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8 results
  • 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.
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  • Responses of inferior colliculus neurons of the anaesthetized, cochlea-ectomized chicken to electrical stimulation of the cochlear nerves were recorded extracellularly. At least three physiologically distinct cell types were found in the central nucleus of the inferior colliculus. Type 1 fired randomly and with a high spontaneous rate, exhibiting a Poisson distribution in the spike interval histogram. Stimulation of either cochlear nerve produced an inhibition lasting 3 to 46 ms (mean of 16.7, n=16). Type 2 exhibited little or no spontaneous activity, and responded to a short stimulus with a single spike or burst of spikes (n=21). Type 3 exhibited regular, spontaneous firing with preferred intrinsic frequencies in the audio range (n=26), usually resulting in multimodal spike interval histograms. Single pulse stimulation of the contralateral nerve reset the firing rhythm, resulting in periodic post-stimulus time histograms (PSTH). The intermodal interval for a PSTH of a type 3 cell was identical to the intermodal interval for a spontaneous interval histogram. A reverse correlation of a random sequence of stimulus pulses with the response spikes revealed preferred frequencies in the input which were similar to the output frequencies seen in post-stimulus time and interval histograms. Thus, these type 3 neurons exhibited both an oscillatory spontaneous and evoked firing pattern, and an intrinsic frequency selectivity which is presumed to give rise to the observed oscillation. These cells were found to be grouped together in a relatively small portion of the inferior colliculus. The location of several type 3 cells was identified using WGA-HRP injected from the recording electrode. These cells were found to be in the core of the central nucleus of the inferior colliculus. These findings suggest the existence of an intrinsic mechanism for frequency filtering and time coding in the CNS.
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  • 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).
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  • 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.
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  • The phase-locked loop (PLL) is an essential building block of modern communication and computing systems. In a wireless communication system, a PLL is almost always used as the local oscillator (LO) that synthesizes the required frequency for data transmission and reception. In wireline and optical communication systems, PLL-based clock and data recovery (CDR) circuits are often employed for the extraction of the clock signal from the incoming data signal, and aligning the recovered clock edge with the incoming data for optimal bit-error rate (BER) performance. Furthermore, in microprocessor and field-programmable gate array (FPGA) systems, PLLs are typically used for clock generation. Although phase-locking is a very mature research topic, its continuous application in modern integrated circuits (ICs) and systems, requires continuous improvement in its performance, power consumption, and manufacturing costs. Analog Type-II PLLs are among the most widely used category of PLLs in CMOS (complementary-metal-oxide-semiconductor) ICs, mainly due to their robustness, superior performance and their well-established theory. However, analog Type-II PLLs require a large area in loop-filter (LF) and employ noisy and difficult-to-design charge-pumps (CPs). All-digital PLLs are also widely used, but they suffer from the strict jitter requirements on time-to-digital converters (TDCs). We propose a Type-I PLL that uses a small LF area, does not require bias-generation circuits or CP, and consumes low power. A pulse-width-modulated (PWM) voltage output from the phase-frequency detector (PFD) is fed to a simple RC single-pole LF. Two major limitations of conventional Type-I topologies – limited lock-range and large reference spur – are overcome by increasing the PFD gain with a combination of a voltage booster and a digital level shifter, and a sample-and-hold (S/H) envelope detector, respectively. Furthermore, a saturated-PFD (SPFD) is proposed to reduce cycle slipping and to further improve the lock-range and lock-time. A proof-of-concept prototype 2.2-to-2.8 GHz PLL occupies a core area of 0.12 mm² in 0.13-μm CMOS and achieves 490 fsrms random jitter, -103.4 dBc/Hz in-band phase-noise, -65 dBc reference spur, 2.5 μs worst-case lock-time while consuming 6.8 mW from a 1.2 V supply.
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  • Phase-locked loops (PLLs) are widely used in telecommunication, radio, and computer applications. This thesis focuses on the study of wide-band PLLs, as they are a critical building block of many wireless and wireline systems. In particular, wide tuning range, low phase noise, and low power are desirable attributes for multi-standard and multi-band communication systems. One of the most critical components in a PLL is the voltage-controlled oscillator (VCO). In this work, two techniques for implementing a wide-tuning-range LC-VCO are presented. As a proof of concept, the techniques are used to design and layout two 13-GHz LC-VCOs, which are fabricated in a 90-nm CMOS technology and successfully tested. One design (Design A) uses two VCO cores and has an extra source-follower buffer while the other (Design B) uses one VCO core with a bank of switched capacitors. The 90-nm CMOS prototypes operate from a supply of 1.2 V. The Design A prototype has a 28.20% tuning range and a phase noise of ‒90.98 dBc/Hz at 1 MHz offset from the carrier, while the Design B prototype has a 24.42% tuning range and a phase noise of ‒94.20 dBc/Hz at 1 MHz offset. This measured performance is comparable with state-of-the-art wide-tuning-range VCOs. The total chip size, excluding pads, is 0.335 × 0.750 mm² and 0.316 × 0.425 mm² for Designs A and B, respectively. It was found that the addition of the source-follower buffer allows the VCO to function at a higher frequency, while the presence of the switched capacitor tends to deteriorate phase noise.
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  • 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.
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  • 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.
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