Rankings and metrics - yet again

I (along with my departmental colleagues) was very happy to see this.  My department does extremely well in a particular ranking scheme (described in the original paper here and implemented online here, though you need to ask for password access) that asks, essentially, what fraction of the papers published by a department fall into the top 10% in terms of impact in an area.  We can debate about the flaws of any ranking scheme (hint:  they're all imperfect, because quantifying scientific quality and impact in a single number is fundamentally wrong-headed).  Still, it is nice to see an approach that agrees well with much intuition (that is, the usual top-10 suspects all look pretty good; schools that don't do much research in an area rank lower) where Rice does well. 

A new kind of solid - why "q-glass" really is weird and interesting

As long-time readers here know, I'm not a big fan of hype and press releases.  While it is very important to let people know what academic scientists and engineers are doing, not everything needs to be trumpeted from the rooftops as a huge breakthrough or a paradigm-altering thunderbolt.  However, this new result (the actual paper is here) is genuinely weird, unexpected, and exciting (at least to me).  The authors claim to have discovered a truly new kind of solid.  Let me break down what this means and why it's surprising.

A solid is a material that resists shear deformation - if you exert a certain force horizontally across the top surface of the material, the material will deform a bit until it's internal forces balance your applied force, and then deformation will reach some constant amount.  (In contrast, a fluid will keep deforming continuously!)  The most ordinary solids people know about are either crystalline (this includes polycrystalline materials made up of many crystal grains) or glasses.  In a crystalline solid, the atoms have taken on highly symmetric spatial arrangements.  That is, the atoms aren't separated by random distances, but integer multiples of certain particular spacings; similarly, crystals are not isotropic - there are particular directions along which atoms are arranged.   In contrast, simple liquids are isotropic, and except for some typical nearest-neighbor distance set by the atomic or molecular size, there is no other spatial ordered arrangement.   When a solid crystallizes from a liquid, it is a collective phenomenon, a phase transition, and this happens on cooling when the free energy of solid phase becomes lower than that of the liquid phase.  Quasicrystals (see 2011 Nobel for Chemistry) are in these senses crystals - their symmetries are just more subtle than those of ordinary crystals.

Glasses (including those made from polymers) are different.  They resist shear, too, but they do not have the long-range, periodic/anisotropic arrangement of constituents seen in crystals.  Instead, upon cooling, glasses become solid (meaning that their viscosity diverges toward infinity) because the constituents become "kinetically hindered".  At the risk of dragging up controversy, the simple description is that there is no true glass phase in the thermodynamic sense - glasses are rigid because the constituents can't readily move out of each others' ways, not because there is some true collective thermodynamic stability (involving free energies) at work. 

The authors of this new work have found something special that they have termed a "q-glass" while looking at what happens in the solidification of a molten mixture of aluminum, iron, and silicon.  In the resulting solids, they find nodules of a new material (Al₉₁Fe₇Si₂, approximately) that is definitely not crystalline or polycrystalline (no preferred lattice spacings; completely isotropic).  At the same time, the material does form out of the melt through a genuine first-order phase transition (!), and therefore appears to be highly ordered in some sense (both distinguishing it from a glass).  It will be very interesting to learn exactly what is going on here, and whether there are other materials that have these peculiar features.

Peer review, tone, and common courtesy

I'm back, though now I'm in the writing-six-things-before-the-term mode, so blogging will likely continue to be sparse.   Several of my friends pointed out this article in the Chronicle of Higher Education regarding the tone of correspondence in the peer review process.  In short, some fraction (from my own experience, I'd say maybe 15%) of "negative" reviews go beyond pointing out issues that need to be corrected to improve the paper and instead are genuinely hostile and nasty - basically tone and phrasing that the reviewer would very likely never have the nerve to use face to face.  Interestingly, those of us who experience the peer review process beat the rest of the world to the observation of a behavior pattern that is now realized to be common on the internet.

I wish I knew the solution to this.  Removing the blindness of the review process is one possibility, though I do worry that the same petty, vindictive people who write reviews like this will then engage in additional unprofessional behaviors toward people that they perceive as slighting them. 

The point of the review process in science is to make sure that correct, clear, original science results get disseminated in the literature.  We are all (allegedly) on the same side.  If people would just adhere to that, then reviews could be much more constructive in tone (e.g., instead of "The authors are just plain wrong", wouldn't it be better to say "I'm concerned that there are some problems with steps 1 through 4"?).

I am worried that there is a general erosion in common courtesy as well.  I know this makes me sound like a grumpy old man, but again there are some people who use electronic communications in general as an excuse for rudeness.  Taking the time to say "please" and "thank you" is never time poorly spent.

Online physics lecture notes

Next week is going to be largely internet-free for me, so don't expect much excitement here until early August.  I do have a few things I want to discuss (thermoelectric effects, the great mess that was San Jose State's experiment in MOOCs, etc.), but that will wait.  In the mean time, I wanted to point out some nice lecture notes I'd found lately on the arxiv and elsewhere.  If people have their own favorite notes to point out online, please do so in the comments.

Neri Merhav has produced a couple of nice sets of notes, written from the perspective of trying to teach very physicsy concepts to electrical engineering students.  This past week he put up these notes about statistical mechanics, and previously he had written this set about the connections between information theory and statistical physics.  I found them both very readable.

Doron Cohen's notes on statistical mechanics and mesoscopics are a bit more mathy and closer to notes than a textbook-style discourse.

Not on the arxiv, but Yoshi Yamamoto's online notes regarding noise and noise processes are great.

Printing at the 180 nm scale??

I stumbled across this post, where it is asserted (with no link, and my google-fu is inadequate) that some group or collaboration at Berkeley is working on the ability to make 180 nm critical dimension transistors via printing (and therefore over really large areas, rather than only on dinner-plate-sized Si wafers).  Can a reader out there point me to who is really doing this, or is this a case of a press office distorting things (e.g., taking a layer thickness and claiming it's a lateral feature size)?

Physics is hard - how much should that worry us?

I'm a bit late to the party, but there have been discussions lately about the number of undergrad STEM majors, including physics, with some gnashing of teeth about overall difficulty. For example, a report by the National Bureau of Economic Research has been interpreted as saying that the main reason students bail on science majors is poor grades. That is, students go in, knowing that science will require more work than majoring in something fluffy, but when many receive tough grades even though they work hard, that's too much for them and they change fields. Chad Orzel does his usual thorough job looking into what the study really says, and it does seem true that tough grading drives some people out of STEM pursuits.

Similarly, there is a new report from the National Academy of Sciences called "Adapting to a Changing World: Challenges and Opportunities in Undergraduate Physics Education". I found the content rather disappointing, in the sense that it didn't seem to say much new. We all know that some approaches can be better under some circumstances than traditional lecture. However, many of those are very labor intensive, and I'm sure that my 50 person class would benefit if it were instead five ten-person classes. More to the point, though, the report specifically claims that hard grades are a major factor in the low participation of women and underrepresented groups in the physics major.

So, is physics unnaturally harsh in its grading, to its detriment? Or is this a question of high school preparation on the one hand, and grade inflation in nonscience majors on the other? I lean toward the latter.

(Note that the NSF has proven that science is hard. Also, here is the paper featured in that article - it's actually very interesting.)

(One other note: no one commented on my three part post about the physics of contacts, and the hit rate on those posts was very low. At the same time, in one 15 minute interval last week my post about "whiskey stones" got nearly 500 page views after it was mentioned in an argument about whiskey on reddit. Guess I should write about other things besides physics if I want more readership:-).

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Contacts III: The search for measurements

In the last two posts I've talked a bit about contact resistances, but I haven't said much of anything about how to infer these experimentally. 

In some sense, the best, most general way to understand contact voltages is through scanning potentiometry.  For example, this paper (pdf - sorry for the long URL) in Fig. 10 uses a conductive AFM tip to look at the local electrostatic potential as a function of position along an organic transistor under bias.  When done properly, this allows the direct measurement of the potential difference between, e.g., the source electrode and the adjacent channel material.  If you know the potential difference and the current flowing, you can calculate the contact resistance.  Even better, this method lets you determine the \( I-V \) characteristic of the contact even if it is non-Ohmic, because you directly measure \(V\) while knowing \(I\).   The downside, of course, is that not every device (particularly really small ones) has a geometry amenable to this kind of scanned probe characterization.

A more common approach used by many is the transmission line method.  In the traditional version of this, you have a whole series of (otherwise identical) devices of differing channel lengths.  You can then plot the resistance of the device as a function of \(L\).  For Ohmic contacts and an Ohmic device, the slope of the \(R-L\) plot gives the channel resistance per unit length, while the intercept at \(L \rightarrow 0\) is the total contact contribution.  This does not tell you how the contact resistance is apportioned between source/channel and channel/drain interfaces (this can be nontrivial - see the figure I mentioned above, where most of the voltage is dropped at the injecting contact, and a smaller fraction is dropped at the collecting contact).  Related to the transmission line approach is the comparison between two- and four-terminal measurements of the same device.   The four-terminal measurement, assuming that no current flows in the voltage contacts and that the voltage probes are ideal, should tell you the contribution of the channel.  Comparison with the two-terminal resistance measurement should then let you get some total contact resistance.  I should also note that, if you know that the channel is Ohmic and that one contact dominates the resistance, you can still use length scaling to infer the \( I-V \) characteristic of the contact even if it is non-Ohmic.

The length scaling argument to infer contact resistances has also been used to great effect in molecular junctions.  There, for non-resonant transport, the usual assumption is that the bulk of the molecule (whatever that means) acts as an effective tunneling barrier, so that conductance should fall exponentially with increasing molecular length (assuming the barrier height does not change with molecular length, an approximation most likely to be true in saturated as opposed to conjugated molecules).  Thus, one can plot \(log G\) as a function of molecular length, and expect a straight line, with an intercept that tells you something about the contact between the molecule and the metal electrodes.  This has been done in molecular layers (see here, for example), and in single molecule junctions (see here, for example).  These kinds of contact resistances can then be related, ideally, to realistic electronic structure calculations looking at overlap between electronic states in the metal and those of the linking group of the molecule.

Hopefully these three posts have clarified a little the issue of contact effects in electronic devices - why they are not trivial to characterize, and how they may actually tell you interesting things.

Contacts, part deux

I will make an argument now that contact resistances are much maligned, and instead of rigorously trying to avoid worrying about them, we should instead look for opportunities (with well defined, reproducible contact interfaces) when they can actually tell us something. I'll punctuate this with some papers from our own group and areas I happen to know, but that's only because those are the examples that come to my mind.

What happens when you try to inject charge from a metal into a hopping conductor - a material with some energy-dependent density of localized states? Many organic semiconducting polymers are such systems. In this situation, an injected charge carrier faces a competition between diffusion away into the channel by hopping, and an attraction to its own image charge in the metal. The rather odd result is that this contact often tends to be Ohmic (in the sense that the contact voltage is directly proportional to the current), but the contact resistance ends up being inversely proportional to the mobility of the charge in the channel. This is true even when the metal Fermi level lies somewhere in the tail of the band (a situation where you would expect a Schottky contact in a nonhopping semiconductor). We ran into this here, and systematically varied the contact resistance by using surface chemistry to adjust the energetic alignment.

In correlated materials, the situation may seem tantalizing yet hopeless. On the one hand, you know something interesting must happen when charge is injected into the material - carriers in the metal are boring, electron-like quasiparticles, while charge excitations in the correlated system could in principle be very different, with fractional charge or spin-charge separation. On the other hand, depending on the bulk properties and ability to make reproducible contacts, it can be very hard to extract useful information from contact resistances in these systems. We did get lucky, and found that in magnetite conduction in both the high temperature (short range ordered) state and in the low temperature (long range ordered) state seems to be through hopping, similar to the description above. I definitely think that there is a lot more to be done in such materials by using contact effects as a tool rather than avoiding them.

In the world of molecular junctions, often one is in the limit where the device is "all contact", in the sense that the "bulk" is only a couple of nanometers and a few atoms. Next time I'll talk about some great measurements by others in these systems, as part of a discussion on how one can measure contact resistance.

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Contacts - annoying or an opportunity

In condensed matter physics, often we are interested in the flow of charge from a "source" electrode, through some material (the "channel", probably a different material than the source), and away into a "drain" electrode.  When the source, channel, and drain are all metals, life is simple.  While there might be some mismatch in the electrical conductivities of the materials, in the end an electron can go from some delocalized (extended, wavelike) state in the source, into such a state in the channel, and then into such a state in the drain, smoothly.   Because of the discontinuity in electronic band structure and dielectric properties, there is some reflection at the interface, leading to a contact resistance.  That is, some fraction of the applied voltage, linearly proportional to the applied voltage, is dropped across the source-channel contact, and some across the channel-drain contract.  This is an example of an ohmic contact.

However, the situation can be more complicated.  If the channel is a crystalline semiconductor, the Fermi level of the metal usually winds up sitting somewhere in the band gap.  If the is appropriate band bending takes place, there can then be an energy barrier (a Schottky barrier) for injection if charge from the metal into the semiconductor.   The spatial width of the barrier depends on the level of doping in the semiconductor, with higher doping leading to a narrower (though not necessarily shorter) barrier.   In this case, the current-voltage characteristics of the contact is not Ohmic, and looks instead like a diode, because the applied bias changes the shape of the barrier.  To avoid this in transistors, the regions of the channel where the source and drain contact it are very highly doped.  Still, in this case we are still assuming that the actual electronic states are extended, delocalized things.

The situation gets more complicated when the channel does not have delocalized states near the Fermi level.  

Usually experiments are designed to mitigate contact effects, either by avoiding measurements of the contact voltages (so-called four terminal measurements) or by making the contact contribution negligible compared to the bulk channel.  However, it turns out that sometimes contact effects can provide valuable insights into charge transport properties in the bulk.  I'll write more soon about this.

Timescales, averaging, and baseball

Please pardon the summer blogging slowdown - it's been a surprisingly busy couple of weeks, between an instructor search, working on papers, proposal stuff, and trying to write more on my big long-term project. 

Thanks to an old friend for pointing me to this link, which does a great job looking at why a knuckleball is so erratic in its flight from pitcher to batter.  For non-Americans:  In baseball, a pitcher throws a ball to a catcher, while a batter attempts to hit the ball.  There are several types of pitches, depending on the pitcher's grip on the ball (which has seams due to the stitching that holds the leather cover on), the throwing motion, and the release.  A fastball can reach speeds in excess of 100 mph (161 kph) and typically spins more than 1000 rpm.  In contrast, a knuckleball can drift by the batter at a leisurely 70 mph yet be nearly unhittable because of its erratic motion.   A knuckleball barely spins, so that it may complete only 1-2 revolutions from leaving the pitcher's hand to reaching the batter.  This means that the positioning of the seams is absolutely critical to determing the aerodynamics of the motion, and no two knuckleballs move the same way.  In physics lingo, a knuckleball has almost none of the orientational averaging that happens in basically every other pitch.  I propose the definition of a new dimensionless parameter, the Wakefield number, \(W\), that is the ratio of the ball's period of revolution to its time-of-flight from pitcher to batter.   A knuckleball is a pitch with \(W \sim 1\).

Come on, PRL editors.

I rarely criticize papers.  I write this not to single out the authors (none of whom I know), nor to criticize the actual science (which seems very interesting) but to ask pointedly:  How did the editors of PRL, a journal that allegedly prizes readability by a general physics audience, allow this to go through in its current form?  This paper is titled "Poor Man’s Understanding of Kinks Originating from Strong Electronic Correlations".  A natural question would be, "Kinks in what?".  Unfortunately, the abstract doesn't say.  Worse, it refers to "the central peak".  Again, a peak in what?!   Something as a function of something, that's for sure. 

Come on, editors - if you are going to let articles be knocked from PRL contention because they're "more suitable for a specialized journal", that obligates you to make sure that the papers you do print at least have titles and abstracts that are accessible.  I'm even a specialist in the field and I wasn't sure what the authors were talking about (some spectral density function?) based on the title and abstract.

The authors actually do a good job explaining the issue in the very first sentence of the paper:  "Kinks in the energy vs. momentum dispersion relation indicate deviations from a quasiparticle renormalization of the noninteracting system."   That should have been the first sentence in the abstract.  In a noninteracting system, the relationship between energy and momentum of particles is smooth.  For example, for a free electron, \( E = p^{2}/2m \) where \(m\) is the mass.  In an ordinary metal (where Fermi liquid theory works), you can write a similar smooth relationship for the energy vs. momentum relationship of the quasiparticles. Kinks in that relationship, as the authors say, "provide valuable information of many-body effects".  

Academic self-sabotage

Ordinarily I wouldn't just post a link, but this article ("Self-Sabotage in the Academic Career: 15 ways in which faculty members harm their own futures, often without knowing it") from the Chronicle of Higher Education is exceptionally good advice for new faculty members.  While several of the points are specific to academia, some are generalizable to any career within a moderately large organization. 

The state of "molecular electronics"

For those interested in the history and current state of "molecular electronics", I refer you to the latest focus issue of Nature Nanotechnology.  Good news for those without subscription access - some good articles are available free of charge:

• Mark Ratner has an historical perspective on the field here.
• Emanuel Lörtscher discusses the challenges of making an actual technology out of these systems. 
• A variety of experts (including me) weigh in with blurbs about where things are and where they are going.
• Sri Aradhya and Latha Venkataraman present an excellent up-to-date review of the field, emphasizing the evolution of measurements beyond just collecting current-voltage characteristics.

Rescheduled, Workshop on Surface Plasmons, Metamaterials, and Catalysis

Three of my colleagues and I are helping to organize a workshop at Rice University on October 21-23, 2013.   (This had originally been planned for May, but sequester-related travel restrictions on the government participants among other things forced a rescheduling.)  The goal of this ARO-sponsored workshop is to explore the opportunities for chemical catalysis arising from recent advances in the fields of metamaterials and plasmonics.  The workshop will bring together scientists from the disciplines of electrochemistry, catalysis, and plasmonics, which have not traditionally had a common platform.





The confirmed invited speakers are:

Rick Van Duyne - Northwestern University
Paul Bohn - University of Notre Dame
Martin Moskovits - University of California, Santa Barbara
Katherine Willets - University of Texas at Austin
Jennifer Dionne - Stanford University
Mark Brongersma - Stanford University
Louis Brus - Columbia University
Harry Atwater - California Institute of Technology
John Yates - University of Virginia
Mengyan Shen - University of Massachusetts, Lowell
Suljo Linic - University of Michigan
Mostafa El-Sayed –- Georgia Tech
Tom Mallouk –- Penn State

Topics include:

• The state of the art in plasmonics, metamaterials, and chemical catalysis
• Areas of catalysis that could benefit from enhanced optical/electromagnetic concepts
• Concepts for nanophotonic- and metamaterials-driven catalysis and heat generation
• Surface nanoengineering to merge nanophotonics and catalysis
• Quantum plasmonics
• Hot electrons driving chemistry
• Chemical sensing using nanophotonic and plasmonic concepts
• Nanophotonic characterization of catalytic structures:  Where do the reactions happen, and how fast?

The deadline for abstract submission is July 21, 2013, and space is limited.  The workshop website is here:  http://PlasEnhCat2013.rice.edu  .

Please feel free to distribute this information to people that would be interested!

What does "heating" mean at the nanoscale?

I've talked before about what physicists mean when they talk about "temperature", and work has me thinking about this a lot these days. Temperature is inherently a statistical concept.  It doesn't really make sense to talk about the temperature of a single electron.  The electron has some momentum (and therefore some kinetic energy), but temperature is not a meaningful concept for a single particle in isolation.  Now, if you have a whole bunch of electrons, you can talk about how many of them have a certain amount of energy.  That distribution of electrons as a function of energy takes on a particular form when the electron system is in thermal equilibrium.  (That is, if the electron system is weakly coupled somehow to an energy reservoir so that energy can be exchanged freely between the reservoir and the electrons.)  When the electron system is in thermal equilibrium with the reservoir, on average no net energy is transferred as a function of time between the electrons and the reservoir; this is what we mean when we say that the electrons and the reservoir have the same temperature.

The situation gets really tricky when a system is driven out of equilibrium.  For example, you can use a battery to drive electrons through some nanoscale system.  When you do that, and you look at different points within the nanoscale system, you will find that, in general, the distribution of the electrons as a function of energy doesn't necessarily look much like the thermal equilibrium case.  So, is there a sensible way to generalize the idea of temperature to quantify how "hot" the electrons are?  The problem is, there are many ways you might want to do this - you are trying to take a potentially very complicated distribution function and essentially summarize it by a single number, some local effective temperature.   A natural direction to go is to consider a thought experiment:  what if you took a reservoir with a well defined equilibrium temperature, and allowed it to exchange energy with the nonequilibrium system at a location of interest.  What reservoir temperature would you have to pick so that there is no net average energy transfer between the system and the reservoir in steady state?   That is one sensible way to go, but in the nano limit the situation can be very tricky, even in the thought experiment.  The details of how the imagined energy exchange takes place can affect the answers you get.  Tough stuff.

Instructor opening at Rice

RICE UNIVERSITY

Wiess Instructorship in Physics and Astronomy

The Physics and Astronomy Department at Rice University invites applications for a one-year instructorship position teaching introductory physics, commencing July/August 2013.  The teaching load is equivalent to two courses per semester.  There would also be opportunities to develop innovative teaching methods and pursue independent research or collaborations with existing research programs (see web page http://physics.rice.edu).  Evaluation of applications will begin immediately and continue until the position is filled. Applicants should send a curriculum vitae, a statement of teaching and research interests, and a list of publications as a single PDF file, and should arrange for three letters of reference to be sent to: vcall@rice.edu with subject line "Wiess Instructorship" (pdf format preferred), or by postal mail to Wiess Instructorship Search, c/o Valerie Call, Physics and Astronomy Department-MS61, Rice University, 6100 Main Street, Houston, TX 77005-1892. Applicants must have a PhD and be eligible to work in the U.S. Rice University is an affirmative action/equal opportunity employer.

Ask me something.

I realized I've never really tried having my readers just post questions for me.   Have at it!

A Scientist Laureate position for the US?

This is at least thought-provoking.  Lamar Smith (R-Texas, chair of the US House science committee, famous for things like this) and Zoe Lofgren (D-CA, about as far from Lamar Smith as I can imagine with the possible exception of Nancy Pelosi) are co-sponsoring a bill that would create a position called Scientist Laureate of the United States.  This person would be appointed by the President following nomination by the National Academy of Sciences, and would be meant to act as an inspirational figure, making public appearances and furthering the cause of science.  This could be a good thing, provided (1) an actual accomplished scientist is chosen, not someone who has to satisfy a political agenda; and (2) the person chosen is charismatic and able to use the bully pulpit effectively.  The Science Laureate should do more than show up at middle schools - they should get major exposure (e.g., late night talk shows; hosting a science program on a major network with actual resources to make it good; having the ear of Congress, perhaps even the limited ability to insist on speaking at a hearing of the House or Senate science-related committees).  (Halftime at the Superbowl is probably out of line.)

While I applaud scientists with great public outreach track records (Neil deGrasse Tyson just spoke at our commencement), that should not be the sole criterion.  If this passes, hopefully Congress will keep in the bit about the NAS making the choice.  Suggestions are invited in the comments.

MOOC followup

Very briefly, here is an open letter by the San Jose State University philosophy department regarding MOOCs (one in particular).   Food for thought.

update:  my colleague Moshe Vardi pointed out his own editorial on this topic.

I don't agree with everything in either of these documents.  I do think it's worth thinking hard about the purpose of MOOCs.  Are they about idealistically providing access to fantastic educational opportunities at very low cost to the student for millions of potential pupils who have an internet connection?  Are they about cynically slashing the operating costs of universities by restructuring the educational experience and potentially eliminating large numbers of faculty jobs?  These are not mutually exclusive.

Fun with single atoms

IBM Almaden research has produced "A Boy and His Atom", a stop-motion movie where the frames are scanning tunneling microscope images of carbon monoxide molecules on the Cu(111) surface.  Here is the "Making of..." movie as well.  Fun stuff.  I was particularly amused by Andreas Heinrich's comment that "if I can get a thousand kids to join science rather than go into law school, I'd be super happy."  Amen :-)