March Meeting - minor update

I'm at the APS March Meeting, and as it is every year, it's really too big.  I've largely been spending much of my time at sessions where my students are speaking or where I'm chairing.  One major point of the meeting is the networking that takes place away from the talks.  Even in these days of modern communications technology, there is still no substitute for sitting down at a table with someone and hashing out a problem with a pen and a pad of paper.  

There are two types of talks:  invited talks (30 min + 6 min for questions) and contributed talks (10 min + 2 min for questions).  Contributed talks are usually most useful to specialists in a field, since with that kind of time constraint it's almost impossible to give an intro to something new.  Occasionally, however, it is possible to learn something new from a contributed talk.  I just saw an excellent one by Kieron Burke of UC Irvine, speaking about how one can get from density functional theory (which sometimes feels like a mysterious black art) and get back to simple Thomas-Fermi theory.  The relevant paper is here, and I really do feel like I learned something.  More later....

Peer review idiocy

In the Wall Street Journal over this past weekend, Peter Berkowitz argues that peer review is a corrupt system with no objectivity and little value.  He says this in connection with the climate science kerfluffle, claiming that peer review makes it easy for scholars to reward friends and punish enemies.  He argues by analogy:  we don't let athletes referee their own games, so why should we allow scholars to do the equivalent?  He does somehow seem aware that there is a real fundamental problem with ditching the system, though - the ability to competently evaluate intellectual work requires actual expertise.  I think Berkowitz gravely underestimates the qualification/expertise problem when it comes to actual physical science.   Sometimes the particular technical areas are incredibly challenging, requiring years of study to appreciate the subtle problems and issues.  Your choice is to let the people who have done such legwork do your evaluations, or to let people without the proper background make decisions.  Those of us who do science know that peer review has its set of problems, but, like democracy, it's the worst system except for all the alternatives.  Harping on its flaws without having a real discussion of these difficulties or offering alternatives is just intellectually lazy.

Narrowcasting.

I know that this will appeal to a differentially small fraction of my readership, but as a native of Pittsburgh of a certain age I am compelled to share this video.  An actual science post will follow tomorrow.  (For those readers that have never spoken with me, no, I do not talk with this kind of accent. But I can.)

Self-promotion, travel, and talking heads

I'm going to be doing some travel this week, and next week is the APS March Meeting in Portland, OR, so blog posts are likely to be thin.  I will try to write a bit about the APS.

In the meantime, I wanted to point out our new paper about shot noise measurements in atomic-scale junctions.  This is the paper I was talking about when posting about (good!) referees.  The referee reports were very helpful in making our presentation much more clear here.

Lastly, with my previous post about being a nanoscale science "talking head", I wanted to point out this terrific video that shows many of the things wrong with TV journalism today.  The dry British sense of humor is tough to beat.

Comedy Central: better science than the Science Channel?

Zapperz wrote an interesting post that links to an article in USA Today about whether Comedy Central (specifically The Daily Show and The Colbert Report) give some of the best real science coverage on television.  They bring on actual scientists (Neil Degrasse Tyson, Sean Carroll, Brian Greene, Lisa Randall, Steven Chu, Bob Park (!)) and have conversations with them that last more than a 30 second sound bite.  I notice that there are no nano folks on the guest lists for either show over the last couple of years.  Clearly this is a clarion call to write a general audience book and try to go on there to promote it; either that, or to try and be their go-to person to debunk outrageous claims about nanotechnological dystopias.

Topological insulators

A very big story in recent years in condensed matter physics is that of topological insulators, and it's a great tale of finding something new "in plain sight".  For something like 70 or 80 years, physicists thought that they had a handle on the insulating state.  Take a large crystalline solid, and ignore electron-electron interactions for the moment.  The allowed electronic states for such a material come in bands, when you look at how they're distributed as a function of energy.  That is, there are many electronic states clustered so close together in energy, separated by energy gaps where there are no allowed electronic states.  Now consider filling up those states with some number of electrons, counting two electrons (one spin up, one spin down) per state.  (This is short-hand for something more sophisticated, but it gets the point across, just as filling up atomic orbitals does a pretty good job describing the periodic table.)  If you end up in the middle of a band, with lots of empty states right next to the filled states in energy, then you have a metal.  If you end up exactly filling a band, then you have either a band insulator (if the energy gap next to the most energetic filled state is several eV) or a semiconductor (if the energy gap is more like 1-3 eV).   Turning on electron-electron interactions can change things a bit, but not too much.  (Interactions can lead to one more kind of insulator, a Mott insulator, in which interactions open up a gap in what would otherwise have been a metallic system.)

Until recently, we thought that this was all there was to it, as far as band insulators go.  It turns out that this is not the case, because of what happens at the boundary of the material (which we have so far been ignoring).  In some band insulators, the surface states (in 3d) or edge states (in 2d) can have special properties.  For example, one could have a situation where (because of spin-orbit coupling + band structure) the right-moving charge carriers have to have their spin pointed in one direction, while the left-moving charge carriers have to have their spin pointed the opposite way.  The result is that these surface states with unusual Dirac-like dispersion are thought to be able to resist back-scattering very effectively.  This means that these surface states may be very interesting for electronics applications, having ballistic properties over long distances.  Moreover, these properties are expected to be rather robust, because they come from the topology of the states, which is not easily disturbed by disorder.  For great reviews of this, see this paper (soon to appear in RMP), this article in Physics Today, and this video.

There is evidence that these states do exist, particularly from surface scattering techniques such as ARPES.  Transport experiments have faced a challenge, however, since many of the candidate materials (Bi₂Se₃, for example) are difficult to grow in sufficient purity that the bulk is actually insulating.  Still, this is exciting stuff, and a new paper on the arxiv (1003.0155) reveals that there may be a whole new class of other materials to play with.  Surprises from nature are always fun.   

Amazing technology.

It's not nano-related, but I found this video fascinating.  Clearly technology has gotten to the point where Hollywood can fake just about anything, often on a TV budget.

Claims of priority

I was looking at this week's Phys Rev Letters, and I saw this paper being highlighted as an editor's suggestion.  Now, I don't know anything about this work, but I was struck by the title, which says explicitly that this measurement is the first of its kind.  This is repeated a few paragraphs into the paper at the end of their introduction.  I thought that claims of "first"s were very strongly discouraged by the editors, as mentioned here, let alone being included in the title, regardless of how well founded the statement.  Was this an oversight, or am I missing something?

High Tc, pseudogaps, broken symmetries

What distinguishes one phase of matter from another?  A physicist would probably say that different phases possess different symmetries.  More specifically, transitions between phases can be described (when going in the right direction) by the breaking of a symmetry.  For example, when water freezes, the continuous rotational and translational symmetry of the liquid (liquid water looks, on average, the same in every direction and at different points within the liquid) are broken, because crystalline ice has a specific lattice (and therefore certain preferred lattice directions, as well as a spatial periodicity).  Solid ice instead has discrete rotational and translational symmetries, rather than continuous ones.

High temperature superconductors have been confounding physicists for 24 years now.  Progress has been made in understanding these complicated materials (typically layered, multicomponent copper oxides with weird oxygen stoichiometries to control the number of mobile charge carriers), but the situation is still a mess.  These compounds have a complicated phase diagram as a function of, e.g., temperature and chemical doping.  The undoped parent compounds are antiferromagnetic insulators.  Over a range of chemical compositions, the ground state is a d-wave superconductor.  Within a good part of that range of composition, at temperatures above the superconducting transition, these materials show a "pseudogap" below some higher temperature, T*.  That is, the number of available electronic states near the Fermi level is depressed compared to what you'd expect for a metal, but not vanishing as you'd expect for a superconductor.  People have been arguing for years about what the pseudogap is - is this a distinct phase?  Is it a precursor to superconductivity (e.g., pairing of electrons w/o long-range coherence), or does it compete with superconductivity?

This recent paper by Louis Taillefer reports the observation of broken rotational symmetry in the pseudogap phase (mainly via the Nernst effect).  The claim is that below T*, the four-fold rotational symmetry (because it's a square lattice) of the electronic properties of the CuO2 planes is broken, and the system becomes electronically anisotropic.  This is important, because it firmly argues that the pseudogap state is a real thermodynamic phase of some kind, and that kind of broken symmetry apparently places strong constraints on possible theories of high Tc.  Not my direct area of expertise, but it looks very interesting.  I'll admit, though, I was surprised by the strong statements made here.  Unless there's way more to this than meets the eye, it's not clear to me why it's justified to claim that we're now much closer to room temperature superconductivity....

Tragic.

Shocking news out of Alabama yesterday, where three faculty members (incl. the chair) of the biology department at UA Huntsville were shot, allegedly by a faculty member involved in a tenure decision.  I'm assuming that there will be a flood of articles and blog posts about this, and probably quite a bit of hyperventilating about tenure and the tenure process.  The fact is, some (thankfully very) small percentage of the population is unbalanced and responds to personal setbacks (real, perceived, or imagined) with violence.  It's a terrible shame, but this sort of thing happens across all occupations.  My condolences to the UAH community and the family and friends of those involved.

update:  ...and I was right.  *Sigh*, Christian Science Monitor.

This week in cond-mat, SQUID edition

Superconducting quantum interference devices, or SQUIDs, are fascinating gadgets.  Take a superconducting loop with two weak links (e.g., tunnel junctions, or constrictions with a lower critical current).   Now thread magnetic flux through the loop.  The superconducting wavefunction, which includes a phase factor that involves the vector potential, must be single-valued around the loop.  That means that the phase factor must return to itself modulo 2 pi going around the loop. The phase factor is proportional to the line integral of the vector potential, which itself is the magnetic flux through the loop.  Therefore, the total magnetic flux through the loop must be quantized.  If the external magnetic field doesn't give an integer number of flux quanta, then the superconductor must generate screening currents around the loop that produce flux and make up the difference.  If you had connected the loop to an external current source and run that external current (which splits itself around the two branches of the loop) up to the edge of the critical current, you would find that the screening currents would drive the loop normal and lead to a detectable voltage drop that is periodic in magnetic flux through the loop.  This periodicity allows SQUIDs to be phenomenally good magnetic field detectors.  One can integrate a tiny SQUID onto a movable probe, and make a scanning SQUID microscope, and do amazing things like figure out the pairing symmetry of high-Tc superconductors.
This week a paper appeared on the arxiv relevant to scanning SQUID microscopy:

arxiv:1002.1529 - Koshnick et al.,  Design concepts for an improved integrated scanning SQUID
Here, Koshnick, together with scanning SQUID experts Kirtley and Moler, lay out ideas that they have in the works for refining the technology of these gadgets.  Neat stuff.

Almost simultaneously, a new paper appeared in Nano Letters on an implementation of an aluminum scanning SQUID microscope.  The basic concept, involving the use of a drawn optical fiber tip as a template for deposition of an aluminum ring and leads, hearkens back to the scanning single-electron transistor charge detector worked on previously by one of the coauthors.

The arxiv blog: a good idea gone awry?

When it first began, I was impressed with the arxivblog.  The anonymous authors did a good and remarkably prompt job of surfing the preprint archive, and posting interesting tidbits, on essentially a daily basis.  Moving to the Technology Review website seemed like it could only be a good thing.  Larger readership, greater outreach to a scientifically literate audience, etc.  Now I have to wonder.  The arxivblog frequently seems to feature theory preprints that are rather far out (alternative theories of gravity; exotic quantum entanglement interpretational issues), and doesn't always make clear just how speculative some of these are.  Moreover, it seems that many of the comments, particularly on these more speculative topics, are, umm, not informative.  So, is the purpose of the arxivblog to showcase exciting new science (which is what the Technology Review usually does), or is it to be "gee whiz"/quantum sure is weird/nanobots-will-save-us-all entertainment?

"Not my job!"

US Secretary of Energy Steven Chu is going to be on "Wait, Wait, Don't Tell Me" this coming Saturday, presumably doing their "Not my job!" game.  For those not in the US, WWDTM is a comedic radio quiz program, and "Not my job!" is a game in which the guest must answer three questions about some subject that is very, very far from their area of expertise.  This should be amusing.

Update:  Here is a link to the relevant part of the show.

Lab mysteries and other annoyances.

One aspect of experimental science that never shows up on TV procedurals (NCIS, CSI) is the "lab mystery" - the simple procedure that's supposed to be a piece of cake, but turns out to be unnecessarily and surprisingly complicated.  Here's an example.  There's a material that is supposed to be photopolymerizable; it starts out as a liquid monomer, and under UV exposure it's supposed to polymerize into a gel.  We have some, and we also have a UV lamp.  As a simple test, we exposed the monomer to the UV for tens of minutes - no response.  I'm sure we'll figure this out, but this sort of thing never happens to Abby or the guys from Mythbusters....  Feel free to leave other examples of lab mysteries in the comments.

update:  Mystery resolved.  In this case, the answer seems to be "more power".   A much (~ 20x) brighter UV lamp works quite well.  The paper we're working from didn't really mention intensities, so I think we can be forgiven.  

Potpourri

The Female Science Professor is doing a great series of posts about interviewing for faculty jobs.  See here, here, and here, with two more parts to come.  On a related note, Massimo has a post about negotiating faculty job offers that is a follow-up to another post by Professor in Training.  I did not bother to re-write my post about applying for faculty jobs this year, and here's an old post I wrote about the interview process.  It also never hurts to remind people about suggestions on how to give talks.

I agree completely with Chad Orzel that Dennis Overbye at the NY Times needs to remember that "physics" extends beyond just high energy theory.

Finally, according to The Onion ("America's Finest News Source"), physics is done.  Guess it's time to re-evaluate that career choice.

Science and spending freezes

Tonight in the State of the Union address, President Obama will supposedly propose a freeze on discretionary federal spending for the next three years.  For those not familiar with the term, discretionary spending leaves out defense and debt service, as well as social security and medicare, but includes NIH, NSF, DOE, NIST, and NASA.  It will be interesting to see if, after a strong start on recovering from the funding morass (cuts in real dollars for several years in a row under the Bush administration and the budgetary mess from two wars), what will happen to federal scientific research support in such a climate.  Note that Pres. Obama is having this year's Intel Science Talent Search winner as a guest at the address.

Wow. Impressive room-temperature single-electron device

Looking at the arxiv this evening, I came across this paper, in which the authors demonstrate a silicon-based single-electron transistor that operates at room temperature.  The device is fabricated from a "finFET", a transistor design put forward for ultrascaled CMOS electronics.  In a finFET, the silicon channel is surrounded on three sides by a wrap-around gate, to achieve comparatively efficient gate coupling.  A single-electron transistor is very different from an ordinary field-effect transistor.  The channel in a SET is an "island" connected via tunnel barriers to source and drain electrodes.  The island has some capacitance, C, and therefore there is an energy cost associated with putting an additional electron on the island given by e²/2C.  If that energy cost is large compared to k_BT, and the tunnel barriers are sufficiently opaque (so that lifetime broadening doesn't smear out the island spectrum), then one can see single-electron charging effects in the conduction.  When the island is very small, one has to worry not just about the Coulomb charging energy, but also about the particle-in-a-box level spacing on the island.  (Note that all of this discussion is assuming that electron-electron interactions can be lumped together simply, via the capacitance.)

The impressive part of this work is just how clean the SET characteristics look at 300 K.  Getting clean SET signatures in conduction requires the thermal energy scale to be at least 20 times smaller than the charging energy scale, and at 300 K that's a tall order!  The data in Fig. 2c are spectacular for a room temperature SET device.  Very very pretty.  If they can figure out how to do this reliably, there are many exciting possibilities.... 

Inelastic electron tunneling spectroscopy

Motivated in part by this recent paper and ensuing conversation here, I thought it might be useful to say a few words about inelastic electron tunneling spectroscopy (IETS).  Mysterious kinks in the current as a function of voltage were first observed over 40 years ago in oxide tunnel junctions between superconductors.  As the voltage passed certain threshold values, the conductance (slope of I vs. V) increased suddenly.  A kink in I vs. V could also be plotted as a step in dI/dV vs. V, or as a peak in d²I/dV² vs. V.  When plotted this way, and converting V into units of energy, Jaklevic and Lamb realized that what they saw looked remarkably like an infrared or Raman spectrum of some organic compound.  They were right - using inelastic electron tunneling, they had measured the vibrational spectrum of organic compounds that had been trapped in their tunnel barrier during the fabrication process.  IETS has undergone a major resurgence in the last decade, in part because of Wilson Ho's group's beautiful demonstration that one can see these effects at the single molecule level, and because it's a way of confirming that fabricated molecular junctions actually contain what they're supposed to.


In IETS, current flows via a second-order tunneling process, in which an electron tunnels on to the vibrational ground state of a molecule, and in the same coherent process tunnels of the vibrationally excited state of that molecule, leaving behind a vibrational quantum of energy.  This can only happen of the voltage applied is large enough to supply the necessary energy; hence the thresholds seen in experiment.  The voltage positions of the features correspond directly with the energies of the modes being excited.  (In the single-electron transistor world, this process would be called "inelastic cotunneling" via vibrationally excited states.)  The requirement that there be a nonzero amplitude for this process gives rise to selection rules, so that not every mode can be pumped this way.  More recently, it's been realized that IETS may not necessarily always lead to simple peaks in d²I/dV² vs. V, because the IETS process can interfere coherently with other tunneling processes.  This is supported by data in the paper mentioned at the top of this post.

IETS is pretty amazing, when you think about it.  Even though the tunneling electrons never "really" occupy the molecule (such a state is classically forbidden due to energy conservation), nonetheless the molecule "feels" the effects of the electrons as they tunnel past.    

Gov. Perry, WTF?

Gee, Gov. Perry, did you ever think that one reason Texas has a hard time recruiting and retaining highly educated workers and high tech companies from outside the state is that you and your appointees have no respect for education at all?!  Someone please explain to me why the Governor's appointees for state school board think they should be able to warp the state history curriculum.  Then explain to me why our state, which is well below average in high school graduation rates and SAT scores, turned down $700,000,000 in federal education stimulus funds.  What's the deal?  I know that there are strings attached to the money, but given the repeated shortfalls in the state education system, should you really look down your nose at this?  Does Perry get kick-backs from all the private schools in the state in exchange for trashing public education?

Someone's going to be annoyed....

I was quite surprised to see this article in Science, talking about the much debated possible merger between Rice and Baylor College of Medicine.  I guess it's a good thing in some sense that my institution is sufficiently high profile that subjects like this make news in international-level publications.  Still, not necessarily the best way for people to learn about Rice.... 

International Conference on Molecular Electronics

I've been spending this week at the International Conference on Molecular Electronics, and it's been very interesting.  Topics have ranged from manipulating isomerization in single molecules using a scanning tunneling microscope to a talk this morning by Michael Grätzel on the latest about dye-sensitized solar cells.  For $2T, it looks like we could produce 20000 sq. km of ~ 10% efficient cells.  (Of course, that doesn't count installation costs, distribution and storage, and finding an area 7 times the size of Rhode Island to cover.)  Discussions have been lively, and it's been fun for me to see how thinking about certain topics has evolved over the few years that I've been working in this field.  For example, it seems like this community now has a much better picture of the relationship between electron transfer as chemists have studied it for decades, and electronic conduction as physicists and electrical engineers typically consider.  Some critical issues remain unresolved, however, including problems of interpretation of certain measurements and reliable theoretical approaches for computing the level alignment between molecular levels and metals.  Much of the physics and chemistry at work in these systems is still fascinating to me.  

Happy New Year

Happy New Year to my readers.  Posts will pick up again in 2010.  In the mean time, you might be amused by a couple of science-y gifts I got this holiday season.  I've got a great science museum-type demo in mind inspired by this desk toy, and no lab should ever be without a sonic screwdriver.  Finally, while not strictly science-related, this is very funny, containing such gems as Super Monkey Collider Loses Funding.  

Arxiv articles I should read

Some recent arxiv articles that I really should find the time to read in depth:

arxiv:0809.3474 - Affleck, Quantum impurity problems in condensed matter physics
Ian Affleck has revised his (rather mathematical) Les Houches lecture notes about quantum impurity problems (typically a single impurity, such as an unpaired electron, in contact with some kind of quantum environment).

arxiv:0904.1933 - Cubrovic, Zaanan, and Schalm, String theory, quantum phase transitions, and the emergent Fermi liquid
This is a Science paper related to my earlier post about the connection between certain quantum gravity models and condensed matter theories.

arxiv:0912.4868 - Heiblum, Fractional charge determination via quantum shot noise measurements
Heiblum is a consummate experimentalist, and this article in honor of Yoseph Imry looks like a great review of this area, particularly recent insights into the subtleties that happen with temperature and bias.

Noise IV

The last kind of electrical noise I wanted to discuss is called 1/f or "flicker" noise, and it's something of a special case.  It's intrinsic in the sense that it originates with the material whose conductance or resistance is being measured, but it's usually treated as extrinsic, in the sense that its physical mechanism is not what's of interest and in the limit of an "ideal" sample it probably wouldn't be present.  Consider a resistance measurement (that is, flowing current through some sample and looking at the resulting voltage drop).  As the name implies, the power spectral density of voltage fluctuations, S_V, has a component that varies approximately inversely with the frequency.  That is, the voltage fluctuates as a function of time, and the slow fluctuations have larger amplitudes than the fast fluctuations.  Unlike shot noise, which results from the discrete nature of charge, 1/f noise exists because the actual resistance of the sample itself is varying as a function of time.  That is, some fluctuation dV(t) comes from I dR(t), where I is the average DC current.  On the bright side, that means there is an obvious test of whether the noise you're seeing is of this type:  real 1/f noise power scales like the square of the current (in contrast to shot noise, which is linear in I, and Johnson-Nyquist noise, which is independent of I). 


The particular 1/f form is generally thought to result from there being many "fluctuators" with a broad distribution of time scales.  A "fluctuator" is some microscopic degree of freedom, usually considered to have two possible states, such that the electrical resistance is different in each state.  The ubiquitous two-level systems that I've mentioned before can be fluctuators.  Other candidates include localized defect states ("traps") that can either be empty or occupied by an electron.  These latter are particularly important in semiconductor devices like transistors.  In the limit of a single fluctuator, the resistance toggles back and forth stochastically between two states in what is often called "telegraph noise". 

A thorough bibliography of 1/f noise is posted here by a thoughtful person.   


I can't leave this subject without talking about one specific instance of 1/f noise that I think is very neat physics.  In mesoscopic conductors, where electronic conduction is effectively a quantum interference experiment, changing the disorder seen by the electrons can lead to fluctuations in the conductance (within a quantum coherent volume) by an amount ~ e²/h.  In this case, the resulting 1/f noise observed in such a conductor actually grows with decreasing temperature, which is the opposite of, e.g., Johnson-Nyquist noise.  The reason is the following.  In macroscopic conductors, ensemble averaging of the fluctuations over all the different conducting regions of a sample suppresses the noise; as T decreases, though, the typical quantum coherence length grows, and this kind of ensemble averaging is reduced, since the sample contains fewer coherent regions.  My group has done some work on this in the past.  

Physics and Industry

I read this column on the back page of this month's APS News, and I think it hits a lot of the right notes, until this paragraph:

Many of the Nation’s physics departments and other departments staffed by physicists should encourage some of their faculty members to take a two or three year sabbatical leave and join the physics staffs of companies wishing to use their skills to strengthen or rebuild their industrial bases. With the expected cutbacks in Federal spending for everything, including scientific research, the physics academic staffs, that already spend far too much of their time writing proposals to compete for Government grants, should help the Nation by joining one of the many companies who really could use their skills to refine their products and introduce the innovations so characteristic of their physics training. In their new industrial positions, the successes of these industrially focused physicists would encourage further enrollments in physics and all related sciences. Meanwhile the Nation’s manufacturing base would be strengthened and rebuilt.

While this is nice in the abstract, I'm trying to imagine how this is any more likely to happen than me getting my own unicorn and a candy-cane tree.  How can an academic physicist with a functioning research group possibly take off for two or three years to work in industry?  What happens to their students?  Their other funding?  What university would actually encourage this, given that they have to have the salary line, lab space, and office space still there, and that they have teaching/service needs?  In an era when companies are loathe to hire permanent research staff and give them proper facilities and resources (allegedly because such things do not maximize (short-term) profits and therefore dilute shareholder value), why on earth would a company want a revolving door of temporary employees that need the same resources as permanent staff but are in continual need of training and business education?

It seems to me that a more realistic approach, if you really want to encourage an industrial R&D resurgence in the US, would focus on tax and policy incentives to convince companies to invest in this stuff.  Discourage ultrashort-term strategies that maximize next quarter's profits rather than ensuring long term health of the company.    Give federal loan guarantees to companies that want to establish research efforts.  I'm 100% certain that if the industrial R&D jobs were there, we would fill them - the problem is that US companies overall have decided that investing in physics doesn't give them a quick stock price boost.  If you want to encourage more interactions between university research faculty and industry, fine.  Give tax breaks for industrial consulting or university research funding by industry.  (Though biomedical research shows that extremely strong coupling between researchers and their profit-motivated funding sources is not necessarily a good thing.)

Noise III

While Johnson-Nyquist noise is an equilibrium phenomenon, shot noise is a nonequilibrium effect, only present when there is a net current being driven through a system.  Shot noise is a consequence of the fact that charge comes in discrete chunks.  Remember, current noise is the mean-square fluctuations about the average current.  If charge was a continuous quantity, then there wouldn't be any fluctuations - the average flow rate would completely describe the situation.  However, since charge is quantized, a complete description of charge flow would instead be an itemized list of the arrival times of each electron.  With such a list, a theorist could calculate not just the average current, but the fluctuations, and all of the higher statistical moments.  This is called "full counting statistics", and is actually achievable under certain very special circumstances.

Schottky, about 90 years ago, worked out the expected current noise power spectral density, S_I, for the case of independent electrons traversing a single region with no scattering (as in a vacuum tube diode, for example).  If the electrons are truly independent (this electron doesn't know when the last electron came through, or when the next one is going through), and there is just some arrival rate for them, then the electron arrivals are described by Poisson statistics.  In this case, Schottky showed that S_I = <(I - < I >)²> = 2 e < I > Amps²/Hz.  That is, the current noise is proportional to the average current, with a proportionality constant that is twice the electronic charge.

In the general case, when electrons are not necessarily independent of each other, it is more common to write the zero temperature shot noise as S_I = F 2 e < I >, where F is called the Fano factor.  One can think if F as a correction factor, but under sometimes it's better to think of F as describing the effective charge of the charge carriers.  For example, suppose current was carried by pairs of electrons, but the pair arrivals are Poisson distributed.  This situation can come up in some experiments involving superconductors.  In that case, one would find that F = 2, or you can think of the effective charge carriers being the pairs, which have charge 2e.  These deviations away from the classical Schottky result are where all the fun and interesting physics lives.  For example, shot noise measurements have been used to show that the effective charge of the quasiparticles in the fractional quantum Hall regime is fractional.  Shot noise can also be dramatically modified in highly quantum coherent systems.  See here for a great review of all of this, and here for a more technical one.

Nanostructures are particularly relevant for shot noise measurements.  It turns out that shot noise is generally suppressed (F approaches zero) in macroscopic conductors.  (It's not easy to see this based on what I've said so far.  Here's a handwave:  the serious derivation of shot noise follows an electron at a particular energy and looks to see whether it's transmitted or reflected from some scattering region.  If the electron is instead inelastically scattered with some probability into some other energy state, that's a bit like making the electrons continuous.)  To see shot noise clearly, you either need a system where conduction is completely controlled by a single scattering-free region (e.g., a vacuum tube; a thin depletion region in a semiconductor structure; a tunnel barrier), or you need a system small enough and cold enough that inelastic scattering is rare.

The bottom line:  shot noise is a result of current flow and the discrete nature of charge, and deviations from the classical Schottky result tell you about correlations between electrons and the quantum transmission properties of your system.  Up next:  1/f noise.   

Interesting times.

It's always interesting to read about your institution in the national media.  It's a pretty good article that captures both sides of the Rice-Baylor question. 

Noise II

One type of electronic noise that is inescapable is Johnson-Nyquist noise.  Roughly a hundred years ago, physicists studying electricity noticed that sensitive measurements showed noise (apparently random time-varying fluctuations in the voltage or current).  They found that the power spectral density of (for example) the voltage noise, S_V, was larger when the system in question was more resistive, and that higher temperatures seemed to make the problem even worse.  Bert Johnson, then at Bell Labs, did a very careful study of this phenomenon in 1927, and showed that this noise appeared to result from statistical fluctuations in the electron "gas".  This allowed him to do systematic measurements (with different resistances at fixed temperature, and a fixed resistance at varying temperatures) and determine Boltzmann's constant (though he ends up off by ~ 10% or so).  Read the original paper if you want to get a good look at how a careful experimentalist worked eighty years ago.

Very shortly thereafter, Harry Nyquist came up with a very elegant explanation for the precise magnitude of the noise.  Imagine a resistor, and think of the electrons in that resistor as a gas at some temperature, T.  All the time the electrons are bopping around; at one instant there might be an excess of electrons at one end of the resistor, while later there might be a deficit.  This all averages out, since the resistor is overall neutral, but in an open circuit configuration these fluctuations would lead to a fluctuating voltage across the resistor.  Nyquist said, imagine a 1d electromagnetic cavity (transmission line), terminated at each end by such a resistor.  If the whole system is in thermal equilibrium, we can figure out the energy content of the modes (of various frequencies) of the cavity - it's the black body radiation problem that we know how to solve.  Now, any energy in the cavity must come from these fluctuations in the resistors.  On the other hand, since the whole system is in steady state and no energy is building up anywhere, the energy in the cavity is also being absorbed by the resistors.  This is an example of what we now call the fluctuation-dissipation theorem:  the fluctuations (open-circuit voltage or short-circuit current) in the circuit are proportional to how dissipative the circuit is (the resistance).  Nyquist ran the numbers and found the result we now take for granted.  For open-circuit voltage fluctuations, S_V = 4 k_BTR V²/Hz, independent of frequency (ignoring quantum effects).  For short-circuit current fluctuations, S_I = 4 k_BT / R A²/Hz.  

Johnson-Nyquist noise is an unavoidable consequence of thermodynamic equilibrium.  It's a reason many people cool their amplifiers or measurement electronics.  It can also be useful.  Noise thermometry (here, for example) has become an excellent way of measuring the electronic temperature in many experiments.   

Noise I

For a while now the fraction of condensed matter physicists that think about electronic transport measurements have been interested in noise as a means of learning more about the underlying physics in systems.  I thought it would be useful to give a sense of why noise is important.  First, what do we mean by noise?  As you might imagine from the colloquial meaning of the term, electronic noise manifests itself as fluctuations as a function of time in either the current through a system (current noise) or the voltage difference across a system (voltage noise).  These fluctuations are distributed about some mean value of current or voltage, so the smart way to characterize them is by taking the average of the square of the deviation from the mean (e.g., <(I - < I >)²>, where the angle brackets denote averaging over time, and I is the current.).  You can imagine that these fluctuations are distributed over all sort of time scales - some might be fast and some might be slow.  The natural thing to do is work in the frequency domain (Fourier transforming the fluctuations), and then you can worry about the power spectral density of the fluctuations.  For current noise, this is usually written S_I, which has units of Amps²/Hz.  If you evaluate S_I at a particular frequency, then that tells you the size of the mean square current fluctuations within a 1 Hz bandwidth about that frequency.  There is an analogous quantity S_V [V²/Hz] for voltage noise.  If the power spectral density is constant over a broad range of frequencies (up to some eventual high frequency cutoff), the noise is said to be "white".  If, instead, there is a systematic trend with a larger power spectral density at low frequencies, the noise is sometimes called "pink".


In any measurement, there might be several kinds of noise that one must worry about.  For example, your measuring equipment might show that the apparent S_I or S_V has several sharp peaks at particular frequencies.  This is narrow band noise, and might be extrinsic, resulting from unintentional pickup.  The classic examples include 60 Hz (50 Hz in many places outside the US) and its multiples, due to power lines, ~ 30 kHz from fluorescent lights, 540-1700 kHz from AM radio, 85-108 MHz from FM radio, etc.  Extrinsic noise is, in physicist parlance, uninteresting, though it may be a major practical annoyance.  There are sometimes intrinsic sources of narrow band noise, however, that can be very interesting indeed, since they indicate something going on inside the sample/system in question that has a very particular time scale.


There are three specific types of noise that are often of physical interest, particularly in nanostructures:  thermal (Johnson-Nyquist) noise, shot ("partition") noise, and 1/f ("flicker") noise.  I'll write a bit about each of these soon.

Fun new CD

They Might Be Giants has a new CD that my readers with kids might enjoy:  Science is Real.  Fun stuff.  This song has long been a favorite of mine.

Cryogenic dark matter detection

Whether this rumor turns out to be accurate or not, the technology used in the CDMS collaboration's dark matter search is quite interesting.  Working down the hall from these folks in graduate school definitely gave me an appreciation for the challenges they face, as well as teaching me some neat condensed matter physics and experimental knowledge.

The basic challenge in dark matter detection is that weakly interacting particles are, well, very weakly interacting.  We have all kinds of circumstantial evidence (rotation curves of galaxies; gravitational lensing measurements of mass distributions; particular angular anisotropies in the cosmic microwave background) that there is a lot of mass out there in the universe that is not ordinary baryonic matter (that is, made from protons and neutrons).  The dark matter hypothesis is that there are additional (neutral) particles out there that couple only very weakly to normal matter, certainly through gravity, and presumably through other particle physics interactions with very small cross-sections.  A reasonable approach to looking for these particles would involve watching for them to recoil off the nuclei of normal matter somehow.  These recoils would dump energy into the normal matter, but you'd need to distinguish between these events and all sorts of others.  For example, if any atoms in your detector undergo radioactive decay, that would also dump energy into the detector material's lattice.  Similarly, if a cosmic ray came in and banged around, that would deposit energy, too.  Those two possibilities also deposit charge into the detector, though, so the ability to identify and discount recoil events associated with charged particles would be essential.  Neutrons hitting the detector material would be much more annoying. 

The CDMS detectors consist of ~ cm-thick slabs of Si (ok) and Ge (better, because Ge is heavier and therefore has more nuclear material), each with an electrical ground plane (very thin low-Z metal film) on one side and an array of meandering tungsten micro-scale wires on the other side.  The tungsten meanders are "superconducting transition edge bolometers".  The specially deposited tungsten films have a superconducting transition somewhere near 75 mK.  By properly biasing them electrically (using "electrothermal feedback"), they sit right on the edge of their transition.  If any extra thermal energy gets dumped into the meander, a section of it is driven "normal".  This leads to a detectable voltage pulse.  At the same time, because that section now has higher resistance, current flow through there decreases, allowing the section to cool back down and go superconducting again.  By having very thin W lines, their heat capacity is very small, and this feedback process (recovery time) is fast.  A nuclear recoil produces a bunch of phonons which propagate in the crystal with slightly varying sound speeds depending on direction.  By having an array of such meanders and correlating their responses, it's possible to back out roughly where the recoil event took place.  (They had an image on the cover of Physics Today back in the 90s some time showing beautiful ballistic phonon propagation in Si with this technique.)  Moreover, there is a small DC voltage difference between the transition edge detectors and the ground plane.  That means that any charge dumped into the detector will drift.  By looking for current pulses, it is possible to determine which recoil events came along with charge deposition in the crystal.  The CDMS folks have a bunch of these slabs attached via a cold finger to a great big dilution refrigerator (something like 4 mW cooling power at 100 mK, for those cryo experts out there) up in an old salt mine in Minnesota, and they've been measuring for several years now, trying to get good statistics.  

To get a flavor for how challenging this stuff is, realize that they can't use ordinary Pb-Sn solder (which often comes pre-tinned on standard electronic components) anywhere near the detector.  There's too high an abundance of a radioisotope of Pb that is produced by cosmic rays.  They have to use special solder based on "galley lead", which gets its name because it comes from Roman galleys that have been sunk on the bottom of the Mediterranean for 2000 years (and thus not exposed to cosmic rays).  I remember as a grad student hearing an anecdote about how they deduced that someone had screwed up and used a commercial pre-tinned LED because they could use the detector itself to see clear as day the location of a local source of events.  I also remember watching the challenge of finding a wire-bonder that didn't blow up the meanders due to electrostatic discharge problems.  There are competing techniques out there now, of course.
Well, it'll be interesting to see what comes out of this excitement.  These are some really careful people.  If they claim there's something there, they're probably right.

Scale and perspective

Well, my old friends at AIG now owe $25B less to the US government.  For those keeping score at home, that's about 4 National Science Foundation annual budgets, or 0.8 NIH annual budgets.  AIG still owes the US government an additional $62B.

Lab conditions

I disagree with this comic, though I can never get my students or our facilities people to back my idea of converting the entire lab into ultrahigh vacuum space.  Sure, spacesuits would be required for lab work, but think of all the time we would save swapping samples and pumping out our evaporator.

Referees

In the world of scientific peer review, I think that there are three kinds of referees:  those that help, those than hinder, and those that are, umm, ineffective.  Referees that are ineffective do an adequate surface job, looking over papers to make sure that there are no glaring problems and that the manuscript is appropriate for the journal in question, but that's it.  Referees that hinder are the annoying ones we all complain about.  You know - they're the ones that send in a twelve word review for your groundbreaking submission to Science or Nature after sitting on it for 6 weeks; the review says little except "Meh." and may even indicate that they didn't really read the paper.  They're the ones that say work is nice but not really original, with no evidence to back up that statement.  They're the ones who sit on papers because they're working on something similar.  

Referees that help are the best kind, of course.  These are the people who read manuscripts carefully and write reports that end up dramatically improving the paper.  They point out better ways to plot the data, or ask for clarification of a point that really does need clarification or improved presentation.  They offer constructive criticism.  These folks deserve our thanks.  They're an important and poorly recognized component of the scientific process.

Obama gets behind science education

Thanks to Phil Plait for pointing this out.  Awesome!

Graphene, part II

One reason that graphene has comparatively remarkable conduction properties is its band structure, and in particular the idea that single-particle states carry a pseudospin.  This sounds like jargon, and until I'd heard Philip Kim talk about this, I hadn't fully appreciated how this works.  The idea is as follows.  One way to think about the graphene lattice is that it consists of two triangular lattices offset from each other by one carbon-carbon bond length.  If we had just one of those lattices, you could describe the single-particle electronic states as Bloch waves - these look like plane waves multiplied by functions that are spatially periodic with reference to that particular lattice.  Since we have two such lattices, one way to describe each electronic state is as a linear combination of Bloch states from lattice A and lattice B.  (The spatial periodicity associated with lattice A (B) is described by a set of reciprocal lattice vectors that are labeled K (K'))

Here is where things get tricky.  The particular linear combinations that are the real single-particle eigenstates can be written using the same Pauli matrices that are used to describe the spin angular momentum of spin-1/2 particles.  In fact, if you pick a single-particle eigenstate with a crystal momentum \hbar k, the correct combination of Pauli matrices to use would be the same as if you were describing a spin-1/2 particle oriented along the same direction as k.  This property of the electronic states is called pseudospin.  It does not correspond to a real spin in the sense of a real intrinsic angular momentum.  It is, however, a compact way of keeping track of the role of the two sublattices in determining the properties of particular electronic states.  

The consequences of this pseudospin description are very interesting.  For example, this is related to why back-scattering is disfavored in clean graphene.  In pseudospin language, a scattering event that flips the momentum of a particle from +k to -k would have to flip the pseudospin, too, and that's not easy.  In non-pseudospin language, that kind of scattering would have to change the phase relationship between the A and B sublattice Bloch state components of the single-particle state.  From that way of phrasing it, it's more clear (at least to me) why this is not easy - it requires rather deep changes to the whole extended wavefunction that distinguish between the different sublattices, and in a clean sample at T = 0, that shouldn't happen.

A good overview of this stuff can be found here (pdf) in this article from Physics Today, as well as this review article.  Finally, Michael Fuhrer at the University of Maryland has a nice powerpoint slide show (here) that discusses how to think about the pseudospin.  He does a much more thorough and informative job than I do here.

Not even wrong.

No, this is not a reference to Peter Woit's blog.  Rather, it's my reaction to reading this and the other pages at that domain.  Wow.  Some audiophiles must really be gullible.

Graphene, part I

Graphene is one of the hottest materials out there right now in condensed matter physics, and I'm trying to figure out what tactic to take in making some blog postings about it.  One good place to start is the remarkably fast rise in the popularity of graphene.  Why did it catch on so quickly?  As far as I can tell, there are several reasons.

1. Graphene has a comparatively simple electronic structure.  It's a single sheet of hexagonally arranged carbon atoms.  The well-defined geometry makes it extremely amenable to simple calculational techniques, and the basic single-particle band structure (where we ignore the fact that electrons repel each other) was calculated decades ago.
2. That electronic structure is actually pretty interesting, for three reasons.  Remember that a spatially periodic arrangement of atoms "picks out" special values of the electron (crystal) momentum.  In some sense, electrons with just the right (effective) wavelength (corresponding to particular momenta) diffract off the lattice.  You can think of the hexagonal graphene lattice as a superposition of two identical sublattices off-set by one carbon-carbon bond length.  So, the first interesting feature is that there are two sets of momenta ("sets of points in reciprocal space") that are special - picked out by the lattice, inequivalent (since the two sublattices really are distinct) but otherwise identical (since it's semantics to say which sublattice is primary and which is secondary).  This is called "valley degeneracy", and while it crops up in other materials, the lattice symmetry of graphene ends up giving it added significance.  Second, when you count electrons and try filling up the allowed electronic states starting at the lowest energy, you find that there are exactly two highest energy filled spatial states, one at each of the two lowest-momentum inequivalent momentum points.  All lower energy states are filled; all higher energy states are empty.  That means that graphene is exactly at the border between being a metal (many many states forming the "Fermi surface" between filled and empty states) and a semiconductor (filled states and empty states separated by a "gap" of energies for which there are no allowed electronic states).  Third and most importantly, the energy of the allowed states near those Fermi points varies linearly with (crystal) momentum, much like the case of an ultrarelativistic classical particle, rather than quadratically as usual.  So, graphene is in some ways a playground for thinking about two-dimensional relativistic Fermi gases.
   
3. The material is comparatively easy to get and make.  That means its accessible, while other high quality two-dimensional electron systems (e.g., at a GaAs/AlGaAs interface) require sophisticated crystal growth techniques.
4. There is a whole literature of 2d electron physics in Si and GaAs/AlGaAs, which means there is a laundry list of techniques and experiments just waiting to be applied, in a system that theorists can actually calculate.
5. Moreover, graphene band structure and materials issues are close to that of nanotubes, meaning that there's another whole community of people ready to apply what they've learned.
6. Graphene may actually be useful for technologies!

Next time I post about this, I'll try to talk more about the special aspect of the valley degeneracy.

Feedback time.

So, readers - any requests for particular topics, or hot tips on things I've missed?

Philip Kim visits Rice; I visit MSU.

Philip Kim visited Rice last week as one of our nanoscience-themed Chapman Lecturers, and it was great fun to talk science with him.  He gave two talks, the first a public lecture about graphene and the second a physics colloquium at a more technical level about how electrons in graphene act in many ways, like ultrarelativistic particles.  It was in this second talk that he gave the first truly clear explanation I've ever heard of the microscopic origin of the "pseudospin" description of carriers in graphene and what it means physically.  It got me thinking hard about the physics, that's for sure.

In the mean time, I spent yesterday visiting the Department of Physics and Astronomy at Michigan State.  They have a very good, enthusiastic condensed matter group there, with three hires in the last couple of years.  It was very educational for me, particularly learning about some of the experimental techniques that are being developed and used there.  Anyone who can measure resistances of 10^-8 Ohms to parts in 10⁵ gets respect!  Thanks to everyone who made the visit so nice.

Mars pictures

These pictures are stunning.  Wow!

Inspirational speech

I can't recall if I've posted this before.  If you're feeling down (e.g., because just about every story in the news today is horrifying to some degree), this might cheer you up.

3He

The lighter helium isotope, ³He, is not something that most people have ever heard of.  ³He is one neutron shy of the typical helium atom, and is present at a level of around 13 atoms per 10 million atoms of regular helium.  Every now and then there is some discussion out there in the sci-fi/futurist part of the world that we should mine the moon for ³He as a potential fuel for fusion reactors.  However, it turns out that ³He has uses that are much more down to earth.  

For example, in its pure form it can be used as the working fluid in an evaporative refrigerator.  Just as you cool off your tea by blowing across the top and allowing the most energetic water molecules to be carried away, it is possible to cool liquid helium by pumping away the gas above it.  In the case of regular ⁴He, the lowest temperature that you can reach this way ends up being about 1.1 K.  (Remember, helium is special in that at low pressures in bulk it remains a liquid all the way down as far as you care to go.)  This limit happens because the vapor pressure of ⁴He drops exponentially at very low temperatures - it doesn't matter how big a vacuum pump you have; you simply can't pull any more gas molecules away.  In contrast, ³He is lighter, as well as being a fermion (and thus obeying different quantum statistics than its heavier sibling).  This difference in properties means that it can get down to more like 0.26 K before its vapor pressure is so low that further pumping is useless.  (You don't throw away the pumped ³He.  You recycle it.)  This is the principle behind the ³He refrigerator.


You can do even better than that.  If you cool a mixture of ³He and ⁴He down well below 1 K, it will spontaneously separate into a ³He-rich phase (the concentrated phase, nearly pure), and a dilute phase of 6% ³He dissolved in 94% ⁴He.  At these temperatures the ⁴He is a superfluid, meaning that in many ways it acts like vacuum as far as the ³He atoms are concerned.  If you pump away the (nearly pure ³He) gas above the dilute phase, more ³He atoms are pulled out of the concentrated phase and into the dilute phase to maintain the 6% solubility.  This lets you evaporatively cool the concentrated phase much further, all the way down to milliKelvin temperatures.  (The trick is to run this in closed-cycle, so that the ³He atoms eventually end up back in the concentrated phase.)  This is the principle behind the dilution refrigerator, or "dil fridge".

Unfortunately, right now there is a major shortage of ³He.  Its price has shot up by something like a factor of 20 in the last year, and it's hard to get any at all.  This is a huge problem for a large number of (mostly) condensed matter physicists, as reported in the October issue of Physics Today (reprinted here (pdf)).  The reasons are complicated, but the proximate causes are an increase in demand (it's great for neutron detectors, which are handy if you're looking for nuclear weapons) and a decrease in supply (it comes from decay of tritium, mostly from triggers for nuclear warheads).  There are ways to fix this issue, but it will take time and cost money.  In the meantime, my sympathies go out to experimentalists who have spent their startups on fridges that they can't get running.

The unreasonable effectiveness of a toy model

As I've mentioned before, often theoretical physicists like to use "toy models" - mathematical representations of physical systems that are knowingly extremely simple, but are thought to contain the essential physics ingredients of interest.  One example of this that I've always found particularly impressive also happens to be closely related to my graduate work.  Undergraduate physicists that take a solid state class or a statistical physics class are usually taught about the Debye theory of heat capacity.  The Debye model counts up the allowed vibrational modes in a solid, and assumes that each one acts like an independent (quantum) harmonic oscillator.  It ends up predicting that the heat capacity of crystalline (insulating) solids should scale like T³ at low temperatures, independent of the details of the material, and this does seem to be a very good description of those systems.  Likewise, undergrads learn about Bloch waves and the single-particle picture of electrons in crystalline solids, which ends up predicting the existence of energy bands.  What most undergrads are not taught, however, is how to think about the vast majority of other solids, which are not perfect single crystals.  Glass, for example.

You might imagine that all such messy, disordered materials would be very different - after all, there's no obvious reason why glass (e.g., amorphous SiO₂) should have anything in common with a disordered polymer (e.g., photoresist).  They're very different systems.  Yet, amazingly, many, many disordered insulators do share common low temperature properties, including heat capacities that scale roughly like T^1.1, thermal conductivities that scale roughly like T^1.8, and particular temperature dependences of the speed of sound and the dielectric function.  To give you a flavor for how weird this is, think about a piece of crystalline quartz.  If you cool it down you'll find a heat capacity and a thermal conductivity that both obey the Debye expectations, varying like T³.  If you take that quartz, warm it up, melt it, and then cool it rapidly so that it forms a glass, if you remeasure the low temperature properties, you'll find the glassy power laws (!), and the heat capacity at 10 mK could be 500 times what it was when the material was a crystal (!!), and you haven't even broken any chemical bonds (!!!).

Back in the early 1970s, Anderson, Halperin, and Varma postulated a toy model to try and tackle this mysterious universality of disordered materials.  They assumed that, regardless of the details of the disorder, there must be lots of local, low-energy excitations in the material to give the increased heat capacity.  Further, since they didn't know the details, they assumed that these excitations could be approximated as two-level systems (TLSs), with an energy difference between the two levels that could range from zero up to some high energy cutoff with equal probability.  Such a distribution of splittings naturally gives you a heat capacity that goes like T¹.  Moreover, if you assume that these TLSs have some dipole-like coupling to phonons, you find a thermal conductivity that scales like T².  A few additional assumptions give you a pretty accurate description of the sound speed and dielectric function as well.  This is pretty damned amazing, and it seems to be a remarkably good description of a huge class of materials, ranging from real glasses to polycrystalline materials to polymers.  

The big mystery is, why is this toy model so good?!  Tony Leggett and Clare Yu worked on this back in the late 1980s, suggesting that perhaps it didn't matter what complicated microscopic degrees of freedom you started with.  Perhaps somehow when interactions between those degrees of freedom are accounted for, the final spectrum of (collective) excitations that results looks like the universal AHV result.  I did experiments as a grad student that seemed consistent with these ideas.  Most recently, I saw this paper on the arxiv, in which Moshe Schechter and P. C. E. Stamp summarizes the situation and seems to have made some very nice progress on these ideas, complete with some predictions that ought to be testable.  This kind of emergence of universality is pretty cool.

By the way, in case you were wondering, TLSs are also a major concern to the folks trying to do quantum computing, since they can lead to noise and decoherence, but that's a topic for another time....

String theory (!) and "bad metals"

I saw a remarkable talk today by Hong Liu from MIT, about quantum gravity and what it has to say about high temperature superconductivity.  Yes, you read that correctly.  It was (at least for a nonexpert) a reasonably accessible look at a genuinely useful physics result to come from string theory.  I doubt I can do it justice, so I'll just give the bare-bones idea.  Within string theory, Maldacena (and others following) showed that there is a duality (that is, a precise mathematical correspondence) between some [quantum theories of gravity in some volume of d+1 dimensions] and some [quantum field theories w/o gravity on the d-dimensional boundary of that volume].  This sounds esoteric - what could it be good for?  Well, we know what we think the classical limit of quantum gravity should be:  Einstein's general relativity, and we know a decent number of solutions to the Einstein equations.  The duality means that it is possible to take what could be a very painful interacting many-body quantum mechanics problem (say, the quantum field theory approach to dealing with a large number of interacting electrons), and instead of solving it directly, we could convert it into a (mathematically equivalent) general relativity problem that might be much simpler with a known solution.  People have already used this approach to make predictions about the strongly-interacting quark-gluon plasma produced at RHIC, for example.


I'd known about this basic idea, but I always assumed that it would be of very limited utility in general.  After all, there are a whole lot of possible hard many-body problems in solid state physics, and it seemed like we'd have to be very lucky for the duals of those problems to turn out to be easy to find or solve.  Well, perhaps I was wrong.  Prof. Liu showed an example (or at least the results), in which a particular general relativity solution (an extremal charged blackhole) turns out to give deep insights into a long-standing issue in the strongly-correlated electron community.  Some conducting materials are said to be "bad metals".  While they conduct electricity moderately well, and their conductivity improves as temperature goes down (one definition of metal), the way that the conductivity improves is weird.  Copper, a good metal, has an electrical resistance that scales like T² at low temperatures.  This is well understood, and is a consequence of the fact that the low-energy excitations of the electrons in copper act basically like noninteracting electrons.  A bad metal, in contrast, has a resistance that scales like T, which implies that the low energy excitations in the bad metal are very complex, rather than electron-like.  Well, looking at the dual to the extremal black hole problem actually seems to explain the properties of this funny metallic state.  A version of Prof. Liu's talk is online at the KITP.  Wild stuff!  It's amazing to me that we're so fortunate that this particular correspondence exists.

Climate change talk

This afternoon we were fortunate enough to have our annual Rorschach Lecture, delivered by Ralph Cicerone, president of the US National Academy of Sciences.  The subject was climate change and its interaction with energy policy, and unsurprisingly to anyone who isn't willfully ignorant, this was a scary talk.  The atmospheric CO₂ data, the satellite-based measurements of accelerating Greenland and Antarctic ice loss, the amazing pace at which China is building coal-fire power plants (roughly 1 GW of electric generating capacity from coal coming on line every 10 days), are all very sobering.  The planet doesn't care, of course, but it sure looks like the human species had better get its act together, and the only way that's going to happen is if we come up with an energy approach that is cheap compared to coal (that includes the possibility of making coal more expensive, of course, but how do you persuade China and India not to burn their cheap, abundant coal?). 

Ahh, Air China

Posting from International Check-in at Beijing International Airport....

I was actually supposed to get home last night, but Air China had other plans.  At least I have quite the story out of it.  I'd originally booked a 2 hour 45 min layover in Beijing, figuring that would be plenty of time.  However, our Hangzhou-Beijing flight was delayed 2 hours.  Then, the pilot made two go-arounds at Beijing, very bumpy (cue the airsick bags and retching noises from fellow passengers), each time getting w/in about 30 feet of the ground, before giving up (due to high winds, I guess), and we diverted to Tianjin.  In Tianjin they kept us on the plane on the tarmac out at the end of their runway for close to 4 hours.  At least the AC worked there.  They ran out of water, and then orange juice.  Finally, they refueled and flew the plane back to Beijing, arriving only 8 hours late.  At least I wasn't alone (two other americans on the flight in the same situation as me), and Air China did, after some convincing, spring for a hotel for the night.

Clearly the simplest possible explanation for this is that I'm destined to make some universe-shattering discovery in the future, the echoes of which are rippling backward in time to try to prevent my return to the US. 

Conference observations so far

This is a nice gathering of people, and the organizers have done a very good job.  More discussion would be nice - the program is very dense.  A few (not very serious) observations:

• I used to think that I was the only condensed matter physicist not working on graphene.  Now I realize I'm the only condensed matter physicist not working on graphene, iron pnictide superconductors, or topological insulators. 
• Chinese ring tones are different than US or European ringtones.
• One speaker inadvertently stumbled on a great, subtle psychological trick:  he used a font for most of his talk that is identical to the font (some Helvetica variant) used by the Nature publishing group for their titles and subtitles.  That font makes everything seem important :-).  He blew this aura of profundity it at the end, though, by switching to comic sans.
• The Chinese groups that have been charging on the iron pnictides must have enormous resources in terms of people and equipment - the rate at which they are cranking out material and data is remarkable.  US materials growers seem very undersupported by comparison.
  
• Laser-based angle-resolved photoemission, in its appropriate regime, is damned impressive.
  

In China this week

I'm off tomorrow for a week-long trip to China, to go to this workshop.  I've never been to China before, so this should be an interesting experience!  I may try to blog a little, but I don't know how internet access will work during the conference.  Hopefully the trip will go more smoothly than the travel arrangements beforehand.  If I ever hear Expedia's "on hold" music again, I may snap.

Update: The trip in was long but problem-free. Blogger access only works through VPN, thanks to the Great Firewall....

Fiber and CCDs

As you've all no doubt read by now, the 2009 Nobel in Physics was awarded to Charles K. Kao, for the development of truly low loss fiber optics (a technology that you're all using right now, unless the internet backbone in your country consists of smoke signals or semaphore flags), and Willard Boyle + George Smith for the invention of the CCD (charge-coupled device, which is the basis for all digital cameras, and has revolutionized spectroscopy).  

The CCD portion makes a tremendous amount of sense.  CCDs work by using local gates on a doped semiconductor wafer to capture charge generated by the absorption of light.  The charge is then shifted to an amplifier and the resulting voltage pulses are converted into a digital signal that can be interpreted by a computer.  The description given in the supporting document (pdf) on the Nobel website is very good.  CCDs have revolutionized astronomy and spectroscopy as well as photography, and the physics that must be understood and controlled in order to get these things to work well is quite rich (not just the charge generation process, but the solid state physics of screening, transport, and carrier trapping).

The fiber optic portion is more tricky, since many people have worked on the development of fiber optic communications.  Still, Kao had the insight that the real limitation on light propagation in fiber came from particular types of impurities, understood the physics of those impurities, guided a program toward clean material, and had the vision to see where this could all lead.  

Certainly there will be grumbling from some that these are <sneer>engineering</sneer> accomplishments rather than essential physics, as if having a practical impact with your science that leads to technology and helps society is somehow dirty, second-rate, or a sign of intellectual inferiority.  That is a terrible attitude, and I'm not just saying that because my bachelor's degree is in engineering.  Trust me:  some engineers have just as much raw intellectual horsepower as high energy theoretical physicists.  Finding intellectual fulfillment in engineering is not some corruption of pure science - it's just how some very smart people prefer to spend their time.  Oh, by the way, the actual will of Alfred Nobel refers to accomplishments that "shall have conferred the greatest benefit on mankind", and specifically mentions "the person who shall have made the most important discovery or invention [my emphasis] within the field of physics".

Finally, this provides yet another data point on just how transformative Bell Labs (and other remarkable industrial R&D labs, including IBM, GE, and others) really was in the physical sciences.  The withering of long-term industrial research will be felt for a long, long time to come.


 

Single atoms in semiconductors

One last post before the obligatory Nobel post tomorrow.

Recently, there has been progress in examining the electronic transport properties of individual dopant atoms in semiconductors.  There are several motivations for this.  First and probably foremost, with increasing miniaturization we are rapidly approaching the limit when the active channel in semiconductor devices will contain, statistically, only a small number of dopants; it makes sense to figure out how these systems work and whether they have any intrinsically useful properties.  Second, these systems are the ultimate small-size limit of quantum dots, even smaller than single-molecule transistors.  Third, since the host materials are extremely well-studied, and quantum chemistry calculations can handle the relevant volumes of material, there is the possibility of realistic, detailed theoretical treatments.  This paper is a great example of treating an individual phosphorus donor in Si as a quantum dot.  This other paper looks at a single arsenic donor, and can see Kondo physics involving the unpaired electron on the donor site interacting with the (valley degenerate) Si conduction electrons.  Very cool stuff!