Less than two years ago, I was reading and writing about a new kind of passive circuit element—the memristor, a companion to the resistor, the capacitor and the inductor. (See the bit-player coverage, a bit-player followup and my American Scientist column.) The principal actors in this story were Leon Chua of Berkeley, who formulated the theory of the memristor 40 years ago, and R. Stanley Williams of Hewlett-Packard, who announced a working device in 2008.
I haven’t been keeping up with memristor news lately, but I did notice that Williams has been making the rounds with a talk on “Mott Memristors, Spiking Neuristors and Turing Complete Computing.” When he came to Harvard last week, I went over to listen in. I discovered that my 20-month-old article is totally out of date.
The memristor I wrote about in 2011 was based on ion drift in titanium dioxide. A layer of TiO2 was fabricated with a conductive doped region and an insulating undoped region. Current flowing through the TiO2 would shift the boundary between the two regions and thereby switch the memristor between conducting and insulating states. The device was bipolar: Currents in opposite directions had opposite effects. It was also nonvolatile: The resistance state would persist even after the current ceased.
The memristor that Williams discussed last week is still recognizable as a member of the same family, but just barely so. 
Again the material is TiO2 or another transition-metal oxide, but there is no mention of doping. And, again, electric currents cause a change in resistance, but the underlying mechanism is quite different. Williams described the device as a cylinder of insulating oxide with a narrow conductive channel down the middle; current flowing through the channel heats the cylinder from the inside out, causing a phase transition that converts surrounding layers of material to the conducting state. Specifically, the heating induces a Mott transition, in which localized electron clouds begin to overlap, bringing on a sudden increase in conductivity.
During the talk I didn’t hear Williams say anything about reversing the phase transition, but other sources indicate that the high-resistance state is restored by applying a second, larger current. Note that this memristor is not a bipolar device: The switching actions can be triggered by currents flowing in either direction.
A heat-induced phase transition sounds like an unlikely mechanism for a modern computing element. When I heard the idea explained, I couldn’t help thinking of computing with a toaster, whose glowing red wires seem ruinously power-hungry, and awfully slow. But Williams says the Mott memristor can be both fast and efficient, because the active volume is very small, with a typical dimension of 30 nanometers. Even though the temperature swing may be as much as 800 Kelvin, the switching time is nanoseconds, and the energy dissipated is femtojoules.
It’s a little disconcerting to see the basic physical principles of the memristor changing so drastically before the device even reaches the market. But I suppose there’s precedent in the early history of the transistor. The transistor announced by Bell Labs in 1948 was a bipolar, point-contact device made from a block of germanium; the world now runs on field-effect transistors imprinted on a silicon surface.
Which brings us to the big, obvious question I wasn’t able to answer two years ago. Will the memristor become the transistor of the 21st century, transforming electronics and computing? Or will it fade away like so many other once-promising technologies, like magnetic bubbles or superconducting Josephson junctions? Or could it find a niche market, the way charge-coupled devices have?
It’s well-known—and it may even be true—that most innovations fail to dislodge the incumbent technology. That’s a reason for betting against the memristor regardless of its particular merits and handicaps. No doubt it’s a smart bet. Nevertheless, I would like to say one thing in support of efforts to develop the memristor and other novelties like it. The mainstream in digital electronics is still focused on taking devices whose operation we understand at micrometer scale and trying to reproduce the same behavior in devices a thousand times smaller. It seems worthwhile trying the opposite strategy as well: Looking at what happens “naturally” at nanoscale, and trying to build something out of it.
After the talk, I dug up a few papers with more details on the recent developments. Two are by members of the Williams group:
Alexandrov, A. S., A. M. Bratkovsky, B. Bridle, S. E. Savel’ev, D. B. Strukov and R. Stanley Williams. 2011. Current-controlled negative differential resistance due to Joule heating in TiO2. Applied Physics Letters 99:202104. (Preprint.)
Strukov, Dmitri B., Fabien Alibart and R. Stanley Williams. 2012. Thermophoresis/diffusion as a plausible mechanism for unipolar resistive switching in metal–oxide–metal memristors. Applied Physics A DOI 10.1007/s00339-012-6902-x. (Preprint.)
The third is a slightly earlier discussion of Mott memristors by a Korean-UCSD collaboration:
Driscoll, Tom, Hyun-Tak Kim, Byung-Gyu Chae, Massimiliano Di Ventra and D.N. Basov. 2009. Phase-transition driven memristive system. (arXiv preprint.)