The organization of behavior: A neuropsychological theory

Extraordinary electron systems can be generated at well-defined interfaces between complex oxides. In recent years, progress has been achieved in exploring and making use of the fundamental properties of such interfaces, and it has become clear that these electron systems offer the potential for possible future devices. We trace the state of the art of this emerging field of electronics and discuss some of the challenges and pitfalls that may lie ahead.

http://science.sciencemag.org/content/sci/327/5973/1607.full.pdf

Oxide Interfaces—An Opportunity for Electronics

Neuromorphic computing has come to refer to a variety of brain-inspired computers, devices, and models that contrast the pervasive von Neumann computer architecture This biologically inspired approach has created highly connected synthetic neurons and synapses that can be used to model neuroscience theories as well as solve challenging machine learning problems The promise of the technology is to create a brain-like ability to learn and adapt, but the technical challenges are significant, starting with an accurate neuroscience model of how the brain works, to finding materials and engineering breakthroughs to build devices to support these models, to creating a programming framework so the systems can learn, to creating applications with brain-like capabilities In this work, we provide a comprehensive survey of the research and motivations for neuromorphic computing over its history We begin with a 35-year review of the motivations and drivers of neuromorphic computing, then look at the major research areas of the field, which we define as neuro-inspired models, algorithms and learning approaches, hardware and devices, supporting systems, and finally applications We conclude with a broad discussion on the major research topics that need to be addressed in the coming years to see the promise of neuromorphic computing fulfilled The goals of this work are to provide an exhaustive review of the research conducted in neuromorphic computing since the inception of the term, and to motivate further work by illuminating gaps in the field where new research is needed

A Survey of Neuromorphic Computing and Neural Networks in Hardware.

Inspired by biological neural systems, neuromorphic devices may open up new computing paradigms to explore cognition, learning and limits of parallel computation. Here we report the demonstration of a synaptic transistor with SmNiO₃, a correlated electron system with insulator-metal transition temperature at 130°C in bulk form. Non-volatile resistance and synaptic multilevel analogue states are demonstrated by control over composition in ionic liquid-gated devices on silicon platforms. The extent of the resistance modulation can be dramatically controlled by the film microstructure. By simulating the time difference between postneuron and preneuron spikes as the input parameter of a gate bias voltage pulse, synaptic spike-timing-dependent plasticity learning behaviour is realized. The extreme sensitivity of electrical properties to defects in correlated oxides may make them a particularly suitable class of materials to realize artificial biological circuits that can be operated at and above room temperature and seamlessly integrated into conventional electronic circuits.

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A correlated nickelate synaptic transistor.

Nonvolatile redox transistors (NVRTs) based upon Li-ion battery materials are demonstrated as memory elements for neuromorphic computer architectures with multi-level analog states, "write" linearity, low-voltage switching, and low power dissipation. Simulations of backpropagation using the device properties reach ideal classification accuracy. Physics-based simulations predict energy costs per "write" operation of <10 aJ when scaled to 200 nm × 200 nm.

Li‐Ion Synaptic Transistor for Low Power Analog Computing

The thickness and origin of the free charge layer which forms at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface is still uncertain By inserting Mn dopants at different distances from the interface we can locate the position of carriers within the ${\mathrm{SrTiO}}_{3}$ surface layers We show that the majority of the carriers in fully-oxygenated samples are confined within 1 unit cell of the interface This confirms the ``polar-catastrophe'' mechanism proposed for this system but the low mobility of these carriers demonstrates the need for improved materials for applications and a more complete understanding of the role of the minority of higher mobility carriers identified

Charge confinement and doping at LaAlO3/SrTiO3 interfaces.

The rare-earth nickelates ($R$NiO${}_{3}$) exhibit interesting phenomena such as unusual antiferromagnetic order at wave vector q $=$ (\textonehalf{}, 0, \textonehalf{}) and a tunable insulator-metal transition that are subjects of active research. Here we present temperature-dependent transport measurements of the resistivity, magnetoresistance, Seebeck coefficient, and Hall coefficient (${R}_{\mathrm{H}}$) of epitaxial SmNiO${}_{3}$ thin films with varying oxygen stoichiometry. We find that from room temperature through the high temperature insulator-metal transition, the Hall coefficient is holelike and the Seebeck coefficient is electronlike. At low temperature the N\'eel transition induces a crossover in the sign of ${R}_{\mathrm{H}}$ to electronlike, similar to the effects of spin density wave formation in metallic systems but here arising in an insulating phase \ensuremath{\sim}200 K below the insulator-metal transition. We propose that antiferromagnetism can be stabilized by band structure even in insulating phases of correlated oxides, such as $R$NiO${}_{3}$, that fall between the limits of strong and weak electron correlation.

Hall effect measurements on epitaxial SmNiO 3 thin films and implications for antiferromagnetism

The rare-earth nickelates (LnNiO3, Ln = lanthanide) are interesting from both fundamental and applied perspectives, but synthesis remains a bottleneck to research due to their thermodynamic instability. Here we report the synthesis of SmNiO3 thin films on oxidized silicon wafers by physical vapor deposition followed by high pressure oxygen annealing at intermediate temperatures. The high pressure annealed films show an insulator–metal transition characteristic of bulk samples. Our experimental observations then allow us to estimate bounds on the phase stability regime, which are particularly useful given the dearth of direct thermodynamic data available for LnNiO3. We examine the limitations of these thermodynamic analyses applied to ultra-thin films. The stabilization of SmNiO3 on a canonical semiconductor template creates opportunities to study the utility of the above room temperature insulator–metal transition (at TIM = 400 K) in electronic devices.

High pressure synthesis of SmNiO3 thin films and implications for thermodynamics of the nickelates

In order to study the fundamental conduction mechanism of LaAlO3/SrTiO3 (LAO/STO) interfaces, heterostructures were modified with a single unit cell interface layer of either an isovalent titanate ATiO3 (A?=?Ca, Sr, Sn, Ba) or a rare earth modified Sr0.5RE0.5TiO3 (RE?=?La, Nd, Sm, Dy) between the LAO and the STO. A strong coupling between the lattice strain induced in the LAO layer by the interfacial layers and the sheet carrier density in the STO substrate is observed. The observed crystal distortion of the LAO is large and it is suggested that it couples into the sub-surface STO, causing oxygen octahedral rotation and deformation. We propose that the ?structural reconstruction? which occurs in the STO surface as a result of the stress in the LAO is the enabling trigger for two-dimensional conduction at the LAO/STO interface by locally changing the band structure and releasing trapped carriers.

https://iopscience.iop.org/article/10.1088/0953-8984/25/17/175005/pdf

Frank Schoofs

Papers

A correlated nickelate synaptic transistor.

Charge confinement and doping at LaAlO3/SrTiO3 interfaces.

Hall effect measurements on epitaxial SmNiO 3 thin films and implications for antiferromagnetism

High pressure synthesis of SmNiO3 thin films and implications for thermodynamics of the nickelates

Carrier density modulation by structural distortions at modified LaAlO3/SrTiO3 interfaces