Friday, September 01, 2006

...and still MORE hype....

I have avoided talking too much about my own research here, with the intent of maintaining a broader perspective on CM and nanoscale physics. However, an example of nano-hype directly related to my own research has come up that I can't just let go (thanks to one of my regular anonymous contributors for pointing out the media aspects of this). Here is a perfectly reasonable theory paper about trying to make single-molecule transistors that operate in a new way. Basically the idea is to somehow (this trivial detail is left as an exercise for the experimentalists, which is actually fine for a theory paper like this) wire up three leads directly to a single small molecule. By varying the voltage on the "gate" lead, the quantum mechanical amplitude for tunneling from the source lead to the drain lead is modulated due to quantum interference. One could imagine (though this isn't discussed in the paper) implementing something like this in a GaAs quantum dot. For example, one could have a little "stub" dot off to the side of a channel connecting the source and drain. If the stub dot was tuned into the Kondo regime via coupling to the channel, then there would be a Fano antiresonance that would suppress source-drain conduction. Same basic idea. Anyway, the concept is sound, and the calculations (though done in some limited approximation on an idealized molecular/lead geometry) show that it's not crazy. Fine.

I have no problem with the science (though experimentally implementing it as conceived will be incredibly difficult). What I do have a problem with is the ensuing media onslought. Read this press release, which got picked up by CNN (broadcast, not the web). Read it all the way through, to the point where the scientist starts talking (I'm not making this up) about little nanobots controlled by computers that use this transistor concept swimming through your bloodstream. AAAAAGGGH! WHY DO PEOPLE DO THIS? Does the Arizona group really think that their paper will have more impact and enable more and better science and technology because of this? Do they think their pending patent on this idea will be more likely to be licensed?
Don't they think that this kind of overreaching hype actually hurts the field in the long run?


Dan.M. said...

Any scientist who mentions the movie "Fantastic Voyage" when talking on the record (e.g., to a reporter) ought to be taken out into the courtyard and shot.

Ok, maybe that's a bit harsh. But you get the idea.

Dave Bacon said...

Hype bad! Back to the physics!

As I understand the paper, the proposed device works by using destructive interference for the two paths around the benzene for the "no current transmision" state and then applies a third lead to cause decoherence and case the "current transmision" state. Why should we believe that this destructive interference effect isn't destroyed by other decoherence mechanisms?

I guess a related question is what fidelities are people achieving and striving for with molecular transistors? Is it okay to have a molecular transistor which fails once every thousand switching events? And then do people imagine using such transistors redundantly to get this failure rate down?

Peter Armitage said...

>Any scientist who mentions the >movie "Fantastic Voyage" when >talking on the record (e.g., to a >reporter) ought to be taken out >into the courtyard and shot.

And Moore's law! I am so sick of nanotalks with Moore's law! I exagerate some, because in some audiences it makes sense to intro with this, but much of the time it is just such a cliche.

While I was at UCLA their CNSI nanoinstitute started these weekly colloquia with high-profile speakers. For the 1st year every one of them started off with Moore's law in the first few slides. Once in a while would be OK, but we got hit with it everyweek and if it wasn't already a tired worn out narrative at the beginning of the year, it certainly was by the end. It got to be a cross-departmental joke. Everything could be related to Moore's law!

>"University of Arizona physicist >Charles A. Stafford said. "Using >electricity to raise and lower >energy barriers has worked for a >century of switches, but that >approach is about to hit the wall."

Torbjörn Larsson said...


I echo the sentiment from my microelectronics experience. Though I learned the hard way that it was used as a practical predictor from stuff like scaling laws to the type and amount of chemicals the next fab generation would consume.

Doug Natelson said...

Regarding David's question about decoherence: the third lead can't just decohere, or you'd only modulate the current from the constructive interference result (2 x the classical conduction of the two paths) to the classical result. Not much of a switch. To be good as a switch, you'd want to go from constructive to destructive interference. That being said, there are real subtle points here. The calculations are at T = 0 if I recall correctly, and truly worrying about decoherence means keeping track of all the various inelastic processes that can happen. Generally, for the isolated molecule I'd say this isn't a problem. It has a very discrete spectrum with large energy gaps, so inelastic processes that change the electronic state should be very suppressed. However, once the molecule is hybridized with the electronic states of the leads, things get much more complicated. At room temperature in a diffusive normal metal, the coherence length is something like 1 nm, which is comparable to the size of the molecule. Could be an issue. The biggest problem, though, would just be making the structure.

Josh said...

This is certainly nothing new. I can recall very similar sorts of hype back in the late 90's when MEMS were the hot item:
In this article from 1998, they predict MEMS submarines swimming through your bloodstream. 8 years later all we really have are DLPs and MEMS air bag sensors. Sure, this hype is ridiculous. However, it seems to be a necessary evil to bring ever scarcer research dollars into an emerging engineering field when the possible applications are still uncertain (ie. think of the laser).