Quantum impurities from Germany

I'm currently at a workshop on quantum impurity problems in nanostructures and molecular systems, sponsored by the Max Planck Institute for Complex Systems here in Dresden. A quantum impurity problem is defined by a localized subsystem (the impurity) with some specific quantum numbers (e.g. charge; spin) coupled to nonlocal degrees of freedom (e.g. a sea of delocalized conduction electrons; spin waves; phonons). The whole coupled system of impurity (or impurities) + environment can have extremely rich properties that are very challenging to deduce, even if the individual subsystems are relatively simple.

A classic example is the Kondo problem, with a localized impurity site coupled via tunneling to ordinary conduction electrons. The Coulomb repulsion is strong enough that the local site can really be occupied by only one electron at a time. However, the total energy of the system can be reduced if the localized electron can undergo high order virtual processes where it can pop into the conduction electron sea and back. The result is an effective magnetic exchange between the impurity site and the conduction electrons, as well as an enhanced density of states at the Fermi level for the conduction electrons. The ground state of this coupled system involves correlations between many electrons, and results in a net spin singlet. The Kondo problem can't be solved by perturbation theory, like many impurity problems.

The point is, with nanostructures it is now possible to implement all kinds of impurity problems experimentally. What is really exciting is the prospect of using these kinds of tunable model systems to study strong correlation physics (e.g. quantum phase transitions in heavy fermion compounds; non-Fermi liquid "bad metals") in a very controlled setting, or in regimes that are otherwise hard to probe (e.g., impurities driven out of equilibrium). This workshop is about 70 or 80 people, a mix of theorists and experimentalists, all interested in this stuff. When I get back I'll highlight a couple of the talks.

Superluminality

Today this blurb from the New Scientist cause a bit of excitement around the web. While it sounds at first glance like complete crackpottery, and is almost certainly a case of terrible science journalism, it does involve an interesting physics story that I first encountered back when I was looking at grad schools. I visited Berkeley as a prospective student and got to meet Ray Chiao, who asked me how long it takes a particle with energy E to tunnel through a rectangular barrier of energetic height U > E and thickness d. He went to get a glass of water, and wanted me to give a quick answer when he got back a couple of minutes later. Well, if I wasn't supposed to do a real calculation, I figured there were three obvious guesses: (1) \( d/c\); (2) \(d/ (\hbar k/m)\), where \(k = \sqrt{2 m (U-E)}/\hbar\) - basically solving for the (magnitude of the imaginary) classical velocity and using that; (3) 0. It turns out that this tunneling time controversy is actually very subtle. When you think about it, it's a funny question from the standpoint of quantum mechanics. You're asking, of the particles that successfully traversed the barrier, how long were they in the classically forbidden region? This has a long, glorious history that is discussed in detail here. Amazingly, the answer is that the tunneling velocity (d / the tunneling time) can exceed c, the speed of light in a vacuum, depending on how it's defined. For example, you can consider a gaussian wave packet incident on a barrier, and ask how fast does the packet make it through. There will be some (smaller than incident) transmitted wavepacket, and if you look at how long it takes the center of the transmitted wave packet to emerge from the barrier after the center of the incident packet hits the barrier, you can get superluminal speeds out for the center of the wavepacket. (You can build up these distributions statistically by doing lots of single-photon counting experiments.) Amazingly, you can actually have a situation where the exiting pulse leaves the barrier before the entering pulse peak hits the barrier. This would correspond to negative (average) velocity (!), and has actually been demonstrated in the lab. So, shouldn't this bother you? Why doesn't this violate causality and break special relativity? The conventional answer is that no information is actually going faster than light here. The wavepackets we've been considering are all smooth, analytic functions, so that the very leading tail of the incident packet contains all the information. Since that leading tail is, in Gaussian packets anyway, infinite in extent, all that's going on here is some kind of pulse re-shaping. The exiting pulse is just a modified version in some sense of information that was already present there. It all comes down to how one defines a signal velocity, as opposed to a phase velocity, group velocity, energy velocity, or any of the other concepts dreamed up by Sommerfeld back in the early 20th century when people first worried about this. Now, this kind of argument from analyticity isn't very satisfying to everyone, particularly Prof. Nimtz. He has long argued that something more subtle is at work here - that superluminal signalling is possible, but tradeoffs between bandwidth and message duration ensure that causality can't be violated. Well, according to his quotes in today's news, apparently related to this 2-page thing on the arxiv, he is making very strong statements now about violating special relativity. The preprint is woefully brief and shows no actual data - for such an extraordinary claim in the popular press, this paper is completely inadequate. Anyway, it's a fun topic, and it really forces you to think about what causality and information transfer really mean.

Kinds of papers

I've seen some recent writings about how theory papers come to be, and it got me thinking a bit about how experimental condensed matter papers come about, at least in my experience. Papers, or more accurately, scientific research projects and their results, seem to fall into three rough groupings for me:

• The Specific Question. There's some particular piece of physics in an established area that isn't well understood, and after reading the literature and thinking hard, you've come up with an approach for getting the answer. Alternately, you may think that previous approaches that others have tried are inadequate, or are chasing the wrong idea. Either way, you've got a very specific physics goal in mind, a well-defined (in advance) set of experiments that will elucidate the situation, and a plan in place for the data analysis and how different types of data will allow you to distinguish between alternative physics explanations.
• The New Capability. You've got an idea about a new experimental capability or technique, and you're out to develop and test this. If successful, you'll have a new tool in your kit for doing physics that you (and ideally everyone else) has never had before. While you can do cool science at this stage (and often you need to, if you want to publish in a good journal), pulling off this kind of project really sets the stage for a whole line of work along the lines of The Specific Question - applying your new skill to answer a variety of physics questions. The ideal examples of this would be the development of the scanning tunneling microscope or the atomic force microscope.
• The (Well-Motivated) Surprise. You're trying to do either The Specific Question or The New Capability, and then all of the sudden you see something very intriguing, and that leads to a beautiful (to you, at least, and ideally to everyone else) piece of physics. This is the one that can get people hooked on doing research: you can know something about the universe that no one else knows. Luck naturally can play a role here, but "well-motivated" means that you make your own luck to some degree: you're much more likely to get this kind of surprise if you're looking at a system that is known to be physically interesting or rich, and/or using a new technique or tool.

Hopefully sometime in the future I'll give an anecdote or two about these. In the mean time, does anyone have suggestions on other categories that I've missed?

Behold the power of google

I am easily amused. They just put up google street-view maps of Houston, and while they didn't do every little road, they did index the driving routes through Rice University. In fact, you can clearly see my car here (it's the silver Saturn station wagon just to the right of the oak tree). Kind of cool, if a bit disturbing in terms of privacy.

This week in cond-mat

Another couple of papers that caught my eye recently....

arxiv:0707.2946 - Reilly et al., Fast single-charge sensing with an rf quantum point contact
arxiv:0708.0861 - Thalakulam et al., Shot-noise-limited operation of a fast quantum-point-contact charge sensor
It has become possible relatively recently to use the exquisit charge sensitivity of single-electron transistors (SETs) to detect motion of single electrons at MHz rates. The tricky bit is that a SET usually has a characteristic impedance on the order of tens of kOhms, much higher than either free space (377 Ohms) or typical radio-frequency hardware (50 Ohms). The standard approach that has developed is to terminate a coax line with an rf-SET; as the charge environment of the rf-SET changes, so does its impedance, and therefore so does the rf power reflected back up the coax. One can improve signal to noise by making an LC resonant circuit down at the rf-SET that has a resonance tuned to the carrier frequency used in the measurement. With some work, one can use a 1 GHz carrier wave and detect single charge motion near the rf-SET with MHz bandwidths. Well, these two papers use a gate-defined quantum point contact in a 2d electron gas instead of an rf-SET. See, rf-SETs are tough to make, are fragile, and have stability problems, all because they rely on ultrathin (2-3 nm) aluminum oxide tunnel barriers for their properties. In contrast, quantum point contacts (formed when a 2d electron gas is laterally constricted down to a size scale comparable to the Fermi wavelength of the electrons) are tunable, and like rf-SETs can be configured to have an impedance (typically 13 kOhms) that can be strongly dependent on the local charge configuration. Both the Harvard and Dartmouth groups have implemented these rf-QPCs, and the Dartmouth folks have demonstrated very nicely that theirs is as optimized as possible - its performance is limited by the fact that the current flowing through the QPC is composed of discrete electrons.

arxiv:0708.0646 - Hirsch, Does the h-index have predictive power?
*sigh*. The h-index is, like all attempts to quantify something inherently complex and multidimensional (in this case, scientific productivity and impact) in a single number, of limited utility. Here, Hirsch argues that the h-index is a good predictor of future scientific performance, and takes the opportunity to rebut criticisms that other metrics (e.g. average citations per paper) are better. This paper is a bit depressing to me. First, I think things like the citation index, etc. are a blessing and a curse. It's great to be able to follow reference trails around and learn new things. It's sociologically and psychologically of questionable good to be able to check on the impact of your own work and any competitor whose name you can spell. Second, Hirsch actually cites wikipedia as an authoritative source on how great the h-index is in academic fields beyond physics. I love wikipedia and use it all the time, but citing it in a serious context is silly. Ahh well. Back to trying to boost my own h-index by submitting papers.

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.