Gabrielse talk

This'll be my last talk description for a while, I promise. Colloquium today was Gerry Gabrielse, talking about their group's latest measurements of the g factor of the electron (really [g/2-1]) and the accompanying inferred value for the fine structure constant. Gabrielse did open his talk with most of this clip, since it's about their work. On a random note, I TAed the first author on that first paper once when he was an undergrad.

Precision measurement physics is extremely impressive in its own way. They measure g to parts in 10^13, and \alpha to parts in 10^10 by doing incredibly precise spectroscopy on a single trapped electron in a magnetic field. To really do this right, they have to get rid of all the relevant black body photons in the microwave range, meaning that they have to cool their cavity down to 80 mK. They also need to account for cavity QED effects - again it's a restricted density of states argument. They get the lifetime for spontaneous emission of a microwave photon from the first excited state to the ground state of their trapped electron to be 260 times what it would be in free space. They achieve this lifetime enhancement by making sure to operate their cavity such that there just aren't any cavity modes available at the right energy for the would-be photon to occupy. A tour de force piece of work. I'm pretty sure that precision measurement like this would drive me bonkers.

Frank Wilczek talk, part two

Frank Wilczek gave his second talk at Rice, "The lightness of being", about the origins of mass and the "feebleness" of gravity. He demonstrated the relative weakness of gravity very effectively by jumping up and down, showing that by using a tiny amount of chemical energy, he could overcome (temporarily) the gravitational attraction of the entire planet. I'll admit that I was a bit disappointed in this talk, in the sense that there was more overlap with yesterday's public lecture than I was expecting. I did come away having learned a new way to think about the origin of the mass of the nucleons, though. Wilczek's most famous contribution to physics is asymptotic freedom of quarks, which can be summarized as this: unlike the other forces that weaken with interparticle distance, the gluon-mediated color charge interaction between quarks grows as the separation between quarks is increased. One result of this is that there are no free quarks - if you try and separate a lone quark, the energies involved in the strong interaction become large enough to favor creation of quark-antiquark pairs. So try to build a nucleon out of three quarks. The quarks have to be pretty localized relative to each other, so that from far away there is no unscreened color charge. Localizing quantum mechanical objects leads to a particle-in-a-box type kinetic energy, though. You can think of this as coming from the uncertainty principle. It's this internal kinetic energy that is the source of 95% of the mass of the proton, via m = E/c^2. Voila - mass comes about due to quantum confinement. "Nano" concepts at work on the "femto" scale.

Another interesting point that Wilczek made: the near-perfect conservation of mass law identified by Lavoisier in chemical reactions is a great example of an emergent law. Strictly speaking, mass isn't conserved - energy is. That's very clear at particle accelerators, where a colliding e-e+ pair can produce particles massing 30000x that of the two electrons. The reason that chemistry doesn't see this effect is very much in the spirit of condensed matter. The excitation spectrum of the bound quarks is very strongly gapped. There are no available excited states of the coupled quark system at the few-eV energies relevant to chemical reactions. This basic idea, that processes can be suppressed because of a lack of available states, is also prevalent in much nanoscale physics.

I asked him about the proton "spin problem", as discussed recently here. At issue is where does the intrinsic angular momentum of the proton come from. Wilczek pointed out that there actually isn't any discrepancy with theory; lattice QCD does give spin-1/2 as the final total. What rubs people the wrong way is that the calculations run counter to most intuition. Rather than that angular momentum coming from the spins of the quarks, it appears that much of it comes from the gluon field. There you have it.

Finally, in the Q&A period, someone asked Wilczek about the possibility of extra dimensions - from context, I assume "large" ones. Wilczek really doesn't like this idea; he favors supersymmetry-driven Planck-scale grand unification. He said that it's hard enough accomplishing that and not running into problems like proton decay, and that pushing unification to lower energies (as would happen in the large extra dimension case) would cause all kinds of difficulties like that. I hadn't heard this said before, and would be curious to know more about it. Presumably the proponents of these extra dimension ideas have thought about this.

Frank Wilczek talk, part one

Frank Wilczek is visiting Rice for two days this week, and is giving two talks. I was fortunate enough to have lunch with him. He's amazingly smart, and extremely versatile. You really don't run into too many people who are conversant on the highest levels of high energy theory (hey, the guy did win a Nobel for asymptotic freedom) and also on the highest levels of condensed matter (he's very interested in non-Abelian statistics and topological quantum numbers in condensed matter systems). His first talk, a public (named) lecture entitled "The Universe is a strange place," was this afternoon. As you might expect from someone as adept at writing physics for a general audience, Wilczek gave a very clear presentation that surveyed modern high energy physics. He discussed ideas relevant from QCD - that most of the mass of nucleons comes from the energy balled up in their constituent quarks and gluons rather than from the rest mass of the quarks. He also emphasized strongly the idea that quarks and other fundamental particles are simply organized, long-lived excitations of underlying quantum fields that are always fluctuating on short time scales (h/mc^2) and length scales (10^-13 cm for nucleons). I hadn't appreciated before that after fixing only three masses (e.g., the K, pi, and b-bbar mesons) lattice QCD nails all the other hadron masses. He talked briefly about dark matter and dark energy, and explained his reasoning for liking supersymmetry. In his words, either the beautiful ideas of supersymmetry are right, leading to unification of the running strong, electroweak, and gravitational couplings, with testable consequences in the form of superpartners detectable at the LHC; or, Nature is cruelly teasing us.

At the very end an audience member asked his opinion on string theory. Wilczek said that string theory was not, properly, a theory - it was not a well-defined set of equations with real predictive solutions (as in QCD). While recognizing the value of aesthetics and symmetry, he clearly understands that the real test of theory is experiment, not intrinsic beauty. (Cue Lubos denouncing Wilczek in 5, 4, 3, ....). He went on to say that it was a collection of very interesting ideas, that it may one day get to an actually predictive form, and that there were only a small number of approaches out there for treating quantum gravity.

Long-term research, companies, and universities

I've posted about this topic before, but Gordon Watts' recent post on the subject of long-term research makes me want to throw this out there again. That, and the disturbing news I heard at the APS March Meeting about a round of layoffs of some of the few remaining physical sciences researchers at Bell Labs. It's terribly depressing: since my time in high school, long-term industrial R&D has been gutted in this country (and in most of the world). "Long-term" now means two years. Companies are under so much pressure to have year-over-year quarterly revenue increases that they blanch at the idea of spending money on something risky that may not lead to a big revenue stream quickly. Maybe that's always been true to some extent, and places like Bell Labs and IBM Research (and RCA and GE Research and GM and Ford Scientific and Westinghouse Research) were all effectively accidental monopolies or near-monopolies when they had major research labs. It's demonstrably much worse now.

More distressing to me is the tacit assumption, mentioned by Gordon, that university research will somehow pick up the slack. That is, federal dollars are more appropriate for this kind of basic work, and companies can always fund university labs to do work for them, too. Anyone who knows how university research actually works can tell you many reasons why this is a bad idea. Apart from low-level practical considerations (publish vs patent? foreign vs. domestic students? export controls?), the big killer here is just one of resources. Back when I was at Bell, if they wanted to they could have put a dozen condensed matter PhDs to work on a problem, along with technical support staff. Given how universities work, with teaching commitments, administrative tasks, student timescales, etc., no university achieve that kind of critical mass.

This week in cond-mat

A few highlights from this week, though brief. I've actually been working on my book rather than writing as much. Coming soon: more about faculty searches (now that I don't have to worry that my comments could give an unfair advantage to any candidate, since we're past the interviewing stage).

cond-mat/0703230 - Karabacak et al., High frequency nanofluidics: an experimental study using nanomechanical resonators
With my mech-E background, I've always liked fluid dynamics and lamented that it gets left out of the typical physics curriculum. This is a nice use of nanomechanical resonators as a means to study fluid motion via the resulting damping of the resonator. Of particular interest is the transition between Newtonian flow (shear stress on a wall given by the product of a viscosity times the velocity gradient at the wall) and non-Newtonian flow (shear stress depending on shear rate, for example; cornstarch in water gets stiff at high shear rates, while mayonnaise gets softer at high shear rates. Both are non-Newtonian fluids).

cond-mat/0703374 - Katsnelson and Novoselov, Graphene: new bridge between condensed matter physics and quantum electrodynamics
This is a good, pedagogical review of a lot of the interesting physics seen in electronic transport in graphene. Because of its band structure, electrons and holes in graphene act rather like ultrarelativistic particles (that is, their energy is approximately linearly proportional to their (crystal) momentum, like photons). The discussion in this paper of the Klein paradox is particularly nice; I hadn't read such a clear summary of it before.

cond-mat/0703247 - Malyshev, DNA double helices for single molecule electronics
This has already come out in PRL. While I'm sure the calculations are reasonable and robust, this is a classic example of a theory proposal that is much easier to talk about than ever actually try. My main problem here is that actually preparing electronic devices from DNA and ending up with a controlled system is incredibly hard. There are compensating ions all over the place; DNA in vacuum or on a surface is not nearly the same thing as in a biological environment, including its conformations. Ahh well.

cond-mat/0703419 - Zhang et al., Noise correlations in a Coulomb blockaded quantum dot
Yet another pretty piece of experimental work from Harvard and Tokyo. Using a combination of tank circuits (RLC resonators), cold voltage amplifiers, and a cross-correlation system, these folks are able to measure shot noise in a Coulomb-blockaded quantum dot. They can use a gate to tune the dot in and out of blockade, and can watch the noise vary from sub- to superPoissonian (that is, are the electrons behaving independently (Poisson statistics for tunneling), avoiding each other (sub-Poissonian), or bunching (super-Poissonian). It all looks so easy, though I know experiments like this are very challenging.

Quote verification?

Last week at the APS, Lars Samuelson closed his nano-related talk with the following quote, reportedly from Albert Einstein: "Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction." Can anyone tell me the primary source of this quote, and whether it's legitimate? I've googled a bit, and all I've found are lists of quotes that appear to have circulated online since the mid 1990s, with no primary source attribution. Since a number of fake quotes propagate online, I want to check this one out. Thanks....

MM2007 - final thoughts

Well, I'm back home from APS. I'll write a bit more about the science over the weekend, but for now, here are some last thoughts on the meeting.

Three things that are frustrating about conferences:

• Speakers that run way over their time. There was an invited talk this morning that was physically very interesting, but the speaker must've run 10 minutes over. The timer goes off - no sign of conclusions. The session chair stands up. No slowing down. The session chair whispers in the ear of the speaker. "I'm concluding." Followed by three more slides.
• Senior people that get your name wrong. Repeatedly. In front of a full room.
  
• Parallel sessions on nearly identical topics at opposite ends of the convention center.

Four good things about conferences:

• Senior people that do cite you, and get your name right.
• Competitors that do similar measurements that complement your work and are nice about it, and good agreement between the independent experiments. (Hurray! Science actually works!)
  
• Former students doing well in their careers.
• Good audiences that ask smart questions.

More MM07

More good physics at the APS meeting, though I'm rapidly approaching the point of mental exhaustion.

There was an invited symposium on silicon nanoelectronics on Wednesday that was very nice - I only saw the first three talks, but they were all good. Steve Lyon from Princeton spoke about his ESR measurements on small numbers of electrons in Si/SiGe heterostructures and dots. Mark Eriksson from Wisconsin gave a good overview of their recent work on trying to get gate-defined quantum dots in Si/SiGe to act as nicely as those in GaAs/AlGaAs. A main point of physics in both of those talks was the effect of valley degeneracy on spin physics in those structures. In bulk Si the bottom of the conduction band is 6-fold degenerate and not located at k=0. In quantum wells or heterojunctions, the degeneracy is partially lifted due to the broken spatial symmetry. Mark and Steve have both been worrying about the size of the splitting in energy between the lowest valley and the next valley, and Mark's work looks like it answers the question in gate-defined dots. The third talk was by my old friend Sven Rogge now from Delft. There he has been working on making measurements on states confined to individual dopant atoms in ultrasmall Si transistors. It's extremely interesting to look at how the hydrogen-like donor wavefunctions hybridize with Si well states when the gate field pulls the electron from the donor toward the well.

Today I've seen two very smooth talks in nanostructures sessions. In the first Amir Yacoby, late of the Weizmann Institute and now at Harvard, showed new work on transport through "double dot" structures made from two metal nanoparticles linked by a small organic molecule. At low temperatures and voltages, the physics is dominated by Coulomb charging effects of the two nanoparticles. They see all kinds of rich Coulomb blockade behavior that can be modeled basically perfectly with only a few free parameters (the capacitances and resistances of the relevant junctions). The second was a talk by Lars Samuelson at Lund. He's one of the big movers and shakers in growing semiconductor nanowires. He gave a full overview of their work on this, which has included some obscene number of high impact publications. People with that kind of productivity are simultaneously impressive and depressing.

Incoherent Ponderer is absolutely right about the graphene thing. I've heard some nanotube folks griping that graphene is the new hotness.

The accidental session chair

I can already tell that I have one big thing in common with my thesis advisor besides our first name: I have a tough time saying 'no' to favors when asked nicely. As a result, I became a session chair this morning when the designated chair didn't show up. Ahh well.

Some neat science that I saw today:

• Buckley Prize talk by Jim Eisenstein, covering his work on liquid crystalline phenomena in high Landau levels of 2d electron systems, and his work on exciton superfluidity in 2d electron bilayers. I want to get him to come to Rice for a Keck seminar or physics colloquium this fall - the physics is really pretty.
• STM experiments by Mike Crommie's group at Berkeley looking at optically induced isomerization switching of azobenzene molecules. Now I know why our own efforts in this direction met with some difficulties. The switching gets quenched in regular azobenzene when the molecule is physisorbed on Au(111). Functionalizing the molecules to weaken their coupling to the metal surface leads to some switching, though even then the cross-section seems to be very small - long exposure to lots of photons = switching of maybe 5% of the molecules.
  

Thoughts from the APS March Meeting

I would live-blog the APS meeting, except that the wireless connection at the Denver convention center is completely dysfunctional. I saw some nice talks today after arriving here, but I'll save science until tomorrow. For now, a couple of remarks:

• $2.97 for a cup of coffee? Seriously?
• What is the deal with the recorded laughter that plays on the escalator up to Exhibition Hall F? Is it supposed to put me in a good mood? It doesn't - it creeps me out. Escalators aren't supposed to be jolly. They're supposed to be escalators.
• There are now a large number of vendors selling cryostats that get down to 100 mK, and at least two cryogen-free models. Maybe Oxford Instruments will be forced to adapt now that they have real competition.
• Overheard in Bush Airport on the way here: "I'm a dentist, and wait 'til you hear about my alternate use for KY jelly!"
  

Jim Carrey and Conan O'Brian: quantum mechanics

This video that Kristen Kulinowski sent me is great. metadatta gets major street cred for figuring out which paper this refers to.