BOOLEAN AND BRAIN-INSPIRED COMPUTING USING SPIN-TRANSFER TORQUE DEVICES

Several completely new approaches (such as spintronic, carbon nanotube, graphene, TFETs, etc.) to information processing and data storage technologies are emerging to address the time frame beyond current Complementary Metal-Oxide-Semiconductor (CMOS) roadmap. The high speed magnetization switching of a nano-magnet due to current induced spin-transfer torque (STT) have been demonstrated in recent experiments. Such STT devices can be explored in compact, low power memory and logic design. In order to truly leverage STT devices based computing, researchers require a re-think of circuit, architecture, and computing model, since the STT devices are unlikely to be drop-in replacements for CMOS. The potential of STT devices based computing will be best realized by considering new computing models that are inherently suited to the characteristics of STT devices, and new applications that are enabled by their unique capabilities, thereby attaining performance that CMOS cannot achieve. The goal of this research is to conduct synergistic exploration in architecture, circuit and device levels for Boolean and brain-inspired computing using nanoscale STT devices. Specifically, we first show that the non-volatile STT devices can be used in designing configurable Boolean logic blocks. We propose a spin-memristor threshold logic (SMTL) gate design, where memristive cross-bar array is used to perform current mode summation of binary inputs and the low power current mode spintronic threshold device carries out the energy efficient threshold operation. Next, for brain-inspired computing, we have exploited different spin-transfer torque device structures that can implement the hard-limiting and soft-limiting artificial neuron transfer functions respectively. We apply such STT based neuron (or ‘spin-neuron’) in various neural network architectures, such as hierarchical temporal memory and feed-forward neural network, for performing “human-like” cognitive computing, which show more than two orders of lower energy consumption compared to state of the art CMOS implementation. Finally, we show the dynamics of injection locked Spin Hall Effect Spin-Torque Oscillator (SHE-STO) cluster can be exploited as a robust multi-dimensional distance metric for associative computing, image/ video analysis, etc. Our simulation results show that the proposed system architecture with injection locked SHE-STOs and the associated CMOS interface circuits can be suitable for robust and energy efficient associative computing and pattern matching

Magnetization dynamics due to field interplay in field free spin Hall nano-oscillators

Spin Hall nano oscillators (SHNOs) have shown applications in unconventional computing schemes and broadband frequency generation in the presence of applied external magnetic field. However, under field-free conditions, the oscillation characteristics of SHNOs display a significant dependence on the effective field, which can be tuned by adjusting the constriction width, thereby presenting an intriguing area of study. Here we study the effect of nano constriction width on the magnetization dynamics in anisotropy assisted field free SHNOs. In uniaxial anisotropy-based field-free SHNOs, either the anisotropy field or the demagnetization field can dominate the magnetization dynamics depending on the constriction width. Our findings reveal distinct auto-oscillation characteristics in narrower constrictions with 20 nm and 30 nm constriction width compared to their wider counterpart with 100 nm width. The observed frequency shift variations with input current and constriction widths stem from the inherent nonlinearity of the system. The interplay between the B_demag and B_anis, coupled with changes in constriction width, yields rich dynamics and offers control over frequency tunability, auto oscillation amplitude, and threshold current. Notably, the spatial configuration of spin wave wells within the constriction undergoes transformations in response to changes in both constriction width and anisotropy. The findings highlight the significant influence of competing fields at the constriction on the field-free auto oscillations of SHNOs, with this impact intensifying as the constriction width is varied.Comment: 25 pages, 11 figure

Modeling and design for energy-efficient spintronic logic devices and circuits

The objective of the proposed research is the modeling and the design of energy-efficient and scalable novel spintronic devices. Over the past two decades, spintronic devices have achieved special status due to their advantages in terms of low-voltage operation, smaller footprint area, non-volatile memory, and compatibility with CMOS technology. To design efficient spin-based systems, researchers require the precise modeling of the physics of nanomagnets, piezoelectrics, thermal noise, and metallic nanowires. Using the models developed during the research, spintronic logic devices comprised of hybrid magnetic and piezoelectric structures are proposed. The delay, energy dissipation, and footprint area of the proposed devices are analyzed. Moreover, the proposed devices are used as building blocks to propose spin-based logic gates, pattern and image recognition circuits, long-range interconnects, interface circuits, and coupled-oscillators. The performance of the proposed circuits is benchmarked against CMOS and other spin-based circuits, which shows improved performance, especially in implementing non-Boolean applications and interface circuits.Ph.D

Spin phenomena in semiconductor quantum dots

This thesis discusses development of new semiconductor quantum dot (QD) devices and materials. Optical spectroscopy of single QDs is employed in order to investigate electronic structure and magnetic properties of these materials. First we realise self-assembled InP/GaInP QDs embedded in Schottky diode structures, with the aim to realise charge control in these nanostructures, which recently provided an important test-bed for spin phenomena on the nano-scale. By varying the bias applied to the diode, we achieve accurate control of charge states in individual QDs, and also characterise the electron-hole alignment and the lateral extent of the exciton wavefunction. Second part of the thesis explores optimum regimes for optically induced dynamic nuclear polarization (DNP) in neutral InGaAs/GaAs QDs. Very efficient DNP under ultra low optical excitation is demonstrated, and its mechanism is explained as the electron-nuclear flip-flop occurring in the second order process of the dark exciton recombination. The final part of the thesis reports on magneto-optical studies of novel individual InPAs/GaInP quantum dots studied in this work for the first time. Here the long-term aim is to realise strong carrier confinement potentially suitable for QD operation at elevated temperatures, e.g. as a single photon emitter. Here we lay foundations for future structural studies of these dots using optically detected nuclear magnetic resonance, and explore regimes for ecient DNP in InPAs dots emitting in a wide range of wavelength 690-920 nm

Advanced Photonic Sciences

The new emerging field of photonics has significantly attracted the interest of many societies, professionals and researchers around the world. The great importance of this field is due to its applicability and possible utilization in almost all scientific and industrial areas. This book presents some advanced research topics in photonics. It consists of 16 chapters organized into three sections: Integrated Photonics, Photonic Materials and Photonic Applications. It can be said that this book is a good contribution for paving the way for further innovations in photonic technology. The chapters have been written and reviewed by well-experienced researchers in their fields. In their contributions they demonstrated the most profound knowledge and expertise for interested individuals in this expanding field. The book will be a good reference for experienced professionals, academics and researchers as well as young researchers only starting their carrier in this field

Ultrafast Spin Dynamics of Next-Generation Nanomagnetic Technologies

Over the past 50 years, our society has experienced a technological revolution that has fundamentally changed the way our world operates. At the heart of this revolution are the computational building blocks that work together to perform mathematical operations and save the results. For many years, the size of the computing elements (e.g. transistors) has been consistently shrunk so that more devices could fit on a chip in order to increase computational power. To provide adequate data storage for the ever-increasing number of computations, the hard-disk drive (HDD) was developed in the 1980s and would forever revolutionize the landscape of memory storage. Today, HDDs still account for a vast majority of the data stored worldwide. These devices store information using the magnetization of nanoscopic domains in a granular magnetic film, however, in recent years it has become increasingly challenging to reduce the size of the domains further without fundamentally changing the HDD. Indeed, the latest iteration of this technology has incorporated lasers into the devices to leverage multiple degrees of freedom in order to achieve higher bit densities. This example highlights a common trend for all next-generation computational technologies – the strong coupling between distinct physical systems must be utilized to sustain the improvements our society has become accustomed to. In order to realize this lofty goal, the physics of nanoscale systems must be well understood to predict their behavior. As our collective understanding of this field continues to flourish, novel effects are found that open doors to previously unimaginable technologies that may usher in a revolution of their own. Indeed, there are both technological and fundamental interests to study nanostructured devices.In this thesis, the time-resolved magneto-optic Kerr effect (TR-MOKE) will be utilized to probe the ultrafast spin dynamics of magnetic films, multilayer heterostructures, and nanostructures. Our experimental observations of these systems are evaluated by combining various field of science and technology, including (but not limited to) condensed matter theory, signal processing, and optics. In doing so, we seek to fully explain the data and to enrich the understanding of these underexplored systems to inform the rational design of next-generation technologies. Specifically, a great deal of attention will be paid to emergent nanotechnologies that leverage the coupling between the magnetic system and either the electronic or mechanical properties of the device to tailor the performance. In this work, a novel method to restore the intrinsic magnetization dynamics and simultaneously improve the magneto-optical response of dense nanomagnet arrays will be presented. Then, our work on the spin dynamics of isolated nanomagnets resonantly excited by microwave-frequency acoustic waves will be reviewed, wherein we show for the first time that the coupling efficiency is ultimately limited by the damping of the magnetic system. In addition, the role of the nanomagnet geometry and the acoustic wavelength will be fully explored to determine critical parameters that govern the dynamic magneto-elastic resonance. Lastly, the development of an optical system to study the interplay between ultrafast all-optical switching and surface acoustic waves will be reviewed

Approaches to Building a Quantum Computer Based on Semiconductors

Throughout this Ph.D., the quest to build a quantum computer has accelerated, driven by ever-improving fidelities of conventional qubits and the development of new technologies that promise topologically protected qubits with the potential for lifetimes that exceed those of comparable conventional qubits. As such, there has been an explosion of interest in the design of an architecture for a quantum computer. This design would have to include high-quality qubits at the bottom of the stack, be extensible, and allow the layout of many qubits with scalable methods for readout and control of the entire device. Furthermore, the whole experimental infrastructure must handle the requirements for parallel operation of many qubits in the system. Hence the crux of this thesis: to design an architecture for a semiconductor-based quantum computer that encompasses all the elements that would be required to build a large scale quantum machine, and investigate the individual these elements at each layer of this stack, from qubit to readout to control