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.