This week in cond-mat

Two papers from the past week, the first of which gives us a chance to discuss one of the on-going controversies in condensed matter physics.
cond-mat/0609301 - Lai et al., Linear temperature dependence of conductivity in Si two-dimensional electrons near the apparent metal-to-insulator transition
For years now, there has been a fairly heated debate about the nature of an apparent metal-insulator transition (as a function of carrier density) seen in various 2d electronic and hole systems. The basic observation, originally made in some Si MOSFETs of impressively high interface quality made in Russia, is that as the 2d carrier density is reduced, the temperature dependence of the sheet resistance changes qualitatively, from a metallic dependence (lower T = lower resistance) at high carrier concentration to an insulating dependence (lower T = higher resistance) at low concentration, with a separatrix in between with nearly T-independent resistance at some critical carrier density. A famous 1979 paper by the "Gang of Four" (Anderson, Abrahams, Licciardello, and Ramakrishnan) on the scaling theory of localization had previously argued that 2d systems of noninteracting carriers all become insulating at T=0 for arbitrarily weak disorder. So, the question is whether the real (interacting) case, with an apparent transition between metallic and insulating states, is profound (that is, a real quantum phase transition) or not (e.g., a percolative transition caused by the system breaking up into disconnected puddles of carriers as the concentration is lowered). There are some interesting pieces of evidence pointing in each direction. This paper weighs in using very nice Si quantum wells in SiGe, showing evidence consistent with a percolative crossover in the conductivity. Anyone out there care to comment on the state of this debate in general? Has there been a really slam dunk experiment out there that I've missed by avoiding this problem?

cond-mat/0609297 - Naik et al., Cooling a nanomechanical resonator with quantum back-action (also available in Nature)
This paper is one I need to read more carefully. These folks have constructed a nanomechanical resonator (operates at about 20 MHz), and are using a superconducting single-electron transistor (SSET) measured at high frequency to detect the resonator's motion. This is a great system for testing ideas about quantum measurement and back-action of the detector on the system being measured. In this case, they find that for the right settings of the SSET detector, they can actually cool the resonator (as determined by the noise temperature of the resonator, inferred from the readout of the detector) using the detector. The claim is that this is analogous to laser cooling in some sense, bit without a closer reading, I don't see how this really works. This shows that I need to think more and read more about this detector back-action business.

Packard Fellows meeting

Sorry for the downtime; the semester has begun, and this past week I also went to the annual meeting of Packard Fellows, which is a wonderful chance to hear technical talks from people working in all sorts of fields of the natural sciences, engineering, and mathematics. I will return with more cond-mat article discussions soon. In the mean time, I wanted to highlight two particular condensed matter talks that I heard at the meeting:

First, Hongkun Park spoke about his recent very interesting work on electronic properties of VO2 nanowires (actually bars - they grow from the vapor phase into long wires with square cross-sections). Some of this has been published. Vanadium dioxide is a weird material. It's supposed to be a Mott insulator at low temperatures, meaning that electronic interactions are so strong that the charges lock into place rather than being free to move around. At higher (not much higher than room) temperature, the material undergoes a first-order structural and electronic phase transition to a metallic state. Prof. Park's group has been playing with these nanowires, and found some amazing phenomena. For example, when the wires are sitting on a surface, the constraint of the surface strain plus the structural phase transition lead to the wires breaking up spontaneously into domains of metallic and insulating regions, and those domains can be (a) imaged with an optical microscope, (b) pushed around by flowing a current, and (c) made to oscillate back and forth because of resistive heating effects. Also, in suspended wires, the metal/insulator phase transition can be incredibly sharp, leading to the possibility of novel temperature sensors. Very neat.
Update: (9/27/06) This has just appeared in Nano Letters.

Second, Kathryn Moler showed her latest work on scanning SQUID microscopy. Basically it's possible to put an incredibly sensitive magnetometer at the very tip of an AFM-like probe, and image magnetic flux with incredible sensitivity. Most recently her group has been looking at superconducting fluctuations in little superconducting ring structures. Imagine putting a small magnetic flux on a superconducting ring. The fact that the superconducting wavefunction has to be single-valued going around the loop implies that magnetic flux through the loop is quantized. That quantization condition is enforced by spontaneous supercurrents in the loop. Well, for narrow loops its possible to be in a regime where rather than set up those currents, it's more energetically favored for the loop to go "normal". This is the Little-Parks effect. Now, if you imagine a split ring that looks just like the loop but isn't a complete circle, that would be superconducting. Can the topology of the ring really deeply affect the microscopic physics in the superconductor? Superconducting fluctuations in the "normal" ring are part of the answer. Again, a neat technique and a very nice piece of physics.

...and still MORE hype....

I have avoided talking too much about my own research here, with the intent of maintaining a broader perspective on CM and nanoscale physics. However, an example of nano-hype directly related to my own research has come up that I can't just let go (thanks to one of my regular anonymous contributors for pointing out the media aspects of this). Here is a perfectly reasonable theory paper about trying to make single-molecule transistors that operate in a new way. Basically the idea is to somehow (this trivial detail is left as an exercise for the experimentalists, which is actually fine for a theory paper like this) wire up three leads directly to a single small molecule. By varying the voltage on the "gate" lead, the quantum mechanical amplitude for tunneling from the source lead to the drain lead is modulated due to quantum interference. One could imagine (though this isn't discussed in the paper) implementing something like this in a GaAs quantum dot. For example, one could have a little "stub" dot off to the side of a channel connecting the source and drain. If the stub dot was tuned into the Kondo regime via coupling to the channel, then there would be a Fano antiresonance that would suppress source-drain conduction. Same basic idea. Anyway, the concept is sound, and the calculations (though done in some limited approximation on an idealized molecular/lead geometry) show that it's not crazy. Fine.

I have no problem with the science (though experimentally implementing it as conceived will be incredibly difficult). What I do have a problem with is the ensuing media onslought. Read this press release, which got picked up by CNN (broadcast, not the web). Read it all the way through, to the point where the scientist starts talking (I'm not making this up) about little nanobots controlled by computers that use this transistor concept swimming through your bloodstream. AAAAAGGGH! WHY DO PEOPLE DO THIS? Does the Arizona group really think that their paper will have more impact and enable more and better science and technology because of this? Do they think their pending patent on this idea will be more likely to be licensed? Don't they think that this kind of overreaching hype actually hurts the field in the long run?

This week in cond-mat

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

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

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

...and speaking of hype....

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

Is it "vision", or is it BS?

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

Statements like that, while realistic, are much less likely to inspire DARPA to hand me $250K/yr for three years than if I said "Within three years we [always use the royal "we" :-) ] will roll out commercial devices using single-molecule switches that operate at room temperature and GHz frequencies." The fact that this is an unrealistic goal is often irrelevant - it shows self-confidence, aggressiveness, and a vision to change the world. I'm reminded of footage of GWB debating Anne Richards for TX governor back in '94. When asked about possibly legalizing gambling in Texas in some form, Gov. Richards gave a very carefully worded, two paragraph response, explaining that this was worth considering provided it was handled correctly and that some of the taxes went to fund education and children's health programs, etc. George Bush's response was "I'm against it. I think it's a bad idea." The short, definitive, ambitious statement often beats nuance and realism - even in science.

There are some in academia (I've been told this explicitly) who view this grantsmanship stuff as an interplay between Big Picture Visionaries, and "Detail People". The Visionaries want to change the world, and often feel hectored by the Detail People, who they perceive as narrow and uncreative. Of course, the Visionaries need Detail People, since they're the ones who actually make things work. What do you all think about this? I think Vision in this context can be dangerously close to hucksterism.

A scam, or self-delusion?

By now you've probably heard about Steorn, a company of dubious provenance (used to be an e-business of some kind back during the .com boom) with no clear technical expertise that placed a full-page ad in The Economist last week. They claim to have developed a device that produces more energy than it takes to run - essentially a perpetual motion machine of the first kind. They go further than that, anecdotally claiming that scientists and engineers at reputable places have tested this gadget and agree that it really does produce energy seemingly from nowhere, but none of those folks have been willing to speak on the record. So, Steorn is trying to put together a "jury" of 12 scientists to test their gizmo. This has many many of the hallmarks of a pseudoscientific scam, complete with an utter lack of technical detail, and the company wanting to decide who does the testing. In fact, they actually won't let the scientists do tests - just examine records of the tests and data. Presumably they'll also say something like "Pay no attention to the man behind the curtain." On the other hand, it's hard to see what they gain by spending close to $200K on an ad, if the net result is a huge pile of negative publicity - I suppose they're just hoping some gullible rich person will believe that The Scientific Orthodoxy is suppressing this incredible breakthrough, and that big investment will follow. Place your bets on whether we'll ever even hear from these folks again....

Recently in cond-mat

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

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

Aphorisms

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

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

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

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

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

Anyone out there got some other good ones?

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

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

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

This week in cond-mat

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

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

hot topics and controversies

As was suggested in a recent comment, now that a nonzero number of condensed matter and nano people are (apparently) reading this blog (at least occasionally), this could be a fun opportunity to have a series of discussions about the hot topics and controversies out there in the world of condensed matter and nanoscale science. The idea would be to take maybe one topic a week, give a relatively gentle introduction to the subject, and then have some discussion, just for fun. This only works, of course, if enough people contribute to make the discussion interesting, rather than just me pontificating (though I suppose that would be de rigour for a blog). As a preamble, I suggest trying to generate a list of topics. Here are a few off the top of my head:

• 2d metal-insulator transition - What is the mechanism for the apparent metal-insulator transition in 2d electron and hole systems at low densities? Is it profound or not?
• High-Tc - what is the mechanism of high temperature superconductivity? What is the ultimate limit of Tc? What is the "bad metal", and what is the pseudogap, really? How important are stripes and checkerboards? Is the phrase "doped Mott insulator" really a generic description of these systems?
  
• Quantum criticality and heavy fermions - Do we really understand these systems? What are the excitations in the "local moment" phase? What is the connection to high-Tc, if any?
• Manganites - What sets the length scale for inhomogeneities in these materials?
  
• Quantum coherence and mesoscopics - Do we really have a complete understanding of mesoscopic physics and decoherence at this point? What about in correlated materials?
  
• Quantum Hall systems - Are there really non-Abelian states at certain filling factors? In bilayers, is there excitonic condensation?
• 1d systems - Is there conclusive evidence of spin-charge separation and Luttinger liquid behavior in semiconductor nanowires? Nanotubes?
• Mixed valence compounds - Is there or is there not charge ordering at low temperatures in Fe3O4, something that's been argued about for literally 60 years now?
• Two-channel Kondo physics - Is there firm evidence for the two-channel Kondo effect and non-Fermi liquid behavior in some physical system?
• Molecular electronics - Is there really improving agreement between experiment and theory? Can novel correlation physics be studied in molecular systems? Can molecules exhibit intrinsic (to the molecule) electronic functionality?
• Organic semiconductors - What is the ultimate limit of charge mobility in these materials? Are there novel electronic correlation effects to be seen? Can one see a metal-insulator transition in these systems?
• Nanomechanical systems - Can we demonstrate true "quantum mechanics", in the sense of a mechanical system that acts quantum mechanically?
• Micro/nano systems to address "fundamental physics" - Can we measure gravity on the 100 nm length scale? Are there experiments with Josephson junctions that can probe "dark energy"?

What am I leaving out? Any other suggestions?

Ahh, missile defense

At the suggestion of my colleague, I want to draw your attention to a very interesting and fun article in today's New York Times (free reg. required). It's about an antimissile laser system developed jointly by the US and Israel. The system works, basically, but is hugely expensive and so large in physical size that deployment is a nightmare. The article is really worth reading, just for the paragraph that begins: "As often happens in the federal development of death rays, parts failed and costs soared."

This week in cond-mat

A couple of new papers on the arxiv that I find particularly interesting....
cond-mat/0607756 - Zarchin et al., Bunching of electrons in transport through quantum dots
The Weizman Institute's work on transport in quantum dots is generally as good as it gets. I've already written about their experimental prowess in measuring shot noise, and this is another example. Shot noise results from the discrete nature of electronic charge. While the current tells you about the average rate at which electrons are flowing through a circuit, there are fundamental fluctuations in that current that describe the temporal correlations between the electrons. For example, if electrons only flowed through the circuit one at a time in perfectly spaced intervals, there would be no noise. On the other hand, if the electrons were Poisson distributed, there would be a classical current noise of 2eI (in units of amps^2/Hz). The authors here looked at shot noise in gate-defined quantum dots on GaAs/AlGaAs 2d electron gas. The authors found a surprising result. In the finite-bias conductance resonances that happen in these kinds of dots (as the source-drain bias is increased to allow access to another charge state for transport), the shot noise was enhanced over this classical result by as much as a factor of 10. This implies that the electrons are bunching up somehow, traversing the dot in bursts. This is quite odd and unexpected.

cond-mat/0607765 - Kitchen et al., Atom-by-atom substitution of Mn in GaAs and visualization of their hole-mediated interactions (also out in Nature)
This is a very nice STM paper by Ali Yazdani's group from Princeton. These folks are able to insert single Mn atoms into the surface of a p-doped GaAs wafer, and watch what happens. This is important because ferromagnetic semiconductors like GaMnAs are a key class of materials for those interested in capitalizing on the spin as well as charge of free carriers. What I really find interesting about these measurements is how very different a dopant atom in this semiconductor system looks from the puffy, hydrogenic picture painted in solid state physics textbooks. These kinds of results always re-emphasize to me that serious STM can't be your hobby - it has to be the main focus of your research effort, or you can't be competitive.

What the...?!

I thought I'd seen it all this evening when I opened my email to find an extensive warning email about laser pointer safety (!) from the SPIE (presumably sent to me because I'm speaking at an upcoming meeting, not because they think I'm a danger to myself and others when armed with a laser pointer). Remember, laser pointers are all fun and games until somebody loses an eye. This warning actually did include the sentence "NEVER stare directly into the beam of a laser pointer!". Whew! Good thing they warned me, in case my advanced degree hadn't given me sufficient critical thinking skills to reason that out for myself. The last line of the email made clear the real reason for sending it. They boldly declaim that any person using a laser pointer at an SPIE event but not adhering to the outlined safety protocols is personally liable in the event of injuries, and the SPIE is not liable. I consider this direct observational proof that our society has too many risk management and personal injury lawyers.

That paled compared to my reaction to this story, though. It would appear the Purdue University has done a thorough and careful investigation of claims of research misconduct in the case of Rusi Taleyarkhan, the scientist who claims to have used sonoluminescence of deuterated acetone to produce table-top-scale fusion. In the spirit of scientific openness and transparency, Purdue has decided to not make public the result of its investigation. So, either Taleyarkhan is legit, and Purdue is content to let his reputation suffer, or they think he's a fraud, but are content not to tell the scientific community, or some mysterious third alternative. What on earth is Purdue's administration thinking with this? Did they assume noone would notice?

This week in cond-mat

Just two papers this time. For the first, I must make a disclaimer: this is certainly not my area of expertise, and I can't really judge the validity of the results, but the topic is very interesting. I also haven't read either of these in any detail - they just look intriguing.

cond-mat/0607492 - Joly et al., Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics.
I vividly remember a great APS meeting talk by Seth Putterman (I think 10 years ago at the big centennial meeting in Atlanta) on basic pieces of table-top physics that we still don't really understand. One that he mentioned was triboelectricity - the separation of charge due to some frictional process. Remember junior high when you were told to rub a lucite rod with rabbit fur to build up a static charge? Amazingly, we still don't really understand the microscopics of this (unless the situation has changed recently. Any enterprising readers out there know anything about this?). Anyway, this paper is about the fluid analog of this. When a fluid containing ions is placed in contact with the walls of a container, the ion distribution is altered. Depending on the microscopic details of the fluid and the wall material, a sub-monolayer of charge can become practially immobilized at the wall (the Stern layer), and beyond that there extends into the fluid a net charge density (decaying exponentially into the fluid on a scale called the Debye length) set by competition between charge screening and diffusion due to concentration gradients (the appropriate diff-eq is the Poisson-Boltzmann equation). All this stuff is very important when worrying about colloidal suspensions, net charge on nanoparticles in solution, electrochemical scanned probe, etc. When fluid is flowing, slippage of the fluid layer right next to the wall can strongly modify the ion concentrations, and this can have big consequences for electrokinetic processes like electro-osmosis and electrophoresis. That's what this paper is on, and it's directly relevant to lots of micro- and nanofluidics work going on, particularly in the lab-on-a-chip community.

cond-mat/0607354 - Qi and Flatte, Current-induced spin polarization in nonmagnetic semiconductor junctions
Kato et al. showed recently that it's possible to build up a net spin polarization in the carriers in a strained nonmagnetic semiconductor (e.g. GaAs) by applying an electric field (and hence driving current into one side of the semiconductor through a junction, and out the other side). Lots of questions were inspired by this - is this a spin-orbit effect? Is this a spin-Hall effect? Now this new paper argues that the effect is neither of these things, and happens even in the absence of spin-orbit effects and for purely spin-independent scattering mechanisms. The trick seems to be that the mobility of carriers ends up depending nontrivially on the spin polarization (see here) for reasons that I don't currently understand. Seems profound enough that I should try to learn about it, though.

A time-saving step

This weekend I'll catch up w/ the cond-mat archive. In the meantime, I wanted to point out one amusing piece of Lubos Motl's latest blog posting:
The previous paragraph also clarifies my style of reading these papers. The abstract has so far been always enough to see that these fundamental gerbes papers make no quantitative comparison with the known physics - i.e. physics of string theory - and for me, it is enough to be 99.99% certain (I apologize for this Bayesian number whose precise value has no physical meaning) that the paper won't contain new interesting physics insights.
This attitude is surprisingly common among physicists. In a graduate seminar course at Stanford, someone else in the class showed our (then pre-)Nobel Laureate theorist professor a paper on high temperature superconductivity. After glancing at the title, author list, and abstract, he tossed the paper face-down on the table, and said, "I don't even have to read this to know that this is crap." Sometimes this approach (or its converse) really does work. I certainly have a list of condensed matter and nano experimentalists whose work I presume to be extremely good, because everything I've ever seen from their research groups has been elegant and solid. However, pre-judging results based on who did the work and what the abstract says is exactly the kind of non-scientific, unobjective attitude that emboldens social science types to argue that science and its findings are largely a social construct, etc., a conclusion that I think is way off base (when I drop my pencil from above my desk, it will fall toward the ground at 9.8 m/s^2, regardless of my sociology, preconceptions, or personal beliefs).

A couple of random things

One of the more popular physics blogs, Cosmic Variance, has an interesting post about rumor mill websites. If you aren't familiar with the concept, rumor mill sites have been around for a number of years associated with physics and astrophysics faculty job searches. The atomic/molecular/optical and condensed matter rumor page is here. Mark over at Cosmic Variance has interesting things to say on the subject.

Also, as a follow up: I did hear back from Phys. Rev. Letters about the possible data falsification that I pointed out to their editors. They heard back from the authors of the paper in question, and say that the authors showed them "raw" data, and that it was some sort of image processing artifact that made all the noise in the relevant images really look identical. Hmm. I'm unconvinced, but the editorial office says they're satisfied. If anyone wants to see the paper in question, contact me.

I'll put up more cond-mat and physics related postings soon; I need to tend to a couple of papers from my students, as well as a not-so-minor crisis involving our cleanroom facility.

Conference proceedings

I'm working on a conference proceedings paper for a meeting at which I'm giving an invited talk next month. So, are conference proceedings papers worth it? Does anyone actually read these things, even the ones published in peer-reviewed form? Or are they part of a borderline sleazy scheme by some professional societies and journal publishers (hint: I'm thinking of one that begins with "Elsev" and ends with "ier") to extort money from cash-strapped libraries for volumes noone ever examines? Also, what is the appropriate ettiquette regarding these? I get the impression that many of my colleagues would have no problem either farming out the writing to a student (even though they wouldn't get first authorship), or just bailing on the whole proceedings altogether (which I confess I've done before, too, when other demands on my writing time get too big). Opinions, anyone?

This week in cond-mat

Just returned from the Electronic Materials Conference. Interesting, and generally much more oriented toward engineering than pure physics, but fun nonetheless. I'll be out of commission for the next week or so, so this blog entry will have to tide over my dedicated readership :-)

cond-mat/0606742 - Camino et al., Transport in the Laughlin quasiparticle interferometer: Evidence for topological protection in an anyonic qubit
In the fractional quantum Hall effect, in very clean two-dimensional electron systems (typically formed at the interface between GaAs and AlGaAs layers) at very low temperatures and particular large magnetic fields, the "normal" metallic state of the electrons is unstable. The particular values of magnetic field are those for which the ratio of magnetic flux through the sample (in units of h/e, the so-called flux quantum) to the density of electrons (number of electrons per cm^2) takes on special values, such as three or five halves (corresponding, respectively, to three flux quanta for each electron, and five flux quanta for each pair of electrons). At these special values of magnetic field, the electrons form a correlated state named after Bob Laughlin, who first wrote down a trial many-body wave function to describe it. In a Laughlin state, the electrons can't be treated as nearly independent, as in a normal metal. Instead, when one tries to probe the electronic system, one finds collective excitations (rather than simple electron-like excitations in a normal metal). These collective excitations have very funky properties: they can have fractional charge (in the three flux quanta per electron case, the excitations have charge 1/3 e) and obey fractional statistics.
Fractional statistics are funky. Swap two electrons, and the total wave function picks up a factor of exp(i pi) = -1. Swap two bosons (like two 4He atoms), and the total wave function of the boson system picks up a factor of exp(i 2pi) = 1. Swap two Laughlin quasiparticles, and the total wave function picks up a factor of exp(i alpha), where alpha depends on precisely which fractional state the system is in. Generically alpha can be anything, earning the nickname anyons for particles that obey such statistics.
This paper looks at conductance oscillations as a function of magnetic field in a patch of Laughlin electron fluid that should exhibit fractional statistics and fractional charge of 1/5 e. The authors claim that these oscillations are surprisingly robust as temperature is increased, and that this is evidence of special stability of that state due to topological considerations. I'm not sure I believe the final conclusions, which seem to depend in great detail on precisely knowing the electron temperature. It's a neat experiment, though, and gives real insight into some exotic quantum effects that people think might be useful for building a quantum computer.

cond-mat/0606802 - Costache et al., Spin accumulation probed in multiterminal lateral all-metallic devices.
The authors in this paper look in detail at the magnetoresistive properties of a little piece of aluminum connected to four separate cobalt electrodes. It turns out fortuitously that each of the four cobalt leads can have its magnetization switched independently of the others, and this lets the authors study effects that arise from pumping certain spin polarizations of electrons into the aluminum island. Since aluminum is a low atomic number material, spin-orbit scattering is pretty weak in there, so electrons can maintain their spin polarization for a while. These experiments require extremely clean interfaces between the Co and the Al to work, and provide concrete numbers for spin lifetimes and diffusion lengths in practical materials.