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.