Link plus a couple of papers

The Incoherent Ponderer has a fascinating analysis up of the statistics of the PhD-to-faculty pipeline in physics. The one thing missing (for lack of a good source of statistics) is how many physics PhDs go on to become faculty in a different discipline. This is increasingly common in this age of interdisciplinary work. For example, while by the IP's rankings Rice only places 1.9 percent of its PhDs as faculty members in top-50 physics departments, I can think of a few who are now faculty in, e.g., EE, Mat Sci, BioE, Chemistry, etc. It would be very interesting to look at the trends over the last twenty or thirty years. One reason for the pedigree effect is that good science is correlated with having cutting-edge resources - as fancier facilities (at least in condensed matter) have trickled down to the masses, so to speak, have things become more egalitarian?

Two more points.... First, I have some nagging doubts about the validity of some of those numbers. I can already count 7 Stanford PhD alumni that I know who have assistant/assoc. faculty positions in top-50 universities. According to the AIP numbers, that's 25% of all of the ones out there. That seems hard for me to believe. Second, Chad Orzel has a very valid observation that goes to the heart of a pathology in our field. 93% of all colleges and universities are not in the top 50. As a discipline I think we do real sociological damage to our students when we brain-wash them into thinking that the only successful outcome of a graduate degree is a tenured job at Harvard. That kind of snobbery is harmful, and probably has something to do with attrition rates. People should not decide that they're failures because R1 academia isn't what they want to do. I thought hard about taking a job offer from a college, and I still resent the fact that some people clearly thought I was loopy for even considering that path.

arxiv:0706.0381 - Fiebig et al., Conservation of energy in coherent backscattering of light
This paper is at once a very nice piece of experimental work, and an example of the kind of argument that I really don't like. In mesoscopic physics, there is a phenomenon known as weak localization for electrons. Consider an electron moving through a disordered medium, and look at one particular trajectory that contains a closed loop (made up of straight propagation pieces and elastic scattering events). Feynman says that the amplitude corresponding to this trajectory is a complex number whose phase is found by adding up the phase from propagation along the straight segments plus the phase shifts from the scattering events. Now consider a second trajectory, identical to the first, but traversing the loop in the opposite direction. It turns out that the amplitudes of these two trajectories interfere constructively for backscattering by the loop. That is, the quantum probability for getting through the loop is below the classical value, and the quantum probability for getting reflected by the loop excedes the classical value. It turns out something very analogous to this can happen for light propagating through a diffusive medium, and this can be the basis for some really cool things, like random lasers (where the back-scattering itself acts like an effective cavity!). The authors of this paper show the physics of this beautifully, but they present it in the form of a straw man argument, saying that the coherent scattering result (with greater than classical backscattering) looks at first glance like it violates conservation of energy. No, it doesn't. It looks like coherent scattering. It doesn't look like a violation of conservation of energy any more than typical diffraction does.

arxiv:0705.4260 - Huang et al., Experimental realization of a silicon spin field-effect transistor
For nearly 17 years people have been trying to make a spin transistor of the type discussed here. The idea is that spins are injected from a magnetically polarized source, traverse a channel region, and then try to leave through a magnetically polarized grain. Depending on the gate electric field, the moving spins precess and either get out of the system or not depending on their eventual alignment relative to the drain magnetization. This has historically been extremely difficult for many reasons, not the least of which are the difficulty in injecting highly polarized carriers into a semiconductor and the annoying fact that spin polarization, unlike charge, can relax away to nothing. Well, this is a pretty convincing demo of a device quite close in concept to the original idea, though it's not a field-effect geometry as first conceived. Very pretty data.

Hype. Again.

Remember this post, where I reported on interesting Shubnikov-deHaas oscillations in very pure high-Tc material? Well, that paper has now come out in Nature. Unsurprisingly, there has been an associated flurry of press, including this article. In case that link doesn't work, I'll spoil the punchline for you:

Canadian physicists have cracked a decades-old mystery surrounding metals that carry electricity without resistance, opening the door for everyday trains that levitate on magnetic fields, ultrapowerful quantum computers and big savings for utilities.
...
Taillefer predicted the discovery would lead to room-temperature superconductors within 10 years, triggering a technological revolution similar to the invention of the transistor.

One of the most promising applications for such superconducting metals is in magnetic levitation trains, which can theoretically run at speeds of up to 500 km/h.
...
Other possible superconducting applications include shrinking MRI machines to the size of laptops, eliminating the 10 to 20 per cent electricity lost from resistance inside power stations and building quantum computers, machines so powerful they would make today's supercomputers resemble mere pocket calculators.

Wow. They get from Shubnikov-deHaas oscillations to room temperature superconductors to maglev trains and quantum computers. I had no idea that getting clean samples could do so much. I'm presuming that most of the fault for this lies in the journalism rather than the scientists, but let this be a cautionary tale.

Annoying conventions

What do you find to be the most annoying conventions in physics? The classic example is the choice (darn you, Ben Franklin) of sign for the charge of the electron. Franklin had a 50/50 chance, and we ended up with the often confusing situation that current flow and particle flow are oppositely directed, that "up" on energy level diagrams corresponds to more negative voltages, etc. [EDIT: I think my earlier statements here about UPS conventions stem from a particular paper that isn't representative; never mind.... ]. Do any of you out there have other examples of really bad/misleading conventions?

This week in cond-mat

Only one quick blurb for now - there have been a number of neat looking papers on the arxiv lately, but I just haven't had time to read them. I am actually making some progress on my book, though.

arxiv:0705.2180 - Martin et al., Observation of electron-hole puddles in graphene using a scanning single electron transistor
A single-electron transistor (SET) consists of an "island" (in this case, a patch of aluminum film) weakly connected by tunnel barriers (in this case, aluminum oxide) to source and drain electrodes (also aluminum films here). Defining the total capacitance of the island to be C, the Coulomb energy cost of adding another electron to the island is E_c ~ e^2/C. If E_c >> kT, the thermal energy scale, and the tunneling resistances of the barriers are >~ h/e^2 (~ 26 kOhms), then the number of electrons on the island is fixed to be an integer. By varying the voltage on a nearby gate electrode coupled capacitively to the island, it is possible to change the average population of the island by one electron at a time. When the gate is set such that the island is just on the cusp of going from an electronic population of n to n+1, the source-island-drain conductance of the device has a peak and is very strongly dependent on that gate voltage. Instead of using a gate electrode, one could use the local electronic environment near the island to modulate the island potential (and hence the conductance). SETs are incredibly good electrometers, able to sense tiny fractions of an electronic charge nearby. Now consider sticking such an SET electrometer on the end of a scanned probe tip (in this case, fabricate it directly on the end of a tapered optical fiber). This is the scanning SET, a wonderful imaging tool developed and refined originally at Bell Labs by people like Harald Hess, Ted Fulton, Bob Willett, Mike Yoo, Amir Yacoby, and Nicolai Zhitenev.

In this paper Amir and colleagues (von Klitzing and company) use the scanning SET to look at graphene near the charge neutrality point as well as in the quantum Hall regime. They can see how the system breaks up into puddles of electron-rich and hole-rich regions with ~ 100 nm spatial resolution. This is a nice application of the S-SET technique, which can be extremely arduous - meeting the temperature requirement for good charge sensitivity requires working at very low temperatures (at least 3He fridge); the SET itself is very fragile and static sensitive; and the scanned probe setup is easy to crash into the sample surface. All in all, a tour de force tool that is unlikely to make its way into common usage any time soon.

FOIA

I got a very surprising email this morning from the NSF. Someone made a Freedom of Information Act request to get a copy of one of my NSF grant proposals. Now, I know that technically this is allowed - in principle, if someone wanted to, they could get (via FOIA) copies of their direct competitor's federal grants (with certain privacy information like social security numbers redacted). However, I've never actually heard of anyone doing this in practice - it's just not cricket, so to speak. The NSF gave me the name of the person, and I'm left to wonder: did they do this just to see an example of a funded proposal? Why didn't they contact me directly? Did they know that NSF was going to tell me about this? It's all perfectly legal, but I find it unsettling, and I can't pinpoint the precise reason. Has this ever happened to anyone else out there?

SCES '07 day 4

Back to the SCES conference this afternoon, after my campus commitments, for the second session on strong correlations in mesoscopic systems. Some highlights:

David Goldhaber-Gordon gave a nice talk about his group's work on using semiconductor nanostructures to engineer the two-channel Kondo effect. The work has been published here and is available on the arxiv here. In the single-channel Kondo problem, a free spin is coupled via tunneling to a single electronic bath. Antiferromagnetic exchange between the spin and the conduction electrons leads to the formation of a singlet at low temperatures - the spin is screened, and the ground state of the system is a Fermi liquid. In the two-channel Kondo problem, a single spin is coupled via tunneling to two independent electronic baths. Each bath tries to "screen" the spin via antiferromagnetic exchange, with the result that the spin is overscreened. The ground state of that system is supposed to be a non-Fermi-liquid, meaning that its low energy excitations don't look like weakly interacting quasiparticles. The hard part about testing this is actually making two truly independent electronic baths. The paper shows a clever implementation that effectively does this, at least over a limited temperature range.

Yong Chen, one of Randy Hulet's postdocs, gave a talk about using cold atoms to study Anderson localization. By sending a laser through frosted glass, they can use the resulting speckle pattern to provide a disordered potential for trapped cold bosonic atoms. They can dial around the strength of the disorder potential by changing that laser's intensity. Then they can play games with the trap potential to test how delocalized the Bose-condensed atoms are (kick the trap and look for resulting oscillations), and independently check for coherence by looking for interference fringes. The preliminary data are pretty exciting.

Ravin Bhatt talked about (theoretical) ways to try and produce ferromagnetism in doped semiconductors containing only nonmagnetic atoms, at very low carrier densities. The trick is to somehow get the system to have more electrons than there are donors. One can imagine doing this with clever modulation doping schemes. No one's pulled it off yet, but it sounds cool and the numerical results look suggestive.

Unfortunately more Rice commitments mean that I won't make it to the last day of the conference tomorrow. Ahh well. It was an interesting meeting.

SCES '07 day 2

Again I only was able to see the morning session today (and will be at Rice until Thursday pm). This means I'll miss the big "BCS@50" plenary session. However, here are a couple of talks that I did get to see....

First, T. Senthil started the day with a talk about spin liquids. This is a theoretically deep concept that I would love to understand better. The basic idea is that one can recast the interacting many-body problem in terms of new excitations of spinons (chargeless spin 1/2 excitations). The cost of doing this is that the spinons have "infinitely nonlocal" statistical correlations. However, these interactions can be made to look simple by introducing some effective gauge "charge" for the spinons and some effective gauge "magnetic field" - then the correlations look like the Aharonov-Bohm effect in this gauge language. If this sounds vague, it's partly because I don't really understand it. The upshot is that the spinons can be fermionic, and therefore have a Fermi surface, and this leads to nontrivial low temperature properties, particularly in systems where the whole weakly interacting quasiparticle picture falls apart. If anyone can point me to a good review article about this, I'd appreciate it.

There were a couple of other strong theory talks. Natan Andrei talked about a general approach to quantum impurities driven out of equilibrium (e.g., as in a quantum dot in the Kondo regime at large source-drain bias). Strong correlations + nonequilibrium is a tough nut to crack. Andrei argued that one can rewrite the problem in terms of scattering of initial states via simple phase shifts, provided that one picks the right (nasty, complicated) basis for the initial states that somehow wraps up the strong correlation effects. This choice of basis is apparently a form of the Bethe Ansatz, which I also need to understand better.

On the experimental side, besides my talk, Gleb Finkelstein from Duke gave a very nice talk about Kondo physics in carbon nanotube quantum dots. The really clever aspect of the work is that, through careful engineering of the contacts to the tube, the actual leads to the dot + the tunnel barriers + the dot itself are all formed out of the same nanotube. As a result the tunnel barriers preserve the special band structure symmetry (SO(4)) of the tube and the leads, leading to profoundly neat effects in transport.

SCES '07, Day 1

Today was the first day of the 2007 International Conference on Strongly Correlated Electron Systems (SCES), hosted this year in Houston jointly by Rice and UH. It's a pretty big meeting, typically with between 600 and 700 participants. Traditionally the meeting has had a very strong European and Asian participation, with a focus on heavy fermion compounds and high-Tc superconductivity. This year, there's an increased inclusion of strongly correlated physics in mesoscopic systems (quantum dots, nanotubes, graphene, single-molecule devices), as well as discussion of model correlated systems based on ultracold trapped atoms and molecules.

Because I'm on an internal search committee for a dean, my semester still hasn't really ended, which means that I'm going to miss a fair bit of the meeting. However, I'll still try to blog a couple of highlights daily. Here are two neat, new (and as yet unpublished) results that I saw this morning.

First, Louis Taillefer spoke about new measurements done in extremely high quality hole-doped YBCO at high magnetic fields (pulsed up to 60 Tesla). This work has been focused on trying to suppress superconductivity with a field in this, the grand-daddy of the high-Tc compounds, and to understand the ground state and possible quantum phase transitions (as a function of doping) of the normal phase. The exciting new result is that Taillefer and collaborators have been able to see Shubnikov-deHaas oscillations in this material for the first time. This is a big deal. First, it tells you that there are some sort of excitations in the normal state that can execute closed cyclotron orbits in the presence of a magnetic field. Since the validity of weakly interacting quasiparticles in the normal state is in significant doubt, this is interesting. Second, the frequency of the oscillations in 1/B reveals the area enclosed by those orbits in k-space - essentially it tells you how big the hole pockets are in the Brillouin zone, and therefore how many mobile holes there are per copper atom. Third, the temperature dependence of the S-deH oscillations lets you infer an effective mass (in this case, about 1.9 free electron masses) for whatever's doing the cyclotron motion. Very neat!

Second, Abhay Pasupathy from Ali Yazdani's group at Princeton showed some beautiful new STM data on BSCCO. The neat thing here is that their superfancy STM is absurdly stable over a big temperature range for days. That means that they can map out the tunneling density of states of the material on the atomic scale as a function of temperature, from deep within the superconducting state to well above the resistively detected Tc. They see that the gap in the density of states indicative of pair formation vanishes nonuniformly over the surface, with local bits persisting to well above the average Tc. They also show that the temperature dependence of the gap as a function of gap size is very different than that in low-Tc materials.

Tenure - some advice.

This past week there was a flurry of science blogging regarding tenure, such as these posts at Cosmic Varience (here and here), Rob Knop's post about his situation, Chad Orzel's commentary, and the Incoherent Ponderer's take on the tenure process here. The IP's take on things summarizes my general thoughts on the tenure process pretty well. In terms of the job pipeline, the biggest cut in population happens when trying to get a faculty position, not at the tenure stage. In reasonable departments, no one is happy when a tenure promotion case fails. Good departments (and schools and universities) try very hard to filter at the hiring level and give their faculty the resources they need to succeed. I can only think of two or three places (in physics anyway) that historically have had a "sink or swim" attitude (that is, hiring a junior person in an area today means that seven years from now the university wants the best senior person in the world in that area - being in-house is not advantage), and I'm not sure that's even true anymore.

In the links above, people are mostly focused on the process and outcomes; I think it would be useful to give some suggestions about how to approach the tenure process from the position of the junior candidate. I am hardly in a position to give too much sage advice about tenure, and what follows below is largely common sense. Obviously the situation is different in various disciplines and at different universities, but here's some basic points that I think should be considered. I'm sure I'll leave things out - feel free to chide me in the comments.

Understand the process. Find out how the tenure process works at your institution. This should be written down in a faculty handbook. Talk to your department chair, your faculty mentor (if your department has such a thing) or senior colleagues. Understand the timeline. Get a sense of the weight that your institution places on the different components of the job (see below). Does the departmental vote carry a lot of weight (as it usually does at Rice, for example), or are the deans or the university promotions and tenure (P&T) committee commonly overriding departmental decisions?

The process probably goes something like this: the candidate is hired for a 4-year tenure-track appointment, with some kind of annual reviews and a more major renewal review in year 3 or 4. (This gives the university a chance to end the process early if there's a major problem with an assistant prof, and forces departments to give some concrete feedback to the assistant prof about how they stand.) In the summer before year 6 (at most places) the candidate is asked to put together a dossier (complete CV, reprints of papers, a summary of funding, a statement about university service, a statement about teaching, a summary of research accomplishments, etc.) and suggest names for external evaluators. The department comes up with additional names for external evaluation, and sends the full dossier to some mix of the external people. Eventually these external letters come back, and the department reads them, puts the whole package together, and there's a vote of the tenured faculty (in October or November) about whether to recommend the assistant prof for tenure. The departmental recommendation then goes to the cognizant dean, and from there to the university P&T committee (which generally would have people from all sorts of disciplines on there, from bio to French lit). Sometimes P&T committees or deans can request more external letters, and they get copies of teaching evaluations, etc., and may meet directly with department chairs. Eventually the P&T committee makes its decisions (in late spring) and the candidate finds out. That decision is finally signed off by the president of the university and the board of trustees.

The research component. To get tenure you need actually need to be getting science done. There's no sure-fire recipe for success here, but let me make a few suggestions:

• Have a mix of projects that range from easier to high-risk/high-reward. Having only one major project can be very risky, particularly if it takes five years to get any results. One key element of getting tenure is that people in your community need to know who you are, what you've done, and what you've been doing that's really yours - new stuff from your professorial position, not rehash of your thesis or postdoc work.
• Make sure that your colleagues know what you're doing. Your colleagues are going to need to understand your work at least on some level, and particularly for hard projects, they will need to have some idea why it may take four years before a paper comes out.
• Have backup plans. High risk things may not succeed (no kidding.). Make sure, for your students' sake and yours, that you have thought out the projects well, so that even if you don't achieve the BIG goal, you are still learning useful things that are worth publishing.
• Have a high attempt frequency for funding. If there's literally only one agency in the world that funds your work, that's risky and unfortunate. Make sure that you know what your options are for funding sources. Call up program officers. Ask to get a chance to serve on review panels - you'll learn a huge amount about writing proposals that way! Know if there are state funding opportunities. Think ahead about private foundations (e.g., Research Corporation).
• Do some self-promotion but don't sell your soul. If your external evaluators don't know who you are, that's the kiss of death. Make sure you give talks at meetings. See what you can do about getting invited to give seminars at other schools. Yes, this is one issue where "well-connected" people really benefit, but if you go to meetings and get to know the people in your field, it's not that bad. Get involved in your own department's seminar series, and invite in people that you'd like to meet and talk to.
• Publish good stuff. This is always the tricky bit, and people joke about the "least publishable unit". Still, holding back everything for the one big Nature paper that may not happen is not necessarily the best strategy, for you or your students.
  

The teaching component. Do a good job teaching. Most universities have resources available to help you - teaching centers that will videotape your lectures, offer suggestions for improved technique, etc. Good teaching can only help tenure in limited ways at a research university, but poor teaching can certainly hurt a borderline case.

The service component. Do a decent job in departmental and university service. Don't let it eat all your time, but get involved in things that matter to you. It's also a good way to get to know your administrators and people in other departments. I'm not suggesting currying favor - just be a solid citizen.

Common sense. People argue about whether blogging can hurt your tenure chances. Blogging is only one example of a public forum, though. Use some common sense. Publicly badmouthing your institution, colleagues, administrators, etc. is not a good idea. (I'm not talking about hushing up legitimate grievances - I'm saying don't antagonize people gratuitously.)

I'm sure I could write more, and will probably update this later. This is some food for thought for now, at least.

The trouble with mercenaries

The trouble with mercenaries is that they can be bought. For example, the state of Texas bribed fair and square - errr, gave $40M and lots of tax incentives - to International Sematech, the big consortium of semiconductor manufacturers, in 2004 in exchange for them staying in Austin. That worked out really well for all concerned: today Sematech announced that they're picking up and moving to Albany because New York offered them more money. If I was Gov. Perry, I'd be pretty annoyed.
Update: Some Sematech people came to Rice yesterday and were grilled a bit about this. They say that the New York business is a parallel operation and won't affect their Texas activities; they also said that the reporting on this was pretty awful. Interesting. I guess time will tell about how much the focus of their work shifts to Albany. Given that the state of NY put over $3B into that setup, it seems like Texas will either have to do something similar, or face a possible eventual slide into secondary status.

An article I'd missed

While I was traveling, the Wall Street Journal ran this article about Bob Laughlin and his tenure as president of KAIST, one of the premiere research universities in South Korea. The article is definitely worth a read. It has some classic understatements:

Dr. Laughlin, a Stanford physicist, is a talkative man quick to express his opinions.

KAIST hired Laughlin to come in and shake things up. When he did, he did so in characteristic Bob fashion, and they reacted negatively. Things went south from there:

In an attempt to assert his control, Dr. Laughlin in December 2005 set out to personally interview and evaluate every one of Kaist's 400 or so faculty members, focusing mainly on the quality of their research projects and academic work. For those professors who agreed to the interviews, he gave them a one-paragraph summary grading their work from "unimportant" to "very important."

Yeah, that may have rubbed people the wrong way.

To be fair to Bob, the leaders of KAIST were crazy to hire him - all issues of personality clashes aside, he'd never managed a group of more than a handful of people, let alone an enormous research institution with a complex bureaucracy, large staff, and huge budget. Surprise: a Nobel prize in physics doesn't automatically imply success in extremely sophisticated management problems. It's also entirely possible that his assigned task was essentially impossible by design. An interesting read, anyway.

NSF grantees conference post mortem

It was useful to network with fellow NSF grantees for the last two days in Reno - an interesting mix of people and projects. A few observations:

• Rice's webmail client is so pathetically slow that it's nearly unusable.
  
• Hotels in Nevada walk a fine line between needing to compete with other hotels on the one hand, and trying to make sure you'd rather sit in the casino than your room on the other.
• "Nuggets" = out. "Highlights" = in.
• "Disruptive technology" = out. "Transformative" = in.
• 4 hours of 5-minute talks in one day is too many, at least for me.
• Some people don't understand the meaning of "Your talk should be three slides."
• Most of the NER projects (high-risk, high-reward one-year single investigator grants) from FY04 in the ECS part of the engineering directorate of NSF actually worked, and were pretty cool.
• The free wifi in the Reno airport makes up for the beeping, blinking slot machines in the terminal.

UPDATE: Rice upgraded their webmail service today. How's that for timing?

Physics intuition and a follow-up

I just had two interesting experiences. First, I spent a couple of hours reading a PhD thesis on a topic that had a strong tie to nonlinear dynamics and chaos. It had all the fun stuff: Poincare sections, phase space localization, etc., in the context of classical elliptical orbits with "kicks" applied as drive. While reading this, I had the realization that I had very little physical intuition for this system, even though it's in many ways an old problem. For example, the statement that, in this 1/r^2 central force problem, trajectories with large angular momentum have less orbital eccentricity did not seem obvious to me - I really had to think about it. Why do I have more intuition for nanoscale and quantum systems than classical central force problems? Because I hardly ever work on the latter. Physical intuition is the intellectual equivalent of muscle mass in some specific group. If I don't exercise the Poisson bracket/Runge-Lenz vector part of my physics brain, it atrophies.

The second experience also relates to intuition. Remember this post? PRL followed up with me last August. They said that they'd looked into my concerns about data manipulation, and that the author (not clear which one they contacted) had shown them "unprocessed" data, and that things looked ok to them. Well, I've been contacted by a colleague at another institution who read my blog post about this, figured out which paper I meant (!), and alerted me to other questionable figures in other publications. Updates as events warrant.

This week in cond-mat

Briefly emerging from my end-of-semester fog, here are some interesting preprints from the past week.

The graphene craze continues unabated. Remember how the superconductivity community descended upon MgB2 and made every superconductivity-related measurement under the sun on the new material in a feeding frenzy? A similar phenomena is taking place with the 2d electron community and graphene. Fortunately, graphene seems to be pretty neat stuff! For example:
arxiv:0704.3165 - Hill et al., Graphene spin valve devices
People have done normal metal contacts to graphene, and superconducting contacts to graphene, so what's left but ferromagnetic contacts to graphene? Unsurprisingly you can use ferromagnetic electrodes to inject spin into graphene, and its such a low-Z material of high purity that both spin-orbit scattering and spin flip scattering from impurities are minimal, leading to real spintronic possibilities in this stuff.

Further exploiting the robustness of graphene even under significant processing:
arxiv:0704.2626 - Huard et al., Transport measurements across a tunable potential barrier in graphene
arxiv:0704.3487 - Williams et al., Quantum Hall Effect in a graphene pn junction
arxiv:0704.3608 - Abanin and Levitov, Quantized transport in graphene pn junctions in magnetic field
Because graphene is a high quality 2d material and can be shifted readily from n and p carriers via doping or gating, it is possible to set up sophisticated structures (npn or pnp junctions; pn junctions) while preserving long mean free paths. The result is rich phenomenology, as seen in the first two (experimental) papers listed here, and analyzed in detail in the third (theory) paper. I'm still waiting for a really unexpected graphene result that isn't readily explained.

Two other papers that involve tunable model systems to examine strong correlation physics:
arxiv:0704.3011 - Bloch et al., Many-body physics with ultracold gases
This is a review article about using cold atoms to look at nontrivial correlation effects. One holy grail in this business is to use strongly interacting cold fermions in a 2d optical lattice to explicitly simulate the Hubbard model (relevant to high-Tc superconductivity), a topic of much interest to one of my faculty colleagues.

arxiv:0704.2614 - Walsh et al., Screening of excitons in single, suspended carbon nanotubes
Carbon nanotubes have 1d band structures, and therefore are subject to strong electron-electron interaction effects and poor screening. The consequence of these interactions is the demise of Fermi liquid theory, and therefore the onset of the fractionalized quasiparticles (spinons and holons) of Luttinger liquid theory. Excitons are also strongly modified in these systems. One way to probe these effects is to change the effective interaction; this is done by using immersion in dielectric media to change the screening of charges, and the effects are probed spectroscopically.

This week in cond-mat, self-indulgent edition

Two cond-mat papers this week, and one other item.

arxiv:0704.1775 - Yu et al., Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions
This paper is by my former student, Lam Yu, now at NIST in Maryland. It is a systematic study of a relatively long-standing problem in the molecular electronics field. Electrons can undergo off-resonant tunneling through molecules; that is, the electrons tunnel from one electrode to the other via the molecule, even though there are no molecular levels aligned with the filled electronic states of the electrodes. You can think of this as a second-order tunneling process: while actually putting an extra electron on the molecule is classically forbidden by conservation of energy, one can consider a second order tunneling process where the electron-on-the-molecule is a virtual intermediate state. If a sufficient bias voltage exists between the source and drain electrodes, and the nuclear wave functions work out right (i.e., Franck-Condon factors), one can have processes where the electron tunnels on to the vibrational ground state of the molecule and tunnels off a vibrationally excited state of the molecule, all in one process. This is the key to inelastic electron tunneling spectroscopy, where such vibrational excitations result in a signature in the IV characteristics of the electrode/molecule/electrode sandwich (nominally a peak in d^2I/dV^2). The problem is, experiments have shown a variety of lineshapes rather than simple peaks. Lam's work shows that the presence of metal ions within the molecular layer can result in complicated lineshapes like those seen in some experiments.

arxiv:0704.0451 - Ward et al., Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy
This paper comes out of my own group. It has been known for some time that metal nanostructures driven at their plasmon resonances can act like little optical antennas, so that the local electromagnetic field near, e.g., metal nanoparticles can be much larger than the incident electromagnetic field. Since Raman scattering (a common vibrational spectroscopy) is a nonlinear optical process that scales like (roughly) the fourth power of the electric field, it is a prime candidate for these enhancement effects. In some geometries, single-molecule Raman sensitivity has been demonstrated. In this paper, we have shown that the electromigrated nanoscale gaps that we use for single-molecule electronic transport experiments are extremely good for surface-enhanced Raman scattering (SERS). We show that one can make these structures in a scalable way, with a high yield, and they show all the hallmarks of few- or single-molecule sensitivity. We're very excited about where we might go with this....

Finally, this past weekend came the first announcement of results from Gravity Probe B, or as my friends and I liked to call it, "The Project that Ate Stanford". GPB is a satellite-based test of general relativity. My thesis advisor remembers Leonard Schiff coming to Cal Tech in 1964 and talking about how GPB was only a couple of years away. What does this have to do with condensed matter? Well, the folks building this thing produced a lot of good science trying to understand thin film niobium superconductors, including a technique using lead balloons (no, really) to produce volumes with magnetic fields smaller than anywhere else in the known universe. Anyway, the finally have announced some results - so far, they've found that GR does a good job (within 1%, anyway) at describing the bending of space near the earth. It would appear that the analysis of the main effect they're trying to see, "frame dragging" of spacetime by the rotation of the earth, is being hampered by annoying systematics in their experiment. Hopefully they'll get it worked out, though they suffer from a sociology of science problem: if they confirm GR, no one will be surprised; if they refute GR, no one may believe the result because the experiment is so complicated. Ahh well.

End of semester and a short break

Just a quick post.... We're approaching the end of the semester, and with several looming thesis defenses, a thesis prize committee commitment, an internal search for an administrative position, and various external deadlines, I will be cutting back on blogging for the next two or three weeks, with the possible exception of "This week in cond-mat" posts....

A paper and a wager

One paper on the arxiv that I need to read more closely: cond-mat/0701728, Sukhorukov et al., Conditional statistics of electron transport in interacting nanoscale conductors. This paper, a collaboration between Rochester, Geneva, and ETH Zurich, looks at the noise properties of transport through a quantum dot in the presence of a charge detector, in this case a quantum point contact (QPC). A QPC is a constriction in a 2d electron gas with a width modulated by gates. If the QPC is tweaked such that it's right on the boundary of pinching off a transverse electronic mode, its conductance can depend strongly on the local charge environment, which acts like an extra gate. By monitoring the conductance of the QPC, the authors can watch tunneling events in the capacitively coupled quantum dot. The presence of an electron on the dot reduces the conductance of the QPC. Like most of the very pretty work to come out of Enslinn's group at ETH, the quantum dot and QPC are defined through local AFM-based surface oxidation of a shallow 2d electron gas in a GaAs/AlGaAs heterostructure. What I don't understand about this paper is a statement in the introduction: "An important property of the QPC charge detector is its noninvasiveness: the system physically affects the detector, not visa-versa." Strictly speaking, this just can't be right. If the quantum dot is capacitively coupled to the QPC sufficiently to modulate the QPC current flow, there has to be back-action of the QPC on the dot charge. While that interaction may be small, it can't be nonexistent, as far as I can see.

An unrelated anecdote: some of you may remember this post, where I talked about Steorn, an Irish company that took out a full-page ad in the Economist magazine looking for scientists to act as a "jury" of some sort and evaluate their "free energy" machine. Well, it would appear that Steorn did manage to find some scientists willing to act as a "jury", whatever that means, and will release some sort of report of their findings this Friday. I actually did communicate with Steorn last fall; while they had some interest in talking to me, they did not come close to answering my questions about how their jury process was supposed to work (e.g., would scientists actually be able to play with the gadget in an off-site laboratory; did Steorn really mean that their machine produced energy, or were they trying to finesse conventional jargon by talking about "coefficients of performance greater than one" (which may mean nothing for a refrigerator, for example)). I'm willing to wager that they have not discovered a loophole in the first law of thermodynamics. It will be interesting to hear what they report, and whether any actual scientists would be willing to stand up and back Steorn's claims.

Update: Sean McCarthy of Steorn informs me that my comments above are inaccurate, and that, indeed, their evaluation will be based on tests specified by their "jury" in an independent laboratory. My apologies for any confusion. This was not clear to me last fall, and I haven't been following this in the interim. I should also point out that my interactions with them were entirely cordial and businesslike.

I reiterate, though, that I think there is zero chance that these folks have been able to circumvent conservation of energy. If they have, I will happily eat my words, as this would be the biggest science story of the century. Extraordinary claims require extraordinary evidence.

Palate cleanser

Alright - after all of the recent seriousness, this blog needs a palate cleanser. How about a few fun links? For example, comic book artist and animator Neal Adams has some, umm, intriguing ideas about geophysics, such as the notion that the earth expanded greatly in diameter in the last 70 million years or so. Oh, and it's not just the earth. Mars, too.

I also recommend some Tom Lehrer music videos, if you haven't seen them before, like this, this, and this.

What I will and won't discuss, and comments

I'm annoyed that I have to make this post, but I guess that I'm going to have to actually set some sort of policy.

In the course of discussions related to the faculty job search process, comments have been made by posters regarding specific people and their job performance. Some of these comments have been about anonymous faculty or candidates; others have not. Some have been by anonymous posters; others have not.

Unfortunately (though understandably), Blogger does not allow me to go in and redact individual names from comments, as far as I know. I can either leave the comment alone or delete it altogether.

Consider this fair warning. If you post a comment that I think is inappropriate, I will delete it. In this case, I think anonymous comments that single out specific people (that is, where the identity of the subject of discussion is unambiguous, even if the full name isn't given) for criticism about their job performance and tenure chances are inappropriate. This policy isn't censorship - you're free to start your own blog and post whatever you want on it. I'm not going to provide a forum for those discussions.

UPDATE: Upon further reflection, I've deleted more comments from the previous thread, scrubbing out potential identifiers. I think I was able to preserve most of the insightful commentary while removing the tawdry gossip. I don't want to moderate posts, but I may have to go in that direction in the future. I wish blogger had an alternative to wiping out comments, but considering that the service is free, I shouldn't complain.

A primer on faculty searches, part III

Here is my long-delayed third post about faculty searches, a follow-up to Part I and Part II. One reason for the delay is that I was chairing a search. That took quite a bit of time, and I also didn't want to give an unfair advantage to any of our candidates who happened to read my blog.

In Part II I'd described the process in fairly complete detail. What I want to do here is give a few pointers to would-be candidates, and answer a couple of questions that people have emailed me in the interim.

• Look over the department webpages before you visit. If the school gives you an advanced or draft copy of your schedule, actually look at the pages of the people you're going to meet. More than likely, most of the people you meet are on the search committee. You want to have some sense of what they do so that you can (a) ask decent questions as you meet them, and (b) pitch your own stuff at the right level. An astro person may not have any idea what "valley degeneracy" is.
• Listen to what your point of contact tells you about who the audience is for your talk(s). No one wants a talk aimed at the wrong level. If you're supposed to give a general colloquium, that usually means that your audience is very broad and may contain undergrads, grad students, and faculty from different subfields. If you're supposed to give a seminar, that usually implies a more specialized audience. Remember, people need to know why they should care about what you're doing.
• Rehearse your talk. Speak clearly. Do not speak super-fast, especially if you use technical terms or people's names.
  
• Listen carefully to physics questions that you're asked, either in the talk or one-on-one. Repeat the question back at the person asking, rephrased slightly to confirm that you know what you're being asked. It's ok to say "I don't know" in response to a question, but don't use that dismissively. If you think of the answer later, make it a point to try to tell the questioner.
• Don't use terms that you don't understand or can't explain. Don't assume that everyone in the audience has heard of the So-and-so Effect. Make sure that you know all the relevant numbers for your work. If you're a theorist and someone asks you how to measure the effect you're calculating, at least have a handwave idea. If you're an experimentalist and someone asks you about errors and uncertainties, make clear that you've thought about those issues.
  
• When discussing budgets, etc., make sure that you know who actually is the point of contact for negotiations like that. In our case it's the department chairperson.
  
• The rule on startup packages is generally "you might as well ask". Show some reasonable judgment, though. A junior person asking for $5M in startup is not reasonable. A junior person asking for 5000 sq.ft. of lab space is not reasonable.
• Find out whether lab renovation costs are counted separately from your startup. That's the case at all the big schools, but some places can be funny about this.
• Ask about the tenure process. Ask about tenure history in that department.
  
• Ask about the department's long-term plan - where are things trending?
• Find out what their schedule is. When do they think they will be wrapping up the search?
  

I've been asked about whether politics can enter decision making in these searches. Someone emailed me who had not been offered a position somewhere despite having multiple first-author papers, and the candidate that had been offered the position had many fewer and less postdoc experience. The short answer is, well-run searches make decisions based on the whole package (departmental needs; research quality; communications ability; personal interactions; etc.). So, it can be hard to say from the outside why a particular committee made particular decisions. Can there be poorly run searches? Can there be searches where one category or need trumps the others, and the candidates don't know about that, and isn't that unfair? Sure, and we don't do that because it's long-term stupid - that's not the way to hire for the future and build up a department. I'm not sure that the situation is worse in academia than in any other profession, though. There is no question that where someone comes from and what their "pedigree" is can have a big impact, as discussed here by the Ponderer, but the process is inherently complicated. Hopefully these postings have clarified things at least a little.

This week in cond-mat

Much as it pains me to admit this, I agree with Lubos Motl about something: Neither of us like the new numerical identifier system launched by Paul Ginsparg and company at the arxiv. Lubos nails both of my complaints. While the old system actually conveyed information (the subject area of the paper and how many papers in that category had been submitted that month), the new system manages to be both cryptic and uninformative. Frankly I don't care how many total papers have been submitted from all categories in the arxiv, and I'm not sure why anyone would. Somehow this reminds me of the apparent desire of the Powers that Be to switch NSF proposal submissions from FastLane, which works extremely well and is easy to use, to grants.gov, which is completely arcane and annoying. Prof. Ginsparg, if you see this, please consider switching back to some incrementally changed form of the old system.

Meanwhile, here's one paper in the old numbering format, for old time's sake, that I thought looked interesting. Perhaps a theorist could take a look at this and tell me if it's as clever and neat as it seems to be.

cond-mat/0703768 - Ostlund, The strong coupling Kondo lattice model as a Fermi gas
The Kondo lattice is a model developed in an attempt to understand the heavy fermion compounds. In this model, there are itinerant conduction electrons, and a lattice of localized unpaired moments (f-shell electrons) representing the ion cores of the rare earth constituents of the heavy fermion material. Under the right conditions, the ground state of these materials is a Fermi liquid, meaning that there are distinct, gapless, electron-like (spin 1/2, charge -e) quasiparticles, but they have an effective mass hundreds of times higher than the free electron mass. The idea is that the conduction electrons have formed fully screened Kondo singlets with the rare earth f-electrons. The true ground state of the Kondo problem is a Fermi liquid, and in this limit the ground state of the Kondo lattice is also a Fermi liquid, though the antiferromagnetic screening of the ion cores leads to the high effective mass. Note that this is rather special - in semiconductors, the effective mass is a single-particle effect that comes from the lattice potential; in these systems, the effective mass is the result of many-body correlations. In this paper, the author explicitly writes down a canonical transformation (read: clever change of variables) that directly maps the Kondo lattice Hamiltonian into that of a weakly interacting Fermi gas. It looks clever to me, but I can't judge it in the context of other theoretical treatments of these strongly correlated systems.

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.

Great talk today

Alain Aspect gave the departmental colloquium today, and his talk was fantastic. He let the audience choose whether to hear about his more recent work on the Hanbury Brown-Twiss experiment with cold atoms, or his very famous work on Bell's Inequalities. By show of hands the packed audience picked the latter, and Aspect gave an extremely clear talk about why local hidden variable theories like the kind desired by Einstein just aren't compatible with quantum mechanics. I know that the talk has been fine-tuned and updated over the years, so the fact that it's polished shouldn't be surprising. Still, it was an impressively well structured colloquium: a good, generally accessible set-up and statement of the problem, a discussion of the experiment and what it means, and conclusions updated to include modern experiments about entanglement and quantum cryptography.

Physics, smarts, and perspective

There's a great post on Cosmic Variance about the "cult of genius" in physics - the myth in our discipline that if you're not supermegabrilliant (Feynman/Einstein/Hawking, as Julianne puts it), you're basically a pedestrian loser. Hand in hand with this is the still-persistent attitude out there that if you get a physics PhD but don't end up a full professor at Harvard, you're a plodder. Read the post and the comments. It's great stuff. It also makes me remember my first real intellectual wake-up call, realizing that I was surrounded by really smart folks and would have to get used to it. First semester, freshman year, taking this class from this fellow, and getting 6 out of 30 on the first exam. The mean was a 9. One real advantage to getting an undergrad degree at a top-tier place is the character-building early realization that there are many people smarter than you. Better to come to that conclusion at 18 than at 22 or 25....

This week in cond-mat

One theory paper, and two experimental papers this time.
cond-mat/0702446 - Poggio et al., Feedback cooling of a cantilever's fundamental mode below 5 mK
Suppose you had a mechanical resonator (mass on a spring). At moderate temperatures you know from the good, old equipartition theorem that the average kinetic energy and average potential energy in the resonator would each equal 1/2 k_{B}T. (Note to self: get LaTeX working in blogger....) At low enough temperatures (k_{B}T < \hbar \omega), you should instead think about the number of vibrational quanta in the resonator. Suppose you could actively damp the resonator - if it's moving toward you, you push back to slow it down. It is possible to effectively cool the resonator this way (though in a Maxwell's demon sense, there's no such thing as a free lunch). How far you can go depends on the noise in your measurement system used for the feedback. In this paper by Dan Rugar's group, they demonstrate that they can cool a Si cantilever from a base temperature of around 4.2 K all the way down to 5 mK, limited by the noise in their feedback system. This is impressive, and of obvious interest to those who want to examine the fundamental quantum properties of mechanical systems (including detector back-action).

cond-mat/0702472 - Kalb et al., Organic small-molecule field-effect transistors with Cytop(tm) gate dielectric: eliminating gate bias stress effects
A persistent problem with organic FETs is that their performance degrades if the gate is biased for long periods. There can be many reasons for this, but one major issue involves the interaction between the semiconductor and the gate dielectric. It is widely believed that in many OFETs charge leaking through the gate dielectric introduces defects and trap states right at the channel interface in the organic semiconductor. Here, Batlogg's group at ETH seems to have found, with collaborators, a fluoropolymer dielectric that doesn't seem to have these problems, and has impressive breakdown strength as well. I'll have to look into getting some.

cond-mat/0702505 - Khodas et al., One-dimensional Fermi-Luttinger liquid
Fermi liquid theory is the standard model of electrons in metals (as well as normal-state liquid 3He). The upshot of FLT is that the quasiparticles of the interacting electron gas look very much like weakly interacting electrons, and have well defined quantum numbers (spin 1/2, charge -e, k-vectors and band indices). In 1d, though, FLT doesn't do well. Luttinger, by assuming that the dispersion E(k) of the carriers around the Fermi points is linear, came up with an exact solution to the 1d problem now called the Luttinger liquid (LL). The LL has some very interesting properties, including separate spin and charge excitations. In this paper, Glazman, Pustilnik, Khamanev, and Khodas consider what happens when the dispersion a the Fermi points is more realistic: linear with a little bit of quadratic correction. This breaks particle-hole symmetry around the Fermi points, and has some profound effects on the structure of the density of states. This is a long paper, and while I think I get the main point, I haven't had a chance to look at it thoroughly. It seems important, though, since the slight nonlinear correction considered here seems very physically reasonable for many systems.

This week in cond-mat

Several papers caught my eye this week; I'll be brief, particularly since I haven't had time to read them in detail. Now that our paper is in and our search is nearing the end, I'll have more time soon. Maybe I'll even get time to work on my book. Anyway....

cond-mat/0702246 - Capelle et al., Energy gaps and interaction blockade in confined quantum systems
The authors consider the general problem of interacting quantum particles confined in a harmonic potential. This could apply to electrons in a small quantum dot, or cold atoms in a magneto-optic trap. They then come up with expressions for the addition energies (how much energy is needed to add one more particle to the confined, interacting system) based on single-particle properties plus the interactions. They predict phenomena analogous to Coulomb blockade for other interacting systems, including some kind of Van der Waals blockade for trapped atoms.

cond-mat/0702259 - Kornyushin, An introduction to the polaron and bipolaron theoretical concepts
This looks like a nice pedagogical derivation of polarons and bipolarons. Should be good for students.

cond-mat/0702332 - Wu et al., Shot noise with interaction effects in single walled carbon nanotubes
This is a typically nice piece of experimental work from the Helsinki group. They've measured shot noise in carbon nanotube devices, and while they have seen interesting quantum coherence effects (Fabry-Perot electronic resonances as have been observed in dc conduction in these systems), they do not see any clear signs of Luttinger liquid physics.

cond-mat/0702348 - Phillips, Mottness
This is a longer article by Phil Phillips on his ideas about the properties and excitations of Mott insulators - materials that are insulating not because their bands are all full, but because strong electron-electron interactions lock the carriers in place. Interesting ideas explained in a compelling way, though theorists have been arguing about this stuff (in particular, the role or lack thereof of Mott physics in, e.g., the normal state of the high Tc compounds) for some time. Prof. Phillips is also the best dressed scientist I've ever met, bar none.

Quantum computing: are we there yet?

(Updated and corrected) As others in the blogging world have pointed out, today is the big day for D-Wave, a privately held, VC-financed Canadian company that plans a public demonstration of a 16 qubit quantum computer. One of the main ideas behind quantum computation is that, because of the way quantum mechanics works, performing a linear number of operations, N, allows you to build up quantum states that can be written as superpositions containing an exponentially large (e.g. 2^N) number of terms. If one can do this and not have decoherence (due to environmental interactions) mess up the superposition states, it is possible to use this property of quantum mechanics to do certain computations much faster than classical computers. Another way to view the power of this quantum parallelism: suppose you want to solve a math problem, and the input is an N-bit binary number. With a generic quantum computer, you can imagine preparing an initial state built out of N qubits that is actually a superposition of all 2^N possible inputs. Your quantum computer could then solve the problem, producing a superposition of all solutions corresponding to those inputs. Readout is the tricky bit, of course, since simple-minded measurement of the final state will only pick out one of those solutions.

There have been many ideas proposed for physical implementations of quantum computers. The requirement that decoherence be small is extremely restrictive. With so-called "fault-tolerant" quantum computation, one can beat down that requirement a bit by using additional qubits to do error correction. In the last few years, there has been great progress in using small superconducting systems as quantum mechanical bits (qubits), either thinking about the charge on small "Cooper pair box" metal islands, or persistent currents in superconducting loops with Josephson junctions. One can do a form of quantum computation using NMR, though the number of effective qubits is strongly limited in molecules. There have been proposals to use tunable hyperfine interactions in phosphorous doped Si to get around that restriction. Some people want to do quantum computation using photons, or through optical manipulations of excitons in semiconductor dots, or directly using individual electron spins in semiconductor nanostructures. The current record (6 qubits) for producing superpositions like the ones I described above, or other related superpositions (8 qubits) has been set using trapped ions.

The D-wave demo is an attempt to do adiabatic quantum computation. The idea is to formulate a problem such that one can start out with the initial data being represented by the ground state (lowest energy state) of a system of interacting qubits. Then one very gently changes the Hamiltonian of the system such that the system never leaves its instantaneous ground state (that's the adiabatic part), but arranges matters so that the solution to the problem is represented by the ground state of the final Hamiltonian. The main proponent of this approach has been Seth Lloyd. Empirically, the D-wave folks are going to use 16 qubits made out of Nb loops and Josephson junctions (as explained here), and they cool this whole mess (128 filtered leads) down to 5 mK in a dilution refrigerator.

There seem to be three big questions here: (1) Is this really quantum computation? It's difficult for me to assess this, as I'm no expert. There seem to be arguments about which problems can really be solved in the adiabatic formulation that's implemented here, and about whether one can actually get significant improvements relative to classical algorithms. (2) Will the demo be fair? The high tech world is no stranger to rigged demos, and in this is a particular black-box affair. One has to trust that the stuff displayed on the screen of the PC controlling the electronics is really being determined by the chip at the bottom of the fridge, and not by some clever software. I'm willing to give them the benefit of the doubt, provided that they let some independent experts play with the system. (3) Why haven't they published everything in the open literature and let outsiders come in to verify that it's all legit? Well, I can't say I really blame them. The paper I linked to up there for their implementation never got into PRL, as far as I can see. I don't see Intel hurrying up to get outside approval for their new gate metallization. If these folks think they can actually get this to work and make a buck at it, more power to them. The truth will out.

A scientific direction that I think is promising

I haven't written too much about my own research on this blog, mostly because I figure that people who really care about it can read my group homepage or my papers. However, there is one area out there that I think has real promise, and I'd like to get other folks thinking about it, at least in general terms.

Electronic transport measurements in nanoscale systems can be considered a kind of spectroscopy. In particular, when a chunk of conducting material is sufficiently small and relatively weakly coupled to leads (call them a "source" and a "drain", after transistor terminology), conduction can be dominated by one or a few specific quantum states of that material. There has been great work done by many groups over the past 15 years or so, looking at these individual electronic states in a bunch of systems, including metal nanoparticles, patches of doped semiconductor, and semiconductor nanowires and nanocrystals. As neat as these systems are, they're all comparatively simple from the electron-electron interaction point of view. With a few exceptions (like Kondo-based physics), you can pretty much work in a single-particle picture. That is, adding one more electron to these systems doesn't drastically change the spectrum of electronic states - the spectrum itself is mostly unchanged except for the population of the states, one of which has increased by 1.

Many interesting materials exist where strong electronic correlations are more important. For example, the high-Tc superconductors in their normal state are often "bad metals" that are not well described by a picture of weakly interacting electrons. There are similar phases in the heavy fermion compounds. Even magnetite (Fe3O4), a comparatively simple compound, has strong correlation effects: it's not really a metal or a semiconductor; it has a room temperature resistivity in the milliOhm-cm range (say 1000 times higher than Cu or Au), and that resistivity increases with decreasing temperature, but not in a simple way as in a semiconductor.

I think it would be very revealing for transport spectroscopy experiments to be performed on nanostructures made from these strongly correlated materials. This won't be easy for many practical reasons (e.g., stoichiometry can be tough to control in nanomaterials; noone knows how to make many of these systems in nanostructured forms yet), but I'm convinced that there is much to learn in such experiments.

Another claim to fame

See this comic? See how, down at the bottom, it says "This comic courtesy of Jeff from Rice U."? That's my grad student, Jeff, who has been having problems with me walking by and having his devices die mysteriously.

My touch with fame

I can't resist posting a link to this article (NY Times, reg. req.), about two friends of mine from college. If you ever see a joke on The Daily Show that involves pretty serious math or science, there's a good chance it was written by Rob. He had a great one a year or two ago involving Venn diagrams....