Superluminal neutrinos - a case study in how good science is done

As many people have now heard, the OPERA collaboration is reporting a very surprising observation.  The OPERA experiment is part of CERN, and is an experiment meant to study neutrino flavor oscillations.  The idea is, the proton beam at CERN creates a beam of neutrinos.  Since neutrinos hardly interact with normal matter, they move in a straight line right through the earth, and pass through the experimental station in Gran Sasso, Italy, where some small fraction of them are then detected.  There are (according to the Standard Model) three flavors of neutrinos, the electron neutrino, muon neutrino, and tau neutrino.  It has been determined experimentally that those flavors are not exact "mass eigenstates".  That means that if you start off with a tau neutrino of particular energy, for example, and let it propagate for a while, it will change into a muon neutrino with some probability that oscillates in time.  Anyway, OPERA wanted to study this phenomenon, and in doing so, they measured the time it takes neutrinos to go from their production point at CERN to the detector in Gran Sasso, using precisely synchronized special clocks.  They also used differential GPS to measure the distance between the production point and the detector to within 20 cm.  Dividing the distance by the time, they found much to their surprise that the neutrinos appear to traverse the distance about 60 ns faster than would be expected if they traveled at the speed of light in vacuum.

So, what could be going on here?  There are a few possibilities.  First, they could have the distance measurement wrong.  This seems unlikely, given the use of differential GPS and the sensitivity (they could clearly see the change in the distance due to a 2009 earthquake, as shown in Fig. 7 of the paper).  Second, they could have a problem in their synchronization of the clocks.  That seems more likely to me, given that the procedure is comparatively complicated.  Third, there is some other weird systematic at work that they haven't found.  Fourth, neutrinos are actually tachyons.  That would be all kinds of awesome, but given how challenging it would be to reconcile that with special relativity and causality, I'm not holding my breath.

Why is this an example of good science?  The collaboration spent three years looking hard at their data, analyzing it many different ways, checking and cross-checking.  They are keenly aware that a claim of FTL neutrinos would be the very definition of "extraordinary" in the scientific sense, and would therefore require extraordinary evidence.  Unable to find the (highly likely) flaw in their analysis and data, they are showing everything publicly, and asking for more investigation.  I want to point out, this is the diametric opposite of what happens in what I will term bad science (ahem.  Italian ecat guys, I'm looking at you.).   This is how real experimental science works - they're asking for independent reproduction or complementary investigation.  I hope science journalists emphasize this aspect of the story, rather than massively sensationalizing it or portraying the scientists as fools if and when a flaw is found.

State of Texas threatens physics departments at smaller public universities

This article is both sad and frustrating.  The coordinating body of the Texas state government that runs the public universities in this state has recommended that a number of places shut down their physics departments.   In particular, this affects two schools near Rice that are historically African American serving, Prairie View A and M and Texas Southern.   (Unfortunately, the article doesn't have a link to the actual Texas Higher Education Coordinating Board recommendations, so I don't have any further information, like which other universities here may be affected.)   

Depressingly updated:  see NY Times story here.

I understand that financial times are tight for the state.  (Look at the "Texas Miracle" in action as we slash the state's education budget.)  The bit that really galls me is the rationale:  enrollment in the upper division courses is small, so we should eliminate the whole department.  This idea that somehow the only valuable and cost effective courses are those with large enrollment is ridiculous, and it seems to have infected the public university system in this state, driven by misguided, bean-counting thinktank types.  If you follow this reasoning all the way, we should only have large service courses, and never have upper division, specialized courses in anything, and of course all of these should be taught by non-tenure-track, non-research-active instructors.  That would surely cut costs.  It would also be a disaster in the long run. As is stated in this article, if you used the same criteria in terms of size of upper division courses across the country, you'd end up shutting down 2/3 of the physics departments in the US, to say nothing of other disciplines.  I can't imagine the situation is any better in, e.g., math, or chemical engineering, or any technical discipline.  I'd also love to see numbers about how much collegiate athletics is net costing the state in public funds, vs. how much it costs to keep these programs going.  Hint: most universities lose money on athletics.

I'd love to try to fix this, but given the politics here (hint:  Rick Perry likes these policies, and his political party controls both houses of the state legislature), it's hard to see a workable path forward.  It's not like this is going to be an honest debate about how to structure the state's higher education system (which we can and should have) - it's an ideological full-court press.   

Think I'm exaggerating?  The superintendent of the THECB, Raymund Paredes, is a close buddy of both Rick Perry and his pal Rick O'Connell, the guy who thinks that a bachelor's degree even in a technical field should be obtainable for $10000 total, period.  You could do that, of course, but it would involve converting our colleges and universities essentially into community colleges or correspondence schools.  I've yet to see any evidence that these guys have an appreciation for science or engineering at all.  They want UT and TAMU to play good football, and they espouse populist rhetoric about wanting to cut costs, but they don't seem to want academic excellence at universities.

Lab habits + data management

The reason I had been looking for that Sydney Harris cartoon is that I was putting together a guest lecture for our university's "Responsible Conduct of Research" course. I was speaking today about data management and retention, a topic I've come to know well over the last year through some university service work working on policies in that area. After speaking, it occurred to me that it's not a bad idea to summarize important points on this for the benefit of student readers of this blog.  In brief:

• Everything is data.  Not just raw numbers or images, but also the final analyzed graphs, the software used to do the analysis, the descriptions of the instrument settings used to acquire the raw numbers - everything.
• The data are the science.  The data are the foundation for all the analysis, model-building, papers, arguments, further refinements, patents, etc.  Protect the data!
• If you didn't document it, you didn't do it.
• Write down everything.  Fill up notebooks.  Annotate liberally, including false starts, what you were thinking when you set up the little sub-experiments or trials that go into any major research endeavor.  I guarantee, you will never, ever in your life look back and say, "I regret that I was so thorough, and I wish I had written down less."  After years of observation, I am convinced that good notebook skills genuinely reduce mean time to thesis completion in many cases.  If you actually keep track of what you've been doing, and really write down your logic, you are less likely to go down blind alleys or have to repeat mistakes.
• You may think that you own your data.  You don't, technically.  In an academic setting, the university has legal title to the data (that gives them the legal authority that they need to adjudicate disputes about access to data, including those that arise in the rare but unfortunate cases of research misconduct), while investigators are shepherds or custodians of the data.  Both have their own responsibilities and rights.  Some of those responsibilities are inherent in good science and engineering (e.g., the duty to do your best to make sure that the published results are accurate and correct, as much as possible), and others are imposed externally (e.g., federal funding agencies require preservation of data for some number of years beyond the end of an award).
• Back everything up.  In multiple ways.  With the advent of scanners, digital cameras, cheap external hard drives, laptops, thumbdrives, "the cloud" (as long as it's better than this), etc., there is absolutely no excuse for not properly backing up data.  To repeat, back everything up.  No, seriously.  Have a backup copy at an off-site location, as a sensible precaution against disaster (fire, hurricane, earthquake, zombie apocalypse).
• Good habits are habits, and must be habituated.  It took me more than 25 years to get in the habit of really flossing.  Do yourself a favor, and get in the habit of properly caring for your data.  Please.

Help finding a Syndey Harris cartoon

I am trying to find a particular Syndey Harris physics cartoon, and google has let me down. The one I'm picturing has an obvious experimentalist at a workbench strewn with lab equipment. There's an angel on one shoulder, and a devil on the other. Anyone who has this cartoon, I'd be very grateful for a link to a scanned version! Thanks.

Single-molecule electric motor

As a nano person, I feel like I'm practically obligated to comment on this paper, which has gotten a good deal of media attention. In this experiment, the authors have anchored a single small molecule down to a single-crystal copper surface, in such a way that the molecule can pivot about the single anchoring atom, rotating in the plane of the copper surface. Because of the surface atom arrangement and its interactions with the molecule, the molecule has six energetically equivalent ways that it can be oriented on the metal surface. It's experimentally impressive that the authors came up with a way to track the rotation of the molecule one discrete hop between orientations at a time. This is only do-able when the temperature is sufficiently low that thermally driven orientational diffusion is suppressed. When a current of electrons is properly directed at the molecule, the electrons can dump enough energy into the molecule (inelastically) to kick the molecule around rotationally. In that sense, this is an electric motor. (Of course, while the rotor is a single small molecule, the metal substrate and scanning tunneling microscope tip are macroscopic in size.) The requirements for this particular scheme to work include cryogenic temperatures, ultrahigh vacuum, and ultraclean surfaces. In that sense, talk in the press release about how this will be useful for pushing things around and so forth in, e.g., medical devices is a bit ridiculous. Still a nice experiment, though.  I continue to find the whole problem of nanoscale systems driven out of thermal equilibrium (e.g., by the flow of "hot" electrons) to be fascinating - how is a steady state established, where does the energy go, where does irreversibility come into play, etc.

Playing with interfaces for optical fun and profit

A team at Harvard has published in Science a fun and interesting result.  When light passes from one medium to another, there are boundary conditions that have to be obeyed by the electromagnetic field (that is, light still has to obey Maxwell's equations, even when there's a discontinuity in the dielectric function somewhere).  Because of those boundary conditions, we end up with the familiar rules of reflection and refraction.  Going up a level in sophistication and worrying about multiple interfaces, we are used to having to keep track of the phase of the electromagnetic waves and how those phases are affected by the interfaces.  In fact, we have gotten good at manipulating those phases, to produce gadgets like antireflection coatings and dielectric mirrors (and on a more sophisticated level, photonic band gap materials).  What the Harvard team does is use plasmonic metal structures to pattern phase effects at a single interface.  The result is that they can engineer some bizarre reflection and refraction properties when they properly stack the deck in terms of phases.  Very cute.  I must confess, though, that since Federico Capasso was once my boss's boss at Bell Labs, I'm more than a little disturbed by the photo accompanying the physorg article.

Supersymmetry, the Higgs boson, the LHC, and all that

Lately there has been a big kerfluffle (technical term of art, there) in the blog-o-sphere about what the high energy physics experimentalists are finding, or not finding, at the LHC. See, for example, posts here and here, which reference newspaper articles and the like. Someone asked me what I thought about this the other day, and I thought it might be worth a post.

For non-experts (and in high energy matters, that's about the right level for me to be talking anyway), the main issues can be summarized as follows. There is a theoretical picture, the Standard Model of particle physics, that does an extremely good job (perhaps an unreasonably good job) of describing what appear to be the fundamental building blocks of matter (the quarks and leptons) and their interactions. Unfortunately, the Standard Model has several problems. First, it's not at all clear why many of the parameters in the model (e.g., the masses of the particles) have the values that they do. This may only be a problem with our world view, meaning the precise values of parameters may come essentially from random chance, in which case we'll just have to deal with it. However, it's hard to know that for sure. Moreover, there is an elegant (to some) theoretical idea called the Higgs mechanism that is thought to explain at the same time why particles have mass at all, and how the electroweak interaction has the strength and symmetry that it does. Unfortunately, that mechanism predicts at least one particle which hasn't been seen yet, the Higgs boson. Second, we know that the Standard Model is incomplete, because it doesn't cover gravitational interactions. Attempts to develop a truly complete "theory of everything" have, over the last couple of decades, become increasingly exotic, encompassing ideas like supersymmetry (which would require every particle to have a "superpartner" with the other kind of quantum statistics), extra dimensions (perhaps the universe really has more than 3 spatial dimensions), and flavors of string theory, multiverses, and whatnot. There is zero experimental evidence for any of those concepts so far, and a number of people are concerned that some of the ideas aren't even testable (or falsifiable) in the conventional science sense.

So, the LHC has been running for a while now, the detectors are working well, and data is coming in, and so far, no exotic stuff has been seen. No supersymmetric partners, no Higgs boson over the range of parameters examined, etc. Now, this is not scientifically unreasonable or worrisome. There are many possible scales for supersymmetric partners and we've only looked at a small fraction (though this verges into the issue of falsifiability - will theorists always claim that the superpartners are hiding out there just beyond the edge of what's measurable?). The experts running the LHC experiments knew ahead of time that the most likely mass range for the Higgs would require a *lot* of data before any strong statement can be made. Fine.

So what's the big deal? Why all the attention? It's partly because the LHC is expensive, but mostly it's because the hype surrounding the LHC and the proposed physics exotica has been absolutely out of control for years. If the CERN press office hadn't put out a steady stream of news releases promising that extra dimensions and superpartners and mini black holes and so forth were just around the corner, the reaction out there wouldn't be nearly so strong. The news backlash isn't rational scientifically, but it makes complete sense sociologically. In the mean time, the right thing to do is to sit back and wait patiently while the data comes in and is analyzed. The truth will out - that's the point of science. What will really be interesting from the history and philosophy of science perspective will be the reactions down the line to what is found.

great post by ZZ

Before I go to teach class this morning, I wanted to link to this great post by ZapperZ about the grad student/research adviser relationship.  Excellent.

Gating and "real" metals.

Orientation week has kept me very busy - hence the paucity of posts.  I did see something intriguing on the arxiv recently (several things, actually, but time is limited at the moment), though.

Suppose I want to make a capacitor out of two metal plates separated by empty space.  If I apply a voltage, V, across the capacitor using a battery, the electrons in the two plates shift their positions slightly, producing a bit of excess charge density at the plate surfaces.  One electrode ends up with an excess of electrons at the surface, so that it has a negative surface charge density.  The other electrode ends up with a deficit of electrons at the surface, and the ion cores of the metal atoms lead to a positive surface charge density.  The net charge on one plate is Q, and the capacitance is defined as C = Q/V.

So, how deep into the metal surfaces is the charge density altered from that in the bulk metal?  The relevant distance is called the screening length, and it's set in large part by the density of mobile electrons.  In a normal metal like copper or gold, which has a high density of mobile (conduction) electrons on the order of 10²² per cm³, the screening length is comparable to an atomic diameter!  That's very short, and it tells you that it's extremely hard to alter the electronic properties of a piece of normal metal by capacitively messing about with its surface - you just don't mess with the electronic density in most of the material.  (This is in contrast to the situation in semiconductors or graphene, by the way, when a capacitive "gate" electrode can change the number of mobile electrons by orders of magnitude.)

That's why this paper was surprising.  The authors use ionic liquids (essentially a kind of salt that's molten at room temperature) to modulate the surface charge density of gold films by something like 10¹⁵ electrons per cm².  The surprising thing is that they claim to see large (e.g., 10%) changes in the conductance of quite thick (40 nm) gold films as a result of this.  This is weird.  For example, the total number of electrons per cm² already in such a film is something like (6 x 10²²/cm³) x (4 x 10^-5 cm) = 2.4 x 10¹⁸ per cm².  That means that the gating should only be changing the 2d electron density by something like a tenth of a percent.  Moreover, only the top 0.1 nm of the Au should really be affected.  The data are what they are, but boy this is odd.  There's no doubt that these ionic liquids are an amazing enabling tool for pushing the frontiers of high charge densities in CM physics....

Topological insulator question

I have a question, and I'm hoping one of my reader experts might be able to answer it for me.  Let me set the stage.  One reason 3d topological insulators are a hot topic these days is the idea that they have special 2d states that live at their surfaces.  These surface states are supposed to be "topologically protected" - in lay terms, this means that they are very robust; something deep about their character means that true back-scattering is forbidden.  What this means is, if an electron is in such a state traveling to the right, it is forbidden by symmetry for simple disorder (like a missing atom in the lattice) to scatter the electron into a state traveling to the left.  Now, these surface states are also supposed to have some unusual properties when particle positions are swapped around.  These unconventional statistics are supposed to be of great potential use for quantum computation.  Of course, to do any experiments that are sensitive to these statistics, one needs to do quantum interference measurements using these states.   The lore goes that since the states are topologically protected and therefore robust, this should be not too bad.

Here's my question.  While topological protection suppresses 180 degree backscattering, it does not suppress (as far as I can tell) small angle scattering, and in the case of quantum decoherence, it's the small angle scattering that actually dominates.  It looks to me like the coherence of these surface states shouldn't necessarily be any better than that in conventional materials.  Am I wrong about this?  If so, how?  I've now seen multiple papers in the literature (here, here, and here, for example) that show weak antilocalization physics at work in such materials.  In the last one in particular, it looks like the coherence lengths in these systems (a few hundred nanometers at 1 K) are not even as good as what one would see in a conventional metal film (e.g., high purity Ag or Au) at the same temperatures.  That doesn't seem too protected or robust to me....  I know that the situation is likely to be much more exciting if superconductivity is induced in these systems.  Are the normal state coherence properties just not that important?

DOE BES CMX PI mtg

Went for the cryptic headline.  I'm off for a Department of Energy Basic Energy Sciences Condensed Matter Experiment principal investigator meeting (the first of its kind, I believe) in the DC area.  This should be really interesting, getting a chance to get a perspective on the variety of condensed matter and materials physics being done out there.  This looks like it will be much more useful than a dog-and-pony show that I went to for one part of another agency a few years ago....

Evolution of blogger spam

Over the last couple of weeks, new forms of spam comments have been appearing on blogger. One type takes a sentence or two from the post itself, and feeds them through a parser reminiscent of ELIZA, to produce a vaguely coherent statement in a comment. Another type that I've noticed grabs a sentence or two from an article that was linked in the original post. A third type combines these two, taking a sentence from a linked article, and chewing on it with the ELIZA-like parser. A few more years of this, and we'll have the spontaneous evolutionary development of generalized natural-language artificial intelligence from blogger spam....

Summer colloquium

Every year at Rice in early August, the Rice Quantum Institute (old website) (shorthand: people who care about interdisciplinary science and engineering involving hbar) has its annual Summer Colloquium. Today is the twenty-fifth such event. It's a day-long miniconference, featuring oral presentations by grad students and posters, by both grad students and undergrad researchers from a couple of REU programs (this year, the RQI REU and the NanoJapan REU). It's a full day, with many talks. It's a friendly way for students to get more presentation experience, and a good way for faculty to learn what their colleagues are doing. I'd be curious to know if other institutions have similar things - my impression has been that this is comparatively unique, particularly its very broad interdisciplinary nature (e.g., talks on spectroscopy for pollution monitoring, topological insulators, plasmons, carbon nanotube composites, batteries) and combination of undergrads and grad students.

Plutonium: a case study in why CM physics is rich

At the heart of condensed matter physics are two key concepts: the emergence of rich phenomena (including spontaneously occurring order - structural, magnetic, or otherwise) in the many-particle limit; and the critical role played by quantum mechanics in describing the many-body states of the system. I've tried to explain this before to lay persons by pointing out that while complicated electronic structure techniques can do an adequate job of describing the electronic and vibrational properties of a single water molecule at zero temperature, we still have a difficult time predicting really emergent properties, such as phase diagram of liquid, solid, and vapor water, or the viscosity or surface tension of liquid water.

Plutonium is an even more striking example, given that we cannot even understand its properties from first principle when we only have a single type of atom to worry about. The thermodynamic phase diagram of plutonium is very complicated, with seven different crystal structures known, depending on temperature and pressure. Moreover, as a resident of the actinide row of the periodic table, Pu has unpaired 5f electrons, though it is not magnetically ordered. At the same time, Pu is very heavy, with 94 total electrons, so that relativistic spin-orbit effects can't be neglected in trying to understand its structure. The most sophisticated electronic structure techniques out there can't handle this combination of circumstances. It's rather humbling that more than 70 years after its discovery/synthesis, we still can't understand this material, despite the many thousands of person-hours spent on it via various nations' nuclear weapons programs.

Einstein, thermodynamics, and elegance

Recently, in the course of other writing I've been doing, I again came to the topic of what are called Einstein A and B coefficients, and it struck me again that this has to be one of the most elegant, clever physics arguments ever made.  It's also conceptually simple enough that I think it can be explained to nonexperts, so I'm going to give it a shot.

Ninety-four years ago, one of the most shocking ideas in physics was the concept of the spontaneous, apparently random, breakdown of an atomic system.  Radioactive decay is one example, but even light emission from an atom in an excited state will serve.  Take ten hydrogen atoms, all in their first electronically excited state (electron kicked up into a 2p orbital from the 1s orbital).  These will decay back into the 1s ground state (spitting out a photon) at some average rate, but each one will decay independently of the others, and most likely at a different moment in time.  To people brought up in the Newtonian clockwork universe, this was shocking.  How could truly identical atoms have individually differing emission times?  Where does the randomness come from, and can we ever hope to calculate the rate of spontaneous emission?

Around this time (1917), Einstein made a typically brilliant argument:  While we do not yet know [in 1917] how to calculate the rate at which the atoms transition from the ground state "a" to the excited state "b" when we shine light on them (the absorption rate), we can reason that the rate of atoms going from a to b should be proportional to the number of atoms in the ground state (N_a) and the amount of energy density in the light available at the right frequency (u(f)).  That is, the rate of transitions "up" = B_ab N_a u(f), where B is some number that can at least be measured in experiments.  [It turns out that people figured out how to calculate B using perturbation theory in quantum mechanics about ten years later.].  Einstein also figured that there should be an inverse process (stimulated emission), that causes transitions downward from b to a, with a rate = B_ba N_b u(f).  However, there is also the spontaneous emission rate = A_baN_b, where he introduced the A coefficient.

Here is the brilliance.  Einstein considered the case of thermal equilibrium between atoms and radiation in some cavity.  In steady state, the rate of transitions from a to b must equal the rate of transitions from b to a - in steady state, no atoms are piling up in the ground or excited states.  Moreover, from thermodynamics, in thermal equilibrium, the ratio of N_b to N_a should just be a Boltzmann factor, exp(-E_ab/k_BT), where E_ab is the energy difference between the two states, k_B is Boltzmann's constant, and T is the temperature.  From this, Einstein shows that the two Bs were equal, was able to solve for the unknown A in terms of B (which can be measured and nowdays calculated), and to show that the energy density of the radiation (u(f,T)) is Planck's blackbody formula.

My feeble writing here doesn't do this justice.  The point is, from basic thermodynamic reasoning, Einstein made it possible to derive an expression for the spontaneous emission rate of atoms, many years in advance of the theory (quantum electrodynamics) that allows one to calculate it directly.  This is what people mean by the elegance of physics - in a few pages, from proper reasoning on fundamental grounds, Einstein was able to deduce relationships that had to exist between different physical parameters; and these parameters could be measured and tested experimentally.  For more on this, here is a page at MIT that links to a great Physics Today article about the topic, and an English translation of Einstein's 1917 paper.  

Slackers, coasters, and sherpas, oh my.

This is mostly for my American readers - be forewarned.

I wrote last year about a plan put forward by Rick O'Donnell, a controversial "consultant" hired by the state of Texas (hint: Gov. Rick Perry, apparent 2012 presidential hopeful, wanted this guy.) to study the way public universities work in Texas. Specifically, O'Donnell came from a think tank that had very firm predetermined concept about higher education: Faculty are overpaid slackers that are ripping off students, and research is not of value in the educational environment. O'Donnell has written a report (pdf) about this topic, and he's shocked, shocked to find that he was absolutely right. By his metrics of number of students taught and research dollars brought in, he grouped faculty at UT and Texas A&M into "Dodgers, Coasters, Sherpas, Pioneers, and Stars". Pioneers are the people who bring in big grants and buy out of teaching. Stars are the people who bring in grants and teach large lecture classes. Sherpas are mostly instructors (he doesn't seem to differentiate between instructors and faculty) who lecture to large classes but don't bring in grants. Dodgers teach small classes and don't bring in grant money. Coasters teach small classes and bring in some grant money.

This is the exact incarnation of what I warned about in comments on my old post. This analysis basically declares that all social science and humanities faculty that teach upper division classes are worthless leeches (small classes, no grants) sponging off the university. People in the sciences and engineering who teach upper level classes aren't any better, unless they're bringing in multiple large research grants. Oh, and apparently the only metric for research and scholarship is money.

Nice. Perry, by the way, also appointed Barbara Cargill to run the state board of education. She's a biologist who wants evolution's perceived weaknesses to be emphasized in public schools, and she also was upset because the school board only has "six true conservative Christians" as members. I guess Jews, Muslims, Buddhists, Hindus, and atheists need not apply.  Update:  It looks like Texas has dodged creationism for another couple of years.  Whew.

What is so hard about understanding high temperature superconductivity?

As ZZ has pointed out, Nature is running a feature article on the history of high temperature superconductivity over the last 25 years. I remember blogging about this topic five years ago when Nature Physics ran an excellent special issue on the subject. At the time, I wrote a brief summary of the field, and I've touched on this topic a few times in the intervening years. Over that time, it's pretty clear that the most important event was the discovery of the iron-based high temperature superconductors. It showed that there are additional whole families of high temperature superconducting materials that are not all copper oxides.

Now is a reasonable time to ask again, what is so hard about this problem? Why don't we have a general theory of high temperature superconductivity?  Here are my opinions, and I'd be happy for more from the readers.

• First, be patient.  Low-T superconductivity was discovered in 1911, and we didn't have a decent theory until 1957.  By that metric, we shouldn't start getting annoyed until 2032.  I'm not just being flippant here.  The high-Tc materials are generally complicated (with a few exceptions) structurally, with large unit cells, and lots of disorder associated with chemical doping.  This is very different than the situation in, e.g., lead or niobium.
• Electron-electron interactions seem to be very important in describing the normal state of these materials.  In the low-Tc superconductors, we really can get very far understanding the normal starting point.  Aluminum is a classic metal, and you can do a pretty good job getting quantitative accuracy on its properties from the theory side even in single-particle, non-interacting treatments (basic band theory).  In contrast, the high-Tc material normal states are tricky.  Heck, the copper oxide parent compound is a Mott insulator - a system that single-particle band structure tells you should be a metal, but is in fact insulating because of the electron-electron repulsion!  
• Spin seems to be important, too.   In the low-Tc systems, spin is unimportant in the normal state, and the electrons pair up so that each electron is paired with one of opposite spin, so that the net spin of the pair is zero, but that's about it.  In high-Tc systems, on the other hand, very often the normal state involves magnetic order of some sort, and spin-spin interactions may well be important.
• Sample quality has been a persistent challenge (particularly in the early days).
• The analytical techniques that exist tend to be indirect or invasive, at least compared to the desired thought experiments.  This is a persistent challenge in condensed matter physics.  You can't just go and yank on a particular electron to see what else moves, in an effort to unravel the "glue" that holds pairs together (though the photoemission community might disagree).  While the order parameter (describing the superconducting state) may vary microscopically in magnitude, sign, and phase, you can't just order up a gadget to measure, e.g., phase as a function of position within a sample.  Instead, experimentalists are forced to be more baroque and more clever.
• Computational methods are good, but not that good.  Exact solutions of systems of large numbers of interacting electrons remain elusive and computationally extremely expensive.  Properly dealing with strong electronic correlations, finite temperature, etc. are all challenges.

Still, it's a beguiling problem, and now is an exciting time - because of the iron compounds, there are probably more people working on novel superconductors than at any time since the heady days of the late '80s, and they're working with the benefit of all that experience and hindsight.  Maybe I won't have to write something like this for the 30th high-Tc anniversary in 2016....

Updated look.

I finally bit the bullet and updated the look of the blog.  I'm still keeping it ad-free, though.

google+

I have a nagging feeling that google+ could somehow be used to significantly increase readership of my blog, if only I was appropriately savvy.  Anyone have any suggestions or thoughts on this?  I don't crave the attention per se, but I'd be fibbing if I said I wasn't jealous of the readership numbers of the folks that blog at, e.g., scienceblogs, discovermagazine.com, or scientificamerican.com.  Larger readership would undoubtedly motivate more writing, too, though that's not necessarily great for my time management....

It's all at the interface. Again.

Over the last decade, there has been a great deal of exciting work in making electronically interesting systems at atomically sharp interfaces between different oxide materials (oxide heterostructures). Analogous efforts at semiconductor-dielectric interfaces have given us the conventional field-effect transistor, something like 10⁹ of which are being used to render this page for you. Likewise, heterointerfaces in compound semiconductor systems (especially the technologically relevant III-V materials like GaAs) have given us two Nobel Prizes and a great deal of quantum electronic fun. Oxides are much trickier beasts from the materials science side, making growth and interfacial control a major challenge. Moreover, with respect to basic science, transition metal oxides can be incredibly rich systems, because in many of them electron-electron interactions lead to competing electronic and magnetic phases, with consequences like the emergence of high temperature superconductivity.

A few years ago, this paper demonstrated that it was possible to get superconductivity at the interface between SrTiO₃ and LaAlO₃, two oxides that are both insulating if perfectly stoichiometric. Still, SrTiO₃ is known to superconduct if highly doped, and therefore this observation, while a great experiment, wasn't hugely shocking, given the existence of a high density electron gas at the STO/LAO interface. More recently, this paper showed that high temperature superconductivity could happen at the interface between a nominally insulating oxide and a metallic (but not superconducting) cuprate related to the high-Tc materials. This past week on the arxiv, a logical successor to these works appeared here. The authors use two nominally insulating oxides (STO again, and CaCuO₂. Because of imperfect stoichiometry at the interface (excess oxygen, apparently), there is a conducting layer at the interface, with a superconducting transition around 50 K (in one sample, though others all show transitions exceeding 25 K). Bearing in mind that this is a preprint (and therefore has not been refereed), it is still very exciting. We are finally approaching the ability to engineer complex materials (not just semiconductors) on the atomic layer level, and this should be an incredible playground for basic science and materials engineering. It'd be great to get plugged into a collaboration working in this area.