Abstractions and Optimizations for Data-driven Applications Across Edge and Cloud

Modern data driven applications have a novel set of requirements. Advances in deep neural networks (DNN) and computer vision (CV) algorithms have made it feasible to extract meaningful insights from large-scale deployments of urban cameras and drone video feeds. These data driven applications, usually composed as workflows, tend to have high bandwidth and low latency requirements in order to extract timely results from large data sources. Other applications may necessitate the use of multiple geographically distributed resources. Such requirements may be driven by data privacy regulations such as the General Data Protection Regulation (GDPR) of the European Union, need for specialized hardware, or as a means of avoiding vendor lock-ins. To support these modern applications, a diverse computing landscape has emerged over the last decade. We have witnessed increasingly powerful Edge computing resources be available in network proximity to the data sources for these applications. The number of Cloud Service Providers (CSPs) has increased along with the regions in which they operate. And finally, the CSPs have supplemented Infrastructure as a Service (IaaS) offerings with modern serverless compute offerings which promise cost benefits as well as lower operational overheads. The availability of choices in compute resources makes it challenging for application developers to manage the lifecycle of their applications – from programming the application, to optimizing it for performance, and finally deploying it. Typically, developers rely on platforms that promise ease of programmability coupled with scalability with minimal developer effort. However, the combination of application requirements and compute resource characteristics makes it challenging for platform designers to make design choices that optimizes the application for programmability and performance. A thorough revisit of existing platforms, abstractions, and optimizations is essential for addressing these challenges. In this thesis, we tackle these challenges with three distinct but related research contributions on scalable platforms, distributed algorithms and system optimizations: (1) We propose Anveshak, a platform that provides a domain specific programming model and a distributed runtime for efficiently tracking entities in a multi-camera network; (2) We design algorithms and heuristics to solve MSP, which co-schedules the flight routes of a drone fleet to visit and record video at waypoints, and perform subsequent on-board Edge analytics; and (3) We develop XFaaS, a platform that allows “zero touch” deployment of functions and workflows across multiple clouds and Edges by automatically generating code wrappers, Cloud queues, and coordinating with the native FaaS engine of a CSP. These platforms, abstractions and optimizations solve different combinations of the problem dimensions, are motivated through real-world applications, and the solutions are validated through detailed experiments on distributed systems. Taken together, this suite of contributions addresses the key gaps highlighted in this dissertation, and help bridge the gap between modern computing resource characteristics and modern application requirements

Structural insights into the organization and channel behavior of Pannexin isoforms

Pannexins are large-pore ion channels structurally related to Connexins and Innexins but remain as hemichannels to release cellular ATP upon activation. Pannexins comprise three isoforms, Pannexin1, 2, and 3, with diverse cellular roles ranging from inflammation, differentiation, and neuropathic pain to ATP release. In this study, we report the Cryo-EM structure of Pannexin3 to draw insights into the effects on channel organization and function compared to the Pannexin1 isoform. The Pannexin3 isoform displays weak ATP binding but shows similar voltage dependence compared to Pannexin1. We also report the structures of Pannexin1 congenital mutant R217H along with a Pannexin1 double mutant W74R/R75D that mimics Pannexin2 pore residues to a resolution range of 3.8-4.2Å. The mutant structures undergo minor structural changes to form a partially closed pore. The ATP binding analysis reveals weak binding affinity of the mutants compared to wild-type Pannexin1. Moreover, the congenital mutant displays altered voltage dependence compared to the wild type. The results signify the vital role of pore-lining residues and their role in affecting pore radius in dictating pannexins' architecture and channel behavior.DB

Investigating spin transport across magnetic insulators and their interfaces

Spin transport across magnetic insulator/heavy metal (MI/HM) interfaces has been a topic of interest in spintronics. The spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR) are two phenomena that have garnered much attention. The SSE studies magnon spin current induced by thermal effects, while SMR investigates the change in HM resistivity due to spin transfer torque at the MI/HM interface. This thesis investigates the use of electrical insulating magnetic materials for spin information transmission at room temperature, with a focus on understanding spin transport phenomena across magnetic insulators and their interfaces. The first part of the thesis presents the work on detecting spin-Hall magnetoresistance (SMR) on a crystalline b-plate of Ho0.5Dy0.5FeO3 (HDFO)/Pt hybrid. The SMR measurements were conducted at various temperatures, ranging from 11 to 300 K. The first set of experiments focused on measuring the angular dependence of SMR at room temperature under fields above and below the critical field, revealing anomalies in the signal. These anomalies were then explained through the simulation of the SMR signal using a simple Hamiltonian model. Further analysis of SMR measurements was conducted under a constant field above the critical field at different temperatures, and the results were discussed. The second part of the thesis describes research on the measurement of SMR and SSE on a polycrystalline Sr3Co2Fe24O41 (SCFO)/Pt heterostructure, a room-temperature magneto-electric multiferroic material. The amplitude of SMR data obtained from two measurement sets shows a non-monotonic behaviour with a sign reversal from negative to positive as the external magnetic field is varied. The observed SMR data in SCFO is analysed using a simple Hamiltonian model. Additionally, longitudinal SSE measurements are performed, which resemble the dc magnetization results at 300 K. In the last part, spin transport (SMR and SSE) was investigated on trilayer devices consisting of MgO/Ni0.8Zn0.2Fe2O4 (NZFO)/NiO/Pt heterostructures with varying NiO thicknesses. SMR (1ω) measurements were conducted at various temperatures, followed by current-induced heating to detect 2ω signal. The lock-in detection technique was used to measure 2ω signals by varying magnetic field, current, and temperature that shows non-sinusoidal SSE signal. This non-sinusoidal SSE signal was attributed to unidirectional anisotropy (UDA), caused by ferrimagnetic/antiferromagnetic exchange coupling, using a simple Hamiltonian model. Overall, this thesis contributes to the advancement of spintronic research by exploring the potential of electrical insulating magnetic materials as carriers of spin information and developing a simple Hamiltonian model for analysing spin-related phenomena in these materials

Transport and criticality in topological systems and spin models

This thesis presents work done on transport in topological insulators and graphene-based systems, and quantum criticality in one- and two-dimensional spin models. In particular we study the following: transport on surfaces of three-dimensional topological insulators in the presence of time-independent and time-dependent barriers, Majorana modes in a one-dimensional topological insulator in proximity with a $s$-wave superconductor, the phase diagram of the Hubbard model on a triangular lattice periodically driven by an in-plane electric field, quantum criticality of a Ising model with three-spin interactions and a transverse field, the origin of spin-orbit coupling in a graphene-WSe$_2$ heterostructure, and a prediction of edge states in trilayer graphene. In the first chapter, we give a brief introduction to the concepts relevant to the rest of the thesis such as topological insulators, superconductivity, Floquet theory for studying periodically driven Hamiltonians, graphene and the spin-orbit coupling terms, quantum phase transitions, and the transverse field Ising model. In the second chapter, we consider a thin-film topological insulator (TI) in which the top and the bottom surfaces are separated by a small distance. The hybridisation between the states on the top and bottom surfaces of this system is characterized by a coupling strength $\lambda$. We study the various features of transport when a potential or magnetic barrier is applied on one of the surfaces. We find that the conductance $G$ of this system oscillates with the barrier strength with the period of oscillations varying with the coupling strength $\lambda$. This gives us an indirect way of estimating the extent of hybridisation in such thin films by looking at the conductance. The period of these oscillations changes from $2\pi$ to $\pi$ as $\lambda$ increases from zero to a value close to the energy of the incident electrons. Next we study the effects of a magnetic barrier, and we find that the conductance reaches a non-zero and $\lambda$-dependent value as the barrier strength is increased. This is in sharp contrast to the behavior of the conductance of a single TI surface where it approaches zero with increasing magnetic barrier strength. We also find oscillations in the case of a magnetic barrier for large barrier widths. The period of these oscillations depends on $\lambda$. In the third chapter, we consider a similar magnetic barrier whose strength is periodically driven in time. We explore the behaviour of the conductance as a function of the driving parameters. Such a barrier can be realised by shining linearly polarised light over a region of width $L$ on the surface of a TI. We find that the conductance of this system exhibits a number of interesting features like prominent peaks and dips as the parameters of the system are varied. This also paves the way to have an optical (electromagnetic) control over the electrical current in such junctions where we can go from a high-conductance regime to a low-conductance regime or vice versa by tuning the amplitude and frequency of the light. We also see that this system can act as a frequency detector or an optically controlled switch as a function of the incident energy of the electron. In the fourth chapter, we consider a model of a TI which is now constricted to a narrow and long strip running along the $x-$direction. We study what happens to the Majorana modes when such a system is placed in proximity to an $s$-wave superconductor. This model hosts a spin-dependent chirality and only has a right-moving spin-up and a left-moving spin-down branch. We find that this leads to a number of unusual features, such as only one zero energy Majorana mode at each end of a finite system, a single Andreev bound state at a Josephson junction instead of two states, and multiple Shapiro steps for particular frequencies of an AC driving. In the fifth chapter, we study a Hubbard model on a triangular lattice at half-filling in the limit of large interaction. At half-filling, this is known to describe a Heisenberg spin Hamiltonian with equal nearest-neighbour couplings. We study the effects of driving this system periodically with an in-plane electric field. Taking the driving to be the perturbation, we find, using Floquet perturbation theory, that the effective Hamiltonian up to third order has two-spin Heisenberg couplings with different magnitudes in the three different directions of the triangular lattice. We also get a three-spin interaction chiral term in the third order with its sign being opposite on up- and down-pointing triangles. We study the ground state phase diagram as a function of the three couplings using exact diagonalization. We find that driving leads to new phases in the system apart from the spiral phase. We have three collinear ordered phases, one coplanar ordered phase, and three disordered (spin-liquid) phases. These phases are distinguished by looking at the peaks of the static spin structure function $S(\vec{q})$ in the Brillouin zone, the ground state fidelity susceptibility, the minimum value of the correlation function $C(\vec{r})$ in real space, and the crossings of the energies of the ground state and first excited state. In the sixth chapter, we consider a one-dimensional Ising model with a three-spin interaction with a transverse field of magnitude $h$. We find that this model has duality and a second-order phase transition at the self-dual point $h=1$. We find from finite-size scaling that the correlation length exponent $\nu$ is close to $0.8$ in this model. Having a dynamical critical exponent $z=1$ and a central charge $c=1$, we find that the model displays weak universality and lies somewhere in the middle of the Ashkin-Teller line of models, with the two extreme limits of the line being the transverse field Ising and four-state Potts models. Unlike the transverse Ising model, our model is non-integrable, with the level spacing statistics being governed by the Wigner-Dyson Gaussian orthogonal ensemble. We also find that this model has a subset of zero energy states which are rather special as they are independent of the value of $h$ and have very low entanglement entropy compared to the states in the neighbourhood of the energy eigenvalues. These states are quantum many-body scars and they violate the eigenstate thermalisation hypothesis (ETH). Chapters $7.1$ and $7.2$ describe works done in collaboration with some experimental groups. In Chapter $7.1$, we study the system of graphene-WSe$_2$ heterostructure where we have a strong proximity-induced spin-orbit coupling. The quantum Shubnikov-de Haas (SdH) oscillations observed experimentally show a beating implying the presence of two closely spaced frequencies. The energy dispersion thus extracted is then studied theoretically using an effective Hamiltonian with all possible spin-orbit couplings present. The Fermi velocity of the sample is about $1.5$ times that of pristine graphene. The data fitting and perturbation calculations show that the spin-splitting energy of nearly $5$ meV comes dominantly from the valley-Zeeman and Rashba spin-orbit couplings in the system. In chapter $7.2$, we study a system of trilayer graphene under the influence of a perpendicular electric field. The non-local and local resistance measurements done in this system show a scaling relation given by $R_{NL} \sim R_{L}^{\alpha}$ with $\alpha =1$ for a range of values of the displacement field. The value of $\alpha$ is seen to be close to 1 up to temperatures around which the bulk gap closes in the system. This strongly suggests that the transport is dominated in this sample by edge modes. We study a theoretical model for trilayer graphene with displacement fields consistent with the experiments, and show that in this regime the valley Chern number is non-zero with a large value of $2.5$ for a given valley and a given spin. We also show that the system host zig-zag edge modes for the displacement fields of interest, although they are not protected from backscattering. A simple resistor circuit model that mimics the inter-valley scattering through dissipation then explains the linear relation between the non-local and local resistances. At the end, we summarise our results and discuss possible future studies in these areas of research

Understanding the mechanisms of polarity establishment and nuclear envelope breakdown in Caenorhabditis elegans embryos

Polarity establishment is critical for the development and stem cell lineages. The one-cell stage of the Caenorhabditis elegans embryo polarizes soon after fertilization. As a result, the first division of the embryo is asymmetric. The parent P0 cell divides into a large AB and a smaller P1 cell. The AB daughter cell specifies the somatic lineage, while the P1 cell lineage forms the germline. Failure to accurately establish polarity results in embryonic lethality. It is well-established that centrosomes are responsible for determining the axis of polarity in the one-cell embryo; however, the identity of the centrosome-associated polarity cue and the precise mechanism of polarity establishment remained unknown. After polarity establishment, the one-cell C. elegans embryo enters in mitotic phase, where the male and female pronuclei expand their size and condense their chromatin. The two pronuclei migrate towards each other, followed by the nuclear envelope breakdown (NEBD) that allows the mixing of the maternal and paternal genomes. Subsequently, the mitotic spindle is assembled, and parental genomes are aligned on the metaphase plate. At the onset of anaphase, the differential cortical pulling forces position the mitotic spindle towards the embryo posterior. Since the position of the mitotic spindle dictates the site for cleavage furrow/cytokinesis, this leads to unequal cell division, producing a larger anterior AB cell and posterior smaller P1 cell. The process of NEBD is conserved in all metazoans that undergo 'open' mitosis and is vital for the accurate segregation of the chromosomes. However, the precise mechanism by which this occurs is poorly understood. In the first part of my thesis, I have characterized the role of conserved mitotic kinase Aurora A in proper polarity establishment in the one-cell C. elegans embryo. In the second part of this work, for the first time, we link the function of phosphatase with the NEBD. We show that the PP2A-B55/SUR-6 (hereafter referred to as B55/SUR-6) is essential for proper NEBD in C. elegans embryos. (1) Aurora A kinase/AIR-1 gradient at the centrosomes ensures singularity in the polarity axis in the one-cell C. elegans embryo Proper cell polarization is vital for generating functional asymmetry within cells, which is crucial for development. In one-cell C. elegans embryo, centrosomes are responsible for polarity establishment, i.e., anterior-posterior body axis formation. Centrosomes are hypothesized to form a protein gradient that diffuses out to the cortex and disassembles the actomyosin network, thereby breaking symmetry and concomitantly establishing distinct domains of anterior and posterior conserved polarity proteins, PAR proteins. Primary candidate/s and the precise mechanism by which the centrosome achieves symmetry breaking remained elusive. We uncovered that RNAi-mediated depletion of conserved mitotic kinase, Aurora A kinase (AIR-1 in C. elegans) in the one-cell embryo disrupts stereotypical actomyosin-based cortical flows that occur at the time of polarity establishment. This misregulation of actomyosin dynamics leads to the formation of two posterior polarity axes. Also, we found that this function of Aurora A in polarity establi= shment is dependent on its kinase activity. Notably, this impact of Aurora A depletion is independent of its central role in microtubule nucleation. Interestingly, centrosome positioning in dictating the posterior polarity axis (or PAR-2 localization) is no longer important when Aurora A is depleted in the one-cell embryo. The mechanism by which Aurora A directs symmetry breaking is likely through direct regulation of RhoA-dependent contractility since we observed rescue in the formation of a single polarity axis in Aurora A (RNAi) embryos that are co-depleted of Rho-GEF, ECT-2. Further, a previous study showed that ECT-2 de-localizes from the posterior cortex at the time of polarity establishment, presumably under the influence of centrosome-mediated polarity cue. Our study shows that in the absence of Aurora A, ECT-2 fails to de-localize from the posterior cortex at polarity establishment, providing a possible explanation for the impaired actomyosin flow seen in Aurora A (RNAi) embryos. In summary, our work has contributed to uncovering an unconventional role of Aurora A kinase in polarity establishment in C. elegans. Thus, we propose that Aurora A gradient at the centrosome is a key for symmetry breaking and thus for ensuring proper polarity set-up. (2) B55/SUR-6 promotes nuclear envelope breakdown in the C. elegans one-cell embryo The nucleus constrains the cell's genetic material by forming a selective barrier to the entry of macromolecules from the cytoplasm. In animal cells, NEBD enables the spindle microtubules to access and attach to the chromosomes within the nucleus during mitosis. Proper chromosome-microtubule attachment ensures faithful segregation of genetic material into the two daughter cells. NEBD is regulated by the activity of critical kinases such as CDK-1, AIR-1 (Aurora A), and PLK-1. While these mitotic kinases are crucial for NEBD, no phosphatase has yet been linked with NEBD at the mitotic entry. Here, we identified B55/SUR-6 as an essential regulatory subunit of PP2A phosphatase critical for timely NEBD in the one-cell C. elegans embryo. We found that in embryos that are depleted for B55/SUR-6, nuclear membrane permeabilization (NEP) is significantly delayed, and nuclear lamin and nucleoporins persist throughout mitosis. As a result, chromosomes' segregation is impaired. Notably, we found that the impact of B55/SUR-6 depletion on NEBD is not because of its effect on cell cycle progression or mislocalization of essential kinases such as PlK-1 or AIR-1. We uncovered that B55/SUR-6 acts redundantly with microtubule-generated pulling forces to promote NEBD efficiently. Further, genetic epistasis experiments suggest that nuclear lamin (LMN-1), but not nucleoporin/s, is the target of B55/SUR-6. Notably, genomically-tagged GFP-B55/SUR-6 localizes to the nucleus before the onset of NEBD, suggesting that B55/SUR-6 nuclear import may directly promote NEBD. Overall, these findings link the PP2A phosphatase complex to a critical process of NEBD in animal cells. In summary, my work has contributed to the mechanistic aspects of the two processes: polarity set-up and nuclear envelope breakdown, which are vital for the establishment and the continuity of life

Stability of wall-bounded compressible shear flows

In this work, the linear stability of compressible shear flows in channels and pipes are studied. A steady fully-developed laminar flow of an ideal gas, driven in a channel and a pipe by a constant body acceleration, is considered as the base state. The base flow profiles, being functions of Mach number, are obtained through numerical solution of the Navier-Stokes equations under steady parallel-flow conditions. Small amplitude normal mode perturbations are added to this flow and temporally growing solutions are studied. The evolution equations for the normal modes, that form a linear eigenvalue problem, are solved numerically by Chebyshev-Pseudospectral method. For both channel and pipe flows, the eigenspectra show presence of compressible higher modes that do not have a counterpart in the incompressible limit. These modes are categorised into two distinct families based on the variation of the real part of their wave-speed with stream-wise wave-number. Numerical studies show the dominant instability at finite Mach numbers to be due to the modes that show a monotonic increase in the real part of the wave-speed with wave-number. These modes become unstable at finite wave-numbers at Mach numbers above a critical value. We have extended the classical stability theorems to compressible flows in bounded domains. A new criteria for the existence of neutral modes is derived which is used to obtain the values of the critical Mach numbers for the stability of the higher compressible modes. In the incompressible limit, a pipe flow is stable to all modal perturbations, but the channel flow is unstable to the Tollmien-Schlichting (T-S) mode. Numerical studies at finite Mach numbers show compressibility to have a stabilising effect on the T-S mode. The critical Reynolds numbers as a function of Mach number are obtained for the all the unstable modes in both channel and pipe flows. A universal scaling of the critical values is shown at high Mach numbers. The critical Reynolds numbers for three-dimensional disturbances are also calculated for a compressible channel flow. It is shown that oblique waves are more unstable than two-dimensional waves with the minimum critical Reynolds number appearing at a specific wave-angle corresponding to a particular Mach number. Numerical calculations of the stability equations are also performed in the inviscid limit where the numerical contour of integration is suitably chosen to avoid the branch point singularity at the critical point. The inviscid limit of the dominant compressible modes in channel and pipe flows compared against the high Reynolds number viscous calculations reveal the instabilities to be viscous in nature. The instability in channel and pipe flow appear due to a change in the viscous wall layer due to the emergence of a critical point very close to the wall. The unstable modes in channel and pipe flows are studied at high Reynolds numbers through an asymptotic analysis. The instabilities in compressible channel flow are categorised into a small wave-number mode, which is the finite Mach number extension of the T-S mode, and finite wave-number modes, which are the dominant compressible higher modes. The asymptotic analysis for the lower and upper branches of the stability curve are performed to obtain the scalings for the wave-number, wave-speed, as well as the wall layer scalings for viscous regularization. An adjoint-based procedure imposing the solvability condition on the first and second correction to the stability equations, is devised to obtain the leading order eigenvalues for the lower and upper branches at high Reynolds numbers. The same asymptotic analysis is performed for the finite wave-number modes of the compressible pipe flow as well. We also study the stability of a compressible flow in a channel with compliant walls. The compliant walls are modelled as spring-backed plates that move in the direction normal to the flow due to the fluid stresses acting at the walls. Wall compliance introduces additional instabilities, referred to as FSI modes, in addition to the Tollmien-Schlichting and compressible higher modes. The numerical studies indicate flow compressibility to have a stabilising effect on the FSI modes, and wall compliance to have a stabilising role on the compressible higher modes. Both flow compressibility and wall compliance are observed to have a stabilising effect on the Tollmien-Schlichting mode. We also calculate the perturbation energy budgets for the different instabilities which allow us to differentiate the different mechanisms of destabilisation of these modes

Linear and Nonlinear Ultrafast Time Resolved Spectroscopy of Topological Insulators, Weyl Semimetals and Semiconducting Nanowires

Ultrafast time resolved spectroscopy has recently gained immense significance due to its potential capability to explore the dynamics of photoexcited carriers in condensed matter systems. The technique has proved to be a very powerful tool in understanding the relaxation process of photogenerated carriers by providing insight into various scattering mechanisms of charge carriers, which, in turn, find significance for practical applications in optoelectronic and photonic devices. On the other hand, condensed matter physics have also witnessed a drastic change over the past few years with the introduction of topological band structure, leading to the discovery of many new materials with extraordinary properties. This has motivated the research community to investigate various scattering mechanisms of charge carriers in topological materials, both in bulk crystals as well as in nanomaterials, where the later further modifies the band structure due to confinement effects. In this work, we report our studies on understanding the carrier relaxation dynamics in topological insulators (Bi_2 Te_3,SnBi_4 Te_7), type -1 Weyl semimetals (TaAs, TaP, NbAs and NbP) and semiconducting nanowires (Te NWs) using ultrafast time resolved spectroscopy. The work mainly covers the linear and second order response of the photoexcited carriers in these materials, studied using time resolved terahertz (THz) spectroscopy and time resolved second harmonic generation (TR - SHG), respectively. The experimental results are then explained using theoretical models emphasizing on different analytical calculations for understanding the carrier relaxation dynamics, thereby giving more microscopic insight into various physical processes

Structural and Thermoelectric Studies of Sb2Te3 and Bi2Te3 Based Chalcogenide Alloys and Nanocomposites

Thermoelectricity is one of the potential solutions for the rapidly increasing energy demand. Thermoelectric generators can turn waste heat into usable energy. Due to their effectiveness in the 300 K to 500 K temperature range, Sb2Te3 and Bi2Te3 are two of the most researched thermoelectric materials. These thermoelectric materials that operate at room temperature can have their thermoelectric performance improved through doping, nanostructuring, orientation engineering, and nanocomposites, among other techniques. Due to their capacity to lower thermal conductivity (κ) while maintaining a high-power factor (PF=S2σ), nanocomposites and doping techniques have garnered the most attention among them. The rate of melt solidification has recently been demonstrated to have the ability to dramatically adjust the thermoelectric characteristics to a greater extent. The thermoelectric characteristics of Sb2Te3/Te nanocomposites and Bi2Te3 alloy were examined in the first section of this thesis. This work has shown that the rate of melt solidification has a substantial impact on the structural and thermoelectric properties. It has been demonstrated that the optimum way to get better thermoelectric performances is with moderate melt quenching rates (normal water and ice water quenching). The thermoelectric properties of nanocomposites made by combining Sb2Te3 and poly methyl methacrylate (PMMA) have been studied in the second section of the thesis. For polymer nanocomposites, thermal conductivity was found to be significantly reduced. A 30% reduction in thermal conductivity has been seen with 5% polymer composites. In the last part of the thesis, the effect of Zn doping on Sb2Te3 has been studied. The prepared powder samples were sintered by spark plasma sintering (SPS). An increase in Zn doping increased the power factor considerably because Zn+2 doping in place of Sb+3 in Sb2Te3 acts as an acceptor. It increases p-type carrier concentration and thereby enhances the electrical conductivity. The thermoelectric figure of merit was found to increase by 12 % for the Zn-doped Sb2Te3. The zT of the SPS sintered Zn-doped Sb2Te3 is increased by 80% compared to the as-prepared Sb2Te3 ingot. The results presented in this thesis demonstrate that the zT of thermoelectric materials can be modulated by using different melt solidification rates, doping, and by forming nanocomposites

Capacity Computation and Coding for Input-Constrained Channels

The setting of the transmission of information over noisy, binary-input, memoryless channels is today well-understood, owing to the work of several information theorists, beginning with Claude Shannon. It is known that it is impossible to transmit information reliably over such channels at rates larger than the fundamental limit that is the capacity of the channel. Moreover, progress made in the last three decades has led to the construction of explicit, practically-implementable coding schemes that achieve rates arbitrarily close to the capacities of such channels. Now, suppose that the inputs of the memoryless channel are required to obey an additional constraint, which stems from physical limitations of the medium over which transmission or storage occurs. What then can be said about the fundamental limits of information transmission over such input-constrained channels, with and without decoder feedback? Is it possible to design good constrained coding schemes of high rate over these channels? If the channel introduces errors adversarially, instead of randomly, how much information can then be sent through, reliably? This dissertation explores answers to such questions. We first derive computable lower bounds on the capacities of runlength limited (RLL) input-constrained memoryless channels, such as the binary symmetric and binary erasure channels (BSC and BEC, respectively), by considering random Markov input distributions that respect the constraint. These bounds unify well-known approaches in the literature, and extend them to the so-called input-driven finite-state channels (FSCs). For the special case of the BEC with a no-consecutive-ones input constraint, we discuss an iterative stochastic approximation algorithm that numerically computes achievable rates that are very close to known upper bounds on the capacity of the channel. We also derive improved analytical lower bounds, for this specific channel. Next, we consider the special case of the $(d,\infty)$-runlength limited (RLL) constraint, which mandates that any pair of successive $1$s be separated by at least $d$ $0$s. We design explicit coding schemes, derived from Reed-Muller (RM) codes, for transmission over binary-input memoryless symmetric (BMS) channels, whose inputs respect the constraint. In particular, we provide constructions using constrained subcodes of RM codes, analytically compute their rates, and derive converse upper bounds on the rates of the largest constrained subcodes of RM codes. We also provide a Fourier-theoretic perspective on the problem of counting arbitrarily-constrained codewords in general linear codes, which can help estimate the rates achievable by using linear codes over input-constrained BMS channels. We illustrate the utility of our method using the somewhat surprising observation that for different constraints of interest, the Fourier transforms of the indicator functions of the constraints are efficiently computable. We then shift our attention to the setting of the $(d,\infty)$-RLL input-constrained BEC in the presence of noiseless feedback from the decoder. We demonstrate a simple, labelling-based, zero-error feedback coding scheme, which we prove to be feedback capacity-achieving, and, as a by-product, obtain an explicit characterization of the feedback capacity. The feedback capacity thus computed is an upper bound on the non-feedback capacity of such a channel. Numerical comparisons made with upper bounds on the non-feedback capacity then reveal that that feedback increases the capacity of such a channel, at least for select values of $d$. Finally, we consider the setting of an input-constrained adversarial channel, where there is an upper bound on the number of bit-flip errors that the channel can introduce, and we seek to design codes that can be recovered with zero error. We present numerical upper bounds on the sizes of the largest such codes, via a version of Delsarte’s linear program. We observe that for different constraints of interest, our upper bounds beat the “generalized sphere packing bounds” that are the state-of-the-art.Prime Minister's Research Fellowship, Qualcomm Innovation Fellowship Indi

Anaerobic digestion characteristics of lignocellulosic feedstocks under solid-state stratified bed mode of fermentation

Lignocellulosic feedstocks have become potential materials for conversion to biomethane but possess varying compositions and densities (18-50kg/m3), requiring alternative approaches for evolving sustainable anaerobic digesters. Solid-state anaerobic digesters such as solid-state stratified bed reactors (SSBR) can take such low-density biomass without extensive pre-processing. However, reactor optimization is needed due to the significant difference in physical and chemical steric properties of lignocellulose. This study attempts to identify appropriate combinations of these properties to identify ideal fermentation conditions beforehand. Temporal changes in physical, chemical, and fermentation properties were recorded as a function of SRT for ten lignocellulosic feedstocks- three dicots land weeds, three dicot tree leaves, and four agro residues in an SSBR, to represent a wide variety of feedstock in the test pool. The rates of degradation described by loss of TS, VS, and gain in bulk density indicate that for most feedstocks, there is an initial rapid weight loss ascribed to loss of extractives and some hemicellulose and an initial rapid gain in bulk density. After this period, decomposition slows down. This turning point, termed the inflection point, appears to be a good representation of economic or ideal SRT (18-25d in dicots and 46-60d in agro residues). The degradation behavior pattern of lignocellulose in SSBR- the extent of degradation, rate of degradation, and achievable average methane production rates could be predicted well by knowing the composition of feedstocks in terms of hot water extractable (HWE), oxalate extractable pectin (Ox), hemicellulose (HC), cellulose (C) and lignin (L) content expressed as ((HWE+Ox+Hc))/((C+L)), with R2=0.78, 0.81, and 0.85, respectively. It is the first time a single parameter has been able to predict multiple degradation behavior for diverse lignocellulose. At the same time, methanogenesis is addressed with a biofilm bed on the digesting feedstock. These digesting (spent) feedstocks had high specific methanogenic assay levels reaching 20L CH4/kg residual TS/d (hydrogenotrophic) and 35L CH4/kg residual TS/d (aceticlastic). Feedstocks with cellulose concentration >27% of TS recorded methane production potential between 7-10.4L/kg feed TS/d through the hydrogenotrophic route of methanogenesis. In addition, the TSMA, HSMA, and ASMA evaluation on SRT at the VS loss inflection point showed a negative correlation with “HWE+Ox+HC” (R2=0.77, 0.77, and 0.56, respectively), while a high concentration of “HWE+Ox+HC” conferred net high degradability to feedstocks. A new approach to seeding- a surrogate to substrate to inoculum ratio (S/I) is proposed for SSAD seeded with solid inoculum source after its SMA has been quantified – “(Kg TS fed/kg digestate TS)” and “SMA/S” (L TSMA required/kg TS fed). These developments show the potential to use new and untried feedstocks (and combinations) while avoiding various intermediate stages of feedstock trials and validation and end-of-life use of spent lignocellulose as high-activity biofilm support. On comparing SSBR with BMP, it was found that SSBR yields reached ≥69% of BMP yield but needed~ 53-73d to catch up. This, however, can be improved by introducing compacted feed and increasing the substrate contact with the methanogens. Methane yields estimated from BMP could be predicted with the formulated parameter- ((HWE+HC))/((C+L+Ox)). It is the first time such a close level of biomethane production potential has been predicted using the physicochemical properties of diverse feedstocks