What's interesting about condensed matter physics

Inspired by this post and this one over at Uncertain Principles, I thought that I should explain what think is interesting about condensed matter physics. Clearly Chad's main observation is that condensed matter has historically had a major industrial impact, but he wants to understand why the science is interesting, and what draws people to it.

Condensed matter physics largely exists at the junction between statistical physics and quantum mechanics. Statistical physics tries to understand the emergence of collective phenomena (whether that's crystalline order, magnetic order, the concept of temperature, or the whole idea of phase transitions and broken symmetry) from a large number of particles obeying relatively simple rules. Throw in the fact that the rules of quantum mechanics are rich and can have profound consequences (e.g., the Pauli principle, which says that no two identical fermions can have identical quantum numbers, leads both to the stability of white dwarf stars and the major properties of most metals), and you get condensed matter physics. It's amazing how many complicated phenomena result from just simple quantum mechanics + large numbers of particles, especially when interactions between the particles become important. It's this richness, which we still do not fully understand, that is a big part of the intellectual appeal of the subject, at least for me.

I will also shamelessly crib Chad's list of points that he likes about AMO physics, and point out that CM physics is also well-described by them:

• "AMO physics is cool because it's the best field for exploring quantum effects." Well, while AMO is a nice, clean area for studying quantum effects, CM is just as good for some topics, and better for others. There's probably just as many people studying quantum computation using solid state systems, for example, as AMO systems.
• "AMO physics is cool because it's concrete." Again, it doesn't get much more concrete that CM physics; it's all atoms and electrons. One fascinating area of study is how bulk properties arise from atomic properties - one gold atom is not a metal, but 1000 gold atoms together are distinctly "metallic". One carbon atom is not an insulator, but 1000 of them together can be a nanodiamond on one hand, or a piece of graphene on the other, How does this work? That's part of what CM is about.
• "Experimental AMO physics is cool because it's done on a human scale." Experimental CM physics is the same way. Sure, occasionally people need big user facilities (synchrotrons, e.g.). Still, you can often do experiments in one room with only one or two people. Very different than Big Science.
• "AMO physics has practical applications." So does CM, and personally that's something that I like quite a bit. The computer and monitor that I'm using right now are applied CM physics.
• "AMO physics provides technologies that enable amazing discoveries in lots of other fields." Again, so does CM. Silicon strip detectors for particle physics, anyone? CCD detectors for all the imaging that the AMO folks do? Superconducting magnets for MRI? Solid-state lasers? Photon-counting detectors for astro?

So, in my opinion most of what Chad says about AMO applies just as well to CM, and I hope I've conveyed a little about what the intellectual interest is behind CM physics. Coming soon: posts on recent papers/preprints.

Rant - updated.

I was going to post about some neat new papers on the arxiv, and mention the very good talk on science policy that I heard today from Norm Augustine, former CEO of Lockheed-Martin and leader of the National Academy committee that wrote the now-famous Gathering Storm report. Instead, I read some news about which I must rant.

Remember the $85B (or, as I like to think of it, the 17 years worth of NSF budgets) that the US government used to "save" reinsurer AIG? Turns out, that wasn't enough. They've already blown through it without actually liquidating their assets as everyone was expecting. Now they've managed to get another $37.8B (7.5 years worth of NSF budgets) from the Federal Reserve. I'm sure they're all done now - after all, they've already spent $440K on a luxury retreat for executives (including $150K for meals and $23K for spa charges) after the $85B bailout. (You know you've gone over the line when the Bush administration calls you "despicable".) In the mean time, the US government is considering taking an ownership stake in a number of banks basically to convince the banks that yes, it's ok to lend money to each other since everyone would be backed by the feds. Of course, that plan may meet with resistance from the banks because it may limit executive compensation for the people who run the banks. Right, because unless we pay top dollar for these geniuses, there's a risk that the banks may not be well-run. Heaven forbid. If this keeps up, it'll be time to invest in tar and feather suppliers. At least I can refer you to a handy guide on how the economic meltdown may affect you.

UPDATE: You've got to be kidding me. AIG is planning another gathering, this time at the Ritz-Carlton in Half Moon Bay, CA, for 150 of their agents. Here's a clue, AIG: When you're so desperate for money that the taxpayers have to keep you afloat, maybe you should, I don't know, consider cutting back on ridiculous luxury expenditures? Don't tell me that you need to pamper your agents or they'll quit. I'm done. I don't care what it does to the global financial system: AIG needs to fail, and their executives need to lose their compensation, and the shareholders of AIG should sue those same executives for the last 10 years worth of compensation.

UPDATE 2: If you want to hear the most lucid explanation I've come across for this whole mess, particularly the problem of credit default swaps, listen to this. It's informative. And scary.

2008 Nobel Prize in Physics

Surprisingly, I haven't seen the usual blogfest of speculation about the Nobel Prize in Physics. The announcement will be this Tuesday. I will throw out my same suggestion as last year, Michael Berry and Yakir Aharonov for nonclassical phase factors in quantum mechanics, though the fact that the prize went to condensed matter folks last year means that this is probably less likely. Another pair I've heard floated every couple of years is Guth and Linde for inflationary cosmology. Any ideas out there?

Open faculty positions

It's that time of year again. My department is conducting faculty searches in three areas: solar physics (which I won't discuss here because it's not my area and I doubt many solar physics types read this), condensed matter theory, and cold atoms/optical lattices theory. There is a joint search committee for the latter two (yes, I'm on it), and here's the ad, which is running in Physics Today and Physics World:

The Department of Physics and Astronomy at Rice University invites applications for two anticipated tenure-track Assistant Professor positions in theoretical physics. One of the positions is in condensed matter physics, with emphasis on fundamental theory, while the other is in ultra-cold atom physics, with a focus on connections to condensed matter. These positions will complement and extend our existing experimental and theoretical strengths in condensed matter and ultra-cold atom physics (for information on the existing efforts, see http://physics.rice.edu/). Applicants should send a dossier that includes a curriculum vitae, statements of research and teaching interests, a list of publications, and two or three selected reprints, and arrange for at least three letters of recommendation to be sent to R. G. Hulet or Q. Si, Co-Chairs, Faculty Search Committee, Dept. of Physics and Astronomy - MS 61, Rice University, 6100 Main Street, Houston, TX 77005 or by email to Valerie Call (vcall@rice.edu). Applications will be accepted until the positions are filled, but only those received by November 15, 2008 will be assured full consideration. The appointments are expected to start in July 2009.

To be completely clear: There are two distinct positions available, and the total number of interviews will reflect this. If you have questions I'll try to answer them or refer you to my colleagues who cochair the committee.

Incredibly pointless paper

This has to be one of the most useless things I've ever seen on the arxiv. Basically, the authors point out that there is absolutely no chance of the helium coolant of the LHC magnet system suddenly deciding to explode. Gee, really?! It is just sad that someone felt compelled to write this.

This paper reminds me of the old Annals of Improbable Research article, "The Effect of Peanut Butter on the Rotation of the Earth".

A subtle statistical mechanics question

A faculty colleague of mine posed a statistical physics question for me, since I'm teaching that subject to undergraduates this semester, and I want to throw it out there to my readership. I'll give some context, explain the question, and then explain why it's actually rather subtle. If someone has a good answer or a reference to a good (that is, rigorous) answer, I'd appreciate it.

In statistical physics one of the key underlying ideas is the following: For every macroscopic state (e.g., a pressure of 1 atmosphere and a temperature of around 300 K for the air in your room), there are many microstates (in this example, there are many possible arrangements of positions and momenta of oxygen and nitrogen molecules in the room that all look macroscopically about the same). The macroscopic states that we observe are those that have the most possible microstates associated with them. There is nothing physically forbidden about having all of the air in your room just in the upper 1 m of space; it's just that there are vastly more microstates where the air is roughly evenly distributed, so that's what we end up seeing.

Crucial to actually calculating anything using this idea, we need to be able to count microstates. For pointlike particles, that means that we want to count up how many possible positions and momenta they can have. Classically this is awkward because position and momentum are continuous variables - there are an infinite number of possible positions and momenta even for one particle. Quantum mechanically, the uncertainty principle constrains things more, since we can never know the position and momentum precisely at the same time. So, the standard way of dealing with this is to divide up phase space (position x momentum) into "cells" of size h^d, where h is Planck's constant and d is the dimensionality. For 3d, we use h³. Planck's constant comes into it via the uncertainty principle. Here's an example of a typical explanation.

Here's the problem: why h³, when we learn in quantum mechanics that the uncertainty relation is, in 1d, (delta p)(delta x) >= hbar/2 (which is h/4 pi, for the nonexperts), not h ? Now, for many results in classical and quantum statistical mechanics, the precise number used here is irrelevant. However, that's not always the case. For example, when one calculates the temperature at which Bose condensation takes place, the precise number used here actually matters. Since h³ really does work for 3d, there must be some reason why it's right, rather than hbar³ or some related quantity. I'm sure that there must be a nice geometrical argument, or some clever 3d quantum insight, but I'm having trouble getting this to work. If anyone can enlighten me, I'd appreciate it!

UPDATE: Thanks to those commenting on this. I'm afraid that I wasn't as clear as I'd wanted to be in the above; let me try to refine my question. I know that one can start from particle-in-a-box quantum mechanics, or assume periodic boundary conditions, and count up the allowed plane-wave modes within a volume. This is equivalent to Igor(the first response post)'s discussion of applying the old-time Bohr-Sommerfeld quantization condition (that periodic orbits have actions quantized by h). My question is, really, why does h show up here, when we know that the minimal uncertainty product is actually hbar/2. Or, put another way, should all of the stat mech books that argue that the h³ comes from uncertainty be reworded instead to say that it comes from Bohr-Sommerfeld quantization?

A mini book review

Recently I acquired a copy of Electrical Transport in Nanoscale Systems by Max Di Ventra, a new textbook aimed at graduate students. I haven't yet had time to read through it in detail, but what I've seen so far is impressive. The book provides a thorough intro to various formalisms appropriate for understanding nanoscale transport, including the usual stuff (Drude, Kubo, Landauer-Buttiker, nonequilibrium Green's function (NEGF)) and other sophisticated approaches that focus on transport fundamentally as a nonequilibrium quantum statistical mechanics problem (dynamic density functional theory, a hydrodynamic approximation for the electron liquid, and a detailed look at the interactions between the electrons and the ions). I also appreciate the effort to point out that truly nanoscale systems really are more complicated and different than "ordinary" mesoscopic systems. The only significant omission (intentional, in large part to avoid doubling the size of the book) is a comparative lack of discussion of strong correlation effects (e.g. Kondo physics). (A good complementary book for those interested in the latter topic is that by Bruus and Flensberg.) It's not exactly light entertainment, but the writing is clear and pedagogical.

Update: By coincidence, Supriyo Datta just put up a nice long review of the NEGF approach. He also has a full book-length treatment written with a very pedagogical focus.

(For those curious about my own book efforts, it's slowly coming along. Slowly.)

Science funding.

This article confirms my previous impressions, and is very depressing. This past week the government promised roughly 200 years worth of the entire NSF annual budget to bail out the banking system. Since 2003 the US government has spent another 200 years worth of the the entire NSF annual budget in Iraq. After two years of "level funding", and the certainty that there will be no real budget passed before the election, what we really need is the prospect of another year of frozen budgets.

In related news, I've come to the realization that my research program is "too big to fail".

Update: I might as well put all of my nonscience stuff in one posting. Looking at the text of the proposed financial bailout bill here, I am aghast because of this section:

Decisions by the Secretary pursuant to the authority of this Act are non-reviewable and committed to agency discretion, and may not be reviewed by any court of law or any administrative agency.

Let me get this straight. The Secretary of the Treasury gets incredibly broad authority to use up to $700 billion to prop up the financial markets in essentially any way he decides is appropriate, and his decisions are explicitly not reviewable ever by anyone, including the judicial branch?! I'm no lawyer, but isn't this, umm, absolutely insane?

Because I'm a big musical nerd...

... I couldn't pass this up. Very well done, though someone should point out to the Obama supporters behind this that things didn't work out too well for most of the characters singing this in Les Miserables.

I will return to actual physics blogging soon, once the immediate disarray settles out.

Ike follow-up

Well, that was interesting. Thankfully we're all fine and our house is undamaged. The prospect of being without power for an extended period continues to suck, to put it bluntly. 90 degree weather, near 100% humidity, and no air conditioning or refrigeration. On the plus side, my university still has power and AC. On the downside, they've disabled access (card keys) to most buildings and water service (i.e. sanitary plumbing) is spotty on campus.

Hurricane Ike

Hello - for those readers who don't know, I live in Houston, which is about to get hit by Hurricane Ike. I'm hopeful that this won't be a big deal, but there's always the chance that I'll be without electricity for a few days. So, blogging may be slow. In the mean time, check out this cool site for following tropical storm systems, and this explanation of how hurricanes are heat engines.

Ahh, the Gulf coast.

You know, I lived the first twenty-nine years of my life without having to pay close attention to stuff like this.

Important online resource

The internet is definitely the best way to keep up with current events. Check here often (look at the link text). (Thanks, Dan.)

Final Packard highlights + amusing article

One of my former professors, Michael Peskin, has a nice article about why the LHC will not destroy the earth. He taught me graduate-level mechanics, and my brain still hurts from his take-home final.

A last few things I learned at the Packard meeting:

• The stickleback is a very useful fish for addressing the question, if natural selection removes variation in phenotypes, then why do we still see so much variation?
• There are structures on the membranes of many cells (the primary cilium; the protein known as rhomboid) that seem to have really profound effects on many cellular processes. Understanding how and why they do what they do demonstrates why systems biology is hard.
• It may be possible to do some kind of "safe" cryptographic key exchange based on functions that are not algebraic (as opposed to usual RSA-type encryption which is based on the asymmetry in difficulty between multiplication and factorization).
• There are deep connections between random permutations and the distribution of the number of prime factors.
• It's possible to run live small animals (zebrafish, c. elegans) through microfluidic assay systems in massively parallel fashion.
• Stem cell differentiation can apparently be influenced by the mechanical properties (e.g., squishy vs. hard) of the substrate. Weird.
• Artificial sieve structures can be very useful for electrophoresis of long segments of DNA.
• There may be clever ways to solve strongly correlated electronic structure problems using tensor networks.
• Natural synthesis of useful small molecules (e.g., penicillin, resveratrol) is pretty amazing. Makes me want to learn more about bacteria, actomycetes, and fungi.
• By clever trading of time and statistics for intensity, 3d superresolution imaging is possible under some circumstances.
• DNA can be used as a catalyst.
• Some bacteria in biofilms secrete molecules that look like antibiotic byproducts, but may actually serve as a way of carrying electrons long distances so that the little buggers far from the food source can still respirate.
• Virus chips are awesome.
• Don't ever get botfly larvae growing in your scalp. Ever.
• Tensegrity structures can be very useful for biomimetic machines.
• Sub-mm arrays are going to be a boon for astronomy.
• It looks like much of the Se and Br in the universe was actually produced by the same compact object mergers that give short gamma ray bursts.
• Dark energy remains a major enigma in physics and astrophysics. It's a big one.
  

Packard highlights

Some things I learned at my final Packard meeting:

• The density of stars in a globular cluster is just absurd - something like 10⁴-10⁶ stars in a volume 10 ly on a side. Wow.
• The joint between the stem and base of a wine glass is a perfect lens for demonstrating the types of Einstein rings that one sees in gravitational lensing.
• In the protoplanetary disk phase of solar system formation, elements get mixed on very rapid timescales (like around 1000 years).
• Bacteria are much better at using ⁴⁰Ca in their metabolism than ⁴⁴Ca, and it's not at all clear how this works kinetically.
• 3-5 million years ago, in the early Pliocene, the global climate is a good test case for comparison with global warming models. Bad news for me: if the trends can really be mapped onto today, the hurricane rate is likely to increase by a factor of two.
• Using isotopic analysis (!), it is possible to put an error bound on how many people the lions in the Fields Museum actually ate: 41 +- 11. Anecdotal evidence had put the number between 15 and 135.
• We're all going to be able to get our genomes sequenced very soon, since the rate at which DNA can be sequenced (base pairs per day, for example) has gone up by five orders of magnitude in the last five years.

More later....

Packard meeting 2008

I'm in Park City for my last annual meeting as a Packard Fellow. As I've said before, I can't speak highly enough of the Packard Foundation. Their support has jumpstarted or otherwise contributed to pretty much my entire research program, and through their annual meetings I've gotten to know meet some fascinating people and hear excellent talks in the sciences, mathematics, and engineering. This is a particularly special meeting because it's the 20th anniversary of the Packard fellowship program, and they've invited back all the previous fellows. I'll try to post some highlights over the next few days.

Follow-ups

First, following up on my earlier post about the field effect.... I'd seen some of this before but had some really good conversations at the workshop in Japan last week about electrochemical gating. The field effect, as I'd said, is a great way of tuning the density of charge carriers at a surface without the disorder associated with chemical doping. By cranking up some gate voltage you can in some sense just rely on the attraction of opposite charges to accumulate carriers, for example. One major limitation to this technique, though, is the amount of charge that you can really get in there using reasonable dielectrics between the gate and the surface of interest. A given insulator can only take a certain amount of electric field across it before leakage current (and eventual breakdown due to damage from "hot" electrons) starts. Calling that limiting field E_max, you can find the maximum gated charge density to be \epsilon_0 \kappa E_max, where \epsilon_0 is the permittivity of free space in SI units (8.85 x 10^-12 Farad/m) and \kappa is the (unitless) relative dielectric constant. If you've got a really good oxide you can get this product up near 10¹³ carriers per square cm. Usually there is a tradeoff - materials with a big \kappa have a smaller breakdown field. One way around this is to use electrochemical gating. Instead of a dielectric, use either a polymer electrolyte or an ionic liquid. If you don't care about speed of response, this is a great idea because you can get a layer of counterions right next to the surface of interest. As a result, you can accumulate carrier densities exceeding 10¹⁴/cm². That's a huge density that can let you do some fun things even in strongly correlated materials, where you're now talking about adding or removing more than one carrier per unit cell.

A very brief follow-up to my post about the weather: Aww, not this crap again. Hurricane Gustav is really starting to look like a potential annoyance. Here's hoping that it hits neither New Orleans nor Houston.

Sad. Just sad.

According to a news article in Nature, there are now only four PhD scientists left at Bell Labs doing basic research. Four. Leaving aside that I know those guys personally, we should all be saddened by this.

I know that in industry there's always a tension between the immediate company bottom line and longer term investment. I can't help but wonder, though, in these days of institutional investors, mutual funds, and huge executive compensation, if we've really screwed up. Let me put it this way.... Once upon a time there were long-term investors who bought, e.g., AT&T, and really cared about whether AT&T was going to be competitive in ten or fifteen or twenty years. Now most stock is held by institutional investors and mutual funds who really don't care whether AT&T exists in ten years - they just care that there's something with a risk/reward profile like that in ten years. Furthermore, the executive compensation system is designed to massively reward short-term results (how much is this quarter's rate of growth greater than last year's? Note that making a big profit isn't enough - you have to be increasing your profit rate, not just the absolute amount of money the company makes.). As far as I can tell, we have effectively removed much of the economic incentive for long-term investment. No wonder any research and development with a 10-year horizon is almost gone from the American technological landscape. The only exceptions, as is often the case, are companies with so much money that a small research investment is negligible and can give decent PR, like Intel, Google, and Microsoft. Yeah, I know that HP Labs still exists, and I know that IBM still has people playing with STM, and I know that Exxon and Dupont and 3M have lots of talented chemists, but it looks like the days are largely gone of having a staff with a critical mass of tens of physics and chemistry PhDs doing cutting edge long-term research in an industrial setting.

Slow blogging....

I'm off to Japan for this week, primarily to speak here, and secondarily to guarantee that I'm a jetlagged wreck for the first week of classes. So, unless I do some Lost in Translation blogging, it'll be a quiet week here.

Cryptophysicists

I think that we need to coin an official term, "cryptophysicist", to describe people who do physics research outside the mainstream. Ronald Mallett is an example of a credentialed cryptophysicist - he wants to build a time machine using circulating optical beams. His tragic motivations aside, this is a scientifically wacky idea - the energy density that you would need in the beams to produce any significant distortion of spacetime is completely unachievable with foreseeable equipment. On the theory side, Harold Puthoff is another example. Puthoff wants to explain things like inertia in terms of interactions between matter and zero-point fluctuations of the electromagnetic field. Mainstream theorists consider this to be a wacky idea for a long list of reasons.

One major difference between cryptophysicists and cryptozooligists is that the public is generally able to perceive that the latter are outside the mainstream. Everyone knows from daily experience that there probably aren't yeti or sea monsters hanging around. Modern physics is abstracted enough from everyday lives and intuition, though, that many people, including some journalists, honestly can't tell when someone's waaay out there. Also, the concept of the lone genius toiling away in obscurity fighting The Scientific Establishment, which makes for good TV, sounds better when applied to a garage tinkerer than to someone camping out looking for the chupacabra. Still, occasionally the biologists do get to have fun with media coverage of this stuff.

The US election and science

For anyone interested in the US presidential candidates' positions on science, I recommend this NPR piece from today's "All Things Considered". What I find most revealing is near the end. Obama supports the goals of the America COMPETES act, which (among other things) was intended to double support for basic science research over the next several years. McCain does not consider science research to be as high a priority, given fiscal constraints. Bear in mind that as a percentage of GDP the US spending on physical science research has been falling for decades, and we've also witnessed the near-death of industrial long-term research in the sciences. I'm not trying to start a political flamewar here - I just wanted to point people to this information.

Update: From the Department of the Obvious, this article from USA Today about general science literacy and cultural perception in the US. How depressing. 44% of those surveyed couldn't name a single scientist, living or dead, that could be a role model for young people. Wow.

Whoa.

This article (thinks, Incoherent Ponderer, for pointing this out and your insightful post) is really distressing. Prof. Plummer is a class act and chaired a session of the APS Focus Topic that Eric Isaacs and I put together for this past March Meeting. I hope Tennessee is able to find the resources to support him fully.

Random samples

Three little items....

First, the Foundational Questions folks have announced their funding recipients. These grants, intended for fundamental questions in physics and cosmology, are supposed to target topics "unlikely to be otherwise funded by conventional sources". I'm a bit skeptical about that, since it sure looks like a number of these people are getting funded for topics that sound just like their regular research. Still, I do think it's a good thing to put some resources into real foundational questions. For the condensed matter people in the audience, our token representation on the list is Subir Sachdev, who is extending his theory formalism to cover black holes (!), and Keith Schwab, who is going to do true quantum mechanics by building micromechanical systems that exist in a quantum regime.

Second, this paper is one of the more bizarre things I've seen on the arxiv lately. It's from some folks trying to find efficient ways to compute pagerank numbers (useful in search algorithms for the web). They claim to have a Schroedinger-like equation that describes the pagerank. I was intrigued, in part because of all the lame joke opportunities this suggested. Funky.

Lastly, it's disappointing to read articles like this one and this one. I don't know what's more sad - that the the Holmdel Bell Labs site is being sold to developers, or that the Wall Street Journal can run multiple long articles about the discord and tribulations at the top of Alcatel-Lucent and not even mention Bell Labs. I can't help but think that the US will really miss industrial long-term research.

Ahh, the weather.

It's that time of year again, when we get a brief break from high-90s (F) heat and high humidity by having a tropical weather system blow through the area. Hopefully this will bring us some much needed rain, but not too much. It's interesting to watch the evolution of computer models here, and also to keep track of historical trends. I never had to think about this stuff growing up in Pennsylvania....

Why the field effect is important.

I'm probably overdue for making a post introducing some physics topic for a nonspecialist audience, so here you go....

What is the field effect? Well, it's the physical process that is used in field effect transistors (FETs), the hundreds of millions of little three-terminal switches that are the basis of the computer you're using right now. In a FET the electrical conduction between two electrodes (a source and a drain) is modulated by applying a voltage to a third electrode (the gate) that is capacitively coupled to (though separated by a thin insulating dielectric layer from) the semiconductor material between the source and drain (the channel). In an ideal (flat band, for the experts) FET, if the gate voltage is adjusted to be positive relative to the source and drain electrodes, a layer of electrons will be capacitively attracted from the source and drain into the channel region. Once there are carriers accumulating in the channel, conduction can take place from the source to the drain. Alternately, one could imagine cranking up the gate voltage to be negative relative to the source and drain, in which case one would expect to accumulate a layer of holes rather than electrons. A device that can really be modulated to have either electrons or holes as the dominant carriers is said to be ambipolar. Of course, real devices are more complicated than this. For example, there can be surface states that live at the semiconductor/dielectric interface close to the gate; the source and drain electrodes may have preferential alignment to either the conduction or valence bands depending on the materials involved; etc. Still, the FET is the basis for modern electronics, and techniques exist for fabricating these little gadgets by the billions in Si with incredible reliability.

That's why the FET is important for industry. Why is the FET important for physics? The FET idea can be rephrased this way: the FET is a way of changing the chemical potential of charge carriers (or, in a related sense, the density of charge carriers) without altering the composition of the material. Think about that for a sec. Chemists understand doping very well. If you're doing chemistry and want to remove some electrons, you add a halogen atom somewhere since those are hugely electronegative and grab an electron. Alternately, you want an extra electron in there? Throw in an alkali metal atom - they love to give up electrons. This kind of doping is commonly done in, e.g., the high temperature superconductors, where the parent copper oxide compound is actually an (antiferromagnetic) insulator that only becomes conducting (and superconducting) if doped chemically. The problem with chemical doping is that adding new atoms to a solid changes its structure and usually increases disorder, since we generally can't control precisely how the extra atoms are distributed. In FET structures, conversely, one can try to tune the carrier density just by turning up a voltage, no chemistry required. In graphene, for example, people have gated readily from having lots of electrons to having lots of holes and back, all in one device. Of course, FET structures do have some serious limitations: they only really put charge onto a surface or interface (rather than the bulk), and it is extremely challenging to get field effect charge densities to be comparable to those achieved easily with chemical doping. Still, the field effect is a tremendous tool for tuning electronic properties without mucking up the disorder or structure. If you're interested in this in more detail, please check out this Rev Mod Phys paper.

Physics virtual swap meet

Now this is a modest proposal whose time has come. It's common that experimental physicists gradually (or not so gradually) accumulate some pieces of equipment over the years that are laboratory "white elephants". These items are typically acquired for some specific research project or direction, and then over the years as research goals and priorities change, they can end up sitting around gathering dust. Surely someone else somewhere could make productive use of these items. Maybe someone should set up a trading post, where we could list these things and arrange reasonable trades or purchases.

Yes, used equipment vendors exist to address these needs, but they're not not always easy to do deal with, and frequently they offer pennies on the dollar. (For example, at Rice we have an 11-year old electron microscope. It's got a problem that is likely to cost about $10K to fix, though annoyingly the microscope vendor refuses to have a reasonable return policy - if we bought the relevant part and that didn't fix the problem, they'd refuse to take it back even for a restocking fee. Two different used equipment vendors have offered around $8K for the whole SEM (!), while they both offer the same item for sale on their sites for more than $100K.) I also know that people buy and sell scientific equipment on E-bay, but that suffers from some of the same problems. The point here isn't for scientists or universities to make money on this - it's to match up scientists/engineers with equipment that they could use and that currently has the wrong home.

Two papers in Nano Letters

Two new papers in Nano Letters caught my eye.

Danilov et al., "Nanoelectromechanical switch operating by tunneling of an entire C60 molecule"
This is a single-molecule electronic experiment, and it's a pretty neat example of using careful measurements to deduce a fair bit of information about a nanoscale system without a direct microscopic imaging probe. In their experiment the authors find bistable switching between two different conducting configurations of a junction thought to contain a single C₆₀ molecule (as inferred by a signature in the electronic conduction of a well-known vibrational mode of the fullerene). Now, telegraph-like switching isn't new by any stretch, and many nanoscale systems exhibit discrete changes in their properties due to motion of one or a few atoms or molecules. Here, by carefully analyzing the voltage and temperature dependence of the switching, they are able to deduce that the most likely mechanism for the change in configuration is motion of the fullerene between two different sites. I'm not sure that I'm 100% convinced by their interpretation, but the data are quite pretty and the analysis is clever.

Stampfer et al., "Tunable graphene single-electron transistor"
In this work, Klaus Ensslin's group starts from a graphene flake (the now-usual scotch-tape approach) and uses standard patterning methods to make a little graphene region connected by narrow graphene constrictions to source and drain electrodes. Using gate electrodes near the constrictions, they could electrostatically shift the local chemical potential there, shifting the graphene from having electrons charge carriers to holes as charge carriers. By further tuning with an additional gate, they could make a complete single-electron transistor - essentially a puddle of confined charges weakly connected by tunnel barriers to larger reservoirs, with the confinement sufficiently strong that the electrostatic energy cost of changing the puddle population by one charge exceeded the available thermal energy. (This was all done at cryogenic temperatures, since the charging energy of the dot was around 3 meV, or about 35 K.). This is a particularly nice, clean example of using graphene and its relatively unique properties as a platform for nanoscale device fabrication. Several groups are getting very good at this, and if efforts to grow large area, high quality, single-layer graphene succeed, there could be some genuine technological implications.

LHC publicity machine

I understand that the folks at CERN feel like it's important for people to be aware of the LHC and get excited about it - at this point, it looks like it's going to be the only game in town in a few years for the frontier of high energy physics. Still, the steady stream of publicity (much of it arguing that they're going to unlock the secrets of the universe, prove string theory, find evidence of extra dimensions, etc.) is getting to be a bit much. Today comes this article discussing the cooldown of the magnets for the collider and the detector. Technologically impressive to be sure, but the whole "colder than deep space" angle is pretty lame - people have been able to reach these temperatures for nearly 100 years, and superconducting magnets are used in thousands of MRI machines the world over. We get it - it's a big machine. If this is the level of publicity hounding that's going on before they even have a single piece of data, the coverage of the actual physics runs is going to be really oppressive.

Scientists, the media, and desperation

I could've predicted this. Given current energy concerns, it's not at all surprising that the various media are ready to give airtime and column space to wacky stories like this one. The temptation must be irresistible: the public is desperate; the story itself is great TV - the lone inventor, persevering in the face of opposition from those stodgy old scientists; they can even put in quotes from the would-be inventor and the scientists and claim to be covering "both sides". You know the drill: "This conventional scientist says that if he drops this pencil it will fall to the ground. Others disagree! The controversy, up next after this commercial message." News flash: sometimes it doesn't make any sense to cover "both sides".

I think the part that frustrates me the most is the misperception by part of the public and some of the media that scientists want these alleged breakthroughs to fail. Nothing could be further from the truth! If someone discovered cheap, inexhaustible energy because of a remarkable revolutionary breakthrough, we'd love it - it'd be the most exciting time in science since the quantum revolution. The problem is, though, that keeping an open mind doesn't mean lowering your scientific standards because you'd like to believe the result. I, for one, am not holding my breath about hydrino power.

slow blogging + pnictide fun

I'll be traveling (work + vacation), so blogging will be slow until July 15 or so.

Before I go, I wanted to point out that the plot continues to thicken regarding the pairing symmetry of the new iron pnictide superconductors. For example, this paper reports scanning SQUID microscopy on a sample of one of the compounds, with no apparent evidence for sign flips in the order parameter that you might expect if the material was, e.g., d-wave like the cuprates. In contrast, this paper argues that scanning tunneling spectroscopy data resemble d-wave expectations. This paper reports photoemission studies showing that the compounds have quite a complicated band structure and suggests that different parts of the Fermi surface may have different phenomenology. That sounds reminiscent of this theory paper, but I haven't read them in detail.

What makes an experiment "good"

Recently I've had some conversations with a couple of people, including someone involved in journalism, about what makes a physics experiment good. I've been trying to think of a good way to explain my views on this; I think it's important, particularly since the lay public (and many journalists) don't have the background to judge realistically for themselves the difference between good and bad scientific results.

There are different kinds of experiments, of course, each with its own special requirements. I'll limit myself to condensed matter/AMO sorts of work, rather than high energy or nuclear. Astro is a whole separate issue, where one is often an observer rather than an experimenter, per se. In the world of precision measurement, it's absolutely critical to understand all sources of error, since the whole point of such experiments is to establish new limits of precision (like the g factor of the electron, which serves as an exquisite test of quantum electrodynamics) or bounds on quantities (like the electric dipole moment of the electron, which is darned close to zero as far as anyone can tell, and if it was nonzero there would be some major implications). Less stringent but still important is the broad class of experiments where some property is measured and compared quantitatively with theoretical expectations, either to demonstrate a realization of a prediction or, conversely, to show that a theoretical explanation now exists that is consistent with some phenomenon. A third kind of experiment is more phenomenological - demonstrating some new effect and placing bounds on it, showing the trends (how the phenomenon depends on controllable parameters), and advancing a hypothesis of explanation. This last type of situation is moderately common in nanoscale science.

One major hallmark of a good experiment is reproducibility. In the nano world this can be challenging, since there are times when measured properties can depend critically on parameters over which we have no direct control (e.g., the precise configuration of atoms at some surface). Still, in macroscopic systems at the least, one should reasonably expect that the same experiment with identical sample preparation run multiple times should give the same quantitative results. If it doesn't, that means (a) you don't actually have control of all the parameters that are important, and (b) it will be very difficult to figure out what's going on. If someone is reporting a surprising finding, how often is it seen? How readily is it reproduced, especially by independent researchers? This is an essential component of good work.

Likewise, clarity of design is nice. How are different parameters in the experiment inferred? Is the procedure to find those values robust? Are there built-in crosschecks that one can do to ensure that the measurements and related calculations make sense? Can the important independent variables be tweaked without affecting each other? Are the measurements really providing information that is useful?

Good analysis is also critical. Are there hidden assumptions? Are quantities normalized in sensible ways? Do trends make sense? Are the data plotted in ways that are fair? In essence, are apples being compared to apples? Are the conclusions consistent with the data, or truly implied by the data?

I know that some of this sounds vague. Anyone more eloquent than me want to try to articulate this more clearly?

New comic

Thanks to Tom for having a link on his page to this. Too true! This is also brilliant. Good to have a laugh, despite the passing of George Carlin. One of my favorite Carlin quotes: [W]e have flamethrowers. And what this indicates to me, it means that at some point, some person said to himself, "Gee, I sure would like to set those people on fire over there. But I'm way too far away to get the job done. If only I had something that would throw flame on them."

No "Singularity" for you.

I wasn't going to even mention the idea of a Singularity, but then the IEEE made a point of dedicating an issue of their magazine to the concept. For those who don't know, the term "Singularity" originates with sci-fi author Vernor Vinge, who has written some compelling novels. Proponents of the concept believe that we live in an era of exponentially accelerating technological change, and that at some point (the Singularity) there will be a complete break in the nature of our species and societies, ushering in what some call a transhumanist future. The technologies typically associated with this idea are (1) Drexlerian molecular nanotechnology, so that we can eliminate scarcity by building anything we want anytime we want via (self-reproducing) nanomachines; (2) immortality via nanotechnological or biochemical control over biological processes that lead to senescence; and (3) strong AI, often including the concept of people uploading their minds to constructed hardware. The thing that continues to surprise me about this idea is that so many people seem to take it so seriously.

Hey, I'm all for optimism, and I'm generally bullish on the future of the species despite current scariness and some scientific arguments, but asserting that we will have a transhumanist utopia in twenty or thirty years is a wee bit of a reach, to put it mildly.

New physics building - suggestions? ideas? horror stories?

My university is in the design phase on a new physics building. This is exciting - first, it's a rare opportunity to design new lab space literally from the ground up. Second, new space will make possible some targeted expansion in the experimental directions in our department as well as in the experimental physicsy part of our electrical and computer engineering department.

Anyone out there have suggestions on building design, particularly with regards to laboratory facilities, utilities, HVAC, electrical service, vibrations, etc.? We're already looking at several recently constructed buildings elsewhere to learn lessons about best practices. If you have thoughts on physics buildings that you think are particularly well done (e.g., the electrical wiring system for the labs at the new nano building at Purdue looks extremely clever and well done), or, conversely, specific examples of design ideas that are lousy in practice or implementation, please post in the comments or email me.

Great scientific workshop

Posting from the scenic Newark International Airport.... I just finished attending the 2008 international workshop, ESPMI-08 - Electronic Structure and Processes at Molecular-based Interfaces, at Princeton University, hosted by Antoine Kahn and David Cahen. For me, this was practically the perfect scientific meeting - about 80 attendees, a mix of theorists and experimentalists, and all the talks were very good and pitched at the right level. I'll write more about this later, but for now, two highlights that show that some things are truly universal.

First, we were having a group discussion about organic photovoltaics and the relevant issues, and it was refreshing to see that everyone, even people who have been thinking about these problems for twenty years, starts out thinking about semiconductor interfaces by drawing the un-coupled materials and then thinking about what happens when they are brought into contact. I know I think this way, but it's reassuring to see that no one can just draw complicated band alignment diagrams freehand.

Second, during this morning's session there was a 1-second brownout/power glitch - the air conditioning shut down and restarted; the computer at the front of the lecture hall rebooted. The part that struck me as amusing was how the Princeton faculty immediately motioned to their grad students/postdocs to run off, or ran off themselves, to check on the lab equipment (particularly the UHV systems). I can totally see myself doing that.

Simple numbers

So, if crude oil futures cost at least $126/42 gallon barrel these days, doesn't that imply that the raw starting material for gasoline already costs (once you factor in the time delay between futures contracts and refining) $3/gallon? This suggests to me that the "correct" price for gasoline in the US should be closer to $5-6/gallon, when the refining catches up with futures. (That doesn't even touch on issues about how much of the crude oil pricing is due to speculation vs. actual supply & demand, or how much of this is due to the effective weak dollar policies of the US central bank.)

Plagiarism at the professional level

Remember my discussion of plagiarism? Remember how a couple of readers didn't seem to thing that this was necessarily that big a deal, particularly if it was "just" background stuff and not actual data? Well, I'd be curious to know what they think of this case. I hope that someone follows through and notifies the editors at the respective journals. Makes you curious about their other publications, doesn't it?

This week in the arxiv: superconductivity update

Summer writing and travel are eating my blogging time a bit, and I've also agreed to write the occasional nano-related blurb for the ACS. While my posting rate has taken a hit, science has continued to march forward, with a lot of exciting new preprints concerning (relatively) high temperature superconductivity. Here's a sampling....

arxiv:0805.4463 - Matsumoto et al., Superconductivity in undoped T' cuprates with T_c over 30 K
This paper is a perfect example of why materials growers are (unfortunately often unsung) heroes in this field. The authors have come up with a new method for growing cuprate compounds of the form T'Re₂CuO₄, where T'Re is a rare earth from the series (Pr, Nd, Sm, Eu, Gd). Historically these compounds were found to be antiferromagnetic insulators - no superconductivity. In this new work the authors argue that these old results were due to interstitial oxygen leading to pair-breaking. Instead, with the new growth + annealing technique, these compounds are found to exhibit superconductivity with transition temperatures as high as 30 K. These subtleties are why one should always be very careful when looking at suggested compositions in new compounds....

arxiv:0805.4630 - Rotter et al., Superconductivity at 38 K in the iron arsenide (Ba_1-xK_x)Fe₂As₂
This is the first paper I've seen (though I may have missed one) that reports superconductivity in a compound related to the new iron arsenide systems but with two iron arsenide layers per unit cell rather than one. Back in the heyday of the cuprates, the same sort of thing happened - people went from compounds with single copper oxide planes to those with multiple planes per unit cell, and transition temperatures went up. Once again we see how rich the materials landscape can be. Update: as anon. in the comments pointed out, this isn't actually the 2-layer version of the compound. Rather, it's analogous to the so-called "infinite layer" version. My mistake.

arxiv:0806.0063 - Wang et al., Very high critical field and superior J_c-field performance in NdO_0.82F_0.18FeAs with T_c of 51 K
Other exciting features of the new iron arsenide superconductors are their extremely high critical fields and critical currents. If the transition temperatures could be raised a bit (say past 77 K) and the compounds could be made in wire form (certainly not easy in the cuprates; unlikely to be simple in these either since like the cuprates they are brittle), this could be a huge deal for high field magnets and other applications of superconductivity.

arxiv:0805.4616 - Chen et al., The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
arxiv:0806.0249 - Matano et al., Spin-singlet superconductivity with multiple gaps in PrO_0.89F_0.11FeAs
These two papers examine two related compounds with different techniques, trying to figure out how the charge carriers in these iron arsenides pair up to form the Cooper pairs that make up the superconducting condensate state. In the former, measurements of Andreev reflection (a process where an electron in a normal metal approaches a superconductor, two electrons actually cross into the superconductor, and a hole is "retroreflected" back into the normal metal, leading to a pronounced feature in the conductance of the metal/superconductor interface) strongly suggest that the samarium compound acts like an ordinary BCS superconductor. That is, each Cooper pair has zero angular momentum (s-wave pairing); this implies that the superconducting gap is uniform in momentum space, with no nodes. In contrast, the cuprates exhibit d-wave pairing, with a superconducting gap that has a four-lobe structure in momentum space and that goes to zero along four particular crystallographic directions.

The second paper uses NMR measurements of the Pr compound to argue instead that there are multiple gaps, and further that the pairing symmetry is p-wave (which has been seen in superfluid ³He and in strontium ruthenate). At first glance, these two results seem to disagree, though (a) they are talking about different materials, and (b) the Andreev measurements are particularly sensitive to the surface, while the NMR measurements are nontrivial to interpret, at least for nonexperts. Well, this is the fun part - stay tuned, and we'll see how this shakes out.

Cold fusion - same old same old.

Once again (and it seems like this happens every couple of years) someone is claiming "success" in a cold fusion experiment. Basically this fellow has made a cell containing some composite of ZrO2 and nanoscale Pd crystals. The claim is that when this cell is filled to moderate pressures (a few bar) with deuterium gas over a couple of days, the cell gets hot (compared to its surroundings) and stays hot for a while (tens of hours), and that ⁴He is detected afterward. Furthermore, the claim is that control experiments with ordinary hydrogen do not produce the long-term heating or helium, and that control experiments without the Pd/ZrO₂ produce no heating at all. People who know next to nothing about nuclear physics argue that the lack of neutrons (from the D+D goes to ³He + n reaction pathway) or gamma rays is fine, since simple p and n counting lets you have D + D goes to ⁴He, despite the fact that the ³He reaction is vastly more favored in ordinary fusion. There continues to be no credible mechanism for getting the D nuclei close enough to each other to get fusion. Now, it's entirely possible that there is weird chemistry going on here, but how come in twenty years of people trying to do this stuff there has yet to be a clean, well-designed experiment done by physicists that is reproducible and actually shows anything interesting? It's grating on many levels that this, an anecdotal discussion of nonconclusive experiments, gets touted online through slashdot, gizmodo, digg, engadget, etc. Extraordinary claims require extraordinary evidence.

This week in the arxiv

Two papers from the past week that caught my eye....

arxiv:0805.3309 - Bunch et al., Impermeable atomic membranes from graphene sheets
This is a nice piece of work from Cornell combining the techniques from three research groups to look at the permeability of single-layer graphene sheets. The authors prepare freely suspended graphene trampolines and apply controlled pressure differences across them. They use scanned probe methods to measure the membrane shape, which ends up being well described by elasticity theory assuming that the elastic modulus for the graphene sheet is about 10¹² Pa (that's big but not unexpected). By watching that shape as a function of time, they can tell how long it takes the pressure inside the chamber (sealed off by the graphene) to equilibrate with the outside environment. Elegant.

arxiv:0805.2414 - Finck et al., Area dependence of interlayer tunneling in strongly correlated bilayer 2d systems at nu(total)=1.
I've written before about two-dimensional electronic systems (2des), and how they are very useful for looking at all sorts of rich physics such as the fractional quantum Hall effect. This experiment looks at a variation on this theme. For a while now it's been possible to make two high quality 2des separated by a thin barrier - thin enough that the charges in one layer can feel the charges in the other layer via the Coulomb interaction. Since like charges repel, if the two layers have the same density of electrons, a favored low energy state would have every electron in the upper layer accompanied by a hole (the absence of an electron) in the lower layer. If the barrier is sufficiently thin, tunneling can take place between the two layers. One fascinating observation has been that this interlayer tunneling, under certain circumstances, can look very much like the kind of Josephson tunneling that one gets between superconductors. One nagging question out there has been whether the very sharp tunneling seen is a bulk effect (and taking place over the whole area where the two layers are tuned to each other) or something else (e.g., an edge effect, like many quantum Hall phenomena). This experiment shows that the tunneling really is proportional to the area, and thus is a bulk effect. This is a tough experiment, requiring great samples, demanding fabrication, and very sensitive measurements at low temperatures.

Public service announcement re: cheating

I want to alert faculty colleagues to a website of which they need to be aware if they teach, particularly undergraduates. I won't link to them since I don't want to drive up their revenue, but it's called cramster.com, and while they bill themselves as a "24/7 study community", what they do is provide links to scanned solution manuals for many many textbooks. What this means is, if you teach a course from a reasonably popular book, you need to be aware that students can and often do buy the homework solutions online. As far as physics goes, they have a rather eclectic assortment. Lots of intro books, and a few major upper level ones (Griffiths; Goldstein; Jackson). If you make up a final exam using problems from the textbook, you're opening yourself up to this problem. If your problem sets contribute a lot to the final grade in a course and you use verbatim problems from the book, again you are almost certainly going to see this on some level. The more you know....

Now that would speed up sample fabrication.

There's no question that one of these would be useful to have in the lab. Check out the whole catalog of their products - fun for all ages.

This week in the arxiv

A couple of interesting papers, two about graphene and one about a weird fluid mechanics effect.

arxiv:0805.1830 - Bolotin et al., Temperature dependent transport in suspended graphene
It's become clear over the last year that a lot of what was limiting the measured electrical transport properties of graphene sheets had to do with interactions between the graphene and the underlying substrate (usually SiO₂). Now multiple groups have started preparing suspended graphene membranes (supported around the edges by oxide) overhanging underlying gate electrodes. By ramping up the current through the suspended membrane, the graphene sheet can be resistively heated in vacuum up to a temperature sufficient to desorb residual contaminants, and electronic properties can be measured without substrate effects. In this paper the Columbia group demonstrates that extremely high mobilities are then possible (well over 100000 cm²/Vs), and by examining the temperature and gate dependence of the conduction they can understand the scattering mechanisms at work as well as residual disorder in the system. Very clean looking data.

arxiv:0805.1884 - Booth et al., Macroscopic graphene membranes and their extraordinary stiffness
The Manchester group has also been very busy. In this paper they show a cute technique to produce large (say 0.1mm in diameter) graphene sheets in a form that's easy to suspend and handle. Basically instead of abrading or cleaving graphite into graphene on top of oxidized Si, they do so on top of Si coated with a layer of e-beam resist. An additional layer of a different sensitivity resist is put on top and patterned, followed by metal deposition. The metal layer forms a frame that goes around the previously identified graphene sheet, and the metal is then used as a seed layer to deposit a more robust Cu layer via electrochemistry. Finally, the original resist layer is dissolved, freeing the graphene+Cu frame for manipulation. They then further study the mechanical properties of these suspended layers, finding that single sheets of graphene are indeed very stiff - much more so than you might think, since they're 1 atom thick. The technique is elegant, and there is one particularly impressive TEM image. Nice SuperSTEM that they have over there in Cheshire.

arxiv:0805.0490 - Amjadi et al., A liquid film motor
Hat tip to arxivblog for pointing this out to me. These folks at Sharif University in Iran have found that DC electric fields can make soap films flow in very interesting and controllable ways. They suggest a few possible mechanisms for this kind of electrohydrodynamic motion, but conclude that none of them are entirely satisfactory. The paper has a minor rendering problem with Fig. 4, but you should definitely watch the movies on their webpage. Very dramatic! Soft CM physics can be inspiring - here's a visually impressive phenomenon that might actually be useful in fluidic applications, and the whole experiment is simple, elegant, and inexpensive. No exotic apparatus required.

The fun parts

In contrast to the previous post, there have been some fun parts of the job lately. Today was commencement, which is always amusing - I get to play dress-up and look like a real academic. If only point 4 in this list was true, then commencement would be much more exciting.

In the lab we've had some genuinely weird data come along, and that can be fun, too. In one kind of structure we're observing a phenomenon that is completely reproducible but for which we have essentially no sensible explanation. We've been messing around with this for a month, and every time we come up with a plan, thinking we know what's going on, nature turns around and proves us wrong. Whatever is going on, it seems interesting. When we figure it out enough to write it up, I'll discuss it further here.

Lastly, after a trip to the movies last week I had the shocking realization that Rice is now partnering with Stark Industries. Sweet. I need to get one of those flying suits.

Copying text without attribution is plagiarism.

Amazingly, there are graduate-level students out there who do not understand this simple, basic fact. When you're writing a scholastic or scientific document, you never copy other people's words - certainly not complete verbatim sentences - without clear attribution and indication that you're quoting someone else. You just don't. Ever. Doing so is plagiarism, and as any kind of professional you should know that it's wrong. Amazingly, some students don't seem to get this point, even when they've been told about this, explicitly, repeatedly, and actually signed documents attesting that they understand this, and when they know that the professor can use this amazing tool called google to figure this sort of thing out.

Just. Don't. Do. It.

Come on, AAAS

I'm a member of the AAAS, in part because I support their various efforts, and in part because I like my subscription to Science. However, at least three times a year, I get junk mail at my house or at my departmental address, asking me if I'd like to join AAAS for the low new-member rate of $99/yr. How can these geniuses not realize that I'm already a member? I have an unusual last name, and they already have both my work and home addresses on file. Can't they tell that Prof. Douglas Natelson and Mr. Douglas Natelson with identical addresses are the same person? They must waste hundreds of dollars in postage and thousands of pieces of paper doing this, since I'm sure I'm not the only one getting these useless mailings. Good grief, folks, just do a sensible search on your mailing database for duplicates.

AMO physics coolness

I saw two things in Science this week that I found quite interesting. First was a mention in Editor's Choice of this paper from my old stomping grounds at Stanford. The arxiv version is here. The idea is another great example of using essentially table-top physics (if you have a large, stainless steel vacuum chamber and lasers on your table) to test the limits of the Standard Model of particle physics, usually the domain of the high energy folks. Here's the story: there are many weird alternatives to the standard model where things like charge quantization (the idea that charge comes in chunks of exactly -e for electrons, and +e for protons, for example) and charge neutrality are approximate rather than exact, due to the breaking of some far out symmetries at very high energy scales. This paper points out that this idea can be tested very precisely (to 1 part in 10²⁸) using interferometry of Bose-condensed atoms. In an optical interferometer, light (consider only one particular color) is split into two beams that take different paths, and then recombined. As light travels on each path, you can figure out how much phase the light waves accumulate by dividing the pathlength by the wavelength (and multiplying by 2 pi if you want your phase to be in radians). The intensity when the beams are recombined is proportional to the cos of the phase difference between the paths. This can be an incredibly precise way of measuring relative path lengths, and is essential to lots of modern technology. In the proposed experiment, the Bose-condensed atoms act like matter waves, and the idea is to do the same thing. However, in quantum mechanics the phase difference that builds up is related not just to the path length, but also picks up a contribution due to the (integrated) difference in (potential) energy (times time, divided by hbar) between the two paths. This is the way AMO and neutron interferometry measurements of gravity work: send waves along paths at different heights and recombine them, and the phase difference will include a contribution proportional to (m g h) where m is the mass of the particles, g is the gravitational acceleration, and h is the height difference. In the proposed experiment the atom waves are sent through regions of different electrostatic potential (voltage). If the atoms aren't exactly neutral, the voltage will couple to their charge and lead to a phase difference that would otherwise be absent. It's very elegant, and may be a way to test advanced high energy ideas without TeV particle accelerators.

The second bit that I read was this article about the race to use cold fermionic atoms trapped in optical lattices as a means of implementing condensed matter models of interesting systems (e.g., the Hubbard model of high-T_c superconductors). The theoretical models are computationally nightmarish to solve exactly, in large part because of the Fermi-Dirac statistics problem that the correct many-body wavefunctions must pick up a minus sign if the positions of any two electrons are swapped. The plan is to implement what are basically analog computers - cold atom systems that can be poked, prodded, and tuned - to map out the solutions. Using tunable model systems to explore strong correlations in quantum matter also happens to be the focus of Rice's Keck Program in Quantum Materials. (One note for regular commenter Sylow: now do you believe me that there is a DARPA program on this?)

Career comments

Well, it's that time of the year again. Lots of blogging (here , here, here, here) about advice to tenure-track faculty (and other interested parties) about the tenure process. I've decided to dust off a post I originally made last May, with a few revisions and additions, to contribute to the discussion.

In terms of the job pipeline, the biggest cut in population happens when trying to get a faculty position, not at the tenure stage. In reasonable departments, no one is happy when a tenure promotion case fails. Good departments (and schools and universities) try very hard to filter at the hiring level and give their faculty the resources they need to succeed. I can only think of two or three places (in physics anyway) that historically have had a "sink or swim" attitude (that is, hiring a junior person in an area today means that seven years from now the university wants the best senior person in the world in that area - being in-house is not advantage), and I'm not sure that's even true anymore.

Generally advice is not in short supply, though good advice can be. Many institutions are setting up official mentoring efforts to ensure that junior candidates have people to talk to about these issues. A colleague of mine found several nice documents online about this issue of advice-giving and receiving. This one (pdf), from the ADVANCE program at the University of Michigan, is particularly good. I am hardly in a position to give too much sage advice about tenure, and what follows below is largely common sense. Obviously the situation is different in various disciplines and at different universities, but here's some basic points that I think should be considered. I'm sure I'll leave things out - feel free to chide me in the comments.

Understand the process. Find out how the tenure process works at your institution. This should be written down in a faculty handbook. Talk to your department chair, your faculty mentor (if your department has such a thing) or senior colleagues. Understand the timeline. Get a sense of the weight that your institution places on the different components of the job (see below). Does the departmental vote carry a lot of weight (as it usually does at Rice, for example), or are the deans or the university promotions and tenure (P&T) committee commonly overriding departmental decisions?

The process probably goes something like this: the candidate is hired for a 4-year tenure-track appointment, with some kind of annual reviews and a more major renewal review in year 3 or 4. (This gives the university a chance to end the process early if there's a major problem with an assistant prof, and forces departments to give some concrete feedback to the assistant prof about how they stand.) In the summer before year 6 (at most places) the candidate is asked to put together a dossier (complete CV, reprints of papers, a summary of funding, a statement about university service, a statement about teaching, a summary of research accomplishments, etc.) and suggest names for external evaluators. The department comes up with additional names for external evaluation, and sends the full dossier to some mix of the external people. Eventually these external letters come back, and the department reads them, puts the whole package together, and there's a vote of the tenured faculty (in October or November) about whether to recommend the assistant prof for tenure. The departmental recommendation then goes to the cognizant dean, and from there to the university P&T committee (which generally would have people from all sorts of disciplines on there, from bio to French lit). Sometimes P&T committees or deans can request more external letters, and they get copies of teaching evaluations, etc., and may meet directly with department chairs. Eventually the P&T committee makes its decisions (in late spring) and the candidate finds out. That decision is finally signed off by the president of the university and the board of trustees.

The research component. To get tenure you need actually need to be getting science done. There's no sure-fire recipe for success here, but let me make a few suggestions:

• Have a mix of projects that range from easier to high-risk/high-reward. Having only one major project can be very risky, particularly if it takes five years to get any results. One key element of getting tenure is that people in your community need to know who you are, what you've done, and what you've been doing that's really yours - new stuff from your professorial position, not rehash of your thesis or postdoc work.
• Make sure that your colleagues know what you're doing. Your colleagues are going to need to understand your work at least on some level, and particularly for hard projects, they will need to have some idea why it may take four years before a paper comes out.
• Have backup plans. High risk things may not succeed (no kidding.). Make sure, for your students' sake and yours, that you have thought out the projects well, so that even if you don't achieve the BIG goal, you are still learning useful things that are worth publishing.
• Have a high attempt frequency for funding. If there's literally only one agency in the world that funds your work, that's risky and unfortunate. Make sure that you know what your options are for funding sources. Call up program officers. Ask to get a chance to serve on review panels - you'll learn a huge amount about writing proposals that way! Know if there are state funding opportunities. Think ahead about private foundations (e.g., Research Corporation).
  
• Do some self-promotion but don't sell your soul. If your external evaluators don't know who you are, that's the kiss of death. Make sure you give talks at meetings. See what you can do about getting invited to give seminars at other schools. Yes, this is one issue where "well-connected" people really benefit, but if you go to meetings and get to know the people in your field, it's not that bad. Get involved in your own department's seminar series, and invite in people that you'd like to meet and talk to.
• Publish good stuff. This is always the tricky bit, and people joke about the "least publishable unit". Still, holding back everything for the one big Nature paper that may not happen is not necessarily the best strategy, for you or your students.
• Get stuff going relatively quickly. Think about the timescales associated with publications and citations. Even if you do the greatest piece of work in your field ever, if you don't get it out the door at least a year or two before your tenure review (that is, a year before letters get sent out to external reviewers), it's going to be very hard for that work to have had much of an impact by the time of the decision.
  

The teaching component. Do a good job teaching. Most universities have resources available to help you - teaching centers that will videotape your lectures, offer suggestions for improved technique, etc. Good teaching can only help tenure in limited ways at a research university, but poor teaching can certainly hurt a borderline case. People joke that getting excessively good teaching evaluations is a sign of misallocated time. That's not necessarily the case. The skills that you learn to be a good lecturer in the classroom overlap quite a bit with the skills you need to present research well - organization, an appreciation for your audience's perspective and knowledge, clarity, etc.

The mentoring component. This is related to both of the above. It definitely helps make the case that you are running a successful research and education enterprise if you can actually graduate students. This means making sure that they are making real progress, publishing papers (and/or patents), and ideally enabling them to land a good job (postdoc or industry) afterwards. This is not just altruistic; it's also enlightened self-interest - if you build a reputation for getting good people out in a reasonable timeframe and with real job prospects, it will help in graduate and postdoc recruiting in the long term. Managing a group isn't easy, and every student is different. If you feel like you're having trouble, definitely find colleagues to ask for advice! Every faculty research mentor has been there.

The service component. Do a decent job in departmental and university service. Don't let it eat all your time, but get involved in things that matter to you. It's also a good way to get to know your administrators and people in other departments. I'm not suggesting currying favor - just be a solid citizen. Becoming known as a pain-in-the-ass on this is not going to help you on any level.

Common sense. People argue about whether blogging can hurt your tenure chances. Blogging is only one example of a public forum, though. Use some common sense. Publicly badmouthing your institution, colleagues, administrators, etc. is not a good idea. (I'm not talking about hushing up legitimate grievances - I'm saying don't antagonize people gratuitously.) Remember, in a practical sense, the tenure decision is based not just on your scientific quality, but on whether you are the kind of colleague that people want to have for the next n years.

Don't panic. At some point, you just have to buckle down and do the work without inducing a psychodrama about the process. If you've been in a graduate program, you've undoubtedly known someone who, rather than actually solving their research problems, spent their time kvetching about how nothing was working. Don't do that to yourself. Remember, you're doing this because you enjoy it intellectually (at least, some of the time!).

Talk this week

First, to the readers of this blog, thanks for the recent trend of posting informative links in the comments. I think that this really adds something to the discussion. One tip: in the comments you're allowed to use html tags, so if you want to post a link with a long URL, you may want to write the html that actually posts the link.

This week we had a fun physics colloquium given by Paul Canfield of Ames Lab and Iowa State University. He spoke about the discovery and characterization of new materials, with a particular emphasis on heavy fermion compounds, but with a significant discussion of MgB₂ as well. His main purpose was to convey how physicists like him think and approach problems, and I think he succeeded. He also had a funny slide called "Periodic Table According to Most Physicists" that looked roughly like this:

H                                       H' (almost like hydrogen)
H'' H'''                  C       H'''' H'''''
                        Si
        Metals       Cu

                     Au

   Elements that may not even be real


         |<---Not on the final exam --->|
         |<---Stuff for bombs  -------->|

Amusing stuff.

Your tax dollars at work.

Like many of my colleagues, I review lots of grant proposals. Recently I was asked to review one for the Department of Energy, and when I said 'yes', they sent me the proposal. By Federal Express. On a CD. Now, you might wonder why, if they don't mind me ending up with this in an electronic format anyway, and if they want me to send in my review electronically, they wouldn't just handle this purely over the web, and save the money and environmental impact of shipping a CD from northern VA to Houston. Ahh well.

Talks this week

I saw some very good talks this week. First up was a physics colloquium by Stuart Parkin from IBM Almaden. In some very real sense, you're reading this because of Parkin - he and his team were the people who first took giant magnetoresistance (GMR) and developed it into a useful technology in the read heads of hard disk drives. The remarkable explosion in data storage capacity over the last decade and a half is largely due to this advance, possibly the best example of true nanotechnology (the film thicknesses involved in spin valves are a few nm) making it out of the lab and into manufacturing and consumer products. Their later work on tunneling magnetoresistance has also now been transferred into hard drive read heads. In fact, TMR heads with MgO tunnel barriers between ferromagnetic layers can have room temperature resistance changes of several hundred percent in the presence of few-Oersted fields like those from drive media. After reviewing all of this at just the right level, Parkin went on to talk a bit about his latest ideas and work on high performance "racetrack" memory. In this idea, a single transistor cell can be responsible for reading and writing tens of bits of memory (as opposed to one in current RAM designs). The bits are stored as domain walls in a ferromagnetic nanowire. The walls can be detected through their local change in the magnetization, and they can be moved by pulsing spin-polarized currents through the ferromagnetic wires. All in all, a great colloquium - one of my colleagues wished that we'd taped it so that we could show it to job candidates as an example of a real general audience colloquium.

There was also a workshop on campus this week about probabilistic and nanoscale computing that featured some nice talks. One of the best was by Tom Theis, head of physical sciences research at IBM, who reviewed their latest developments and the future of the field-effect transistor from his perspective. Anyone who has alternative ideas in mind about computing technologies really needs to do their homework by listening to someone like Theis, who has perspective about the science as well as the economic and manufacturing issues.