaining 11 sites had evidence of high-severity fires only. At the site level, mean importance values of canopy trees were more even among coniferous species and greater for Pseudotsuga menziesii at mixed-severity sites. The greater numbers of veteran trees and discontinuous age structures were also significant indicators of mixed-severity fire histories. In a second study, I crossdated tree ages and fire-scar dates for 172 sites and tested whether historic fire occurrence depended on inter-annual to multi-decadal variation in climate (Chapter 3). Eighteen fires between 1646 and 1915 burned during drought years, with a weak association to El Niño phases and the negative phase of the Pacific Decadal Oscillation. Fire frequency varied through time, consistent with climate drivers and changes in land use at continental to inter-hemispheric scales. No fire scars formed since 1915, although potential recorder trees were present at all sites and climate was conducive to fire over multiple years to decades. Thus, the absence of fires during the last century can largely be attributed to active fire suppression. Improved understanding of the drivers of the historic mixed-severity fire regime enhances scientifically-based restoration, conservation, forest and wildfire management in the Park and surrounding montane forests.
    Data Types:
    • Other
  • Back-propagation is a popular method for training feed-forward neural networks. This thesis extends the back-propagation technique to dispersive networks, which contain internal delay elements. Both the delays and the weights adapt to minimize the error at the output. Dispersive networks can perform many tasks, including signal prediction, signal production, channel equalization, and spatio-temporal pattern recognition. For comparison with conventional techniques, a dispersive network was trained to predict future values of a chaotic signal using only its present value as an input. With adaptable delays, the network had less than half the prediction error of an identical network with fixed delays, and about one-quarter the error of a conventional back-propagation network. Moreover, a dispersive network can simultaneously adapt and predict, while a conventional network cannot. After training as a predictor, the network was placed in a signal production configuration, where it autonomously generated a close approximation to the training signal. The power spectrum of the network output was a good reproduction of the training signal spectrum. Networks with fixed time delays produced much less accurate power spectra, and conventional back-propagation networks were unstable, generating high-frequency oscillations. Dispersive networks also showed an improvement over conventional techniques in an adaptive channel equalization task, where the channel transfer function was nonlinear. The adaptable delays in the dispersive network allowed it to reach a lower error than other equalizers, including a conventional back-propagation network and an adaptive linear filter. However, the improved performance came at the expense of a longer training time. Dispersive networks can be implemented in serial or parallel form, using digital electronic circuitry. Unlike conventional back-propagation networks, they can operate in a fully pipelined fashion, leading to a higher signal throughput. Their implementation in analog hardware is a promising area for future research.
    Data Types:
    • Other