1,721,090 research outputs found
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
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
Transport experiments in electron waveguides
No full textT-shaped electron waveguide structures have been fabricated on AlGaAs/GaAs heterojunctions using electron-beam lithography and a shallow wet-chemical etch. The electrical width and carrier density of the narrow channels are estimated from their magnetoresistances. Two types of devices have been investigated at temperatures down to 30mK. T devices made on insulating substrates show conductance oscillations as the length of the stub is changed by depleting it with a Schottky gate on the surface. Although some of the frequencies observed fall in the range of expected frequencies, fringing fields from the gate complicate the interpretation of the oscillations. The effects of these fringing fields are greatly reduced by changing the substrate to P+ GaAs. Using the P+ substrate as a backgate, oscillations are observed in fixed-length T devices as the carrier density is varied. The observation of similar oscillations in straight wires implies that scattering in the waveguide is producing universal conductance fluctuations which mask the effects of device geometry. The conductance fluctuations are analyzed to estimate the phase-breaking length. Fabrication methods which produce waveguides free from disorder must be perfected before clear geometrical resonance can be observed in long narrow waveguide-type structures
Coulomb and spin blockade effects in nanoscale electronic transport
No full textTheoretical efforts to evaluate the current-voltage I(V) characteristics of nano-scale devices have mainly employed the non-equilibrium Green\u27s function (NEGF) technique, coupled with appropriate Self Consistent Field (SCF) methods to account for electron-electron interactions. While the NEGF formalism allows a full quantum mechanical treatment of the transport problem, the application of SCF methods is questionable especially in the case where electron interactions dominate. Such cases are often encountered when considering electronic transport through quantum dots and ultrashort molecules, and pose considerable theoretical challenge. One such case is the regime of Coulomb blockade (CB) where the device is strongly interacting but weakly coupled to contacts. In this work, our focus is on interpreting notable transport experiments and novel transport effects in this regime. The NEGF-SCF formalism is highly preferred due to the relative ease with which atomistic electronic structure calculations can be coupled with transport processes, since the compuatational scaling of the one-particle Hamiltonian is linear. However, Coulomb blockaded devices considered here are very difficult to model using the one-particle Hamiltonian, but can be understood very well in terms of the multi-particle space viewpoint. Two types of devices are considered here: (a) Molecular electronic devices: In the first part of this work, we use the multi-particle Hilbert (Fock) space of the exact diagonalized many-electron Hamiltonian of various molecular structures. Using a kinetic equation in Fock space, we identify experimental signatures in the I(V) characteristics of such weakly contacted molecules arising directly from the many-particle excitation spectrum. This approach successfully explains various non-trivial features in the I(V) characteristics that are hard to realize within the NEGF-SCF framework. We then extend this approach to a simpler and more intuitive R-C equivalent by employing an incoherent sum of the many-particle excitations, that aids in understanding the observed experiments. (b) Quantum Dot Devices: The latter part of this work focusses on novel spin correlation and scattering effects in double quantum devices. In several notable experiments, spin correlation effects have revealed novel transport signatures such as regions of multiple Negative Differential Resistances (NDR) in the I(V) characteristics and hysteresis effects. We develop a generic mechanism for multiple NDRs to occur due to spin blockade and develop a theory for hysteretic bistability that is induced via feedback from hyperfine scattering of electron spins with the nuclear spins. The resulting Dynamic nuclear polarization forms the essence of various spin-based quantum information schemes that are currently being explored. A deep understanding of non-linear effects in strongly correlated systems will in the future contribute crucially toward theoretical progress in diverse fields where far-out-of-equilibrium dynamics are involved
Spin circuit representation of electronic transport in materials with spin orbit coupling
Get PDFModern nanomagnetic devices involve materials and phenomena featuring both spin and charge transport. SPICE compatible spin circuits with 4-component voltage and current (1 for charge and 3 for spin) have been developed to represent this emerging class of devices. However there has not been much work on circuit representation for materials with high spin-orbit coupling (SOC) which are becoming increasingly important with the discovery of giant spin Hall effect (GSHE) and topological insulators. In this work we describe a spin circuit representation for 3D bulk materials like Tantalum or Tungsten exhibiting GSHE, which has received extensive attention recently due to their potential applications to write units in memory. This work shows how this circuit representation leads to many established results in a straightforward way, while providing a versatile tool for the numerical analysis of complex geometries. Next, we move onto new type of materials called topological insulators where we develop theoretical models for electron transport, benchmark them against available experimental data and make interesting predictions that can be tested experimentally, some aspects of which have recently received experimental support. We believe that this approach is applicable not only to topological insulators but to 2D interfacial channels with SOC in general
Electronic transport in molecular wires
No full textIn this work we describe a theoretical model that we have developed for describing electronic transport in organic molecules sandwiched between two metallic contacts. Such molecules have been synthesized recently, and have potential applications in molecular electronics. We will begin with equilibrium properties and describe how the location of the Fermi energy can be fixed relative to the molecular energy levels. Next we will discuss the factors affecting the low bias resistance. By combining the Landauer approach with the Friedel sum rule, we relate the low bias conductance of the molecule to the energy level broadening and the charge transferred into or out of the molecule. Possible methods for enhancing the molecular conductance will be discussed. Finally we will discuss the current-voltage characteristics obtained at large bias and show that a third parameter, the voltage division factor, plays an important role. We will show that the experimentally observed conductance spectrum (dI/dV versus V) for a series of different molecules can all be understood in terms of three parameters, namely, the energy level broadening, the equilibrium Fermi energy and the voltage division parameter
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
Zero field spin splitting in indium gallium arsenide/indium aluminum arsenide heterostructures
No full textThe InGaAs/InAlAs heterojunction is a promising material system for high speed device applications and opto-electronic integration. From beating effects observed in Shubnikov-de Haas (SdH) oscillations it is demonstrated that the degeneracy of the conduction band is lifted in zero magnetic field in this material system. The SdH oscillations observed in two pseudomorphic samples and one lattice matched sample show clear beats with up to six null points for magnetic fields B less than 1T. This field is low enough that the effect of non-parabolicity is deemed insignificant. Zero field spin splittings of 1.5-2.75meV were obtained for the three samples. Two mechanisms produce zero field spin splitting in semiconductor heterostructures: (i) a k-cube term arising from crystal inversion asymmetry and (ii) the Rashba term due to interface-spin-orbit-interaction. It is demonstrated that in this material system only one of these mechanisms is dominant and that it is probably the Rashba term. From investigation of the spin splitting in a tilted magnetic field, negative g-factors for 2-3 are obtained for these materials
- …
