Gravity

There has been a good deal of talk lately about gravity. We're all taught early on in our science education about the remarkable insight of Isaac Newton, that the force that causes, e.g., apples to fall from trees is, in fact, the same force that keeps the moon in orbit about the earth (or rather about a common center of gravity relatively close to the center of the earth). The Newtonian gravitational constant, G, is the least precisely known of all the fundamental constants, in part because gravity is a shockingly weak force and therefore difficult to measure. (As I demonstrated to my freshmen students, gravity is so weak that even with the feeble muscles in my legs I can jump up in the air in defiance of the opposing force of the gravitational pull of the entire earth.) More frustrating than the difficulty in precision measurement of G is the fact that different research groups using different techniques come up with experimental estimates of G that differ by surprisingly large amounts. This paper (published last week in Phys. Rev. Lett.) is another example. The authors sweated over the details of their systematic uncertainties for two years before publishing this result, which disagrees with the "official" CODATA value for G by 10 sigma (!). This is a classic showcase for the art, elegance, and necessary attention to detail required in precision measurement physics.

Also making many waves during 2010 is this paper by Erik Verlinde. The claim of this paper is that gravity is emergent, rather than a "real" force. It's been argued since Einstein published general relativity that gravity is different at a deep level than traditional forces. GR says that we should think of gravity as a deformation of spacetime due to the presence of stress/energy. Freely falling particles always travel on geodesics (locally straight lines), and those geodesics are determined by the distribution of mass and energy (including that due to spacetime deformation). In the appropriate limit, GR reduces to Newtonian gravity. Verlinde, striking out in a completely different direction, argues that one can start from very general considerations, and gravity emerges as an "entropic" force. An entropic force is an apparent force that results from the tendency of matter and energy to explore all available microscopic states. For example, a polymer will tend to ball up because there are many more microscopic states that describe the polymer wadded up than extended. Pulling on the two ends of the polymer chain to straighten it out will require overcoming this entropic tendency, and the result is a tension force. Verlinde argues that gravity arises similarly. I need to re-read the paper - it's slippery in places, especially on what underlying background assumptions are made about time and space, and what really plays the role of temperature here. Still, intriguing food for thought, and it's elegant that he can get both something GR-like and something Newtonian to fall out of such an analysis.

Regardless of how you may feel about Verlinde's speculations and the difficulty of measuring G, at least you can laugh in shocked disbelief that these people are serious.  (I should be careful making jokes.  Knowing Rick Perry, they'll start pushing this in Texas public schools next year.)

Two for the price of one.

I had noticed (and it was also pointed out by a colleague) the essentially simultaneous publication of this paper and this paper (which appear to have been submitted within a week of each other as well).  In both papers, the authors have created short-channel graphene-based transistors in a clever way. They take a conductive nanowire (doped GaN in the Nano Letters paper; CoSi in the Nature paper), coat it with thin aluminum oxide via atomic-layer deposition, and then lay it down on top of a piece of exfoliated graphene. Then they evaporate Pt on top of the device. On either side of the nanowire, the Pt lands on the graphene, making source and drain electrodes. The nanowire shadows part of the graphene (the channel), and then the nanowire itself acts as the gate. This is a nice, self-aligned process, and the resulting graphene devices appear to be very fast (the Nature paper has actual high frequency measurements). Looks like they managed to get two papers in good journals for the price of one technique advance.

Arguing from authority? Hawking, you're supposed to be better than that.

In Saturday's Wall Street Journal, there was an article by Stephen Hawking and Leonard Mlodinow clearly designed as a naked promotion of their new book.  In the article, they argue that modern physics removes the need for a divine being to have created the universe.  Religious arguments aside (seriously, guys, is that particular argument even news anymore?), one thing in the article especially annoyed me.  Toward the end, the authors state:

As recent advances in cosmology suggest, the laws of gravity and quantum theory allow universes to appear spontaneously from nothing. Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist. It is not necessary to invoke God to light the blue touch paper and set the universe going.

Our universe seems to be one of many, each with different laws.

You know what's wrong with this? It states, as if it is established fact, that we understand cosmology well enough to declare that universes spontaneously self-create. It states that the multiverse is a prediction of "many" theories, implying strongly that it's on firm ground. The problem is, this isn't science. It's not falsifiable, and in its present form it's not even close to being falsifiable in the foreseeable future. Seriously, name one PREdiction (as opposed to retrodiction) of these cosmological models, or more seriously, the multiverse/landscape idea, that is testable. Don't claim that our existence is such a test - the anthropic principle is weak sauce and is by no means evidence of the multiverse. Man, it annoys me when high profile theorists (it always seems to be theorists who do this) forget that physics is actually an experimental science that rests on predictive power.

This won't end well, because it's blindingly idiotic.

According to the Chronicle of Higher Education, my Texas A&M colleagues up the road in College Station now get the privilege of being evaluated based on their bottom-line "financial value" to the university.  Take how much money the professor brings in (including some $ from tuition of the number of students taught), subtract their salary, and there you go.  This raises problematic points that should be obvious to anyone with two brain cells to rub together.  First, I guess it sucks to be in the humanities and social sciences - you almost certainly have negative value in this ranking. Congratulations, you leeches who take salary and don't bring in big research funding!  Second, it firmly establishes that the service contributions of faculty to the university are worthless in this ranking scheme.  Third, it establishes that the only measure of your educational contribution is how many students you teach - purely quantity, so if you teach large intro classes you're somehow valuable, but if you teach smaller upper division courses, you're less valuable.  Gee, that's not simplistic at all.  Now, the article doesn't actually say how these rankings will be used, but I'm having a hard time imagining ways that this metric is a good idea.

Silicon oxide and all that.

It's been a busy week work-wise; hence the low rate of blogging.  However, I would be remiss if I failed to talk about the science behind a paper (on which I am a coauthor) that was mentioned on the front page of the New York Times yesterday.  A student, Jun Yao, co-advised by my colleagues Jim Tour and Lin Zhong, did a really elegant experiment that has gotten a lot of attention, and the science is pretty neat.  Here's the deal.  Lots of people have done experiments where they've seen what appears to be nonvolatile switching of the electrical resistance in various nanoscale systems (e.g., junctions in nanotubes and other nanomaterials).  That is, what is observed is that, with the use of voltage pulses, the electrical resistance of a device may be programmed to be comparatively high or comparatively low, and that state is preserved for a looooong time.  Long story short:  sometimes this behavior has nothing in particular to do with the nanoscale system being studied, and really results from the properties of the underlying or nearby silicon oxide, which is generally treated as inert and boring.  Well, as people in the Si industry can tell you at length, it turns out that silicon oxide isn't necessarily inert and boring.   What Jun showed via some elegant cross-sectional transmission electron microscopy is that when big voltage pulses are applied across small distances, it is possible to modify the oxide, effectively doing electrochemistry, and turning some of the oxide back into Si nanocrystals.  When those nanocrystals give a hopping path from one electrode to the other, the device is "on".  When that path is broken, the device is "off".  The resulting nanocrystals themselves are quite small, on the order of a few nm.  Hence the excitement about possibly using this approach for very dense, nonvolatile memory.  There are, of course, a great many engineering issues to be overcome (there's no need to tell me about that in the comments....), but it is definitely a pretty science result. 

The wisdom of combining complementary techniques

In the September issue of Nature Materials, I have a News and Views piece about a really neat article by Sakanoue and Sirringhaus of the Cambridge University organic electronics group. My apologies to those without subscriptions - here's a brief summary:

Transport in organic semiconductors is generally poor when compared with that in inorganic semiconductors. Disorder and purity are major concerns, and electronic conduction (parametrized by the mobility of the charge carriers) very often is thermally activated, so that decreasing temperature leads to an exponential worsening of charge transport. This is in contrast to the situation in clean, nice materials like Si or GaAs, when lowering T leads to improving mobility, as scattering of carriers by thermal phonons is reduced. The Cambridge investigators have successfully made transistors from high quality spin-cast films of TIPS-pentacene, a small molecule organic semiconductor. These films actually do show improving conduction as T is reduced down to 140 K. At high source-drain electric fields and high carrier densities, transport becomes pretty temperature independent down to cryogenic temperatures.

Most importantly, however, the Cambridge group has also done "charge modulation spectroscopy" - optical spectroscopy measurements on the films as well as on the molecules in solution. By combining the optical measurements with the transport experiments, they are able to make rather strong statements about how localized the charge carriers are. They can thus rule out exotic physics or voltage-driven metal-insulator transitions as the origin of the good conduction regime.

This work shows the power of combining complementary techniques. Relying only on transport, we had made similar arguments here. However, the addition of the optical data greatly enhances the scientific arguments - what we had argued as "consistent" is totally nailed down here, thanks to the additional information from the spectra.

Deep thoughts....

Pondering introductory mechanics has made me think again about some foundational issues that I've wondered about in the past.  Mach's Principle is the idea, put forward by Ernst Mach, that the inertial properties of matter depend somehow on the distribution of matter at far away points in the universe.  The classic thought experiment toted out to highlight this idea is "Newton's bucket".  Imagine a bucket filled with water.  Start rotating the bucket (relative to the "fixed stars") about it's central axis of symmetry.  After transients damp away due to viscosity of the water, the water's surface will have assumed a parabolic shape.  In a (non-inertial) frame of reference that co-rotates with the bucket, an observer would say that the surface of the liquid is always locally normal to the vector sum of the gravitational force (which wants to pull the liquid down relative to the bucket) and the (fictitious, and present because we're working in a rotating frame) centrifugal force (which is directed radially outward from the rotation axis).  [In an inertial frame of reference, the water has arranged itself so that the gradient in hydrostatic forces provides the centripetal force needed to keep the water rotating about the axis at a constant radius.]  This rotating bucket business, by the way, is a great way to make parabolic mirrors for telescopes.

Mach was worried about what rotation really means here.  What if there were no "fixed stars"?  What if there were no other matter in the universe than the bucket and liquid?  Moreover, what if the bucket were "still", and we rotated the whole rest of the universe about the bucket?  Would that somehow pull the liquid into the parabolic shape?  This kind of thinking has been difficult to discuss mathematically, but was on Einstein's mind when he was coming up with general relativity.  What does acceleration mean in an otherwise empty universe?  There seems to be reason to think that what we see as inertial effects (e.g., the appearance of fictitious forces in rotating reference frames) has some deep connection with the distribution of matter in the far away universe.  This is very weird, because a central tenet of modern physics that physics is local (except in certain very well defined quantum mechanical problems).    

The thing that's been knawing away at the back of my mind when thinking about this is the following.  There is a big overall dipole moment in the cosmic microwave background.  That means, roughly speaking, that we are moving relative to the center-of-mass frame of reference of the matter of the universe.  We could imagine boosting our velocity just so as to null out the dipole contribution to the CMB; then we'd be in an inertial frame co-moving with the overall mass distribution of the universe.  If inertial properties are tied somehow to the overall mass distribution in the universe, then shouldn't the center-of-mass frame of reference of the universe somehow be special?  Some high energy theorist may tell me this is all trivial, but I'd like to have that conversation.   Ahh well.  It's fun that basic undergrad physics can still raise profound (at least to me) issues. 

Memories and The Mechanical Universe

As I get ready to teach honors mechanics to first-year undergrads, I have been scouting the web for various resources.  I ran across the complete series run of The Mechanical Universe (streaming for residents of the US and Canada), a great show that I remember watching on PBS occasionally when I was in high school.  It's based on first-year physics at Cal Tech, and each episode opens and closes with David Goodstein lecturing to a class in an auditorium.  It's very well done, and the computer animation was exceptionally good and informative, considering it was produced in the mid-1980s.  Thanks, Annenberg Foundation, for making this show available!  (Funny sequel of sorts:  I actually had the pleasure of meeting Prof. Goodstein in 2003, and for some irrational reason I was surprised that he didn't look exactly the same as he had in 1984....)

What I missed, plus book recommendations

I'm finally back from travel, just in time to immerse myself in prep for the upcoming semester. It's hard to believe that classes start in 10 days.

While I was away from blogging, it looks like I missed some fun posts. For example, the Japanese group that made the first major discovery of the iron pnictide superconductors has found that sake (or something in sake) boosts superconductivity in a related compound.  Chad Orzel did a pretty nice job posting about superconductivity as well, though I might do a different post later about this.  He also had a post prompted by a reader demanding to know why all statistical physics courses are lame.  (The answer is, of course, that the reader had never taking stat mech from me :-).  Ahem.  Perhaps not.)  Along related lines, Charles Day at Physics Today has started a blog, which I will add to the blogroll at right.  Glad to see that he leaps into discussing why he likes condensed matter physics.  I also missed the excitement about the proposed proof that P != NP.  The discussion online about the would-be proof is very impressive - it's always nice to see Fields medalists blogging, especially when they write as well as Terence Tao. 

One final remark for now.  I strongly recommend reading The Alchemy of Air and The Demon Under the Microscope.  These are terrific, interesting books, and they really do a great job of making science (in this case chemistry) as exciting as any novel.  Many thanks to Paul Chirik for recommending them to me.

A cool application and more travel

Apologies for the long break between posts.  It's been an incredibly hectic summer, and I'm about to go on a last big trip before the school year starts (and I get to teach honors intro mechanics to ~ 90 frosh - should be exciting, at least).

Before I go, I wanted to point out a very cool application of micromachining and computing power.  There are many consumer electronic devices now that contain within them a little 3-axis accelerometer made by micromachining techniques, like this one.  The basic gadget consists of a micromachined "test mass" (typically a block of Si) suspended on (silicon) springs.  When the whole device is accelerated, the test mass "lags behind" because of its inertia, just as you get pushed back into the seat of your car when the car accelerates forward.  Through (often) capacitive sensing, the displacement of the test mass can be transduced into a voltage that the chip then outputs.  If the displacement can be detected along three axes, voila, you have a 3-axis accelerometer.  This is the widget that tells the Nintendo Wii how you've been swinging the controller, and it tells iPhones and other similar toys how to orient their displays.  With added sophistication, it's also possible to make micromachined gyroscopes.  They aren't true gyros that spin.  Rather, they're micromachined resonators (like tuning forks of particular shapes), and rotation leads to Coriolis forces that twist the resonator in a way that can be detected.  (For Wii aficionados, that is how the "Wii Motion Plus" works.)  Then you can get angular accelerations, too.

What is the point of this discussion?  Well, some people at Microsoft Research had a great insight.  You can put a sensor like this on a digital camera.  If the acceleration data is logged when a picture is snapped, then it is possible to retroactively unblur photos (at least, pictures that were blurry because the camera was moving).  This is the slickest thing I've seen in a while!     

Why there has been no Carl Sagan or Brian Greene of condensed matter physics

It's impossible to be a condensed matter physicist that cares about outreach and scientific literacy, and not think about why condensed matter physics has taken such a back seat, comparatively, in the popularization of science.  It is easy to argue that condensed matter physics has had more direct impact on the daily lives of people living in modern, technological societies than any other branch of physics (we could get into an argument about the relative impacts of the transistor and the laser, but I think the CM folks would win).  So, how come there are specials and miniseries on PBS and Discovery Channel about string theory, the LHC, cosmology, and astrophysics with considerable regularity, people like Stephen Hawking, Brian Greene and Neil DeGrasse Tyson show up on The Daily Show, and the closest condensed matter gets to the public consciousness is a BBC special from several years ago about the Schon scandal?  Is it just that there is no charismatic, telegenic champion of the cause?  I think it's more than that.

First, there is the issue of profundity.  High energy physics makes an obvious play toward people's desire for answers to Big Questions.  What is mass?  What is everything made out of?  How many dimensions are there?  How did the Universe begin, and how will it end?  Likewise, astrophysics talks about the history of the entire Universe, the birth and death of stars, the origin of galaxies, and literally heaven-shaking events like gamma ray bursts.  Condensed matter physics has a much tougher sell.  In some ways, CM is the physics of the everyday - it's the reason water is wet, metals are shiny, diamond is transparent and sparkly, and the stuff in sand can be used to make quasimagical boxes that let me write text read all over the world.  Moreover, CM does look at profound issues (How does quantum mechanics cross over into apparently classical behavior?  How do large numbers of particles interacting via simple rules give rise to incredibly rich and sometimes amazingly precise emergent properties?), just ones that are not easy to state in a five word phrase.

Second, there is the problem of accessibility.  CM physics is in some sense an amalgam of quantum mechanics and statistical mechanics.  People do not have everyday experience with either (at least, the vast majority don't realize that they do).  It's very challenging to explain some of the very nonintuitive concepts that crop up in condensed matter to lay-people without either gross oversimplification or distortion.  There can be a lot of overhead that must be covered before it's clear why some CM questions really are interesting.  An awful lot of CM issues literally cannot be seen by the naked eye, including atoms.  Of course, the same can be said for quarks or colliding neutron stars - this is not an insurmountable problem.

Third, there is perceived relevance.  This is complementary to profundity.  People are naturally interested in Big Questions (the origins of the stars) even if the answers don't affect their daily lives.  People are also naturally interested in Relevant Questions - things that affect them directly.  For example, while I'm not that into meteorology, I do care quite a bit about whether Tropical Storm Bonnie is going to visit Houston next week.  Somehow, people just don't perceive CM physics as important to their daily existence - it's so ubiquitous that it's invisible.  


These issues greatly constrain any attempt to popularize CM physics....

Wow - look what I missed!

I did some travel + have a busy period at work, and what happens?  Scienceblogs implodes, and Chad Orzel laments something I've worried about for a long time:  the difficulty of explaining the importance (and basic coolness) of condensed matter physics to a general audience.  As for the former, serves 'em right for not inviting me to participate -- kidding!  There are enough talented people involved that they'll be fine, and as Dave Bacon points out in his linked post above, mixing up new networks of people interested in communicating science is probably a net good thing.  I do think it's a shame, though, that some interesting blogs have seemed to fade away (Incoherent Ponderer, Angry Physicist, you are missed.).  Regarding the second topic, I do want to point out a previous post I made about topological insulators (the strawman topic of Chad's post), and once I dig out from under work, I'll write more about why condensed matter is particularly difficult to popularize, and thoughts on how to get around those inherent challenges.

Symmetries and level-appropriate teaching

This fall I'm going to be teaching honors introductory mechanics to incoming undergraduates - basically the class that would-be physics majors take.  Typically when we first teach students mechanics, we start from the point of view of forces and Newton's laws, which certainly parallels the historical development of the subject and allows students to build some physical intuition.  Then, in a later class, we point out that the force-based approach to deriving the equations of motion is not really the modern way physicists think about things.  In the more advanced course, students are introduced to Lagrangians and Hamiltonians - basically the Action Principle, in which equations of motion are found via the methods of variational calculus.  The Hamiltonian mechanics approach (with action-angle variables) was the path pursued when developing quantum mechanics; and the Lagrangian approach generalizes very elegantly to field theories.  Indeed, one can make the very pretty argument that the Action Principle method does such a good job giving the classical equations of motion because it's what results when you start from the path integral formulation of quantum mechanics and take the classical limit.

A major insight presented in the upper division course is Noether's Theorem.  In a nutshell, the idea is that symmetries of the action (which is a time integral of the Lagrangian) imply conservation laws.  The most famous examples are:  (1) Time-translation invariance (the idea that the laws of physics governing the Lagrangian do not change if we shift all of our time parameters by some amount) implies energy conservation.  (2) Spatial translation invariance (the laws of physics do not change if we shift our apparatus two feet to the left) implies conservation of momentum.  (3) Rotational invariance (the laws of physics are isotropic in direction) implies conservation of angular momentum.  These classical physics results are deep and profound, and they have elegant connections to operators in quantum mechanics.

So, here's a question for you physics education gurus out there.  Does anyone know a way of showing (2) or (3) above from a Newton's law direction, as opposed to Noether's theorem and Lagrangians?  I plan to point out the connection between symmetry and conservation laws in passing regardless, but I was wondering if anyone out there had come up with a clever argument about this.  I could comb back issues of AJP, but asking my readers may be easier.  

Science and communication

I've tended to stay away lately from the arguments about scientists-as-communicators that seem to flare up periodically.  This recent editorial by Chris Mooney, about how scientists who actively listen to the general public do a better job of communicating and affecting policy, was simultaneously informative and yet blindingly obvious in some ways.   (Here's a shock:  making it clear to an audience that you're listening to their concerns and considering them seriously gets better results than talking down to them or ignoring them dismissively.)  Chad Orzel followed up with a very well-written (as usual) post about scientists and communication skills that is, like Mooney's, really common sense in the end.  (Here's another shock:  not everyone is Carl Sagan or Neil DeGrasse Tyson, and sometimes our scientific and academic institutions do not value public communication as much as they do utter dedication to scientific research.)  

Many people in the general public do have some baseline interest in science and engineering issues, even if they don't label them as such.  Lots of people watch Mythbusters.  Lots of people read about nutritional information or medical research quasiobsessively.  Many people do care about space, and climate, and the environment, and energy, and electronics, and so forth, even if those concerns are not the top of their list all the time.  There is a thirst for information, and this is all good for society.  I do want to point out one additional issue that seems to get neglected to some degree in this discussion, however.  There are people out there who either don't know what they're talking about (the MD who somehow has a column on the Huffington Post who periodically spouts off utter pseudoscientific nonsense), or actively are pushing misleading or inaccurate information (members of the TX Board of Education who grossly mischaracterize the nature of science).  Scientists can do as much as possible to "market" ourselves and communicate our enthusiasm and willingness to have an honest and open dialog about scientific issues.  However, when anti- or pseudo-science can command at least as big a bully pulpit, and when education and time make it difficult for the average person to discriminate between gold and dross, it's an up hill struggle.  Add in to this the mainstream media's love of controversy ("Some say that the earth goes around the sun, but others disagree.  Let's look at both sides of this issue!"), and the situation can get downright depressing.

Edit:  I realize I left out two other confounding factors:  (1) Scientists who end up distorting actual science beyond recognition in a misguided attempt at popularization (Michio Kaku is an example); and (2) Scientists who are so aggressively arrogant and obnoxious that they only hurt their own cause.

Helium

Helium consumption is still a problem.  A real problem.

Lies, damned lies, and lying statistics

This is not physics, but it is interesting.  According to fivethirtyeight.com (a blog run by Nate Silver that specializes in statistical analysis of political polls), a major polling firm is being sued for fraud by liberal blog DailyKos.  The grounds for the suit:  this report, which details a number of statistical anomalies suggesting that the polling firm was either cooking their numbers, or outright making them up.  Bonus:  one of the guys who did this analysis is Mike Weissman, a condensed matter physicist who retired from UIUC.  Weissman is an expert on noise measurements, author of this highly cited review article.  He's a guy who knows statistics.

Excitons

A reader emailed me and asked if I had done a posting about excitons.  Looking back, I see that I haven't, so here is an attempt to rectify the situation.  As I've written previously, condensed matter physicists are fond of giving specific names to excitations of solid state systems when those excitations have well-defined quantum numbers (and are in that sense "particle-like").  An exciton comprises an electron and a "hole" bound together by the attractive Coulomb interaction (since an electron has charge -e and a hole has charge +e).  It is the (negative!) binding energy of the exciton that makes it different than a generic "electron-hole" excitation in which an electron is kicked out of an occupied state (leaving behind a hole) and into a previously empty state.

Excitons can exhibit very rich physics.  In a 3d crystalline system, excitons can be very analogous to hydrogen-like atoms, or more accurately, positronium, the bound state of an electron and a positron.  One can think of the electron and hole as having center-of-mass momentum, and having an exciton wavefunction that describes the relative displacement of the electron and hole, which would look like a hydrogenic orbital (s-like, p-like, etc.).  Like positronium, the electron and hole can annihilate each other and emit a photon.  Several important features crop up, however, due to the fact that the exciton exists within a solid host.  For example, one cannot ignore the screening of the electron-hole Coulomb interaction by the surrounding host.  One approximation commonly shown in textbooks is to treat this screening by using the bulk (relative) dielectric constant of the host material when solving for the exciton wavefunctions.  As a result, the exciton is much larger, spatially, than positronium - say 5 nm in extent rather than 0.5 nm.  (Note that this had better be true!  Otherwise the assumption that the bulk material can screen the interaction would not be internally consistent....)  Large excitons like this are called Wannier excitons.  In contrast, if the screening is relatively weak, the exciton can be small compared to a unit cell of the crystal.  Such a small exciton is called a Frenkel exciton.

Furthermore, the electron and hole parts of the exciton wavefunction are really "built" out of the Bloch wave electronic states of the solid.  In a semiconductor, the hole states "live" in the valence band, while the electron states live in the conduction band.  Hole states often exhibit stronger spin-orbit effects, and as a result, confinement can affect the exciton energy levels quite strongly.

Excitons may be produced by the absorption of light of appropriate energy, and therefore are of intense interest in photovoltaic research.  The comparatively strong screening in traditional semiconductors that gives large exciton spatial sizes also leads to modified binding energies.  Wannier exciton binding energies in materials like silicon can be on the order of 10 meV (as opposed to electron volts for positronium!).  In materials with weaker screening (with Frenkel-like excitons), the exciton binding energy can be higher, more like hundreds of meV.  These binding energies are of critical importance.  In a silicon pn junction, for example, the built-in electric field due to the junction is large enough to rip apart any light-produced excitons - that's how charge separation happens in a silicon solar cell.  In organic semiconductors, in contrast, the binding energies are stronger, and built-in fields are too weak to take apart excitons.  Thus, there are no homojunction organic solar cells, and this is one of a number of reasons why organic photovoltaics is challenging. 

Travel + interesting review article

I'm traveling this week, so blogging is thin. I did want to point out an interesting review article from the arviv: arxiv:1006.3736, Force-detected nuclear magnetic resonance: Recent advances and future challenges, Poggio and Degen. This article take a look at the progress over the years in this micro mechanical approach to incredibly sensitive spin measurements.

Kavli Prizes for Nanoscience

This post is a bit late, but real life has been busy recently. The Kavli Foundation recently announced their 2010 Kavli Prize for Nanoscience, which they awarded to Don Eigler and Nadrian Seeman, for "their development of unprecedented methods to control matter on the nanoscale". As in their previous 2008 award to Louis Brus and Sumio Iijima, this prize is richly deserved by the awardees.

Don Eigler ran the scanning tunneling microscopy (STM) research group at IBM Almaden, where he and co-workers constructed incredibly stable STMs that functioned in ultrahigh vacuum and at low temperatures. With the resulting stability and surface cleanliness, Eigler et al. were able to demonstrate manipulation of matter on the atomic scale, giving us several of the most iconic images in nanoscience. Eigler's intellectual progeny have gone on to many faculty positions and trained generations of practitioners in the art and science of working at the atomic scale.

Nadrian Seeman had the foresight to realize what an incredible toolkit nature has provided for us in the form of DNA. While most people are familiar with double-helix structure of DNA, Seeman and co-workers developed techniques to make nanoscale DNA building blocks that can assemble into complex, three-dimensional structures. This is DNA as a construction tool rather than DNA as a carrier of genetic information. Who knows what the end result will be of this capability - I have been very impressed by some related work.

Nanomechanical mass sensing in fluid

The idea of using mechanical resonators as mass sensors is an old one, and one that may be explained to a first-year physics undergrad. The (angular) frequency of a mass on a simple Hooke's Law spring is (k/m)^0.5, where k is the spring constant. Change the mass, and the resonant frequency changes. With the development of micromachining techniques, there has been a great deal of interest in using tiny, high frequency resonators (e.g., doubly clamped Si beams) as mass sensors. One can have a metal wire along the resonator, and in the presence of a dc magnetic field perpendicular to the wire, an ac current may be used to apply a driving force to the structure. This is the same principle used to move the filament back and forth in those cheesy old flicker light bulbs. By measuring the induced voltage along the wire as it moves through the static magnetic field, the resonator's motion may be detected. Michael Roukes' group at Cal Tech been enthusiastic about the possibility of achieving sensitivities high enough to resolve a single atomic mass unit (1.66 x 10^-27 kg).

There are many situations where one would love to have great mass detection capabilities in a liquid environment (e.g., to detect the binding of some cancer marker). The problem is, if you immerse a mechanical resonator in a liquid, viscous damping completely kills your sensitivity by damping the resonance. An old acquaintance of mine from graduate school, Scott Manalis at MIT, has come up with a solution to this problem. Don't put the resonator inside liquid; rather, put liquid inside the resonator. His group has been making mechanical resonators with micro (and now nano)fluidic flow channels inside them. In their latest work, they report a sensitivity of 30 attograms. I think this is very elegant, and a tour de force fabrication exercise.

Demagnetization cooling

I've been meaning for some time to write a post about demagnetization cooling, a technique that is readily explained in an undergrad stat mech class, but has to be seen to be believed.  I was finally inspired to write this post by seeing this preprint.  Here's the basic idea.  Start with an ensemble of magnetic moments in what we will call a "demag stage".  The sample of interest will be thermally connected to this demag stage.  When I worked on this stuff in grad school, we used the nuclear magnetic moments of copper nuclei, but it's also possible to use electron magnetic moments in a paramagnetic salt of some kind.  Anyway, apply a large magnetic field to these magnetic moments (spins) while attached to a refrigeration source of some kind. It's energetically favorable for the moments to align with the applied field. When they flip to align, the energy that is released is carried away by the refrigerator.  Likewise, in the case of a metal like copper, the ramping up of the magnetic field can generate heat via eddy currents; that heat is also carried away by the refrigerator.  Now, once the spins are basically aligned, unhook the thermal connection between the demag stage and the refrigerator, and gently lower the applied magnetic field.  What happens?

First, the formalistic explanation.  Basic statistical physics tells us that the entropy of an ensemble of magnetic moments like those in our demag stage is only a function of the ratio B/T, where B is the applied magnetic field and T is the temperature of the moments.  If we are gentle in how we lower B, so that the entropy remains constant, that implies that lowering B by a factor of two also lowers T by a factor of two.  When I first did this as a grad student, it seemed like magic.  We thermally isolated the demag stage (plus sample), and I used an ancient HP calculator to tell a power supply to ramp down the current in a superconducting magnet.  Voila - like magic, the temperature (as inferred via the capacitance of a special pressure transducer looking at a mixture of liquid and solid ³He) dropped like a stone, linear in B.  Amazing, and no moving parts!  

So, physically, what's really going on, and what are the limitations?  Well, the right way to think about the ensemble of magnetic moments is as an entropic "sink" of energy.  Equilibrium statistical physics is based on the idea that all microscopic states of a system that have the same total energy are equally likely.  When you create an ensemble of 10²³ magnetic moments all pointed in the same direction (that is, with an aligned population much greater than what one would expect in equilibrium based on the new value of B), the most likely place for thermal energy in your system to go is into flipping those spins, to try and bring the aligned population down and back into the new equilibrium.  That means that heat will flow out of your sample and out of, e.g., the lattice vibrations of the demag stage, and into flipping those spins.  The fortuitous thing is that for reasonable numbers of moments (based on volumes of material) and accessible initial values of B and T, you can get lots of cooling.  This is the way to cool kilogram quantities of copper down to tens of microKelvin, starting from a few milliKelvin.  It's a way to cool a magnetic salt (and attached sample) down from 4.2 K to below 100 mK, with no messy dilution refrigerator, and people sell such gadgets.  

There are practical limitations to this, of course.  For example, there is no point in reducing the external B below the value of the effective internal magnetic field due to spin-spin interactions or impurities.  Also, when demag-ed, the system is a closed box with a finite (though initially large) heat capacity.  Any measurement done on an attached sample will dump some heat into the stage, even if only through stray heat leaks from the rest of the world, limiting the amount of time the stage and sample remain cold before needing another demag cycle.  Finally, and most relevant to the preprint linked above, there are real issues with establishing thermal equilibrium.  For example, it is not hard to get the nuclei of copper to have a much lower effective temperature than the conduction electrons, with an effective equilibration time longer than the demag-ed spin system can be kept cold.  In other words, while the nuclei can get very cold for a while, the electrons are never able to reach similar temperatures.  Still, the whole concept of cooling through demagnetization is very interesting, and really brought home to me that all the abstract concepts I'd learned about entropy and spins had real consequences.   

Workshop: Negotiating the Ideal Faculty Position

For the last few years I have been involved with Rice's ADVANCE program, a NSF-supported initiative designed particularly to increase the number of women faculty members in the sciences and engineering.  This FSP recent post reminded me that now is the right time to advertise ADVANCE's upcoming workshop on negotiating the ideal faculty position, and this blog is one way to reach a wide audience.  Potential participants need to apply, since space is limited.  (Last year there were 1100 applications and 65 slots.)  View this as a way to practice a job talk in a friendly, constructively critical environment, and to discuss issues that come up in the faculty job hunt (e.g., how the process works; picking letter-writers; lab space + startup packages).  This is a way to talk to knowledgeable people in yours and related areas who are not your mentors or direct potential employers.    

Tidbits.

• While modern communications tools are definitely improving, there is still no substitute for actually sitting down with collaborators at a table with pen and paper, and hashing things out face-to-face.   I just returned from a quick trip to talk with some theorist colleagues, and it was a great way to get a lot accomplished in a relatively short period of time.  Much higher bandwidth than repeated emails.
• If you're ever invited to write a review article, and you have any concerns about the quality of the journal or the publisher, don't ignore your instincts.  My postdoc and I just went through a painful experience along these lines - in the end, everything's going to be fine (with another publisher!), but the original publisher (I'll name names some other time) was amazingly incompetent.  You'd think, for example, that a journal editor would have an email system that actually accepts attachments, particularly if their web-based system is utterly fubar.
• A follow-up to my recent post about IR CCDs....  Anyone out there have experience with the MOSIR-950?  It's actually a Si CCD with a special front end that makes it sensitive from 950nm out to around 1700 nm.
• I'm very tempted to buy this.  (If you have never seen the tv show Lost, you won't get this.)

This will be a big new story.

Craig Venter's company appears to have succeeded in creating a synthetic genome and getting it into an emptied-out (prokaryotic bacterial) cell, essentially changing the cell into a new species. This is going to be huge. Of course, we still don't actually understand what everything in that custom genome does, exactly - much of it is copied from another bacterium species. Still, it's an amazing achievement that one can design (on a computer) a DNA sequence, stitch it together via various methods, and get a cell to "run" that software.

IR CCD arrays for spectroscopy?

The charge-coupled device, or CCD, was the gadget behind part of this past year's Nobel prize in physics.  Far and away, the most common CCDs out there are based on silicon, and these devices are highly efficient from the visible out to the near-infrared, with efficiency really taking a major hit at wavelengths longer than about 1100 nm.  One advantage of CCDs is that generally their total efficiency is high:  an incident photon stands a good chance of producing some charge on a pixel, and that charge can be collected well, so that getting a "count" on a particular pixel requires only a couple of photons.  It turns out that one can also get CCDs based on InGaAs, a semiconductor with a smaller band gap than Si, and therefore sensitive to longer wavelengths, typically from around 950 nm out to 1700 nm or so.  I have been thinking about trying to get such a gadget for a few reasons, Raman spectroscopy in particular, and I would welcome reader recommendations.  For our application we really would like something with CCD-like sensitivity (as opposed to a linear array of photodiodes, which is considerably cheaper, but requires on the order of 100 photons to produce a single "count").  Feedback would be greatly appreciated.  I know that Princeton Instruments sells one gadget (though really for imaging rather than spectroscopy), and Newport appears (from press releases)  to offer something with more pixels, though it doesn't show up on their website....

Scale and perspective II

The title of this post harkens back to a previous example of stellar corporate governance.  Today the CEO of BP made the statement that "The Gulf of Mexico is a very big ocean. The amount of volume of oil and dispersant we are putting into it is tiny in relation to the total water volume". While that is literally true, as a physicist I have to ask, is that the right metric? I mean, are we worried about the total fraction of Gulf of Mexico that is oil? No, because everyone knows that the relevant point of comparison is not the total volume of water, but the point at which the oil content is having catastrophic effects on the environment.  We can gain some perspective by comparing with other oil spills. According to experts who have viewed the (long delayed by BP) video of the leak, the flow rate of oil is somewhere around 70000 barrels a day, or about 1 Exxon Valdez disaster (I think everyone sane agrees that it was a real mess) every four days. This has been going on for three weeks. Arguing that "the ocean is really big so this isn't that much of a problem" is just wrong.

update:  It's increasingly clear that BP is far more worried about their liability than about actually fixing the problem.  Check out this quote from the NY Times:  

BP has resisted entreaties from scientists that they be allowed to use sophisticated instruments at the ocean floor that would give a far more accurate picture of how much oil is really gushing from the well.

“The answer is no to that,” a BP spokesman, Tom Mueller, said on Saturday. “We’re not going to take any extra efforts now to calculate flow there at this point. It’s not relevant to the response effort, and it might even detract from the response effort.” 

Right, because good engineering solutions have nothing at all to do with accurately understanding the problem you're trying to solve. Idiots.

What do fancy research tools really cost at a university?

Over the years, I've become convinced that there are lies, damned lies, and cost accounting.  What I mean by this is that "true costs" for various items in a business or at a university (a type of nonprofit business, after all) are sometimes allocated in whatever way is necessary to bolster a particular argument at hand.  If those making an argument want something to look like a bargain, no problem, there's a way to do the accounting for that.  If those making an argument want to make something look so expensive that it's economically unattractive, no problem, there's a way to do that, too.  I remember as a postdoc when the part of Lucent Technologies that dealt with real estate argued (apparently successfully) that they should get rid of the simple general stockroom because somehow having the square footage allocated to that use was losing money.  So they shut down the stockroom, and had a couple of hundred PhD scientists and engineers spending their (expensive) time ordering 4-40 screws from McMaster Carr online or over the phone instead of just walking upstairs and grabbing some.

Let's take an electron microscope as a test case.  Suppose a university or company buys an SEM for $350,000 (for the sake of round numbers).  How much should they charge, fairly, for its use?  Let's assume that this is a shared tool and not just sitting in one person's lab.  This microscope and associated hardware take up something like 100 ft² of floor space.  The microscope also needs electricity (say 1 kW) and cooling water.  Now, a university is unlikely to charge a department or faculty member "rent" on the floorspace, but a large company may decide to "tax" a business unit for space at some rate.  The electricity and cooling water are likely part of a university's or business's "overhead".  Overhead charges are assessed when it's difficult to trace a particular designated responsible source for certain kinds of costs-of-doing-business.  For example, the overhead rate at my institution is 52.5%.  That means that for every $1 of direct research cost (say a grad student's salary), the university charges my research account (and therefore the funding agency) $1.525.  That "extra" $0.525 goes to cover the university's costs in, e.g., keeping the lights on in my lab, the air handlers running for my fume hoods, and the road paved outside my building.

If the university or business wants to maintain the electron microscope, they probably want to buy an annual service contract for, say, $25K.  Now, in the absence of a staff person to run the system, you might think that a reasonable user fee would then be $25K divided by the number of hours the system is used (say 2000 hours per year).  Not so fast - you have to charge overhead.  Moreover, the university or business may decide to depreciate the SEM.  That means that they may have an interest in replacing the SEM eventually, so they are allowed to tack on a depreciation cost, too.  For our example, a typical depreciation schedule would be seven years, so in addition to the actual maintenance cost, they would tack on, in this case, $50K per year.  There are major federal rules about depreciation.  For example, you can't buy something with a federal grant (e.g., a NSF "Instrumentation for Materials Research" grant) and then also depreciate it - that would be like double-billing the government, and that's not allowed.

If the university or business does have some fraction of a staff person responsible for the instrument, it may be fair (depending on the discussion) to consider a fraction of that person's salary (plus fringe benefits [e.g. health insurance] plus overhead) as a cost to be recovered as well.  

So, the next time you are paying $30/hour for access to an electron microscope, and you're wondering where on earth that figure came from, now you have at least some idea.   You can also see how administrations can sometimes argue that they "lose money" on research - they cannot always recover the costs that they put into things (e.g., the actual overhead income may not cover the utility costs), and sometimes they choose not to  (e.g., by not charging rent for space).  This is all stuff about which I was blissfully ignorant back in my student days.

Amateur economics

Perhaps a more economically savvy and inclined reader could comment, but is it fair to say that some fraction of the recent decline of the US stock markets (excepting dramatic short-term spikes like the one this afternoon between 14:30 and 15:00 EDT) is not a "real" decline, but a reflection of the increased value of the dollar relative to the euro?  From what I can see, the euro has fallen about 8.5% against the dollar since mid March, and the US financial markets are actually down about 4% (mostly in the last week or two) over the same time period.  Naively, if dollars are worth more, one should see "deflation" on the dollar-denominated stock markets, I would guess....

Manageable-sized, LaTeX-happy .eps figures

In much of physics, LaTeX is the standard for typesetting scholarly work, including research papers and theses.  Traditionally, when working with figures in LaTeX documents, the preferred format is encapsulated postscript (.eps).  There are any number of ways to produce figures in .eps format, but some people seem to have recurring problems doing this with economical file sizes.  For example, today b/c I am on a committee reviewing doctoral theses for a departmental award, I had to download a thesis that was around 50 MB in size, entirely because the figures were unnecessarily huge.  Over the years I've come up with a few different ways of making small, good (in the sense that they render nicely and LaTeX likes them) .eps files.  Here's a quick how-to.  Mostly this is from the point of view of a windows user, by the way.  (I have both PCs and Macs, fwiw).

On a pc, I cannot recommend gsview and ghostscript strongly enough - they're essential tools.  There are linux versions of these as well, of course.  In general, if a postscript file opens cleanly in gsview, you're going to be fine w/ LaTeX compatibility.  gsview is also perfect for redefining bounding boxes, grabbing individual pages from a multipage postscript file, converting .ps into .eps, and other related tasks.    Another set of tools worth having is ImageMagick.  Very helpful for converting between formats.

Option 1:  Use an application that can natively export nice .eps (that is, vector format rather than automatically using bitmapping) with no attached preview.  For example, Origin can do this, as can Matlab or gnuplot if properly set up.  On the Mac, inkscape is a great vector drawing and editing piece of software.  Adobe (who invented postscript, as far as I know) has programs like Illustrator and Photoshop that can do this.  The former seems better at producing economical output.  The latter, without careful intervention, produces bloated, bitmapped, preview-laden junk.  

Option 2:  Use a generic postscript printer driver that can print to a file in .eps format.  Adobe has one for pc that lives here.  Using this, you can do things like use powerpoint as a cheesy compositing tool to draw something or put together multiple images, and then print a particular slide to an .eps file.  The result will be LaTeX-friendly, but not necessarily economical in file size.

Option 3:  Produce an image in a different, nicely economical format and then use a "wrapper" to produce a .eps file.  Here's one example.  Suppose I have a huge bitmap file.  I can use my favorite software (imagemagick, or even MS Paint or powerpoint) to save the image as .jpg or .png.  Then I can use jpeg2ps (in the former case) or bmeps (in either case) to produce a .eps file that is only slightly larger than the originating image.  

This last option provides a way out of the annoying situation of having a huge (say 10 MB) .eps file produced by some other application (like Matlab).  You can open the offending .eps file in gsview, and try to copy the on-screen image (zoomed as needed) to the buffer (that is, click on the displayed image and hit ctl-c on a windows pc).  Then paste the buffer into either paint or powerpoint, and export it as a .png file (nice format - no compression!).  Once you have the .png file, run bmeps to produce a new .eps, and you're all set.  Your 10 MB old .eps file can end up as a 70 kB new .eps file.  This wrapper strategy is also the one recommended by the arxiv folks.

This is by no means exhaustive, but if it saves anyone the pain of having monster .eps files that warp the final documents, then it was worth posting.  (I suspect that someone will comment about how things like this are one reason that journals are drifting toward MS Word.  Word carries with it many, many other problems for scientific writing, in my opinion, but I'd rather not get into a debate on the subject.)    

The unexpected - the fun part of experimental science

There's an old quote from Isaac Asimov that is very true:  "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny...'."  In my group, we recently had an experience that supports this, and it highlights what I think is some of the most fun you can have as an experimental scientist:  trying to use the tools at your disposal to learn as much as you can about what's behind some unexpected and surprising phenomenon.

The story starts out several years ago.  We'd been having some nice success making single-molecule electronic junctions and using them as model systems to study a particular piece of physics, the Kondo effect.  Our theorist colleagues pointed out that these molecular devices, unlike typical semiconductor quantum dots, might give us an opportunity to study a particularly rich and interesting piece of physics called a quantum phase transition, because molecular devices are easier to attach to ferromagnetic electrodes.  The idea, not directly germane to this story, is that an unpaired electron on the molecule is torn between two competing "baths" of excitations.  On the one hand, the unpaired electron can undergo Kondo processes with the conduction electrons of the electrodes.  On the other hand, spin waves in the ferromagnetic electrodes can also talk to the unpaired electron.  By varying a gate voltage appropriately, the hope was to tune from one limit (the Kondo regime) into the other (expected to be a more exotic "non-Fermi liquid" state).  

Anyway, for various reasons, it became clear that working with palladium electrodes might be a good place to start.  Palladium is almost ferromagnetic.  That means that it has long-lived spinwave-like excitations (paramagnons).  At the same time, it's chemically friendlier (less prone to forming magnetically complicated oxides) than common ferromagnetic metals like iron, nickel, or cobalt.  So, step zero of this project would be to make some bare palladium tunnel junctions (just two pointy palladium electrodes, without any molecule bridging them) and make sure that they're simple and boring, as expected.  After all, we'd looked at literally thousands of gold tunnel junctions like this, and if properly made (so that you don't have extra metal nanoparticles around), they are dull as dirt:  current-voltage (I-V) curves that are nearly linear and essentially temperature-independent.

Surprise!  My postdoc, Gavin Scott, found that Pd tunnel junctions are very much not boring.  While they look dull at, say, 10 K, if they are cooled down to lower temperatures, all kinds of sharp features appear in their differential conductance (dI/dV as a function of V).  The features appear at voltages symmetric around V = 0, and they evolve with temperature in a very interesting way.  In fact, if you look at the temperature dependence of those features, it looks very much like what you see for the temperature dependence of the order parameter in a "mean field" phase transition.  We spent months trying various things, turning all the easily turned "knobs" like temperature, magnetic field, gate voltage, etc.  One striking trend is the observation that, looking at all of our devices on one set of axes, the voltages where the conductance features appear extrapolate to zero when the conductance of the junction approaches e²/h.  In other words, when the metal tips touch, the whole effect goes away.   

It's been science-as-puzzle-solving, trying to figure out what could be going on here.  We came up with many possible explanations, and tried to come up with ways to test the possibilities, eliminating the ones that didn't fit.   For example, the data look (qualitatively) rather like superconductor tunnel junctions, but the quantitative values (specifically, the relationship between the voltage scale and the temperature scale) are far, far away from numbers that would make sense for a superconductor.  In the end, we think that the most likely physics ingredient is the onset of magnetic order at the tips, though that's not a perfect explanation by any means.  It is clear, though, that Pd is special - other metals (Au, Ni, Pt) just don't seem to show this.  The paper is out here (email me if you want a copy).  Hopefully others will get interested.  Suggestions are always appreciated.  In the meantime, it's a good example of how sometimes systems that you think are dull can surprise you, and how science is supposed to work.

Nanotechnology-enabled sensing

Last May, I participated in a very well run and informative workshop on nanotechnology for sensing purposes.  The workshop was run by the National Nanotechnology Initiative (and associated Federal agencies).  The report that we ended up producing is now available for download.  Here is a direct link to the pdf.  Other NNI reports and information may be found here.

A word of caution re: departmental rankings

The US News rankings of US graduate programs are out again, and I've heard a fair bit of discussion about them.  My department, for example, went up slightly in the rankings, while the chemistry department here slipped a few spots.  I want to point out something that US News makes no effort to broadcast:  The US News graduate rankings are a popularity contest.  What I mean is, the US News rankings are the result of an opinion survey taken of department chairs, not the result of actual quantitative metrics like publication rates, citation rates, research funding, major awards, graduation rates, or the like.  Essentially the rankings give you a snapshot of the perception of the community of department chairs in a discipline, not an actual real ranking of some defined quality.  This has some consequences.  For example, perceptions are very hard to change, so it's unlikely that there will be lots of movement on these rankings unless there are exceptional circumstances (e.g., a particular department wins a couple of Nobel prizes out of the blue).  It is distressing to me to see how much importance some people (prospective students on the one hand, administrators on the other) place on rankings that measure reputation rather than something truly quantitative.

The NRC, by contrast, does survey real data like those mentioned above.  Unfortunately, they seem to be in a mode of continuously delaying the release of their "decadal survey".  Anyone have any more insight into why that is taking so long? 

An ethical dilemma

Here is a scientific ethical dilemma that came up in conversation recently.  What do you do if you get a positive referee report, but it's clear from the comments that the referee completely misunderstood your manuscript, or basically had no clue at all?  Do you point this out to the editors?  I'm actually a bit surprised that this never seems to come up, given how often people complain about the complementary case of a clueless referee that bashes a paper because of a lack of understanding....

This week in cond-mat

Three interesting pieces of physics (among many) from the arxiv this week:

arxiv:1004.0546 - Mak et al., Atomically thin MoS₂:  A new direct-gap semiconductor
The electronic structure of ordinary semiconductors is usually presented, in textbooks, in the context of band theory, neglecting electron-electron interactions and assuming infinitely large crystals.  In the case of the layered dichalcogenide, MoS₂, the structure of the bulk is that of an indirect gap semiconductor.  The highest filled (single-particle) electronic states (at the top of the valence band) are labeled by wavevectors k that are near zero.  That is, the wavelike electronic states have very long wavelengths.  The (energetically) lowest empty states happen to have k values that are away from zero.  That means, for example, that the energetically cheapest way to kick an electron from the valence into the conduction band requires enough momentum (probably made up via a phonon) to make up the difference in k vectors.  The authors of this paper have observed something interesting:  as this layered material is made thinner and thinner, down toward the atomic limit, the band structure changes.  The finite-k conduction states go up in energy relative to the k = 0 conduction band states, so that the material becomes instead a direct gap system.  Neat stuff.


arxiv:1004.1233 - Cabrera et al., Oblique propagation of electrons in crystals of germanium and silicon at sub-Kelvin temperature in low electric fields
This paper also involves the concept of indirect-gap semiconductors.  Silicon and germanium are both indirect gap systems, so that the lowest energy electronic states in the conduction band live in "valleys" far from k = 0.  That means that if processes that allow intervalley scattering (such as inelastic interactions w/ phonons) are turned off, the motion of conduction electrons in real space has to reflect these valleys.  Of course, the structure of the valence band in k space is quite different than the conduction band.  That means that holes will propagate differently in real space under the same circumstances.  These folks, part of the CDMS collaboration who have been using cryogenic Si and Ge crystals as dark matter detectors, have completed a clean study of this.  Great stuff.

arxiv:1004.1202 - Cheng and Robbins, Defining contact at the atomic scale
As my students and I have encountered, particularly in a paper currently out for review, sometimes it can be very challenging to define what we mean when we say two pieces of material are touching at the one atom or two atom level, or what me mean when we say they're not touching, but instead are a certain distance apart.  These authors take a hard look at this topic in terms of the forces between the two sides.

Medical physics FTW

Blogging will be on hold for a few days, as I'm going to have an appendectomy very soon. Thank you, medical physicists, for developing the wicked cool CT machine (and blech-tasting contrast agents) key to confirming my diagnosis! Thanks, too, to the scientists and engineers that gave us wireless Internet.....

UPDATE! I had the surgery this morning and it seemed to go smoothly. Barring weirdness, I should be back up to some kind of speed in a couple of more days. Thanks for the sentiments!

Superconductivity induced in a ferromagnet

Four years ago, this paper caught my attention.  The authors had made a structure with superconducting NbTiN contacts on top of a CrO₂ film, with the intent of studying how superconductivity leaks into the chromium dioxide.  The "leakage" of superconductivity into a non-superconducting metal is called the proximity effect.  In a normal metal, the proximity effect extends over a spatial scale comparable to the coherence length, the distance that the electrons can travel before their quantum mechanical phases become scrambled due to inelastic processes (such as electron-electron scattering, or spin-flip scattering from magnetic impurities).  The coherence length in a normal metal can be quite long at low temperatures - say a micron in a clean normal metal at 1 K.  

Now, CrO₂ is not a normal metal.  Rather, it is a half-metal, an extreme limit of an itinerant ferromagnet, where all of the mobile charge carriers have the same spin polarization.  This is important, because ordinary ("s-wave") superconducting correlations rely on pairing up electrons with opposite spins and momenta.  If only one spin polarization is allowed, that should preclude any s-wave superconductivity.  Practically speaking, in a typical ferromagnet with some magnetic exchange characterized by an exchange energy U, one can define an exchange length (in a diffusive material, given by sqrt(\hbar D/U), where D is the diffusion constant for the electrons) over which these correlations should die.  For a strong ferromagnet, one finds that the exchange length is very short - a few nanometers or less.  Knowing this, one would not expect to see any proximity induced superconductivity in a ferromagnet over longer distances.  That's why this paper was surprising - the authors did see evidence of long-range (hundreds of nm) superconductivity in the ferromagnetic oxide.  This implies some kind of unusual superconductivity in the ferromagnet - either p-wave pairing (when each pair of electrons in the superconducting material has one quantum of orbital angular momentum), or some more exotic state ("odd-frequency pairing", for the experts).

Several years passed, and no one reproduced this result.  Until now.  The authors of this new paper see the same sort of thing, and they try to explain in detail why this has been so hard to reproduce.  The short version:  CrO₂ is a pain to work with.

Interestingly, there have been other signs of similar effects within the last year.  For example, the Birge group at Michigan State has reported long-ranged proximity superconductivity induced in cobalt layers, though careful engineering of the contacts was required.  Likewise, a Penn State collaboration has seen proximity superconductivity in Co nanowires hundreds of nm long.  It's nice to see so much progress in this area lately.

Slightly missing the point

I am, of course, pleased that Nature Materials ran a news item about our recent paper.  However, they appear to have missed the point a bit.  What we were trying to point out was that plotting data in scaled coordinates (current normalized by temperature to some power vs. voltage normalized by temperature, in this case) can be misleading.  In this particular case, plotting temperature-independent data in this way on a log-log plot can make it look like the data all collapse onto some universal curve (with deep implications).  In fact, the data themselves aren't doing any such thing - the apparent collapse is due to a flawed plotting procedure.  Ross McKenzie got this point immediately.  Ahh well.  Bottom line: be very careful when plotting "scaled" quantities, to make sure that you're not biasing yourself toward a particular conclusion.

Three years of hindsight

Back in February, 2007, I mentioned that I thought that making and measuring nanostructures directly out of strongly correlated materials was a promising research direction.  I still think so, in part because of the experiences my lab and others have had in the mean time.  As I've mentioned before, strongly correlated materials are those where we can no longer get away with our (often miraculously good) simple single-particle description of electronic structure that ignores the electron-electron interaction.  Examples of correlated materials include the high temperature superconductors and various transition metal oxides.  In the high T_c case, single-particle band structure says that the undoped parent compound should be a metal, when it's found instead to be an antiferromagnetic insulator.  Strongly correlated materials often display a rich variety of interesting electronic states as well as phase transitions between them.

Why nanostructures?  Three main reasons.  First, when some strongly correlated materials go through electronic (and structural, sometimes) transitions, they display inhomogeneities.  For example, when vanadium dioxide goes through its metal-insulator transition (from below) near 341 K, metallic domains nucleate and grow, eventually encompassing the whole material.  Nanostructures can allow you to access such materials on the scale of individual domains, as was done here and here in VO₂, and here in a colossal magnetoresistance oxide.  

Second, nanospaced electrodes allow you to use electric field as an interesting perturbation, and to tell the difference between effects driven by voltage (or energy) and those actually driven by field.  For example, in this paper (very influential and highly cited), the authors found that a charge-ordered oxide could be kicked out of an insulating state and into a low resistance state when around 1000 V was applied across a 1mm crystal of the material.  That's cool, and prompted much work, but as an experimentalist one always wonders whether the 10⁶ V/m electric field is responsible (which would be particularly interesting), or whether the energy available to the electrons is actually doing some kind of chemistry or damage to the material.  In a nanostructured system, one could get the same electric field by applying 100 mV across electrodes separated by 100 nm, getting big fields without big available energies.  We've done work along these lines in Fe₃O₄, and have had quite a bit of fun with it.

Third, nanospaced electrodes are a way of driving systems far from equilibrium and potentially measuring them under those conditions.  When an ordinary electron is injected into a correlated material, it can be thought of as a superposition of the "natural" low-energy excitations of the correlated system.  For example, in a Luttinger liquid, the injected carrier "fractionalizes" into a spinon (spin-1, chargeless mode) and a holon (charged, spinless mode).  Truly nano-spaced measurements may be a means of catching this process in action, though it won't be easy!

I still think that this is a fun and exciting area, and interest appears to be growing.

March Meeting wrap-up

The March Meeting is over, and overall it was good, as usual, and far too large, as usual.  Clearly the favorite topics this year were topological insulators, graphene, and iron pnictide superconductors.  On Thursday I did see a very good invited session on scanned probe microscopy, including examinations of vortices in the iron pnictides (among other things) and careful measurements of spin excitations at the atomic scale (that last being a substitute talk by Prof. Hla of Ohio University).  I had good discussions with colleagues from other places, got some good ideas for different experimental techniques, and looked at all the gadgets being hawked by vendors.  Clearly the era of the cryogen-free dilution refrigerator is upon us, if you can actually get any ³He, have a spare couple of hundred thousand dollars, and can support a 6-10 kW compressor.  I was disappointed by turnout at an invited session that I'd helped organize (more in a separate post), but it was up against a session with talks by three Nobel laureates.  On the return flight, I had a fun time talking with the neighboring passenger, the drummer for Ra Ra Riot, on his way to a gig in Austin.  Good to be home, though.

Physics for Everyone

This was the title of an invited session yesterday at the APS meeting.  Ivan Schuller put together a very interesting collection of talks, with subjects of broad interest to a large audience.  Ray Orbach gave an updated version of his presentation about the global energy challenge, including a strong plea that the US re-start reprocessing spent nuclear fuel (an idea I've held for a long time, so clearly it has merit :-) ).  Ted Postol spoke about his ideas on ballistic missile defense.  Charles Falco presented his recent work on converting digital cameras from conventional visible light photography to infrared (and recently ultraviolet) wavelengths, and using those cameras to look at art works.  Fascinating stuff.  Eugenie Reich gave a half-hour talk to a packed room about Hendrik Schön (see this pdf for the whole megillah), based on her book.  No revelations, and surprisingly little discussion of co-author/collaborator responsibilities.  She did have some amusing graphs, like the one displaying Schön's publications vs. time next to the Lucent stock price vs. time.  Finally, the session closed with a fun talk by Alan Nathan about the physics of baseball.  These kinds of sessions are a great feature of large meetings, though they must be a lot of work to put together.