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Chaotic dynamics in nanoscale NbO2 Mott memristors for analogue computing


At present, machine learning systems use simplified neuron models that lack the rich nonlinear phenomena observed in biological systems, which display spatio-temporal cooperative dynamics. There is evidence that neurons operate in a regime called the edge of chaos1 that may be central to complexity, learning efficiency, adaptability and analogue (non-Boolean) computation in brains2,3,4,5,6,7. Neural networks have exhibited enhanced computational complexity when operated at the edge of chaos2, and networks of chaotic elements have been proposed for solving combinatorial or global optimization problems8. Thus, a source of controllable chaotic behaviour that can be incorporated into a neural-inspired circuit may be an essential component of future computational systems. Such chaotic elements have been simulated using elaborate transistor circuits that simulate known equations of chaos9,10,11,12, but an experimental realization of chaotic dynamics from a single scalable electronic device has been lacking5,6,13. Here we describe niobium dioxide (NbO2) Mott memristors each less than 100 nanometres across that exhibit both a nonlinear-transport-driven current-controlled negative differential resistance and a Mott-transition-driven temperature-controlled negative differential resistance. Mott materials have a temperature-dependent metal–insulator transition that acts as an electronic switch, which introduces a history-dependent resistance into the device. We incorporate these memristors into a relaxation oscillator14 and observe a tunable range of periodic and chaotic self-oscillations15. We show that the nonlinear current transport coupled with thermal fluctuations at the nanoscale generates chaotic oscillations. Such memristors could be useful in certain types of neural-inspired computation by introducing a pseudo-random signal that prevents global synchronization and could also assist in finding a global minimum during a constrained search. We specifically demonstrate that incorporating such memristors into the hardware of a Hopfield computing network can greatly improve the efficiency and accuracy of converging to a solution for computationally difficult problems.

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Figure 1: Device structure and static behaviour.
Figure 2: Experimental dynamical behaviour.
Figure 3: Simulations of thermal fluctuations and relaxation oscillator dynamics.
Figure 4: Chaos-aided global minimization.

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The research is in part based on work supported by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via contract number 2017-17013000002. We thank L. O. Chua for reviewing the manuscript, discussions and data analysis. We also thank G. Gibson for discussions and insights.

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All authors contributed to the conception of the idea, design of experiments, construction of the model, data analysis and writing of the manuscript. S.K. primarily set up experiments and collected experimental data. S.K. and J.P.S. together ran the simulations of the models. R.S.W. conceptualized the static model, the inclusion of thermal noise in the dynamical model, and determined the relevance of chaos in computational systems. J.P.S. had the specific idea of using chaos for accelerating solutions in Hopfield networks.

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Correspondence to Suhas Kumar or R. Stanley Williams.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks W. Ditto, Z. Toroczkai and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Kumar, S., Strachan, J. & Williams, R. Chaotic dynamics in nanoscale NbO2 Mott memristors for analogue computing. Nature 548, 318–321 (2017).

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