Recent ACS + cond-mat

A couple of interesting recent results - a busy summer has really cut into my non-essential paper-reading, unfortunately.

One sideline that has popped up with the recent graphene feeding frenzy is trying to understand its optical properties. I don't mean anything terribly exotic - I mean just trying to get a good understanding of why it is possible, in a simple optical microscope, to see any optical contrast from atomically thin single layers of graphene. Papers that have looked at this include:
arxiv:0705.0259 - Blake et al., Making graphene visible
arxiv:0706.0029 - Jung et al., Simple approach for high-contrast optical imaging and characterization of graphene-based sheets
doi:10.1021/nl071254m (Nano Lett., in press) - Ni et al., Graphene thickness determination using reflection and contrast spectroscopy
UPDATE: Here's another one:
doi:10.1021/nl071158l (Nano Lett., in press) - Roddaro et al., The optical visibility of graphene: interference colors of ultrathin graphite on SiO2
It all comes down to the dielectric function of graphene sheets, how that evolves with thickness, and how that ultrathin dielectric layer interacts optically with the oxide coating on the substrate.

Another paper that looks important at a quick read is:
doi: 10.1021/nl071486l (Nano Lett., in press) - Beard et al., Multiple exciton generation in colloidal silicon nanocrystals
To excite the charge carriers in a (direct gap) semiconductor optically typically requires a photon with an energy exceeding the band gap, Eg, between the top of the valence band and the bottom of the conduction band. If an incident photon has excess energy, say 2Eg, what ordinarily happens is that a single electron-hole pair is produced, but that pair has excess kinetic energy. It's been shown recently that in certain direct-gap semiconductor nanocrystals, it's possible to generate multiple e-h pairs with single photons. That is, a photon with energy 3Eg might be able to make three e-h pairs. That's potentially big news for photovoltaics. In this new paper, Beard and coauthors have demonstrated the same sort of effect in Si nanocrystals. This is even more remarkable because bulk Si is an indirect gap semiconductor (this means that the because of the crystal structure of Si, taking an electron from the top of the valence band to the bottom of the conduction band requires more momentum than can be provided by just a photon with energy Eg). At a quick read, I don't quite get how this works in this material, but the data are pretty exciting.

Texas and education

Governor Perry, why did you have to go and ruin my week? It's bad enough that the Texas Republican Party platform explicitly declares that "America is a Christian nation" - so much for not establishing a preferred religion. Now our governor has gone and appointed a creationist anti-intellectual to be the head of the state board of education. Frankly I don't care what his personal religious beliefs are, but I am extremely bothered that the governor has appointed a man who believes that education and intellectualism are essentially useless ("The belief seems to be spreading that intellectuals are no wiser as mentors, or worthier as exemplars, than the witch doctors or priests of old. I share that scepticism.") to run the state educational system. Great move, Governor. Ever wonder why it's hard to convince high tech industry to create jobs here?

Ob: Potter

This is the obligatory Harry Potter post. Yes, I read the 7th book, and while it's got a few narrative problems (characters sometimes behaving in deliberately obtuse ways for dramatic necessity - like nearly every episode of Lost), on the whole it was a satisfying wrap-up of the series. If you don't care about spoilers, here is a great parody of the whole thing (via Chad Orzel).

This week in cond-mat

It's been a busy summer, hence the sparseness of my recent postings. Here are a couple of papers that caught my eye this past week.

arxiv:0707.1923 - Hogele et al., Quantum light from a carbon nanotube
Here the authors do careful time-resolved photoluminescence experiments on individual single-walled carbon nanotubes. By studying the time distribution of photon production, they can get insights into the exciton (bound electron-hole) dynamics that lead to light emission. They find evidence that photons are produced one-at-a-time in these structures, and that multiphoton processes are strongly suppressed. Perhaps nanotubes could be useful as sources of single photons, strongly desired for quantum cryptography applications.

arxiv:0707.2091 - Quek et al., Amine-gold linked single-molecule junctions: experiment and theory
This is a nice example of a mixed experiment/calculation paper in molecular electronics that actually has an interesting point. Very pretty experimental work by Venkataraman et al. at Columbia has shown that NH2-terminated molecules form better-defined contacts with Au electrodes than the conventional thiol (sulfur)-based chemistry. For example, looking at huge data sets from thousands of junction configurations, benzene diamine glommed into a Au break junction has a well-defined most likely conductance of around 0.0064 x 2e^2/h. Now theory collaborators have done a detailed examination via density functional theory of more than a dozen likely contact geometries and configurations for comparison. The calculations do show a well-defined junction conductance that's robust - however, the calculations overestimate the conductance by a factor of seven compared to experiment. The authors say that this shows that DFT likely misses important electronic correlation effects. Hmmm. It's a neat result, and now that they mention it, the almost every non-resonant molecular conduction calculation I've ever seen based on DFT overestimates the conduction by nearly an order of magnitude. The only underestimates of molecular conduction that come to mind are in the case of Kondo-based mechanisms, which can strongly boost conductance and are always missed by ordinary DFT.

This is just silly.

I got an email about an audio conference about faculty recruiting titled "How to Recruit Gen X Faculty Members". I shouldn't pre-judge, and I should be glad that anyone is trying to improve the faculty recruiting process, but it's sad that anyone needs to be told this stuff. The premise is this:

The era when colleges and universities could rely on prestige and a little cash to recruit top academic talent is gone. Increasingly, up-and-coming faculty talent is from Generation X, the much derided and little understood generation that is much more than the Gap-employee stereotype you heard about a decade ago. This generation has a different set of work priorities, and colleges that understand these priorities stand a better chance of landing the best candidates and keeping them.

Riiiggght. It must be because of their generational culture, not the fact that two income families are vastly more common now, and there are many more women faculty candidates then forty years ago, etc. The topics to be covered include:

Why prestige and tenure may not matter as much to this generation as previous generations, and what that means for recruiting.

The importance of being "family friendly" and how job candidates judge that now that all colleges are claiming that they are.

How Gen X professors view hierarchy and what that means in the context of departments.

The importance of transparency and collegiality.

So, basically we can sum this up in a few words that generalize beyond the university setting: People don't want to work at places where they will be treated poorly. People may want to actually have lives outside of their jobs, and like to work at places that understand that. Smart, educated people don't like being told what to do by people who are clueless just because the clueless have seniority. People don't like it when their employers are rude or have obscure, byzantine policies. My goodness, those Gen X slackers are totally unreasonable.

Organic Microelectronics workshop

I just spent two days at the 3rd Annual Organic Microelectronics Workshop, meeting this year in Seattle. The workshop, sponsored jointly by the ACS, MRS, IEEE, and APS, was really very good - about 90 participants, and most of the big movers in the field. The talks were a great mix from the very applied (e.g. trying to optimize solvent conditions to avoid the coffee ring problem when inkjet or gravure printing solution-processable organic semiconductors) to the basic physics and chemistry of these materials. Among the things that I learned:

• Among the single-crystal organic semiconductors, rubrene is truly special in a number of ways. The most important point from the perspective of understanding electronic transport is that it can be made particularly pure, and oxidation in this material is reversible, unlike, e.g., pentacene.
• With polymer electrolytes, it is possible to make field-effect devices with gated surface charge densities exceeding 10^14 carriers/cm^2. I'd seen a couple of papers on this, and it's looking very impressive as a technique.
• Clever phase separation tricks can produce self-assembling organic devices that encapsulate themselves within a protective coating.
  
• RFID tags from Si are very very cheap.
• When developing a manufacturing process, "'Good enough' is good enough, and 'better' is not necessarily better."
  

This ought to be fun.

Looks like those folks at Steorn are going to do a 'demo' of their alleged free energy machine. I think I can safely predict (a) Steorn will claim success; (b) the reporting will generally give them the benefit of the doubt and "report the controversy"; and (c) we will not cure all the world's energy needs with magnet-based machines that violate the first law of thermodynamics.

UPDATE: Wow - it turns out that I'd overestimated Steorn. They couldn't get their demo to work. Apparently they'd decided to ignore back-ups, rehearsals, and contingency planning in addition to the laws of physics. So, was this self-deception, the long con, a postmodern publicity stunt designed to show how effectively they could market vaporware, or something else?

four interesting ACS journal articles

Here are four recent articles ACS journals, two from Nano Letters and two from JACS, that made an impression on me.

Dattoli et al., Fully transparent thin-film transistor devices based on SnO2 nanowires
The authors of this paper have made fully functional n-type FETs based on lightly doped tin oxide nanowires with indium tin oxide source, drain, and gate electrodes, and the performance of these FETs is reasonable when compared with the ones currently driving the pixels in your flat panel display. Since the entire FET structure is very transparent in the visible, this could have some significant applications in display technologies.

Angus et al., Gate-defined quantum dots in intrinsic silicon
People have been making Coulomb blockade devices out of puddles of gate-confined two-dimensional electron gas for nearly two decades now. Mostly this has been done at the GaAs/AlGaAs interface, and more recently it's been achieved in nanotubes, semiconductor nanowires, and SiGe heterostructures. The authors of this work have managed to do this nicely at the Si/SiO2 interface in a MOSFET. What this really shows is how well the interface states at that junction are passivated, how nicely the authors can make gates without messing up the surrounding material, and that properly made Ohmic contacts in Si FETs can operate well down to cryogenic temperatures. This could be a very important paper if one can build on it to manipulating electron spins in these dots - unlike GaAs structures, there should be many fewer nuclear spins to worry about for effects like hyperfine-induced decoherence of electron spins.

Albrecht et al., Intrinsic multistate switching of gold clusters through electrochemical gating
Lots of people in the molecular electronics community have pointed out the similarities and differences between three-terminal (electrostatically gated) molecular devices and solution-based electrochemical oxidation/reduction experiments in electron transfer. These authors are some of the only experimentalists out there that I have seen really delving into this, trying to unravel how the electrochemical case really works. This experiment is analogous to the Coulomb blockade experiment of the preceding paper, but performed using an STM in an electrochemical medium, with ligand-protected gold clusters playing the role of the quantum dot.

Shim et al., Control and measurement of the phase behavior of aqueous solutions using microfluidics
This isn't particularly deep, but it sure is cool. Microfluidics has come a long way, and the extremely nice properties of polydimethylsiloxane (PDMS) have been a big help. That's the transparent silicone rubber used for many microfluidic applications, as well as being related to the silicone used for soft contact lenses and breast implants. The authors here have carefully used the water and gas permeability of thin PDMS layers to control the concentrations of solutes in water-based solutions, allowing them to do things like gently make supersaturated conditions to control crystallization of proteins. We're just at the leading edge of the potential applications for these kinds of systems.

This week in cond-mat

Two good review articles in the last week appeared on cond-mat....

arxiv:0706.3015 - Bibes et al., Oxide spintronics
This is a nice overview of recent developments in using transition metal oxides, which often exhibit strong electronic correlations, for measurements and devices involving spin. This includes materials like the manganites (colossal magnetoresistance oxides), half-metals (magnetite, CrO2), magnetically doped oxides (TiO2, ZnO) as wide-band gap dilute magnetic semiconductors, and new multiferroic materials (ferroelectricity + magnetic order all wrapped up in one system). Good stuff.

arxiv:0706.3369 - Saminadayar et al., Equilibrium properties of mesoscopic quantum conductors
Despite being rendered in some species of pdf that my viewer finds nearly unreadable, this is a very nice article all about equilibrium quantum effects in nanostructures comparable in size to the electronic phase coherence length. This includes persistent currents in small metal and semiconductor loops. These persistent currents (flowing without dissipating!) result in part from the requirement that the electronic phase be single-valued when traversing a loop trajectory in a coherent manner. The persistent currents are very challenging to measure, and as far as I know there continues to be controversy about whether the magnitude and sign of the resulting magnetic moments is consistent with theory.

ACS journal articles

One reason why I've been writing up arxiv preprints rather than published articles in PRL/APL/Science/Nature is that the APS Virtual Journals do a very good job of aggregating articles from those sources. The Virtual Journal of Nanoscale Science and Technology in particular is one of my favorite places to look for nano-themed condensed matter work. One unfortunate flaw of the virtual journals, however, is that they do not have a nice agreement in place to let them include links to articles published in ACS journals. That's really too bad, since an awful lot of very neat results have been showing up there, particularly in Nano Letters, and I suspect that the new longer-paper ACS Nano is going to be of similar high quality. So, I'm going to try pointing out a couple of JACS/Nano Lett/ACS Nano articles that catch my eye every week or two.

Prolific theorists

How do they do it? No, really. How can some theorists be so prolific? I know they're not constrained by little things like having to get experiments to work, but surely it takes a certain amount of intellectual effort and creativity (or at least, supervision of students and postdocs, or correspondence with collaborators at other institutions) to produce a decent paper. At a little before the midpoint of the year, I can think of two CM theorists who have already produced, between the two of them, 23 preprints on the arxiv. That's something like one paper every 2.5 weeks for each of these people. Wow.

Grand challenges

As a condensed matter blogger, I am obligated to comment on the new report out from the National Research Council, titled "Condensed-Matter and Materials Physics: the Science of the World Around Us". This report is intended to list grand challenges for the discipline in the coming decade(s). I agree with the title, of course. As I wrote when I started this blog, while high energy physics and astrophysics grab much of the cachet and popular attention, it's very hard to dispute that condensed matter physics has had a much more direct impact on the daily lives of people living in developed societies. The transistor, the solid-state laser, and magnetic data storage are three prime examples of technologies that originated from condensed matter physics.

I haven't read the full report yet, but I had read the interim report and know several of the people who put this thing together. I think the substance is definitely there, though I do wonder if the summary suffers because of the decision to write the grand challenges in language for the consumption of the lay public. The challenges are:

1. How do complex phenomena emerge from simple ingredients? Phrased this way this challenge sounds rather naive; the whole point of condensed matter physics is that rich phenomena can be emergent from systems with many (simply) interacting degrees of freedom. Still, this gets to the heart of the discipline and many outstanding questions. Why can one material system exhibit metallic behavior, superconductivity, and antiferromagnetic insulating order with only minor tweaks in composition? Figure that one out, and win a trip to Stockholm.
2. How will the energy demands of future generations be met? This is clearly not the purview of condensed matter alone, but there is little doubt that our discipline can contribute here. Photovoltaic materials, supercapacitor and battery electrodes, catalytically active materials, light/strong composites, novel superconductors for transmission.... There are any number of reasons why investing in CMMP is an intelligent component of a sound energy policy.
3. What is the physics of life? This is really a biophysics question, though certainly condensed matter physics is closely relevant. At the very least, the principles and methods of condensed matter physics are highly likely to play roles in unraveling some of the basic questions in living systems (e.g., How does the chemical energy released in the conversion of ATP to ADP actually get translated into mechanical motion in the protein motor that turns the flagellum of a bacterium?).
   
4. What happens far from equilibrium and why? This is a good one. Equilibrium statistical mechanics and its quantum form are tremendously useful, but nonequilibrium problems are very important and there exists no general formulation for treating them. Heck, any electronic transport measurement is a nonequilibrium experiment, and beyond linear response theory life can get very complicated. Add in strong electronic correlations, and you are at the frontiers of some of the most interesting work (to me, anyway) going on right now.
   
5. What new discoveries await us in the nanoworld? Wow - this one really sounds like a sixth-grade filmstrip title. I would've preferred something like, "What new physics will be found when we control materials on the nanoscale?" The ability to manipulate and engineer systems with precision approaching the atomic scale lets us examine systems (e.g., single quantum impurities; candidate qubits) that can reveal rich physics as well as possible applications to technology.
6. How will the information technology revolution be extended? I don't know.... While this is certainly a useful goal of CMMP, and this point clearly encompasses exciting physics relevant in quantum computation as well as things like plasmonics and nanophotonics, I'm not sure that this is really a physics grand challenge per se - more of an engineering challenge.
   

So, what's missing? Well, I'm sure people will make suggestions in the comments, but here's one from me (though I'm sure that the NRC panelists consider this to be subsumed under point 1 above): Is there an efficient and exact general computational method for finding the ground state of the general strongly-interacting, strongly correlated many-electron problem? Basically I want something better than DFT that handles strong correlations. That would definitely be a grand challenge, though it's way too detailed ("physicsy") to fit the structure used in the above list.

The report also emphasizes the fact that research funding in the physical sciences, particularly CMMP, is lagging that in other nations these days, and that this is probably not to our competitive advantage. The demise of long-term industrial R&D in the US has not helped matters. None of this is news, really, but one major purpose of reports like this one is to send a message to Congress. Hence the use of non-physicsy language for the challenges, I'm sure.

Albany Nanotech

I returned today from a 1-day visit to Albany Nanotech, the absolutely enormous joint venture between SUNY Albany and a whole slew of collaborators, including International Sematech. In terms of facilities, this place is unparalleled. They have multiple photolithography tools for 300mm wafer processing, including standard (in-air, capable of 65 nm features), immersion (using the refractive index of very pure water to shrink the wavelength, allowing features down to 33 nm), EUV (reflective optics, 13.6 nm wavelength source, one of only two such systems in the world), and e-beam. They have every etching, deposition, polishing, and characterization tool you can think of. 80000 ft^2 of cleanroom space. I confess: I have facility envy. No other university could pull this off - this is an unprecedented confluence of industrial investment, educational initiative, and gobs of state funding, and seems to me like a sustainable model, at least for the next decade or more. No wonder Sematech is shifting lots (most?) of their operations to Albany.

This week in cond-mat

Two more papers that look interesting.

arxiv:0706.0792 - Koop et al., Persistence of the 0.7 anomaly of quantum point contacts in high magnetic fields
One of the neatest results (in my opinion) in mesoscopic physics is the appearance of conductance quantization in quantum point contacts, first shown in the late 1980s. The basic idea is simple. Start with a two-dimensional electron gas such as that formed at the interface between GaAs and modulation-doped AlGaAs. Metal gates on top of such a structure can be used to deplete the electron gas in particular places. Two closely spaced gates may be used to create a narrow constriction between two large reservoirs of 2d electron gas. As the constriction width is reduced until it is comparable to the Fermi wavelength of the confined electrons, the conductance through the constriction is quantized (at zero magnetic field) in integer multiples of G0 = 2e^2/h, the quantum of conductance (about 1/(13 kOhms)). That is, each spatial mode (each transverse subband of the constriction) can transport e^2/h worth of conductance per spin degree of freedom. Indeed, at very large magnetic fields, the conductance is quantized as integer multiples of G0/2, as one would expect if the different subbands are spin-split due to the Zeeman effect. This is all well explained by single-particle theory and the Landauer-Buttiker picture of conduction through small systems. In very clean quantum point contacts, additional structure is seen at 0.7 G0 - this is the so-called 0.7 anomaly. In the presence of a little bit of in-plane magnetic field, this approaches 0.5 G0, and therefore looks like there is some spontaneous spin-splitting, and this is a many-body effect that is the result of some kind of electron-electron correlation physics. This paper is an extensive study of 14 such point contacts, fully mapping out their magnetic field dependence and nonequilibrium (large bias voltage) properties.

arxiv:0706.0906 - Clark et al., Nonclassical rotational inertia in single crystal helium
The controversy over whether 4He has a true supersolid phase continues. This week this article appeared in Science, summarizing a number of recent experiments, and strongly suggesting that single crystals of pure 4He should not show a real supersolid phase - basically the claim is that the effects ascribed to such a phase are really due to disorder (glassy 4He at grain boundaries between crystals? 3He impurities somehow?). Now comes this paper from Moses Chan's group, arguing from new experiments that even carefully nucleated and grown single crystals of 4He show evidence of supersolid behavior (in the form of a nonclassical moment of rotational inertia). Hmmm. Neat, clever experimental design. It'll be interesting to see how this all pans out.

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.