## Wednesday, August 30, 2006

### This week in cond-mat

Two papers that are fun to talk about because of more than just the science:

cond-mat/0608576 - Klimczuk et al., Superconductivity in Mg_10 Ir_19 B_16
This is just a typical example of the kind of neat stuff that can come out of a really outstanding solid-state chemistry group. Bob Cava, formerly of Bell Labs and these days at Princeton, is an impressive materials chemist who has been involved in the discovery and synthesis of an ungodly large number of new materials. The one in the title of this paper is apparently one of a whole new family of superconductors. If someone told me that room temperature superconductivity was discovered, my first guess at the grower would probably be Cava. Just having someone like this on your campus can really make things happen, just like having a fantastic MBE grower. Of course, the total number of people like this who are this successful is very small. You can't just be edisonian - you have to have impressive insight into the chemistry and materials science issues, and you have to have access to the appropriate characterization tools.

cond-mat/0608492 - Hirsch, Do superconductors violate Lenz's Law?
Jorge Hirsch is a very interesting guy. He's very much a political activist, a person interested in developing useful metrics for measuring academic performance, and a condensed matter theorist with his own ideas about superconductivity.
When a (type I) superconductor is brought into a region of magnetic field, the superconductor develops screening currents to exclude the magnetic flux. Those currents flow within a penetration depth of the surface of the material, and the result is essentially perfect diamagnetism - this is called the Meissner effect. When those currents get set up, a torque is exerted on the lattice of the superconductor. Basically the paired electrons making up the supercurrent have some orbital angular momentum about the axis of the magnetic field. Since total angular momentum is conserved, the ions of the lattice have to pick up angular momentum going the other way, so that the total remains zero. Hirsch claims (and for fun, is trying to take bets on this to finance an experiment) that there is a big difference between the bring-a-superconductor-into-a-field case, and the cool-through-the-superconducting-transition-in-a-field case. He argues that the torques on the lattice in those two cases should be in opposite directions. I think he's wrong - at the very least, his treatment of this problem is waaaaay to simple. Anyone?

## Monday, August 28, 2006

### ...and speaking of hype....

You know, I like Neil deGrasse Tyson - he does a huge amount for public outreach about science, and has even appeared on the Colbert Report (which I would love to do, since I think Colbert's observations about truthiness and wikiality have a lot to do with science today). However, this article is exactly the sort of hype-ridden malarky that really hurts science in the long run. Our risk of being swallowed by a black hole is negligibly small, and the science in the article is appallingly dumbed down (black holes were long thought to be roughly stationary? With respect to what, exactly?). While it may get people's attention, implying that it's likely that roving black holes in our neighborhood are going to kill us all is not the best way to get science in the public eye.

## Thursday, August 24, 2006

### Is it "vision", or is it BS?

In a comment to my previous post, Alison Chaikin tries to put the Steorn business in perspective, pointing out that many grant proposals contain an awful lot of highly improbable exaggeration of potential, too. This is going to sound self-righteous, but I'll say it anyway: I really wonder sometimes if I hamper my own academic impact (defined, say, by funding levels, citations, publications in glossy journals) because of my low tolerance for bullshit. For example, our single-molecule transistor work is really nice science, with a good mix of physics and chemistry. However, when I give talks, I try to point out that, at least as implemented now, these devices are very unlikely to be good for high speed, high performance computers. There are some reasons to be optimistic, and there remains a large amount of great basic science as well as engineering to do before we can really assess whether these gadgets will, in something like their present form, be technologically useful.

## Tuesday, August 22, 2006

### Recently in cond-mat

Two recent papers that I find particularly interesting (both of which have now come out in print as well)....
cond-mat/0603442 - Sela et al., Fractional shot noise in the Kondo regime (also PRL 97, 086601 (2006)).
As I've discussed before, shot noise is noise that results from the fact that charge comes in discrete chunks. For strongly correlated systems, when the low energy excitations of the system can't be nicely described as single quasiparticles that act like "free" electrons, there can be dramatic signatures in the shot noise. These authors argue that such an effect should be present in the shot noise that results when current flows through a quantum dot in the Kondo regime - that is, when an unpaired spin on the dot is strongly entangles with the conduction electrons of the leads via higher order tunneling processes. The claim is that the effective charge of the carriers measured via shot noise is actually 5/3e, rather than simply e. This would be very neat.

cond-mat/0608459 - Koppens et al., Driven coherent oscillations of a single electron spin in a quantum dot (also Nature 442, 766 (2006)).
Once again, the Kouwenhoven group at Delft turns out a gorgeous piece of experimental work. This time, not only do they succeed in electrically measuring single-electron spin resonance. They go further, and demonstrate that they can coherently manipulate the spin, placing it into, e.g., a superposition of "up" and "down", and watching the Rabi oscillations back and forth. Wow. This is a real tour de force experiment, when you consider that the whole system needs to work at mK temperatures.

## Wednesday, August 16, 2006

### Aphorisms

Just returned from a conference at which I somehow managed not to hurt anyone with my laser pointer, and I picked up a couple of aphorisms from Tom Jackson, an EE professor at Penn State:

Jackson's 2nd rule of engineering (paraphrased): Don't argue with idiots; bystanders have a hard time telling the difference.

Jackson's 1st rule of engineering: Don't polish turds.

These brought to mind a couple of favorites from grad school:

Rogge's rule: When soldering, there is no such thing as too much flux.
O'Keefe's contradiction: Too much flux makes solder run like piss.
Salvino's rule: Any hose may be connected to any other hose with the appropriate hose clamp.
Gilroy's maxim: Graduate school is the process of continually lowering your expectations.
Natelson's variation: Graduate school is the process of continually increasing your cynicism.

Anyone out there got some other good ones?

## Sunday, August 13, 2006

### This week in cond-mat (mini-version + digression)

One particular paper caught my eye this week:
cond-mat/0608243
- Nakamura et al., Low-temperature metallic state induced by electrostatic carrier doping in SrTiO3.
The authors of this paper have managed to solve, at least well enough to do the experiment, the surface processing and ohmic contact challenges to make a field-effect transistor on the surface of a n undoped strontium titanate single crystal. At high enough gate voltages, they can accumulate enough carriers in the channel to drop the sheet resistance of the 2d charge layer well below the resistance quantum (~ h/2e^2 ~ 13 kOhms), and see metallic temperature dependence of the channel conductance (that is, the conductance improves with decreasing temperature). Anytime someone does this sort of thing with a new material system it's interesting, and SrTiO3 is particularly noteworthy because it's a perovskite (crystal structure not that different from high Tc materials), it's an incipient ferroelectric (very large dielectric constant as T decreases), and when doped at moderate levels, it's been known to superconduct. Field-effect "doping" is a very nice tool for studying this sort of physics, because the carrier density can be changed without introducing the disorder that comes with chemical doping. I'm actually a co-author on a forthcoming Reviews of Modern Physics paper about this general topic.

Now that you've glanced at that preprint, take a look at this PRL. Those folks have been looking at conduction in a semiconducting polymer, poly(3-hexylthiophene), and claim to observe a metal-insulator transition. The data are very pretty, but I just don't see how the interpretation matches the data well. These folks argue that, because the temperature dependence of the (highly nonlinear) conduction that they measure (at large source-drain voltage) gets weaker with increasing gated charge, and approaches temperature-independence, they are seeing a metal-insulator transition. It seems that the picture is: for high quality polymer films, the potential minima from disorder are relatively shallow, and when the potential is sufficiently tilted (by source-drain), and the deeper minima are filled (by large gated charge), then one can get tunneling (rather than thermal activation) out of the minima, and temperature-indep. conduction. This may well be right, but I really object to calling this a metal-insulator transition. There is no true transition here, and never does conduction improve with decreasing T, as in a metal. Again, the data are good, but the title and language are, to me, an example of wordsmithing. (Full disclosure: one reason this rubs me the wrong way is that in our own work we saw similar weakening of T-dep. several years ago. I would never have thought of calling this a transition to a metallic phase.)

## Tuesday, August 08, 2006

### This week in cond-mat

Two papers for now....
cond-mat/0608069 - Zhou et al., First direct observations of Dirac fermions in graphite
This paper is also in press at Nature Physics. The authors take angle-resolved photoemission spectroscopy (ARPES), and apply it to high purity graphite. ARPES is a very impressive technique - a really nice (highly collimated, bright, well-controlled energy - like from a synchrotron) x-ray beam is incident in a carefully controlled geometry on a sample, and the photoelectrons kicked out of the material are detected in an angularly resolved way. Applying conservation of momentum and energy lets one use this method to extract (2d) band structure information about the material. In high Tc compounds, for example, ARPES has contributed greatly to the understanding of "Fermi Arcs" and so forth. Anyway, these folks look at graphite, and find that massless Dirac fermions really do describe well the 2d band structure of this material. They also see some "boring" carriers in there, with parabolic dispersion (that is, energy proportional to the square of carrier momentum, indicating that the effective mass is a well-defined concept). Finally, they see signs that impurities and defects lead to electrons sitting in there. So, the electronic transport physics in this stuff is "rich", meaning very complicated. This is a good example of applying a highly refined tool to a new (yet very old) material system.

cond-mat/0608159 - Sellier et al., Transport spectroscopy of a single dopant in a gated silicon nanowire
The authors here have done a very elegant experiment. They've taken doped Si on insulator, and etched it to form an "island" with source, drain, and gate leads. That island contains a single dopant atom, and by performing low temperature conductance measurements, including significant magnetic fields, they've been able to look at two charge states of that single dopant, and compare with long-held models (D0 and D- configurations) of how dopants sit in Si. The single arsenic donor acts like an extremely small quantum dot, having electron addition energies exceeding 15 meV. This is the kind of experiment that is conceptually simple, but actually doing the work has real experimental challenges.

## Wednesday, August 02, 2006

### hot topics and controversies

As was suggested in a recent comment, now that a nonzero number of condensed matter and nano people are (apparently) reading this blog (at least occasionally), this could be a fun opportunity to have a series of discussions about the hot topics and controversies out there in the world of condensed matter and nanoscale science. The idea would be to take maybe one topic a week, give a relatively gentle introduction to the subject, and then have some discussion, just for fun. This only works, of course, if enough people contribute to make the discussion interesting, rather than just me pontificating (though I suppose that would be de rigour for a blog). As a preamble, I suggest trying to generate a list of topics. Here are a few off the top of my head:
• 2d metal-insulator transition - What is the mechanism for the apparent metal-insulator transition in 2d electron and hole systems at low densities? Is it profound or not?
• High-Tc - what is the mechanism of high temperature superconductivity? What is the ultimate limit of Tc? What is the "bad metal", and what is the pseudogap, really? How important are stripes and checkerboards? Is the phrase "doped Mott insulator" really a generic description of these systems?
• Quantum criticality and heavy fermions - Do we really understand these systems? What are the excitations in the "local moment" phase? What is the connection to high-Tc, if any?
• Manganites - What sets the length scale for inhomogeneities in these materials?
• Quantum coherence and mesoscopics - Do we really have a complete understanding of mesoscopic physics and decoherence at this point? What about in correlated materials?
• Quantum Hall systems - Are there really non-Abelian states at certain filling factors? In bilayers, is there excitonic condensation?
• 1d systems - Is there conclusive evidence of spin-charge separation and Luttinger liquid behavior in semiconductor nanowires? Nanotubes?
• Mixed valence compounds - Is there or is there not charge ordering at low temperatures in Fe3O4, something that's been argued about for literally 60 years now?
• Two-channel Kondo physics - Is there firm evidence for the two-channel Kondo effect and non-Fermi liquid behavior in some physical system?
• Molecular electronics - Is there really improving agreement between experiment and theory? Can novel correlation physics be studied in molecular systems? Can molecules exhibit intrinsic (to the molecule) electronic functionality?
• Organic semiconductors - What is the ultimate limit of charge mobility in these materials? Are there novel electronic correlation effects to be seen? Can one see a metal-insulator transition in these systems?
• Nanomechanical systems - Can we demonstrate true "quantum mechanics", in the sense of a mechanical system that acts quantum mechanically?
• Micro/nano systems to address "fundamental physics" - Can we measure gravity on the 100 nm length scale? Are there experiments with Josephson junctions that can probe "dark energy"?
What am I leaving out? Any other suggestions?