Bad science, bad science journalism: the EmDrive

No, NASA has not discovered warp drive.  There is a huge amount of media attention (here, here, here, for examples of relatively mainstream media) being given to a claim that a NASA team has successfully tested a gadget called the EmDrive.  The claim is that one can take a conical microwave resonator (picture the cavity that is your microwave oven, only shaped like a truncated cone rather than a rectangular box), fire up microwaves to drive the resonant modes, and the cone will experience a steady thrust in one direction (the direction of the fat end of the cavity).  There are multiple alleged explanations for this, ranging from botched thinking about special relativity to really bizarre word-salad about virtual particles, the quantum vacuum, and "warp fields".

Let me explain why this is bad science, terrible science journalism, and highly problematic.

First, the science.  Our theory of electricity and magnetism is arguably the best understood, most precisely tested theory we have, both in its classical limit (the limit relevant for your microwave oven) and in its quantum limit (the limit relevant for things like calculating the magnetic moment of the electron, something that we can do to more than 14 decimal places!  According to that theory, a closed microwave resonator does not generate thrust (surprise surprise).  Given over 100 years of tests of classical E&M, it's going to take more than one poorly documented experiment, not published, to convince scientists that something exotic is going on.  Extraordinary claims require extraordinary evidence, and this just isn't it.  Moreover, claims that exotic quantum vacuum effects or "warp fields" are somehow relevant here are just on their face absurd!  The energy densities, the materials involved, none of this couples to exotic quantum vacuum physics any more than my microwave oven does.  This is like arguing that by accelerating a simple dielectric like a piece of plastic, I should see electron-positron pair production and warped spacetime.  It's nonsense.

What would it take to convince me?  How about a thoroughly documented experiment done by someone with credibility in precision measurement, for a start.

As for science journalism:  The number of outlets who uncritically pass along something like this is appalling.  What's worse, they distort it even more - the third link up top not only claims that this is a reactionless drive, but that it will allow faster-than-light travel.  What the hell?  (Yes, I know that the Daily Fail is third-rate fish-wrap.)  I fully expect to see a CNN story about this, and it will be terrible.  This will propagate in the media for several days, and they will portray it as some underdog inventors showing that the Scientific Establishment is wrong, or they'll present this as an actual scientific controversy, when in fact the burden is all on the experimenters to show that their work (which flies in the face of decades of contrary evidence) is right.  Hey, IFLS:  You should be ashamed of yourselves for your coverage of this.  Good grief - I thought part of your message was that people should, I don't know, think critically!

Why is this problematic?  It's an issue because people don't trust science, in part because they end up reading uncritical bull like this and come away thinking that science is either a dodge, a scam, or entirely a matter of opinion, when in fact it's an approach to thinking critically about the world that has made possible all of modern technology and medicine.

Anecdote 2: Life in a lab - the Demon Liquefier From Hell

I know this will come as a shock to many of you (ahem), but when I was a kid I watched a lot of Star Trek reruns.  Even in middle school one story-telling trope that seemed phony to me was the way Scotty (and Kirk) could tell just from the sound and feel of the ship whether something was wrong with the engines or environmental controls.   Years later, as a grad student in the Osheroff lab, I realized that this was actually one of the more realistic bits of writing and characterization in the show.

Our lab focused on ultralow temperature physics.  We ran experiments using dilution refrigerators (also see here), and these each required multiple vacuum pumps running continuously (in our case, each fridge needed a helium-leak-tight, sealed, mechanical "roughing" pump, a big conventional mechanical pump (for the "1K pot"), and a large diffusion pump as a "booster").   The mechanical pumps were housed in a cabinet in a room one floor below the main lab, and even with that kind of distance and insulation they provided a continuous background hum to the room.  That basement room also contained our group's helium liquefier, an ancient beast of a machine (a twin is shown here) that took in recycled helium gas from our experiments, cooled it by using pistons to drive a big flywheel, and then liquefied it by squirting it through a tiny, cold orifice.  The liquefier provided something between a wheeze and a heartbeat to the lab, a steady state "pachooka pachooka" sound with a repetition period of around one second when it was working well.  The muffled version of this noise also permeated the lab.  After being in the group for a few months, I understood completely where Scotty was coming from.  It was deeply disturbing to walk into the lab and realize that something, somewhere was amiss because the sound or extremely subtle floor vibrations felt "off".  

The liquefier (officially the Demon Liquefier From Hell [DLFH], or The Liquef--ker) was a formative part of our lab's grad school experience.  Running the system, which predated any serious automated controls, required some amount of fiddling in the best of times, interpreting half a dozen cryptic gauges ("inches of water" as a pressure unit?  Really?), with the only useful diagnostic being whether the liquid level in the big helium storage dewar is increasing or not.  A period preventative maintenance every few months meant replacing press-fit bearings, cleaning amazingly stinky phenolic parts, and worrying that we would bend a cam "wrist" and be out hundreds of dollars for a spare as well as having the system be down for a week.  Even before helium prices rose dramatically, recycling helium was a good idea if you could do it.   One of the most depressing calculations you could do as a student in our lab, as you were listening to the intake purifier blow moisture like a sad sneeze and wondering why the hell the DLFH wasn't making liquid, was to compare the cost of your time, recycled helium, and externally purchased helium, and realize that it was clearly financially smart for your adviser to use you to maintain the system.

The DLFH was certainly educational.  I learned a lot about engines and big mechanical systems.  I learned that it is only marginally cheaper to build a heavy crate and ship via an express carrier than it is just to buy a plane ticket for a 130 kg flywheel.  I learned what it feels like to take a jolt of 208 V (not recommended) and that yelped curses from that room could still be heard up in the lab.  To this day I still reflexively shudder a bit when I hear that "pachooka" sound when I visit a place with a similar gadget.  

Anecdote 1: The Qual

A key aspect of a good graduate education is realizing, more than ever, that to be competitive you'll have to raise your game.

My cohort of physics grad students arrived at Stanford in a sunny, dry September of 1993, and we were an interesting bunch.  Four out of the twenty of us were Russian (or from the recently former Soviet Union), and for this story it's important to understand that these folks were incredibly well-prepared in terms of academic physics training.  Growing up in the Soviet system, they basically decided for you when you were something like 14 years old if you were going to be trained as a physicist.  We all got together at a mixer in a crummy graduate apartment, and I still remember a bunch of us standing around the drinks table, chatting about our undergrad schools and what we'd studied.  One person had been a kicker for the Northwestern football team!  One person had been into rock climbing and had done a fun summer program at Los Alamos.  Then one of the Russians said that he'd studied conformal field theory.  For fun.  Kind of set the stage a bit.

At the time the department had a "qualifying exam" that was one of a series of tasks students had to complete in order to (eventually) receive doctoral candidacy.  In this case, the qual was a two-day, six hours each day, written exam with a total of eight problems, basically on advanced Stanford-level undergrad material, administered early in the fall quarter.  Two of the questions were "general physics", meant to test your ability to think on the fly and reason quantitatively as a physicist - these tended to be hard, since they didn't really seem like the kinds of questions you're usually asked in a standard undergrad physics class.  As I later learned from serving as the student rep on the department's qual committee, the point of the test was not to act as a filter to weed out weak students, or some kind of check on admissions.  The intent, at least for the 30% of the faculty who really thought this was a good idea, was that this was an assessment tool.  For example, if you passed overall but did badly on the quantum question, you would be strongly encouraged to think about taking (or grading) the undergrad quantum course.  You had two tries to pass the written exam, and if you were well prepared, you were strongly encouraged to give it a shot as soon as you got started in the program - why wait?  A strong showing on the qual could also ease the process of finding a rotation slot with a would-be thesis adviser.  Still, like any formal exam when the stakes are high, the process was fraught with tension.

Getting a really good qual exam together is very challenging, particularly if you want the problems to be solvable yet not be rehashed from books or other common sources.  This particular year, Bob Laughlin was chairing the qual committee, and he had lost patience with some of his colleagues and decided to put together much of the exam himself.   (Laughlin is a well-known, larger-than-life person who figures in a couple of other stories I may get around to telling.)  The previous year he'd written a question about heat capacity and thermal conductivity involving the cooking of a pot roast.  This problem was sufficiently infamous that he thought it would be funny to write another problem our year about pot roast (though he spelled it "potroast", prompting one Russian to ask, "Vot is this 'po-tro-ast'?").  He wrote a question spoofing "Brilliant Pebbles" (pdf!), a missile defense concept that he found completely ridiculous and impractical.  The exercise was about "brilliant pot roast", with the idea of de-orbiting 2 kg pieces of beef as kinetic kill weapons to take out missiles.   This included giving your opinion and a physics justification of whether the pot roast would splatter on the outside of the missile or punch a cartoonish pot roast-shaped hole through the missile.  He concluded the problem by saying "Don't worry if the numbers you find for this are absurd.  We'll just delete them and replace them with happier numbers.  This is called 'government science'."

We took the test in a big lecture room in one of the buildings ringing Stanford's main quad.   Chalkboards up front, lots of wood, afternoon sunlight slanting through narrow windows near the high ceiling.  The room had somewhat shallow tiered seating and long, curved tables rather than desks, so that everyone taking the exam (probably 30 people or so) could spread out and have plenty of room.   Stanford's honor code meant that the exam was unproctored, but Laughlin was sitting outside doing some reading, in case we had questions about the wording of the test.

Around 5 hours into day 1 (if I recall correctly), Laughlin came into the room, looking somewhat agitated.  "May I have your attention please?  It's been brought to my attention that there is a typographical mistake on the exam."

[groan from frustrated, tired students]

"On the time-dependent quantum problem, these two frequencies \( \omega_{0} \) and \( \omega \) are both supposed to be \( \omega_{0} \).  It may not be analytically solvable as written."

[angry muttering from bitter, aggravated students who had been wasting critical time on this]

"No," says a clear, Russian-accented voice from the back of the room, the same fellow who had studied conformal field theory, "Is difficult, but can be solved.  Have done."

[combination of disbelief, resignation, and semi-desperate laughter from the crowd]

Welcome to physics grad school.

Anecdotes from grad school and beyond

I've been thinking about what a more general audience likes to read in terms of science writing beyond descriptions of cool science.  Interesting personalities definitely have appeal.  Sure, he was a Nobel Laureate, but my guess is that much of Feyman's popularity originates from the fact that he really was a "curious character" and a great story-teller.  I'm not remotely in the same league, but in my scientific career, going back to grad school, I've been ridiculously fortunate to have had the chance to meet and interact with many interesting people.  Some of the stories might give a better slice-of-life feel for graduate science education and a scientific career than you'd get from The Big Bang Theory.  I'm going to start trying to write up some of these anecdotes - my apologies to friends who have heard some of these before....

Several items - SpaceX, dark matter, Dyson spheres, Bell Labs, and some condensed matter articles

There are a number of interesting physicsy science stories out there right now:

• SpaceX came very very close to successfully landing and recovering the first stage of their Falcon 9 rocket yesterday.  It goes almost without saying that they are doing this because they want to reuse the booster and want to avoid ruining the engines by having them end up in salt water.  I've seen a number of well-intentioned people online ask, why don't they just use a parachute, or set up a big net to catch it if it falls sideways, etc.  To answer the first question:  The booster is designed to be mechanically happy in compression, when the weight of the rocket is pushing down on the lower parts as it sits on the pad, and when the acceleration due to the engines is pushing it along its long axis.  Adding structure to make the booster strong in tension as well (as when it gets yanked on from above by parachute drogue lines) would be a major redesign and would add mass (that takes away from payload).  For the second question:  The nearly empty booster is basically a thin-walled metal tube.  If it's supported unevenly from the side, it will buckle under accelerations (like hitting a net).  Good luck to them!
• It would appear that there is observational evidence that dark matter might interact with itself through forces that are not just gravitational.  That would be very interesting indeed.  Many "simple" ideas about dark matter (say photinos) are not charged, so real dark-dark interactions beyond gravity could limit the candidates to consider.  I'm sure there will be papers on the high energy part of the arXiv within days claiming that string theory predicts exactly this, regardless of what "this" is.
• A Penn State group did a study based on WISE data, and concluded after surveying 100000 distant galaxies that there are only about 50 that seem to emit "too much" in the infrared relative to expectations.   Why look for this?  Well, if there were galaxy-spanning civilizations capable of stellar-scale engineering projects, and if they decided to use that capability to build Dyson spheres to try to capture more than 10% of the star-radiated power in the galaxy, and if those civilizations liked temperature ranges near ours, then you would expect to see an excess of infrared.  So.  Seems like galaxy-spanning civilizations that like to do massive building of Dyson spheres and similar structures are very rare.  I can't say that I'm surprised, but I am glad that creative people are doing searches like this.
• Alcatel-Lucent, including Bell Labs, is being purchased by Nokia.  If anyone knows what this means for Bell Labs research at the combined company, please feel free to post below.  
• One interesting article I noticed in Nature Physics (sorry for the paywall) shows remarkably nice, clean fractional quantum Hall effect (FQHE) physics in ZnMgO/ZnO heterostructures.  The FQHE tends to be "fragile" - the 2d electron system has to be in a material environment so clean and perfect that not only can an electron make many cyclotron orbits before it scatters off any impurities or defects, but that kind of disorder has to be weak compared to some finicky electron-electron interactions that are at milliKelvin scales.   The new data shows FQHE signatures at "filling fractions" (ratios of magnetic field to electron density) that correspond to some comparatively exotic collective states.  Neat.
• There is a special issue of Physica C coming out in honor of the remarkable (and very nice guy) Ted Geballe, a pioneer in superconductivity research.  I really don't like Elsevier as a publisher, so I am not going to link to their journal.  However, I will link to the arXiv versions of all the articles I've found from that issue:  "What Tc Tells", "Unconventional superconductivity in electron-doped layered metal nitride halides", "Superconductivity of magnesium diboride", "Superconducting doped topological materials", "Hole-doped cuprate high temperature superconductors", "Superconductivity in the elements, alloys, and simple compounds", "Epilogue:  Superconducting materials, past, present, and future", and "Superconducting materials classes:  Introduction and overview".  Good stuff by some of the big names in the field.

The Leidenfrost Effect, or how I didn't burn myself in the kitchen

The transfer of heat, the energy content of materials tied to the disorganized motion of their constituents, is big business.  A typical car engine is cooled by conducting heat to a flowing mixture of water and glycol, and that mixture is cooled by transferring that heat to gas molecules that get blown past a radiator by a fan.  Without this transfer of heat, your engine would overheat and fail.  Likewise, the processor in your desktop computer generates about 100 W of thermal power, and that's carried away by either a fancy heat-sink with air blown across it by a fan, or through a liquid cooling system if you have a really fancy gaming machine.

Heat transfer is described quantitatively by a couple of different parameters.  The simplest one to think about is the thermal conductivity \(\kappa_{T}\).  If you have a hunk of material with cross-sectional area \(A\) and length \(L\), and the temperature difference between the hot side and the cold side is \(\Delta T\), the thermal conductivity (units of W/m-K in SI) tells you the rate (\(\dot{q}\), units of Watts) at which thermal energy is transferred across the material:  \( \dot{q} = \kappa_{T} A \Delta T/L\).

Where things can get tricky is that \(\kappa_{T}\) isn't necessarily just some material-specific number - the transport of heat can depend on lots of details.  For example, you could have heat being transferred from the bottom of a hot pot into water that's boiling.  Some of the energy from the solid is going into the kinetic energy of the liquid water molecules; some of that energy is going into popping molecules from the liquid and into the gas phase.  The motion of the liquid and the vapor is complicated, and made all the more so because \(\kappa_{T}\) for the liquid is \(>> \kappa_{T}\) for the vapor.  (There is a generalized quantity, the heat transfer coefficient, that is defined similarly to \(\kappa_{T}\) but is meant to encompass all this sort of mess.)  If you think about \(\dot{q}\) as the variable you control (for example, by cranking up the knob on your gas burner), you can have different regimes, as shown in the graph to the right (from this nice wikipedia entry).  

At the highest heat flux, the water right next to the pan flashes into a layer of vapor, and because that vapor is a relatively poor thermal conductor, the liquid water remains relatively cool (that is, because \(\kappa_{T}\) is low, \(\Delta T\) is comparatively large for a fixed \(\dot{q}\)).    This regime is called film boiling, and you have seen it if you've ever watched a droplet of water skitter over a hot pan, or watched a blob of liquid nitrogen skate across a lab floor.  The fact that the liquid stays comparatively cool is called the Leidenfrost Effect.   This comparatively thermal insulating property of the vapor layer can be very dramatic, as shown in this Mythbusters video, where they show that having wet hands allows you to momentarily dip your hand in molten lead (!) without being injured. Note that this demo was most famously performed by Prof. Jearl Walker, author of the Flying Circus of Physics, former Amateur Scientist columnist for SciAm, and inheritor of the mantle of Halliday and Resnick.  The Leidenfrost Effect is also the reason that I did not actually burn my (wet) hand on the handle of a hot roasting pan last weekend.

This heat transfer example is actually a particular instance of a more general phenomenon.  When some property of a material (here \(\kappa_{T}\)) is dramatically dependent on the phase of that material (here liquid vs vapor), and that property can help determine dynamically which phase the material is in, you can get very rich behavior, including oscillations.  This can be seen in boiling liquids, as well as electronic systems with a phase change (pdf example with a metal-insulator transition, link to a review of examples with superconductor-normal metal transitions ).  

submerged due to grant deadline

Fear not, a new post is coming soon, but for now I'm trying to finish off a proposal.

America's "obsession with STEM education" is neither an obsession, nor is it dangerous

I'm late to the party about Fareed Zakaria's piece in the Washington Post titled "Why America's Obsession with STEM Education is Dangerous".  Zakaria is a smart guy, and I recognize that he has a book to sell, but this article is rhetorically frustrating:  He demolishes a serious straw man.  He wants people to be aware of the importance of a broad-based education, and he is apparently worried (or claiming to be for the sake of getting attention) that the US is culturally too focused on STEM and not enough on the other things, like creativity, the arts, and teaching people how to write well.

He is absolutely right that a broad-based education is generally a good idea, and that teaching people actual critical thinking and writing skills and an appreciation for things beyond math and science is also good.  However, I don't think you'll find any reasonable person advocating for purely technical educations with no cultural appreciation and ignoring teaching people how to communicate.   It's easy to demolish an argument that no one is making.  I could write 500 words about how it's crazy for people to drive themselves into crushing debt to get degrees that fail to teach them anything beyond rudimentary writing skills, but that would not be an assault on liberal education.

In two key respects, Zakaria has missed the boat.  First, while there is basically zero chance that we are going to abandon broad-based education in the US, it does seem like there is a far more real danger that we are trending away from science and rationality (c.f. vaccines, evolution, climate science).  Second, and here he was much closer to right, there is a danger in viewing absolutely all public investment in people (via education) and research purely in terms of short-term economic benefit - essentially eschewing basic research or basic education in favor purely of applied research and vocational training of obvious economic benefit to the country.   Frankly, there are people out there who truly do not believe in public education, period, and that's much scarier to me than an imagined attack on the value of the humanities as a component of an education.

The physics of drying your hands

We've all been there:  You wash your hands after using the restroom facilities, and turn away from the sink only to find one of those sad, completely ineffectual, old-style hot-air hand dryers bolted to the wall.  You know, the kind with the info graphic shown to the right (image credit:  nyulocal.com).  Why do these things work so poorly compared to paper towels?  What insight did Excel and Dyson have that makes their systems so much better?

It all comes down to the physics of trying to dry your hands.  At a rough estimate, the surface area of your hands is around 430 cm².   If your hands, when wet, are coated on average by a layer of water 100 microns thick (seems not crazy), that's a total volume of water of 4.3 cm³.  How can you get that water off of you?  One approach, apparently the one pursued by the original hot air dryers, is to convert that water into vapor.  Clearly the idea is not to do this by raising the temperature of your hands to the boiling point of water.  Rather, the idea is to flow hot, dry air over your hands, with the idea that the water molecules in question will acquire the necessary latent heat of vaporization (the energy input required to pull water molecules out of the condensed (liquid) phase and into the vapor phase) from their surroundings - the dry air, your hands, etc.  This "borrowing" of energy is the principle behind evaporative cooling, why you feel cold when you step out of the shower.

[A digression in fancy thermodynamic language:  When liquid water is in contact with dry air, the chemical potential for the water molecules is much higher in the liquid than in the air.  While the water molecules are attracted to each other via hydrogen bonds and polar interactions, there are so many many more ways that the water molecules could be arranged if they were diluted out into vapor in the air that they will tend to leave the liquid, provided each molecule can, through a thermal fluctuation of some sort, acquire enough energy to sever its bonds from the liquid.  The departing molecules leave behind a liquid with a lower average total energy, cooling it.  Note that water molecules can come from the vapor phase and land in the liquid, too, depositing that same latent heat per molecule back into the liquid.  When the departure and arrival processes balance, the vapor is said to be at the "saturated vapor pressure", and evaporative cooling stops.  This is why sweating a whole bunch on a super humid day does not cool you off.]

Back to your hands.  Converting 4.3 cm³ of water into vapor requires about 9700 Joules of energy.  If you wanted to do this with the heat supplied by the hot air dryer, and to do it in about a minute (which is far longer than most people are willing to stand there rubbing their hands as some feeble fan wheezes along), the dryer would have to be imparting about 160 W of power into the water.  Clearly that's not happening - you just can't get that much power into the water without cooking your hands!  Instead, you give up in disgust and wipe your hands discreetly on your pants.

In contrast, paper towels use thermodynamics much more effectively.  Rather than trying to convert the water to vapor, paper towels take great advantage of (1) the very large surface area of paper towels, and (2) capillary forces, the fact that the liquid-solid surface interaction between water and paper towel fibers is so attractive that it's energetically favorable for the water to spread out (even at the cost of increasing more liquid-vapor interface) and coat the fibers, soaking into the towel.  [Bonus physics lesson:  the wet paper towel looks darker because the optical properties of the water layer disfavor the scattering processes on micron-scale bits of fluff that tend to make the towel look white-ish.]  Yes, it takes energy to make paper towels, and yes, they must then be disposed.  However, they actually get your hands dry!

What about Excel and Dyson?  They realized very clearly that trying to vaporize the water on your hands is a fool's errand.  Instead, they try to use actual momentum transfer from the air to the water to blow the water off your hands.  Basically they accelerate a stream of air up to relatively high velocity (400 miles per hour, allegedly, though that sounds high to me).  That air, through its viscosity, transfers momentum to the water and that shear force drives the water off your hands.  They seem to have found a happy regime where they can blow the water off your hands in 10-15 seconds without the force from the air hurting you.    The awesome spectacle of those good dryers just shows how sad and lame the bad ones are by comparison.

Cleanrooms - what is new and exciting?

Cleanrooms - basically climate-controlled, dust-mitigated environments filled with equipment useful for micro/nanoscale fabrication and associated characterization - are a staple of modern research universities.  What kind of tool set and facilities you need depends on what you're trying to do.  For example, if you want to teach/do research on the fabrication of high performance Si transistors or large-scale integrated circuits, you probably want a dedicated facility that deals primarily with Si CMOS processing.  That might include large-area photolithography or wafer-scale e-beam lithography or nanoimprint lithography tools, evaporators/sputtering systems/PECVD/RIE/ALD systems able to service 150 mm or 200 mm substrates, and you might want to keep non-Si-friendly metals like Au far far away.  On the flip side, if you are more interested in supporting microfluidics or MEMS work, you might be more interested in smaller substrates but diverse materials, and tools like deep etchers and critical point dryers.

We're about to embark on a cleanroom upgrade at my institution, and I would appreciate input from my relevant readers:  What in your view is the latest and greatest in micro/nanofab tools?  What can't you do without?  Any particularly clever arrangements of facilities/  Assume we are already going to have the obvious stuff, and that we're not trying to create a production line that can handle 200 mm substrates.  Conversely, if you have suggestions of particular tools to avoid, that would also be very helpful.  Insights would be greatly appreciated.

Brief items, public science outreach edition

Here are a couple of interesting things I've come across in terms of public science outreach lately:

• I generally f-ing love "I f-ing love science" - they reach a truly impressive number of people, and they usually do a good job of conveying why science itself (beyond just particular results) is fun.  That being said, I've started to notice lately that in the physics and astro stories they run they sometimes either use inaccurate/hype-y headlines or report what is basically a press release completely uncritically.  For instance, while it fires the mind of science fiction fans everywhere, I don't think it's actually good that IFLS decided to highlight a paper from the relatively obscure journal Phys. Lett. B and claim in a headline that the LHC could detect extra spatial dimensions by making mini black holes.  Sure.  And SETI might detect a signal next week.  What are the odds that this will actually take place?  Similarly, the headline "Spacetime foam discovery proves Einstein right" implies that someone has actually observed signatures of spacetime foam.  In fact, the story is the exact opposite:  Observations of photons from gamma ray bursts have shown no evidence of "foaminess" of spacetime, meaning that general relativity (without any exotic quantumness) can explain the results.   A little improved quality control on the selection and headlines particularly on the high energy/astro stories would be great, thanks.
• There was an article in the most recent APS News that got me interested in Alan Alda's efforts at Stony Brook on communicating science to the public.  Alda, who hosted Scientific American Frontiers and played Feynman on Broadway, has dedicated a large part of his time in recent years to the cause of trying to spread the word to the general public about what science is, how it works, how it often involves compelling narratives, and how it is in many ways a pinnacle of human achievement.  He is a fan of "challenge" contests, where participants are invited to submit a 300-word non-jargony explanation of some concept or phenomenon (e.g., "What is a flame?", "What is sleep?").  This is really hard to do well!  
• Vox has an article that isn't surprising at all:  Uncritical, hype-filled reporting of medical studies leads to news articles that give conflicting information to the public, and contributes to a growing sense among the lay-people that science is untrustworthy or a matter of opinion.  Sigh.
• Occasionally deficit-hawk politicians realize that science research can benefit them by, e.g., curing cancer.  If only they thought that basic research itself was valuable.

"Flip chip" approach to nanoelectronics

Most people who aren't experts in the field don't really appreciate how amazing our electronic device capabilities are in integrated circuits.  Every time some lithographic patterning, materials deposition, or etching step is performed on an electrically interesting substrate (e.g., a Si chip), there is some amount of chemical damage or modification to the underlying material.  In the Si industry, we have gotten extremely good over the last five decades at either minimizing that collateral damage, or making sure that we can reverse its effects.  However, other systems have proven more problematic.  Any surface processing on GaAs-based structures tends to reduce the mobility of charge in underlying devices, and increases the apparent disorder in the material.  For more complex oxides like the cuprate or pnictide superconductors, even air exposure under ambient conditions (let alone much lithographic processing) can alter the surface oxygen content, affecting the properties of the underlying material.

However, for both basic science and technological motivations, we sometimes want to apply electrodes on small scales onto materials where damage from traditional patterning methods is unavoidable and can have severe consequences for the resulting measurements.  For example, this work used electrodes patterned onto PDMS, a soft silicone rubber.  The elastomer-supported electrodes were then laminated (reversibly!) onto the surface of a single crystal of rubrene, a small molecule organic semiconductor.  Conventional lithography onto such a fragile van der Waals crystal is basically impossible, but with this approach the investigators were able to make nice transistor devices to study intrinsic charge transport in the material.  

One issue with PDMS as a substrate is that it is very squishy with a large thermal expansion coefficient.  Sometimes that can be useful (read this - it's very clever), but it means that it's very difficult to put truly nanoscale electrodes onto PDMS and have them survive without distortion, wrinkling, cracking of metal layers, etc.  PDMS also really can't be used at temperatures much below ambient.  A more rigid substrate that is really flat would be great, with the idea that one could do sophisticated fab of electrode patterns, and then "flip" the electrode substrate into contact with the material of interest, which could remain untouched or unblemished by lithographic processes.

In this recent preprint, a collaboration between the Gervais group at McGill and the CINT at Sandia, the investigators used a rigid sapphire (Al2O3) substrate to support patterned Au electrodes separated by a sub-micron gap. They then flipped this onto completely unpatterned (except for large Ohmic contacts far away) GaAs/AlGaAs heterostructures.  With this arrangement, cleverly designed to remain in intimate contact even when the device is cooled to sub-Kelvin temperatures, they are able to make a quantum point contact while in principle maintaining the highest possible charge mobility of the underlying semiconductor.  It's very cool, though making truly intimate contact between two rigid substrates over mm-scale areas is very challenging - the surfaces have to be very clean, and very flat!  This configuration, while not implementable for too many device designs, is nonetheless of great potential use for expanding the kinds of materials we can probe with nanoscale electrode arrangements.

Tunneling two-level systems in solids: Direct measurements

Back in the ancient mists of time, I did my doctoral work studying tunneling two-level systems (TLS) in disordered solids.  What do these words mean?  First, read this post from 2009.   TLS are little, localized excitations that were conjectured to exist in disordered materials.  Imagine a little double-welled potential, like this image from W. A. Phillips, Rep. Prog. Phys. 50 (1987) 1657-1708.

The low temperature thermal, acoustic, and dielectric properties of glasses, for example, appear to be dominated by these little suckers, and because of the disordered nature of those materials, they come in all sorts of flavors - some with high barriers in the middle, some with low barriers; some with nearly symmetric wells, some with very asymmetric wells.   These TLS also "couple to strain" (that's how they talk to lattice vibrations and influence thermal and acoustic properties), meaning that if you stretch or squish the material, you raise one well and lower the other by an amount proportional to the stretching or squishing.

When I was a grad student, there were a tiny number of experiments that attempted to examine individual TLS, but in most disordered materials they could only be probed indirectly.   Fast forward 20 years.  It turns out that superconducting structures developed for quantum computing can be extremely sensitive to the presence of TLS, which typically exist in the glassy metal oxide layers used as tunnel barriers or at the surfaces of the superconductors.  A very cool new paper on the arxiv shows this extremely clearly.  If you look at Figure 2d, they are able to track the energy splittings of the TLS while straining the material (!), and they can actually see direct evidence of TLS talking coherently to each other.  There are "avoided crossings" between TLS levels, meaning that occasionally you end up with TLS pairs that are close enough to each other that energy can slosh coherently back and forth between them.   I find this level of information very impressive, and the TLS case continues to be an impressive example of theorists concocting a model based on comparatively scant information, and then experimentalists validating it well beyond the original expectations.   From the quantum computing perspective, though, these little entities are not a good thing, and demonstrate a maxim I formulated as a grad student:  "TLSs are everywhere, and they're evil."

(On the quantitative side:  If the energy difference between the bottoms of the two wells is \(\Delta\), and the tunneling matrix element that would allow transitions between the two wells is \(\Delta_{0}\), then a very simple calculation says that the energy difference between the ground state of this system and the first excited state is given by \(\sqrt{\Delta^{2} + \Delta_{0}^{2}}\).  If coupling to strain linearly tunes \(\Delta\), then that energy splitting should trace out a shape just like the curves seen in Fig. 2d of the paper.)

Table-top particle physics

We had a great colloquium here today by Dave DeMille from Yale University.   He spoke about his group's collaborative measurements (working with John Doyle and Gerry Gabrielse at Harvard) trying to measure the electric dipole moment of the electron.  When we teach students, we explain that as far as we have been able to determine, an electron is a truly pointlike particle (infinitesimal in size) with charge -e and spin 1/2.  That is, it has no internal structure (though somehow it contains intrinsic angular momentum, but that is a story for another day), and that means that attempts to probe the charge distribution of the electron (e.g., scattering measurements) indicate that its charge is distributed in a spherically symmetric way.

We know, though, that from the standpoint of quantum field theory like quantum electrodynamics that we should actually think of the electron as being surrounded by a cloud of "virtual" particles of various sorts.   In Feynman-like language, when an electron goes from here to there, we need to consider not just the direct path, but also the quantum amplitudes for paths with intermediate states (that could be classically forbidden), like spitting out and reabsorbing a photon between here and there.   Those paths give rise to important, measurable consequences, like the Lamb shift, so we know that they're real.  Where things get very interesting is when you wonder about more complicated corrections involving particles that break time reversal symmetry (like B and K mesons).  If you throw in what we know from the Standard Model of particle physics, those corrections lead to the conclusion that there actually should be a non-zero electric dipole moment of the electron.  That is, along its axis of "spin", there should be a slight deficit of negative charge at the north pole and excess of negative charge at the south pole, corresponding to a shift of the charge of the electron by about \(10^{-40}) cm.  That is far too small to measure.

However, suppose that there are more funky particles out there (e.g., dark matter candidates like the supersymmetric particles that many people predict should be seen at the LHC or larger colliders).  If those particles have masses on the TeV scale (that'd be convenient), there is then an expectation that there should be a detectable electric dipole moment.  DeMille and collaborators have used extremely clever atomic physics techniques involving optical measurements on beams of ThO molecules in magnetic and electric fields to look, and they've pushed the bound on any such moment (pdf) to levels that already eliminate many candidate theories.

Two comments.  First, this talk confirmed for me once again that you really have to have a special kind of personality to do truly precision measurements.  The laundry list of systematic error sources that they considered is amazing, as are the control experiments.  Second, I love this kind of thing, using "table-top" experiments (for certain definitions of "table") to get at particle physics questions.   Note that the entire cost of the whole experiment over several years so far as been around $2M.  That's not even a rounding error on the LHC budget.  Sustained investing at a decent level in this kind of work may have enormous bang-for-the-buck compared with building ever-larger colliders.

March Meeting, days 1 and 2

I am sufficiently buried in work, it's been difficult to come up with my annual March Meeting blog reports.  Here is a very brief list of some cool things I've seen:

• Jen Dionne from Stanford showed a very neat combination of tomography and cathodoluminescence, using a TEM with tilt capability to map out the plasmon modes of individual asymmetric "nanocup" particles (polystyrene core, gold off-center shell).
• Shilei Zhang presented what looks to me like a very clever idea, a "magnetic abacus" memory, that uses the spin Hall effect in a clever readout scheme as well as a spin transfer torque way to flip bits.
• I've seen a couple of talks about using interesting planar structures for optical purposes.  Harry Atwater spoke about using plasmons in graphene to make tunable resonant elements for, e.g., photodetection and modified emissivity (tuning black body radiation!).  My former Bell Labs department head Federico Capasso spoke about using designer dielectric resonator arrays to make "metasurface" optical elements (basically optical phased arrays) to do wild things like achromatic beam steering.
• Chris Adami had possibly the most ambitious title, "The Evolutionary Path Toward Sentient Robots".  Spoiler:  we are far from having to worry about this.
• Michael Coey spoke about magnetism at interfaces, including a weird result in CeO2 nanoparticles that appears to have its origins in giant orbital paramagnetism.
• There was a neat talk by Ricardo Ruiz from HGST about the amazing nanofabrication required for future hard disk storage.  Patterned media (with 10 nm half-pitch of individual magnetic islands) looks like it's on the way.

There were a number of other very nice talks.  It's pretty clear that I could have spent the whole meeting so far at "beyond graphene" 2d materials talks and things to do with spin-orbit coupling.  A bit more tomorrow.

Brief items + the March APS Meeting

This has been an absurdly busy period for the last few weeks; hence my lower rate of posting.  I hope that this will resolve itself relatively soon, but you never know.  I am going to the first three days of the March APS meeting, and will try to blog about what I see and learn there, as I have in past years.

In the meantime, a handful of items that have cropped up:

• If you go to the APS meeting, you can swing by the Cambridge University Press table, and pre-order my nano textbook for a mere $64.  It's more than 600 pages with color figures - that's a pretty good deal.  They will have a couple of bound proof copies, so you can see what it looks like, to a good approximation.  If you teach a senior undergrad or first-year grad sequence on this stuff and think you might have an interest in trying this out as a text, please drop me an email and I can see about getting you a copy.  (My editor tells me that the best way to boost readership of the book is to get a decent number of [hopefully positive] reviews on Amazon....)
• On a related note, you should really swing by the Cambridge table to order yourself a copy of the 19-years-in-the-making third edition of Horowitz and Hill's Art of Electronics.  I haven't seen it yet, but I have every reason to think that it's going to be absolutely fantastic.  Seriously, from the experimental physics side, this is a huge deal.
• This is a fun video, showing a "motor" made from an alkaline battery, a couple of metal-coated rare-earth magnets, and a coil of uninsulated wire.  It's not that crazy to see broadly how it works (think inhomogeneous fields from a finite solenoid + large magnetic moment), but it's cool nonetheless.
• Here's an article (pdf) that's very much worth reading about the importance of government funding of basic research.  It was favorably referenced here by that (sarcasm mode = on) notorious socialist organization (/sarcasm), the American Enterprise Institute.

Centers, institutes, and all that

One hallmark of the modern research university is the proliferation of Centers and Institutes, groupings of investigators outside the hierarchy of traditional academic departments.  I'd like to explain a bit about what these entities are, what (in my view) makes an effective one, and some challenges that these organizations face.  There is a great deal of variance across universities with these terms; the version I'm going to describe is mostly what we have at my home institution.  Your mileage may vary, and I'd like to hear your thoughts on what makes a great center or institute.

An Institute is an organization that draws members from across different departments (indeed, often from across different Schools such as Natural Science and Engineering), with a strong, usually broad, thematic focus, and with an annual budget for staff and programs that comes largely from internal university funds.  Institutes support programs that benefit their membership.  Examples of programs include:  seminar series; topical workshops and conferences, including interaction with companies, political entities, or the media; visitor programs; educational forays such as interdisciplinary graduate programs, training grants, research experience for undergraduates or teachers, K12 outreach days; endowed postdoctoral fellowships; etc.  An Institute is meant to act as a a catalyst or enabling structure to bring together researchers with a common intellectual interest, to foment new and support existing collaborations, and to further research activity in that area.  At some universities, an Institute may have its own building or administer shared infrastructure.

A Center is usually a smaller, more focused group of researchers that is often expected to be financially self-supporting through multi-investigator external funding.  (Sometimes these are called Laboratories.)   Centers often exist within or are founded as the result of institutes.  A Center likely concentrates on a portfolio of specific research-related projects, rather than having broad programmatic efforts like an Institute.  The US NSF sponsors a number of center programs (MRSECs, ERCs, STCs, CCIs, and formerly NSECs), as does the US DOE (EFRCs).

An effective Center is almost self-defining:  It is able to accomplish focused research goals and to raise sufficient external resources to be self-supporting at least on the several year to decade timescale.  A good Center is able to identify and adapt to new research opportunities, while realizing which avenues are becoming played-out and should be set aside - basically, effective self-criticism while encouraging creativity through seed projects to generate new activity.   Longevity and research productivity metrics are two ways to assess the utility of a Center.

An effective Institute needs to serve its members by successfully supporting and carrying out its programs.  Because many Institutes have very broad programmatic goals, this requires serious "buy-in" - a decent fraction of the membership have to be willing to invest their time and energies to ensure that these programs are a success.  This only happens when the people involved really believe in the efforts and can see that they and the institution derive real benefit from the work.   This means that the Institute has to be responsive to the needs of its membership.   At the same time, the university has to assess (via research and funding metrics) the impact of the Institute and its programs, since the university has to decide whether the internal resources of the Institute could have been better spent elsewhere.

Both Institutes and Centers can be vulnerable to budgetary problems (internal and external, respectively) and to lack of engagement by membership.  At most places (the University of Chicago seems to be a big exception, since there Institutes have a lot of power) an Institute can be particularly exposed in tight times, since departments and much of their budgets are explicitly necessary for the reaching mission of the university, while Institutes are often viewed as elective or discretionary expenses.  In terms of engagement with members, like many organizations, Institutes and Centers succeed by succeeding and fail by failing.  You can't force people to collaborate, but once some do arise, productive collaborations lead to further productive collaborations.  Overall, Centers and Institutes appear to be key components of successful research universities.  It's not clear how these organizational structures (and their associated programs) will fare if we are in a long era of declining federal funding and internal cost cutting.

2d metallicity at low temperatures - a nice new result

To quote this blog from about 8.5 years ago (!): 

For years now, there has been a fairly heated debate about the nature of an apparent metal-insulator transition (as a function of carrier density) seen in various 2d electronic and hole systems. The basic observation, originally made in some Si MOSFETs of impressively high interface quality made in Russia, is that as the 2d carrier density is reduced, the temperature dependence of the sheet resistance changes qualitatively, from a metallic dependence (lower T = lower resistance) at high carrier concentration to an insulating dependence (lower T = higher resistance) at low concentration, with a separatrix in between with nearly T-independent resistance at some critical carrier density. A famous 1979 paper by the "Gang of Four" (Anderson, Abrahams, Licciardello, and Ramakrishnan) on the scaling theory of localization had previously argued that 2d systems of noninteracting carriers all become insulating at T=0 for arbitrarily weak disorder.

So, there has been a long-simmering controversy about why some 2d systems (electrons or holes) seem to show a really metallic temperature dependence of their conductance at low temperatures.  This dependence, where the conductivity apparently increases by, say, a factor of 2 from \(T =\) 4.2K down to 0.1 K, takes place over a temperature range where the scattering of electrons by lattice vibrations (the mechanism responsible for the increase in conductivity of ordinary metals as they are cooled from room temperature down to cryogenic temperatures) is supposed to be all finished.   I mentioned this as an ongoing controversy in '06 and again in '12.  What is going on here?

There is a new preprint from Bruce Kane and colleagues at Maryland that clarifies things considerably, in my view.  Kane, probably best known for proposing a quantum computing scheme involving individual phosphorus donors in Si, is a very clever experimentalist.  He has developed a method of creating field-effect transistors,where the conducting channel is the hydrogen-terminated surface of a Si wafer, and the gate dielectric is vacuum (!).  Using these devices, his group has been able to look at the apparent metallicity in both electrons and holes in the same system.  They find that the improvement in conduction at low temperatures has to do with the screening of charged impurities by the conducting system (and for the experts:  in Si the electrons are able to do this better than the holes because there are 6 conduction band valleys, while there is no valley degeneracy for the holes).  This doesn't directly get to the "fundamental" question about whether the true, zero-temperature ground state is insulating in a real, interacting system, but it does go a long way toward demonstrating why the conductivity still has a metallic change with temperature even though phonons should be out of the picture.

Updated: Advice on choosing a grad school

Over the last week I've run into a couple of readers of this blog who pointed out that many people never find older posts (unless they happen to use google with just the right search terms), and that it might be valuable to re-run updated versions of some of those, particularly the ones geared toward career advice.  This makes lots of sense, given how long this blog has been running and how readership has evolved.  So, here is the first of these updated re-runs (from 2011):  Advice on choosing a grad school. 

This is written on the assumption that you have decided, after careful consideration, that you want to get an advanced degree (in physics, though much of this applies to any other science or engineering discipline).  This might mean that you are thinking about going into academia, or it might mean that you realize such a degree will help prepare you for a higher paying technical job outside academia.  Either way,  I'm not trying to argue the merits of a graduate degree.

• It's ok at the applicant stage not to know exactly what you want to do.  While some prospective grad students are completely sure of their interests, that's more the exception than the rule.
• If you get the opportunity to visit a school, you should go.  A visit gives you a chance to see a place, get a subconscious sense of the environment (a "gut" reaction), and most importantly, an opportunity to talk to current graduate students.  Always talk to current graduate students if you get the chance - they're the ones who really know the score.  A professor should always be able to make their work sound interesting, but grad students can tell you what a place is really like.
• I know that picking an advisor and thesis area are major decisions, but it's important to realize that those decisions do not define you for the whole rest of your career.  I would guess (and if someone had real numbers on this, please post a comment) that the very large majority of science and engineering PhDs end up spending most of their careers working on topics and problems distinct from their theses.  Your eventual employer is most likely going to be paying for your ability to think critically, structure big problems into manageable smaller ones, and knowing how to do research, rather than the particular detailed technical knowledge from your doctoral thesis.  A personal anecdote:  I did my graduate work on the ultralow temperature properties of amorphous insulators.  I no longer work at ultralow temperatures, and I don't study glasses either; nonetheless, I learned a huge amount in grad school about the process of research that I apply all the time.
• Always go someplace where there is more than one faculty member with whom you might want to work.  Even if you are 100% certain that you want to work with Prof. Smith, and that the feeling is mutual, you never know what could happen, in terms of money, circumstances, etc.  Moreover, in grad school you will learn a lot from your fellow students and other faculty.  An institution with many interesting things happening will be a more stimulating intellectual environment, and that's not a small issue.
• You should not go to grad school because you're not sure what else to do with yourself.  You should not go into research if you will only be satisfied by a Nobel Prize.  In both of those cases, you are likely to be unhappy during grad school.  
• I know grad student stipends are low, believe me.  However, it's a bad idea to make a grad school decision based on a financial difference of a few hundred or a thousand dollars a year.  Different places have vastly different costs of living - look into this.  Stanford's stipends are profoundly affected by the cost of housing near Palo Alto and are not an expression of generosity.  Pick a place for the right reasons.
• Likewise, while everyone wants a pleasant environment, picking a grad school largely based on the weather is silly.
• Pursue external fellowships if given the opportunity.  It's always nice to have your own money and not be tied strongly to the funding constraints of the faculty, if possible.  (It's been brought to my attention that at some public institutions the kind of health insurance you get can be complicated by such fellowships.  In general, I still think fellowships are very good if you can get them.)
• Be mindful of how departments and programs are run.  Is the program well organized?  What is a reasonable timetable for progress?  How are advisors selected, and when does that happen?  Who sets the stipends?  What are TA duties and expectations like?  Are there qualifying exams?  Where have graduates of that department gone after the degree?  Know what you're getting into!
• It's fine to try to communicate with professors at all stages of the process.  We'd much rather have you ask questions than the alternative.  If you don't get a quick response to an email, it's almost certainly due to busy-ness, and not a deeply meaningful decision by the faculty member.  For a sense of perspective:  I was traveling yesterday, and during that time my email queue expanded by about 50 messages, not counting all the obvious spam I deleted. 

There is no question that far more information is now available to would-be graduate students than at any time in the past.  Use it!  Look at departmental web pages, look at individual faculty member web pages.  Make an informed decision.  Good luck!

What are liquid crystals?

Once you accept the idea that the simple, microscopic interactions between bits of matter can lead to the emergence of dramatic collective properties when large numbers of particles are concerned, it's not surprising to realize that there are many different ways that large ensembles of particles end up organizing.  As mentioned previously, a true liquid is a system where the average distance between the particles is comparable to the particle size, but the particles are in constant motion and there is no particular long-range order to the way the particles are arranged.

New possibilities present themselves if the particles have some kind of "internal degree of freedom".  For example, think of the particles not as little featureless billiard balls, but as elongated objects.  Now we can consider having the orientation of all the particles have some long-range correlation.  A liquid crystal is an emergent phase when the particles are close together and there is not 3d spatial order in the arrangement of particle positions, but there is order in the orientations of the particles.  In nematic liquid crystals, the centers of mass of the particles are completely spatially disordered, but there is long-range order in their orientation. For example, they could all be pointing the same direction, indicated by the not-so-cleverly-named vector, the director.  Cholesteric liquid crystals have some twist or chirality to the particle orientation.  In smectic liquid crystals, the particle centers of mass are actually spatially ordered in one direction, but not in the other two (i.e., you can think of stacks of layers of particles, with particles free to move within each layer).  The wiki page about liquid crystals gets into the history of these systems, and here is a nice webpage that classifies them.  Liquid crystals are very useful because their directed nature gives them anisotropic optical properties, and if the objects in question are polar molecules, it is possible to reorient them electrically.  This combination enables many technologies, almost certainly including the display device you're using to read this.

There was a time when I was somehow skeptical that all these phases were "real" thermodynamic phases.  I was used to solids, liquids, and gases, and I'd learned about "hard" condensed matter phases like ferromagnets and superconductors that dealt with emergent properties of the electron gas.  Somehow these liquid crystal things didn't seem like the same sort of thing to me.  Then I read the really great book by Chaikin and Lubensky, and saw things like the figure at right (from G. S. Iannacchione and D. Finotello, Phys. Rev. E 50, 4780 (1994)).  The figure shows the specific heat of a liquid crystal (in some nanopores) as it goes through a thermally driven transition between the nematic and isotropic phases, as a function of scaled temperature, \(t \equiv (T/T_{\mathrm{c}})-1\).  This kind of sharp, divergent feature and scaling as a function of temperature are hallmarks that show these phases and their transitions are every bit as real as any other thermodynamic phase, even though the materials are squishy.