681 research outputs found
Dr. Monti Datta – Faculty Author Interview
No full textDr. Monti Datta, Assistant Professor of Political Science, discusses his forthcoming new book, Anti-Americanism and the Rise of World Opinion. Drawing from a wealth of research data, interviews and surveys of social media, this book directly examines pro- and anti-American views and asks what we can learn about the nature and impact of world opinion. By treating anti-Americanism as a case study of public opinion at work, Professor Datta reveals how we can better understand the relationship between global citizens and their political leaders, and concludes that anti-Americanism does in fact substantially impact US security, as well as its economic and political interests
Fundamentals of Nanoelectronics
No full textContributed by Supriyo Datta and Behtash Behinaein of Purdue University, this course aims " to convey the conceptual framework that underlies this microscopic viewpoint using examples related to the emerging field of nanoelectronics." All lectures are available through some sort of video platform (Breeze and mp4s) and cover topics such as Quantum of Conductance, Ohm's Law, Covalent Bonds, Bandstructure, Density of States, and much more. For visitors who cannot watch the videos, most lectures are available in pdf form. There is also a link to all homework assignments, handouts, and exams, complete with solutions. This is an excellent resource for nanotechnology educators looking to bolster classroom material with lecture videos or to gather ideas about assignments, projects, and exams for classroom use
Spin transport in lateral structures with semiconducting channel
No full textSpintronics is an emerging field of electronics with the potential to be used in future integrated circuits. Spintronic devices are already making their mark in storage technologies in recent times and there are proposals for using spintronic effects in logic technologies as well. So far, major improvement in spintronic effects, for example, the `spin-valve\u27 effect, is being achieved in metals or insulators as channel materials. But not much progress is made in semiconductors owing to the difficulty in injecting spins into them, which has only very recently been overcome with the combined efforts of many research groups around the world. The key motivations for semiconductor spintronics are their ease in integration with the existing semiconductor technology along with the gate controllability. At present semiconductor based spintronic devices are mostly lateral and are showing a very poor performance compared to their metal or insulator based vertical counterparts. The objective of this thesis is to analyze these devices based on spin-transport models and simulations. At first a lateral spin-valve device is modeled with the spin-diffusion equation based semiclassical approach. Identifying the important issues regarding the device performance, a compact circuit equivalent model is presented which would help to improve the device design. It is found that the regions outside the current path also have a significant influence on the device performance under certain conditions, which is ordinarily neglected when only charge transport is considered. Next, a modified spin-valve structure is studied where the spin signal is controlled with a gate in between the injecting and detecting contacts. The gate is used to modulate the rashba spin-orbit coupling of the channel which, in turn, modulates the spin-valve signal. The idea of gate controlled spin manipulation was originally proposed by Datta and Das back in 1990 and is called ‘Datta-Das’ effect. In this thesis, we have extended the model described in the original proposal to include the influence of channel dimensions on the nature of electron flow and the contact dimensions on the magnitude and phase of the spin-valve signal. In order to capture the spin-orbit effect a non-equilibrium Green\u27s function (NEGF) based quantum transport model for spin-valve device have been developed which is also explained with simple theoretical treatment based on stationary phase approximation. The model is also compared against a recent experiment that demonstrated such gate modulated spin-valve effect. This thesis also evaluates the possibility of gate controlled magnetization reversal or spin-torque effect as a means to validate this, so called, ‘Datta-Das’ effect on a more solid footing. Finally, the scope for utilizing topological insulator material in semiconductor spintronics is discussed as a possible future work for this thesis
Modeling of spin transport in MTJ devices
No full textSpin-Transfer Torque Random Access Memory (STT-RAM) is a promising candidate for the next generation universal memory technology, with a combination of non-volatility, high endurance, high speed, and better scalability compared to existing memory technologies. The key building block of the operation of STT-RAM is the Magnetic Tunnel Junction (MTJ) device, which uses spin polarized currents instead of magnetic fields to read/ write magnetic bits. While the read process is largely determined by the charge current, the spin current is critical to the write process. This thesis presents a Non-equilibrium Green\u27s Function (NEGF) based transport model for MTJ devices that allows us to model both charge and spin currents accurately. Although there are several experiments describing the bias and switching behavior of MTJ, none of the existing theoretical models correctly provide quantitative agreement with experiments. Using our model, we show: (1) good agreement with diverse experimental measurements like resistance and spin torques; (2) provide simple explanation for bias dependences of the torques; and (3) propose an asymmetric STT device, which has the potential of switching in non-reciprocal way. Furthermore, we present an analytical expression for the spin current starting from the current operator expression; we discuss about the formalism of Non-Equilibrium spin current in the coherent transport regime and discuss initial results for a possible approach to developing a compact model for STT-RAM cell integrable with CMOS circuits
Tapping Economies of Scale and Scope in Consumer Cooperation - A Case Analysis of Possible Cooperation among selected Cooperatives
No full textBecause of its narrow and negative perspective of safeguarding the interests of only poor consumers against unethical practices of the private traders, consumer cooperation in India seems to have failed, except probably in some isolated pockets. A number of social welfare functions like poverty alleviation and public distribution of essential items of consumption have been imposed on them at the cost of their basic economics. With the basic micro and macro-economic rationale for consumer cooperatives as a positive form of economic organization being lost sight of, they seem to be facing enormous problems both historically as well as currently in a era of economic liberalization. Their worries seem to have been compounded with the threat of impending competition from large private enterpriss - both domestic and foreign, which highlights the need for evolving strategies to rectivy their systemic weaknesses and tackling the competition head on. This case has attempted to document just such an initiative through a round table conference with several doyens of the consumer cooperative movement in India such as Warana Bazar and Amalsad Mandali as well as some fledging consumer cooperatives from West Bengal which are already in existence for some time or contemplating entry into this field. The roundtable conference organized in the spirit of Cooperation among Cooperatives attempted to evolve strategies to capture economies of scale and scope in order to take on the competition, as well as to facilitate dissemination of ideas and information across the country.
Hardware Implementation of Autonomous Probabilistic Computers
No full textConventional digital computers are built using stable deterministic units known as “bits”. These conventional computers have greatly evolved into sophisticated machines, however there are many classes of problems such as optimization, sampling and machine learning that still cannot be addressed efficiently with conventional computing. Quantum computing, which uses q-bits, that are in a delicate superposition of 0 and 1, is expected to perform some of these tasks efficiently. However, decoherence, requirements for cryogenic operation and limited many-body interactions pose significant challenges to scaled quantum computers. Probabilistic computing is another unconventional computing paradigm which introduces the concept of a probabilistic bit or “p-bit”; a robust classical entity fluctuating between 0 and 1 and can be interconnected electrically. The primary contribution of this thesis is the first experimental proof-of-concept demonstration of p-bits built by slight modifications to the magnetoresistive random-access memory (MRAM) operating at room temperature. These p-bits are connected to form a clock-less autonomous probabilistic computer. We first set the stage, by demonstrating a high-level emulation of p-bits which establishes important rules of operation for autonomous p-computers. The experimental demonstration is then followed by a low-level emulation of MRAM based p-bits which will allow further study of device characteristics and parameter variations for proper operation of p-computers. We lastly demonstrate an FPGA based scalable synchronous probabilistic computer which uses almost 450 digital p-bits to demonstrate large p-circuits
On Spin-Inspired Realization Of Quantum and Probabilistic Computing
No full textThe decline of Moore’s law has catalyzed a significant effort to identify beyondCMOS devices and architectures for the coming decades. A multitude of classical and quantum systems have been proposed to address this challenge, and spintronics has emerged as a promising approach for these post-Moore systems. Many of these architectures are tailored specifically for applications in combinatorial optimization and machine learning. Here we propose the use of spintronics for such applications by exploring two distinct but related computing paradigms. First, the use of spin-currents to manipulate and control quantum information is investigated with demonstrated high-fidelity gate operation. This control is accomplished through repeated entanglement and measurement of a stationary qubit with a flying-spin through spin-torque like effects. Secondly, by transitioning from single-spin quantum bits to larger spin ensembles, we then explore the use of stochastic nanomagnets to realize a probabilistic system that is intrinsically governed by Boltzmann statistics. The nanomagnets explore the search space at rapid speeds and can be used in a wide-range of applications including optimization and quantum emulation by encoding the solution to a given problem as the ground state of the equivalent Boltzmann machine. These applications are demonstrated through hardware emulation using an all-digital autonomous probabilistic circuit
Electrical transport in heterojunctions between unconventional superconductors: Application of the Green function formalism
No full textThe primary objective of this work is to develop suitable techniques for the analysis and design of electronic devices based on semiconductor-superconductor heterostructures involving both conventional superconductors (like Niobium) and the high-T\sb{c} superconductors (like YBCO, BSCCO). Much of the earlier theoretical work in this field was focused on simple idealized geometries. Our work has led to a sophisticated model that can be used to analyze arbitrary shaped mesoscopic structures with superconducting elements. This is a very powerful analytical tool that can include the effects of impurities, boundaries, phase-breaking, complicated band-structures and unconventional order parameters. It is now believed that high-T\sb{c} superconductors have an order parameter that is fundamentally different from those in the conventional superconductors. Most importantly it seems quite likely that the order parameter changes sign for electrons with different k-vectors. This makes high-T\sb{c} superconductors very sensitive to the presence of surfaces and boundaries which mis electronic states with different k. We have used our method to understand these effects by comparing our theory with experiments done on different high-T\sb{c} junctions
Information processing with spin-coupled multi-magnet networks
No full textThe speed and efficiency of information processing in conventional charge based field effect transistors have progressed dramatically in the last 50 years due to scaling of transistor dimensions. However, the fundamental scaling limits of this technology are threatening to halt this progress; leading to an enormous interest in alternative computation schemes and devices. All-spin logic (ASL) is one such alternative approach to information processing where information is stored in the magnetization of nanomagnets coupled by spin coherent channels. We developed an experimentally benchmarked spin-coupled multi-magnet simulator to investigate two major aspects of ASL operation. Firstly, we evaluated the energy (E) delay (τ) performance metric of ASL switches and found that similar to transistors, Eτ = Q2R where R is the resistance of the device and Q, total charge supplied by the power supply in the switching process is proportional to the number of Bohr Magnetons comprising the magnet. Secondly, we found that the inbuilt non-reciprocity in ASL allows cascading them to form spin-coupled multi-magnets networks (SMN) where universal logic gates, ring oscillator and other interesting circuits can be implemented. These circuits seem difficult to implement experimentally at this time, since the large magnets used today require very high threshold spin signal to switch. Hence, for ease of experimental implementation, we proposed a different class of devices which could operate at sub-threshold bias; but still rely on the spin torque based interaction between nanomagnets. We have also developed a generic framework that can be used to understand the results of such sub-threshold experiments on spin-coupled multi-magnets networks.
All spin logic: Modeling multi-magnet networks interacting via spin currents
No full textThe increasing level of power dissipation in today\u27s transistors, due to their continued downscaling, has led to an interest in alternatives to charge-based electronics for information processing. All-spin logic (ASL) represents one such new approach where the roles of charges and capacitors in CMOS are now played by spins and magnets. Available experiments utilizing this principle show operating voltages of the order of few tens of milli-volts, far below today\u27s transistors. However, before an alternative logic scheme — like ASL — can be employed to build logic circuits, certain characteristics have to first be exhibited at the device level such as directionality of information transfer, implementing universal logic gates, cascading and fan-out. In order to devise and analyze ASL based strategies that can incorporate these device characteristics, this report first introduces a novel 4-component Spin-Circuit formalism, which is then coupled to an existing model for magnetization dynamics. This coupled model can simultaneously describe two distinct physical phenomena: (1) spin torque switching of magnets and (2) generation and transport of non-collinear spin currents in spin diffusive channels. The model is first benchmarked against available experimental data and is then used to provide key insights at the ASL device level, such as how to incorporate inbuilt directionality of information transfer and to propose scaling laws. Towards the end of this report, the model is extended to simulate multi-magnet ASL networks interacting via spin currents. In particular, examples of an ASL ring oscillator and a universal NAND gate are presented, which form the basis for designing large scale ASL circuits
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