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.