Abstract
Information processors process information in a variety of ways. The human brain processes information through a highly interconnected system of neurons and synapses, while a digital computer processes information by having a binary switch toggle on and off in response to a stream of binary bits. The “switch” is the most primitive unit of the modern computer. The better it is (faster, more energy efficient, more reliable, etc.), the more advanced is the computer hardware. Energy efficiency, however, is more important than any other attribute, not so much because energy is costly, but because too much energy dissipation prevents increasing the density of switches on a chip that is necessary to make the chip increasingly more powerful. Reducing dissipation entails radically new and often revolutionary approaches for implementing the switch. One such approach is to encode digital bit information in the spin polarization of a single electron (or ensemble of electrons) and then using two mutually antiparallel polarizations to represent the binary bits 0 and 1. Switching between the bits can be accomplished by simply flipping the polarizations of the spins, which takes very little energy. Such switches are extremely energy efficient if designed properly, but they are somewhat slower than traditional transistor-based switches and can be more error prone. This paper discusses the pros and cons of spin-based switches and introduces the reader to the most recent advancements in information processing predicated on encoding information in electron spin polarization.
Highlights
The major drawback of Single Spin Logic (SSL) is that it requires cryogenic operation because (i) exchange interaction between spins confined in semiconductor quantum dots is very weak, and yet it has to exceed the thermal energy kT manyfold in order to have small error probability p (see (10)); (ii) higher temperatures increase the spontaneous spin flip rate 1/T1 dramatically and increase the extrinsic error probability pextrinsic rapidly (see (9))
Reference [89] and the later work by our group have shown that the total energy dissipated per bit flip in hybrid spintronic/straintronic memory is about 400kT at room temperature if we switch in ∼1 ns
We have outlined recent developments in spinbased architectures for logic and memory, focusing on nanomagnetic computing where shape-anisotropic nanomagnets act as binary switches for both logic and memory
Summary
Information processors (computers, cell phones, digital watches, personal communicators, etc.) pervade our everyday lives. The convention used in this article is valid) that as long as one-half of the Zeeman splitting caused by the local magnetic fields that orient the spins in the input dots greatly exceeds the exchange coupling energy, that is, hA = hB J and J > Z/2, where Z is the Zeeman splitting due to the global magnetic field, the T-F gate can be implemented. Reference [5] showed that the energy dissipated in a NAND gate operation is approximately gμB|Bglobal| which happens to be the energy difference between the two antiparallel spin states in any isolated dot that is not subjected to any external field other than the global field. It has been shown that graphene nanoflakes can implement SSL-type logic gates with much higher exchange interaction strength (2J = 180 meV) which allows roomtemperature operation with a bit error probability p = e−2J/kT = 0.1% [58]. Recent demonstration of field effect transistors with 6 nm gate length [59] shows that lithography is advancing to the level where such challenges can be met
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