Rozprawy doktorskie na temat „Research Subject Categories – TECHNOLOGY – Electrical engineering, electronics and photonics”

In this thesis, we propose a coordination of the rst and last mode connectivity in a multi- modal transportation system. In particular, we consider a one-shot problem wherein the passengers must be transported to or from a common location before a fixed deadline. We consider a macroscopic model, wherein we model a region of interest by a graph and consider flows of vehicles and volumes of passenger demand and vehicle supply. We then consider the problem of operator's profit maximization through optimal pricing and allocations of feeder vehicles given the knowledge of demand and supply distributions. Due to the speci c nature of the problem, we can first determine the optimal prices, subsequent to which the overall problem becomes a linear program. We first study the problem of one-shot feed-in". Given the large scale or scope of the problem and the need for a near real-time implementation, we rst seek to reduce the problem size. We propose an o -line route elimination algorithm that given a route- set returns another reduced" route-set by eliminating the routes that would never be used in an optimal solution irrespective of the demand and supply distributions. In simulations on a 24 node graph, our proposed route reduction algorithm reduced the number of optimization variables to nearly one third of the original number. Such a reduced route-set could then be utilized for faster computation of the optimal solutions of the feed-in problem when the demand and supply distributions are revealed. We then analyse the supply optimization problem for one-shot feed-in for a given demand distribution. In this problem, given a total supply, we are interested in optimally distributing the supply so as to maximize the operator's pro ts. With this formulation, we give the closed form expression of the \absolute maximum pro ts" of the operator over all supply distributions given the demand distribution. Next, we go on to show that the one-shot feed-out" problem is equivalent to the supply optimization problem for one-shot feed-in" and that similar results can be drawn using the equivalence analysis. Finally, we propose a simple framework for determining the prices. With this framework, we are able to analyse the cost of a rst or last mode feeder service relative to the best alternative transportation at which the feeder service becomes pro table. We demonstrate our analytical results with a suite of simulations, including a com- parative study of our model with respect to a macroscopic single-depot single-window routing problem.

Multilevel inverters (MLI) are the most preferable alternatives for medium to high voltage applications. It finds in many applications like traction drives, fans, blowers, HVDC and FACTs etc. It offers many attractive features over a conventional 2-Level inverter like lower voltage stress across the power switches, lower dv=dt across the load terminals, lesser EMI, lesser current and voltage total harmonics distortion (THD) etc. In contrast to a 2-Level inverter, the output of a MLI produces many discrete steps which makes the output voltage nearer to an ideal sine wave. This allows to reduce the switching frequency in a MLI which in turn control the losses in the power switches in a MLI. All these features make MLI superior compared to a 2-Level inverter in every aspect. In modern days, the application of MLI in the field of medium to high power AC motor drives becomes indispensable. There are many basic MLI schemes like Neutral Point Clamped (NPC), Flying Capacitor (FC) and Cascaded H-Bridge (CHB) inverters. These basic inverter scheme have many advantages and disadvantages over one another. To mitigate the disadvantages of the basic MLI schemes, hybrid MLI schemes have been presented in the literature. Also, the multilevel operation can be achieved using dual fed inverter schemes where two inverters are being fed from either side of the open-end load. With the increase in levels, basic MLI schemes require more number of components like clamping diode (in case of NPC inverters), floating capacitors (in case of FC inverters) and isolated DC-sources (in case of CHB inverter). The main objective behind developing a new MLI scheme is to optimize the component counts and also increasing the reliability. In this thesis, 5-Level reduced component count MLI schemes are proposed using a single DC link. Usage of a single DC-link in a MLI scheme enables easy four quadrant operation. Furthermore, the proposed schemes are more reliable compared to the basic MLI schemes. In case of a fault in any of the floating bridges, the schemes can still be operated at rated power of the drive. All the floating capacitor in these schemes are charged by the phase current by employing a hysteresis controller. This feature eliminates the need of an extra pre-charging circuit for the inverter. The floating capacitors are balanced in each PWM cycle which enables to use low value capacitor even for high power applications. For many safety critical applications it is extremely important that the reliability of electric drives to fulfil certain safety norms, especially when human lives, environmental damages or important economic losses are involved. Power electronic converters play an important role in electrical drives where certain fault conditions such as open- or shortcircuit can lead to potentially dangerous situations. In conventional multilevel inverters, abnormal condition arises when a fault occur in the power switch, gate driver or any of the discredited wire of the inverter. Reliable fault tolerant MLI schemes are usually achieved by providing overrated or more reliable components, using redundant design or adopting automatic changes in the control strategy in the event of partial failures in the converter. In this thesis, a 5-Level fault tolerant scheme is presented using reduced number of components compared to the basic MLI schemes. In the proposed scheme, if the fault occurs in any of the power switch in the floating bridges, the inverter can operate the drive without any interruption. After isolating the fault, the drive can be operated at the full rated power. The faulty part of the inverter can be isolated seamlessly during drive operation without introducing any transient in the phase voltage or current. This is possible because the the supplied power to the load remains same before the fault and after isolating the fault. Experimental results in the normal operating condition and the fault mode of operation are presented for the inverter scheme. A capacitor balancing algorithm is proposed using space vector redundancy and the capacitors are balanced in a PWM cycle during the inverter operation at steady and transient states. The operation of the inverter and the capacitor voltage balancing scheme is validated through experiments at different frequencies of operation of the inverter. The basic and most popular multilevel inverter schemes are NPC, FC and CHB. With increase in the number pole voltage level, the NPC inverter requires more number of clamping diodes. Also, the NPC inverter suffers from the neutral point balancing problem at higher modulation indices. The neutral point voltage control becomes more difficult with increase in the number of stacked DC-link capacitors. On the other hand, a FC inverter requires more number of floating capacitors. The balancing of all the floating capacitors by switching state redundancy requires a complex balancing algorithm. It also causes an increase in switching loss. In case of a CHB inverter, more number of isolated DC sources are required, which makes the system bulky and costlier. Also, the usage of multiple DC-links make the regeneration operation of the drive difficult. In this thesis, a reduced switch count (only using six switches per phase) 5-Level inverter scheme is proposed using a single DC link for OEIM drive. In this scheme two inverters are feeding OEIM from either side of the open stator windings. One inverter is a 3-Level FC inverter and this inverter is supplied with a DC-link. It is the source of the active power for the OEIM drive. The other inverter is a capacitor fed 2-Level inverter. The DC-link to the capacitor voltage in the floating bridge is maintained at a ratio of 4:1. The inverter scheme is controlled such that the 2-Level capacitor fed bridge acts as a switched capacitive filter. So the active power being fed by the bridge is zero which maintains the capacitor voltage irrespective of the load power factor. The generalization method is also presented in the thesis to extend the proposed scheme to a n-Level inverter. Another remarkable feature of the proposed scheme is the fault tolerant capability. In case of a failure in the capacitor-fed floating bridge, the OEIM drive can still be operated with the other inverter after bypassing the faulty bridge. To validate the proposed scheme, the simulation and experimental results are presented during the drive operation at steady and transient states. The fault tolerant capability of the inverter scheme is verified by performing experiments during the fault in the capacitor fed bridge. The faulty bridge is isolated from rest of the inverter without interrupting the drive operation. A conventional 2-Level or a multilevel inverter operating in linear modulation range in hexagonal space vector PWM, generates strong fundamental component and the harmonics at the side band of the switching frequency. To obtain more fundamental voltage at the phases, the inverter schemes need to operate in over-modulation region (i.e. generated peak phase fundamental voltage greater than 0.577 times DC-link voltage). Inverter operating in over-modulation region produces strong of component of fundamental with the lower order harmonics (i.e. 5th, 7th, 11th etc.). This lower order harmonics produces lower order torque pulsation in the motor shaft. Also, these harmonics causes extra losses in the motor windings and decreases the efficiency of the drive system. The maximum peak fundamental component can be obtained from an inverter during square mode of operation is 0.637 time DC-link voltage. At this time (i.e. six-step mode operation) inverter produces strong lower order harmonic components. In this thesis a hybrid MLI scheme is proposed to extend the linear modulation range to the full base speed irrespective of the load power factor without introducing lower order harmonics in the phase voltage. This is achieved by boosting the DC-link voltage with a charged capacitor. At the same time during PWM operation, the capacitor voltage is controlled by using redundant vectors. Linearly increasing the modulation range to the full base speed from 0.577 to 0.637 times DC-link voltage, is proposed for the first time in this work with PWM control throughout the modulation range irrespective of the load power factor. Due to linearization of the extended modulation range, the phase voltage does not contain any lower order harmonics. The inverter operation and capacitor balancing technique is analyzed thoroughly. The simulation and experimental results for unity and non-unity power factor loads are presented in steady and transient states. A 10kW, 50Hz, 415V, 3-phase induction motor is used and phase coil group end connections are connected appropriately to configure the machine as a 3-phase open-end winding induction motor. To perform experiments, open loop v=f and closed loop field oriented controllers are employed. 100A, 1200V IGBT semiconductor half-bridge modules (SKM-100GB-12T4) are used as the power switch for the inverter. Opto-isolated gate drivers (M57962L) with desaturation protection are used for driving the IGBT switches. The controller is implemented in a TMS320F28335 DSP. The analog signals (capacitor voltages and phase currents) are fed to the ADC channel of the DSP. The generated PWM pulses along with the capacitor voltage status and the current polarity are sent to a FPGA (Xilinx SPARTAN-3 XC3S200). Based on these data received from DSP, the FPGA produces the gate signals for all the IGBT switches. A dead time of 2 S is provided in between complementary gate pulses. With the advantages like reduced switch count, fault tolerant capability, single DC supply requirement, extension of linear modulation range, linear control of the inverter over the entire modulation range, lesser dv=dt stresses on devices and motor phase windings, inherent capacitor balancing, the proposed inverter schemes can be considered as good choice for medium voltage, high power motor drive applications.

Transmission of bulk electric power from the generating stations to the load centres can be carried out only through high voltages transmission lines. One of the main issues in the design and maintenance of extra and ultra-high voltage transmission system is the phenomenon named corona. It is the local electrical breakdown of air in the vicinity of the line conductors and hardware. Even though the design and dimensions of these elements are made considering the corona onset, surface abrasions arising either during installation or operation can lead to intolerable corona. Apart from producing some insignificant chemical reactions and noticeable acoustic noise, they can be a significant source of electromagnetic interference. In the early days, this interference was of concern only to radio and television receptions, however, with extensive use of wide frequency bands for modern applications, it has assumed prime importance. The EMI due to the transmission line corona has been extensively studied and reliable empirical formulas have been proposed. The basis for all the earlier studies was the experimentally measured corona currents. This approach fails for new line designs especially with higher and higher voltages being employed due to non-availability of experimental data. A second approach assumed corona current to be injected into the line and subsequent analysis was carried out based on transmission line model. However, there were assumptions made on the mode of corona current injection into the conductor and the frequency range involved were also not adequate for the modern-day applications. Applicability of transmission line model for analysis is also questionable. From a theoretical perspective, the coupling of the field produced by corona to the conductor was hardly investigated and the total field produced by the corona itself was not quantified. In order to address these serious lacunae, the present work was taken up and it can be considered as the first leap towards the correct picturization, as well as, quantification of the problem. The field produced by the electron avalanche involves noticeable retardation effects. In the literature, only the field produced by arbitrarily moving point charge of fixed strength is given by the Heaviside-Feynman equation. On the contrary, the avalanche involves an arbitrarily moving charge of time varying strength at its head with trailing positive charge, which is almost stationary. Starting from the basics, an analytical expression for the electric field due to an arbitrarily moving point charge of time varying strength is derived which forms a fundamental contribution to Electrodynamics. This is extended to deduce an expression for the total electric field due to an avalanche for the very first time. Suitable validation of the expression is provided through numerical simulation of electric field integral equation. Corona discharge is a complex phenomenon having many distinctly different modes which differ in their visual, as well as, electrical characteristics. Innumerable electron avalanches contribute to the measured corona current with their space-charge acting as a moderator. Therefore, in order to model the corona on conductors, an indirect approach based on linear system theory is proposed. An equivalent spatio-temporal dipole distribution was obtained to produce the measured current on the conductor. The general expression derived for the isolated avalanche is extended for this purpose. Using the above, the means of induction, spatial decay rate of corona current in the close range, its propagation mode and field produced by both avalanche/equivalent dipole and that due to induced current in the conductor, have all been investigated and quantified. In summary, the contributions made in this work are more of fundamental in nature and would be of significant interest to the high voltage power transmission line, as well as, to the communication engineers.

Machine localization of sound sources is necessary for a wide range of appli- cations, including human-robot interaction, surveillance and hearing aids. Robot sound localization algorithms have been proposed using microphone arrays with varied number of microphones. Adding more microphones helps increase the localization performance as more spatial cues can be obtained based on the number and arrangement of the microphones. However, hu- mans have an incredible ability to accurately localize and attend to target sound sources even in adverse noise conditions. The perceptual organi- zation of sounds in complex auditory scenes is done using various cues that help us group/segregate sounds. Among these, two major spatial cues are the Interaural time difference (ITD) and Interaural level/intensity difference(ILD/IID). Popular algorithms, for binaural source localization, model the distributions of ITD and ILD in each frequency subband us- ing Gaussian Mixture Models (GMMs) and perform likelihood integration across the time-frequency plane to estimate the direction of arrival (DoA) of the sources. In this thesis, we use ITDs and show that the localization performance of a GMM based scheme varies across subbands. We pro- pose subband selection and subband weighting schemes in order to exploit the subband reliability for localization. Source localization results demon- strate that the proposed schemes perform better than uniformly weighing all subbands. In particular, the best set of weights closely correspond to the case of selecting only the most reliable subband. We also propose a new binaural localization technique in which templates, that capture the direction-speci c interaural time di erence patterns, are used to localize sources. These templates are obtained using histograms of ITDs in each subband. DoA is estimated using a template matching scheme, which is experimentally found to perform better than the GMM based scheme.

The multiferroics are an important class of multifunctional material which simultaneously possess spontaneous ferroelectric polarization and magnetic ordering. If there exists a coupling between the ferroelectricity and the magnetic ordering, the materials are known as magneto electric (ME) multiferroic materials. The coupling between the magnetic and electric order parameters allows to tune the magnetic properties by an electric field and vice versa. Multiferroic materials are promising candidate for designing new spintronic devices, advanced sensors, high density ferroelectric memory devices and the emerging category of four-state memory devices. In multiferroic memory devices, data can be written electrically using its ferroelectric property and can be read magnetically without causing any Joule heating. Depending on the origin of ferroelectricity and magnetic orderings, multiferroics can be divided into two categories: type I and type II multiferroics. The type I multiferroics have different sources of ferroelectricity and magnetism. On the other hand, ferroelectricity is induced by the magnetic ordering in type II multiferroic materials and they have a strong ME coupling. However, even after extensive investigations into different families of compounds, a multiferroic material with high-enough polarization and magnetization suitable for practical applications has not been realized yet. In order to overcome this problem, composite multiferroics are designed by combining a ferroelectric and a ferromagnetic material. Recently composite multiferroics have drawn significant attention due to its enormous design flexibility which can be used for a wide range of applications. In this thesis, a thorough study of the structural, electrical, and magnetic properties of multiferroic BiFeO3 and CuO epitaxial thin films is carried out. BiFeO3 is a type I multiferroic material with a perovskite distorted rhombohedral (R3c) crystal structure. It is ferroelectric (TC = 1123 K) and G-type antiferromagnetic (TN = 643 K) at room temperature. Antiferromagnetism in BiFeO3 arises from the Fe sublattice having d5 configuration whereas ferroelectricity appears due to the directional orientation of 6s lone pair electrons of the Bi3+ ion. We observed that the crystal structure of BiFeO3 thin film gets altered depending on lattice misfit stress caused by the substrate which in turn modifies its magnetic properties through strong magneto-structural coupling. Furthermore, a signature of magneto-(di)electric coupling and exchange bias effect were observed between the BiFeO3 and SrRuO3 layers of a heterostructure. On the other hand, CuO is a type II multiferroic material where ferroelectricity is generated between 213 K and 230 K due to incommensurate spiral magnetic spin ordering along its crystallographic ‘b’ axis. We found that CuO thin films can be grown in the direction of its static polarization axis by proper choice of substrate and the temperature dependent magnetic properties of CuO thin films vary depending on its crystallographic orientations due to strong magneto-structural coupling. Chapter 1 provides a general introduction to various physical phenomena, such as ferroelectricity, ferromagnetism, antiferromagnetism, multiferroicity, magneto-electric coupling, and different magnetic interactions, like Dzyaloshinskii-Moriya interaction, and exchange bias effect. Basic concepts of impedance spectroscopy, dielectrics and perovskite structures are also discussed. General introductions of different materials, which are studied in the thesis, and the motivation of choosing them are incorporated at the end of the chapter. Chapter 2 contains the description of thin film growth technique and different steps of device fabrication process. Different characterization techniques, the instruments used for the characterizations and the working-principle of those instruments have been summarized in the chapter. Chapter 3 focuses on the variation of magnetic properties and crystal structure with the thickness of BiFeO3 thin films. BiFeO3 thin films of different thicknesses, ranging from 16 nm to 60 nm, were grown on (001) SrTiO3 substrate by PLD technique. Detailed x-ray diffraction studies show that the 16 nm, 20 nm and 30 nm films have “R-like” crystallographic phase with an out-of-plane lattice parameter of 4.06 Å whereas the 45 nm and 60 nm films have “R-like” and ‘T-like” crystallographic phases simultaneously. The “T-like” phase has an out-of-plane lattice parameter of 4.65 Å and a c/a ratio of 1.25, resembling a tetragonal crystal structure. Off-specular reciprocal space mapping and azimuthal φ scan show that the “T-like” phase deviates from an ideal tetragonal crystal structure by a monoclinic tilt. The occurrence of the “T-like” phase is associated with the formation of a very thin layer of parasitic Bi2O3 phase which appears in between two film-thicknesses of 30 nm and 45 nm and BiFeO3 grows in “T-like” phase thereafter. High lattice mismatch between Bi2O3 phase and BiFeO3 phase causes more distorted unit cell in “T-like” phase with a high c/a ration. Parasitic Bi2O3 phase appears because of slightly higher partial oxygen pressure used during the growth which prevents the formation of the parasitic ferrimagnetic γFe2O3 phase in the films. Moreover, our XPS studies confirmed that the films contain Fe3+ only without any trace of Fe2+ within a resolution of few atomic percentages and the magnetic signals measured in our experiments are entirely from the BiFeO3 phase. The saturation magnetizations of the films were found to increase with decreasing thickness. At room temperature, the saturation magnetization of a 16 nm-thick BiFeO3 thin film is 87 emu/cc but it goes down to 9 emu/cc when the thickness increases to 60 nm. Moreover, it was observed that the 16 nm thick film is magnetically more anisotropic in comparison to the 60 nm thick film and there is an apparent out-of-plane magnetic hard axis in the 16 nm film. Summarizing the results obtained from the films with different thicknesses, it can be concluded that the vanishing magnetic anisotropy is related to the structural transformation of the film. Chapter 4 provides a detailed study of the variation of magnetic properties of a BiFeO3 thin film with its crystal structure. BiFeO3 thin films of different thicknesses were grown on orthorhombic (001) NdGaO3 substrate. In-depth x-ray diffraction studies and off-specular reciprocal space mapping show that a 15 nm thick BiFeO3 film grows with monoclinic crystal symmetry (Cm) with an out-of-plane lattice parameter of 4.187 Å on the NdGaO3 substrate. The crystal structure was further verified by the TEM studies which showed a good agreement with the results obtained from x-ray diffraction studies. To probe the ferroelectric nature of the monoclinic BiFeO3 film, piezo response force microscopy was performed. It was found that the oppositely oriented ferroelectric domains have 180° phase contrast and a phase vs. voltage hysteresis loop gets generated when the domains are switched between two antiparallel directions. DC magnetic measurements at room temperature showed that the saturation magnetization of the 15 nm film with Cm crystal symmetry is as high as ~250 emu/cc. Experimental evidence confirmed that the films are free from all magnetic parasitic phases and the high saturation magnetization comes solely from the BiFeO3 phase. For comparative study, BiFeO3 films of similar thickness were deposited on (001) SrTiO3 under identical conditions which grew in “R-like” crystal structures. We saw that “R-like” BiFeO3 films have saturation magnetization 2.5 times lower (~100 emu/cc) than that of the film with Cm structure grown on NdGaO3. Our observation was further supported by density functional theory calculations which show that BiFeO3 has a ferromagnetic ground state in the Cm crystal phase. The theoretically obtained magnetic moment is 266 emu/cc which is very close to magnetization values found experimentally. Chapter 5 deals with the magnetic interaction and the magneto-electric coupling between the BiFeO3 and SrRuO3 layers of a heterostructure. BiFeO3/SrRuO3 heterostructures were grown on (001) SrTiO3 substrate by PLD technique. The ferroelectric nature of the top BiFeO3 layer was probed by out-of-plane piezo response force microscopy technique. Temperature dependent magnetization measurements of the heterostructure show a sharp ferromagnetic to paramagnetic transition at 160 K which arises from the bottom SrRuO3 layer. Therefore, the heterostructure is ferroelectric and ferromagnetic below 160 K. Magnetic interactions between the two layers were investigated by isothermal magnetic hysteresis loop (M-H) measurement in a SQUID magnetometer. The M-H measurements at 10 K showed a two-step magnetic hysteresis loop which implies that magnetic moments of the SrRuO3 layer get pinned by the magnetic interaction between the two layers. During magnetization reversal process, the pinned magnetic moments switch at a higher magnetic field and generate the second step of the hysteresis loop whereas the first step appears at a lower magnetic field during the switching of the free SrRuO3 moments. The amount of the pinned SrRuO3 moments depends on the thickness of the BiFeO3 layer as the magnetic properties of a BiFeO3 thin film are related to its thickness. Moreover, evidence of the exchange bias effect was also found in the heterostructure. Field-cooled M-H measurement shows that the second step of the hysteresis loop shifts in two opposite directions along the magnetic field axis depending on the polarity of the cooling field whereas the first step doesn’t respond to the cooling field. This confirms that the exchange bias effect is directly related to the pinned magnetic moments of the SrRuO3 layer. The total amount of pinned moment and hence the exchange bias effect reduces with increasing temperature and disappears completely above 100 K. A strong coupling between the electrical properties of the BiFeO3 layer and the magnetic properties of the SrRuO3 layer was also observed in the heterostructure. To carry out electrical measurements, interdigitated gold electrodes were fabricated on the BiFeO3 layer of the heterostructure by standard photolithography, magnetron sputtering, and lift-off procedure. Temperature dependent resistance and reactance measurements of the heterostructure at different frequencies show anomalies at ferromagnetic TC of the bottom SrRuO3 layer. Moreover, temperature dependent capacitance measurement at 0 T and at 5 T magnetic fields also showed anomalies near 160 K which indicate that the electrical properties of the heterostructure are affected by the magnetic transition of the SrRuO3 layer. Furthermore, impedance spectroscopy measurements were carried out at different constant temperatures and the corresponding Nyquist plots were fitted with an equivalent circuit model. Remarkably, the capacitance and resistance of the equivalent circuit corresponding to the BiFeO3 layer of the heterostructure, show anomalies at 160 K. Absence of any dielectric anomaly at 160 K in pure BiFeO3 confirms that the observed ones appear because of the magnetic phase transition of the bottom SrRuO3 layer. Therefore, the BiFeO3/SrRuO3 heterostructure has ferroelectric and ferromagnetic properties along with a strong magneto-electric coupling between the layers which can be a promising candidate for the composite multiferroic. Chapter 6 describes a correlation between the crystal structure and magnetic properties of CuO thin film. CuO thin films were grown on (001) SrTiO3, (110) SrTiO3, and (111) Si substrate with MgO buffer layers by PLD technique. On (110) SrTiO3 substrate, CuO thin films grow along [010] direction, which is the direction of ferroelectric polarization of CuO, but growth direction becomes [111] when (001) SrTiO3 substrate is used. The CuO film becomes polycrystalline when it is grown on (111) Si substrate. To find the in-plane epitaxial relations between the substrate and the two layers, cross-sectional TEM of the heterostructure grown on (110) SrTiO3 was carried out. HRTEM images showed very sharp interfaces between the layers indicating high-quality growth of the heterostructure. The epitaxial relations were deduced from the SAED pattern and the FFT pattern of the HRTEM images. Distinctly different temperature dependent magnetic properties were found for three differently oriented CuO films. Two anomalies at 213 K and 230 K are clearly visible in temperature dependent magnetization (M vs. T) plot of the heterostructure with (010) CuO film which are associated with the two magnetic transitions of CuO. On the other hand, no such anomaly was observed in M vs. T plot of the heterostructure with (111) CuO film. The heterostructure with polycrystalline CuO film shows a very weak magnetic anomaly at 230 K in its M vs. T plot. It can be concluded from our studies that the contrasting magnetic behaviours of these three heterostructures are due to the difference in epitaxial orientations of the CuO layers. Moreover, CuO thin films can be successfully grown in the direction of static ferroelectric polarization which is the ‘b’ axis of its monoclinic crystal structure. Chapter 7 concludes with general findings pertaining to various observations made in the different chapters. Prospects for future work are briefly outlined in this chapter.

The feasibility of isolation of layered materials and arbitrary stacking of different materials provide plenty of opportunities to realize van der Waals heterostructures (vdWhs) with desired characteristics. In this thesis, we experimentally demonstrate the tunability of optical and electrical characteristics of transition metal dichalcogenides (TMDs), a class of layered materials, using their vdWhs. Monolayer (1L) TMDs exhibit remarkable light-matter interaction by hosting direct bandgap, strongly bound excitonic complexes, ultra-fast radiative decay, many-body states, and coupled spin-valley degrees of freedom. However, their sub-nm thickness limits light absorption, impairing their viability in photonic and optoelectronic applications. The physical proximity of layers in vdWhs drives strong interlayer dipole-dipole coupling resulting in nonradiative energy transfer (NRET) from one layer (donor) to another (acceptor) under spectral resonance. Motivated by the high efficiency of NRET in vdWhs, we study the prospect of enhancement of optical properties of a 1L-TMD stacked on top of strongly absorbing, non-luminescent, multilayer SnSe2 whose direct bandgap is close to exciton emission of 1L-TMDs – MoS2 and WS2. We show that NRET enhances both single-photon and two-photon luminescence by one order of magnitude in such vdWhs. We also demonstrate a new technique of Raman enhancement driven by NRET in vdWhs. We achieve a 10-fold enhancement in the Raman intensity, enabling the observation of the otherwise invisible weak Raman modes. We establish the evidence for NRET-aided photoluminescence (PL) and Raman enhancement by modulating the degree of enhancement by systematically varying multiple parameters - donor material, acceptor material, their thickness, physical separation between donor and acceptor by insertion of spacer layer (hBN), sample temperature, and excitation wavelength. We also use the above parameters to decouple the effects of charge transfer and optical interference from NRET and establish a lower limit of the NRET-driven enhancement factor. We significantly modulate the strength of NRET by controlling the spectral overlap between 1L-TMD and SnSe2 through temperature variation. We show a remarkable agreement between such temperature-dependent Raman enhancement and the NRET-driven Raman polarizability model. We emphasize the advantages of using SnSe2 as a donor and elucidate the impact of various parameters on the PL enhancement using a rate equation framework. This NRET-driven enhancement can be used in tandem with other techniques and thus opens new avenues for improving quantum efficiency, coupling the advantages of uniform enhancement accessible across the entire junction area of vdWhs. Further, we study the role of NRET in photocurrent generation across vdWhs by designing a vertical junction of SnSe2/multilayer-MoS2/TaSe2. We report the observation of an unusual negative differential photoconductance (NDPC) behaviour arising from the existence of NRET across the SnSe2/MoS2 junction. The modulation of NRET-driven NDPC characteristics with incident optical power results in a striking transition of the photocurrent's power law from sublinear to a superlinear regime. These observations highlight the nontrivial impact of NRET on the photoresponse of vdWhs and unfold possibilities to harness NRET in synergy with charge transfer. The stacking angle between the individual layers in vdWhs provides another knob to tune their properties. The emergence of moiré patterns in twisted vdWhs creates superlattices where electronic bands fold into a series of minibands, inducing new phenomena. We experimentally demonstrate the PL emission from the moiré superlattice-induced intralayer exciton minibands in twisted TMD homobilayers using artificially stacked 1L-MoS2 layers at minimal twist angles. We also show the electrical tunability of these moiré excitons and the evolution of distinct moiré trions. We experimentally discern the localized versus delocalized nature of individual moiré peaks through different regimes of gating and optical excitation. Further, we discuss the gate-controlled valley coherence and resonant Raman scattering of moiré excitons. These experimental results provide unique insights into the moiré modulated optical properties of twisted bilayers. Next, we focus on tuning the electrical characteristics of vdWhs to realize ambipolar injection, which is useful for LED and CMOS applications. vdW contacts offer atomically smooth and pristine interfaces without dangling bonds, coupled with a weak interaction at the interface. Such contacts help to achieve a completely de-pinned contact close to the Schottky-Mott limit. We demonstrate the weakly pinned nature of a vdW contact (TaSe2) by realizing improved ambipolar carrier injection into few-layer WS2 and WSe2 channels (compared to Au). Backward diodes offer a superior high-frequency response, temperature stability, radiation hardness, and 1/f noise performance than a conventional diode. We demonstrate a vdWh based backward diode by exploiting the giant staggered band offsets of the WSe2/SnSe2 junction. The diode exhibits an ultra-high reverse rectification ratio of ~2.1*10^4 up to a substantial bias of 1.5 V, with an excellent curvature coefficient of ~37 V^{-1}, outperforming existing backward diode reports. We efficiently modulate the carrier transport by varying the thickness of the WSe2 layer, the type of metal contacts employed, and the external gate and drain bias. We also show that the effective current transfer length at the vertical junction in vdWhs can be as large as the whole interface, which is in sharp contrast to the smaller transfer length (~100 nm) in typical metal-layered semiconductor junctions. The results from this thesis widen the horizon for practical electronic, photonic, and optoelectronic applications of vdWhs.

The lightning return stroke forms one of the severest natural sources of electromagnetic interference for ground-based and airborne systems. Many physical fields are involved in this complex physical phenomenon. Several pertinent aspects are somewhat unclear, and it is not practical to conduct the field measurements to resolve them. One such important aspect, which is of practical relevance, is the influence of soil's electrical properties on the stroke current evolution and the fields in the soil. It formed the genesis of the present work. The collection of the required data from on-field measurements would be nearly impossible, and hence suitable theoretical approach was considered. For that, an appropriate model for the return stroke is necessary. Among different models for the lightning return stroke, only the 'Self-consistent return stroke' model is found to be suitable. This model employs a macroscopic electrical representation of the underlying physical phenomenon and accounts for the associated dynamic electric field to emulate the stroke current evolution. However, in the past works, only perfectly conducting earth was considered, and it relied on the time-domain thin-wire formulation to evaluate the associated dynamic electromagnetic fields. On the other hand, a more realistic representation of the soil, with its frequency-dependent and non-linear parameters, is required for the present work. This necessitated a suitable adoption of the domain-based 'Finite difference time domain' (FDTD) method for field computation. It turned out that, in an FDTD framework, the modeling of the channel and its corona sheath, soil-ionization, and soil-dispersion is a challenging exercise. For the simulation, a straight vertical channel of 5 km is considered. A complex-frequency-based PML (perfectly matched layer) is employed to truncate the problem domain. The high aspect ratio of the channel does not permit the application of standard FDTD update equations with a realistic spatial discretization. The conventional subcell approach, generally used to model thin-wire structure in an FDTD framework, was also not usable for two reasons. Firstly, the channel has a dynamic conductivity, and secondly, the presence of corona-sheath surrounding the channel produces a typical field profile in the region. The channel in the soil and the non-linear ionization around it also posed a similar problem. A ‘Modified subcell approach’ was developed to handle the lightning channel, which is one of the essential contributions of the present work. In the ‘Modified subcell approach’, the spatial field variation is computed at each time step, taking into account all the relevant field contributions in the respective region. The radial current produced by the charge deposited in the corona sheath is also be taken into account separately. The frequency-dependent conductivity and permittivity of the soil require a convolution in time domain formulation. This would require a repeated calculation of the integral over each cell, a forbidden task. Based on one of the recent literature, a suitable simplification is adopted, thereby drastically minimizing the computational requirement. The soil ionization, a strongly field-dependent phenomenon, required a different set of developments. Each cell is divided into subgrids to account for the local field variation and the dynamic conductivity profile. The developed FDTD formulation is deployed to investigate the role of soil's electrical properties on the stroke current evolution and the field in the soil using the self-consistent return stroke model. For the first time, it is shown that the soil's electrical conductivity has some noticeable influence on the stroke current magnitude (up to about 45 %), and the ionization phenomenon in soil tends to reduce this influence . It is shown that the current magnitude varies most for a low magnitude fast-rising current as the soil ionization is minimal for these cases. On the other hand, for high-level slow-rising currents, the ionization process significantly matures, and as a result, the dependence of current magnitude on soil resistivity is reduced substantially. It is noted that the effect of soil permittivity and the frequency-dependent soil parameters on the return-stroke current is minimal. From the results of the detailed simulation, it is found that soil resistivity also affects the field in the soil significantly. The field in the air is increased with decreasing soil resistivity, and the increase is primarily due to the increase in channel current magnitude. For the field in the soil, in addition to modulating the field magnitude, soil resistivity also affects the temporal nature, with the field becoming peakier for lower resistivity. A comparison of the computed field demonstrates that the field is underestimated significantly by the prevalent quasi-static approach, and the difference increases with the radial distance from the channel. The frequency-dependency of the soil's conductivity, and permittivity to a lesser extent, significantly reduces the field in the soil. It is also seen that the current concentration near the surface due to skin-effect is altered at later periods by the field produced by the channel current. The presence of a second layer of lower resistivity at a shallow depth, on the other hand, effectively controls the current and field in the top layer. It is also shown that the field for a strike to a mountain can depend significantly on the mountain height. In summary, significant contributions have been made in the present work towards the FDTD formulations for modeling lightning phenomena and assessing the soil’s electrical parameters on lightning stroke current evolution and the resulting field .