Monday, September 29, 2014
Penny-wise, pound-foolish: Krulwich blog
I just read that NPR is getting rid of Robert Krulwich's excellent science blog, allegedly as part of cost-cutting. Cost-cutting? Really? Does anyone actually think that it costs a huge sum of money to run that blog? Surely the most expensive part of the blog is Robert Krulwich's time, which he seems more than willing to give. Seriously, we should find out what the costs are, and have a kick-starter project to finance it. Come on, NPR.
Thursday, September 25, 2014
The persistent regional nature of physics
In the 21st century, with the prevalence of air travel, global near-instantaneous communications, and active cultures of well-financed scientific research on several continents, you would think that the physics enterprise would be thoroughly homogenized, at least across places with similar levels of resources. Sure, really expensive endeavors would be localized to a few places (e.g., CERN), but the comparatively cheap subfields like condensed matter physics would be rather uniformly spread out.
Strangely, in my (anecdotal, by necessity) experience, that doesn't seem to be the case. One area of my research, looking at electronic/optical/thermal properties of atomic and molecular-scale junctions, has a very small number of experimental practitioners in the US (I can think of a handful), though there are several more groups in Europe and Asia. Similarly, the relevant theory community for this work, with a few notable exceptions, is largely in Europe. This imbalance has become clear in terms of both who I talk with about this work, and where I'm asked to speak. Interestingly, there are also strong regional tendencies in some of the citation patterns (e.g., European theorists tend to cite European experimentalists), and I'm told this is true in other areas of physics (and presumably chemistry and biology). I'm sure this has a lot to do with proximity and familiarity - it's much more likely for me to see talks by geographically proximal people, even if it's equally easy for me to read papers from people all over the world.
Basically, physics areas of pursuit have a (surprising to me) large amount of regional specialization. There's been a major emphasis historically on new materials growth and discovery in, e.g., Germany, China, and Japan compared to the US (though this is being rectified, in part thanks to reports like this one). Atomic physics w/ cold atoms has historically been dominated by the US and Europe. I'm sure some of these trends are the result of funding decisions by governments. Others are due to the effect of particularly influential, talented individuals that end up having long-lasting effects because the natural timescale for change at universities is measured in decades. It will be interesting to see whether these inhomogeneities smooth out or persist over the long term.
Strangely, in my (anecdotal, by necessity) experience, that doesn't seem to be the case. One area of my research, looking at electronic/optical/thermal properties of atomic and molecular-scale junctions, has a very small number of experimental practitioners in the US (I can think of a handful), though there are several more groups in Europe and Asia. Similarly, the relevant theory community for this work, with a few notable exceptions, is largely in Europe. This imbalance has become clear in terms of both who I talk with about this work, and where I'm asked to speak. Interestingly, there are also strong regional tendencies in some of the citation patterns (e.g., European theorists tend to cite European experimentalists), and I'm told this is true in other areas of physics (and presumably chemistry and biology). I'm sure this has a lot to do with proximity and familiarity - it's much more likely for me to see talks by geographically proximal people, even if it's equally easy for me to read papers from people all over the world.
Basically, physics areas of pursuit have a (surprising to me) large amount of regional specialization. There's been a major emphasis historically on new materials growth and discovery in, e.g., Germany, China, and Japan compared to the US (though this is being rectified, in part thanks to reports like this one). Atomic physics w/ cold atoms has historically been dominated by the US and Europe. I'm sure some of these trends are the result of funding decisions by governments. Others are due to the effect of particularly influential, talented individuals that end up having long-lasting effects because the natural timescale for change at universities is measured in decades. It will be interesting to see whether these inhomogeneities smooth out or persist over the long term.
Tuesday, September 23, 2014
Hype, BICEP2, and all that.
It's been a few years since I've written a post slamming some piece of hype about nanoscience. In part, I decided that all this-is-hype posts start to sound the same and therefore weren't worth making unless the situation was truly egregious or somehow otherwise special. In part, I also felt like I was preaching to the choir, so to speak. That being said, I think the recent dustup over the BICEP2 experiment is worth mentioning, as an object lesson.
- If the BICEP2 collaboration had only posted their paper on the arxiv and said that the validity of their interpretation depended on further checks of the background by, e.g., the PLANCK collaboration, no one would have batted an eye. They could have said that they were excited but cautious, and that, too, would have been fine.
- Where they (in my view) crossed the line is when they orchestrated a major media extravaganza around their results, including showing up at Andre Linde's house and filming his reaction on being told about the data. Sure, they were excited, but it seems pretty clear that they went well beyond the norm in terms of trying to whip up attention and recognition.
- While not catastrophic for science or anything hyperbolic like that by itself, this is just another of the death-by-1000-cuts events that erodes public confidence in science. "Why believe what scientists say? They drum up attention all the time, and then turn out to be wrong! That's why low fat diets were good for me before they were bad for me!"
- Bottom line: If you are thinking of staging a press conference and a big announcement before your paper has even been sent out to referees, please do us all a favor and think again.
Thursday, September 18, 2014
When freshman physics models fail
When we teach basic ac circuits in second semester freshman physics, or for that matter in intro to electrical engineering, we introduce the idea of an impedance, \(Z\), so that we can make ac circuit problems look like a generalization of Ohm's law. For dc currents, we say that \(V = I R\), the voltage dropped across a resistor is linearly proportional to the current. For reactive circuit elements and ac currents, we use complex numbers to keep track of phase shifts between the current and voltage. Calling \(j \equiv \sqrt{-1}\), we assume that the ac current has a time dependence \(\exp(-j \omega t\). Then we can say that the impedance \(Z\) of an inductor is \(j \omega L\), and write \(V = Z I\) for the case of an ac voltage across the inductor.
Where does that come from, though? Well, it's really Faraday's law. The magnetic flux through an inductor is given by \(\Phi = LI\). We know that the voltage induced between the ends of such a coil is given by \(-d\Phi/dt = L (dI/dt) + (dL/dt) I\), and in an ordinary inductor, \(dL/dt\) is simply zero. But not always!
Last fall and into the spring, two undergrads in my lab (aided by two grad students) were doing some measurements of inductors filled with vanadium dioxide powder, a material that goes through a sharp first-order phase transition at about 65 \(^{\circ}\)C from a low temperature insulator to a high temperature poor metal. At the transition, there is also a change in the magnetic susceptibility of the material. What I rather expected to see was a step-like change in the inductive response going across the transition, and an accompanying step-like change in the loss (due to resistive heating in the metal). Both of these effects should be small (just at the edge of detection in our scheme). Instead, the students found something very different - a big peak in the lossy response on warming, and an accompanying dip in the lossy response on cooling. We stared at this data for weeks, and I asked them to run a whole variety of checks and control experiments to make sure we didn't have something wrong with the setup. We also found that if we held the temperature fixed in the middle of the peak/dip, the response would drop off to what you'd expect in the absence of any peak/dip. No, this was clearly a real effect, requiring a time-varying temperature to be apparent, and eventually it dawned on me what was going on: we were seeing the other contribution to \(d\Phi/dt\)! As each grain flicks into the new phase, it makes a nearly singular contribution to \(dL/dt\) because the transition for each grain is so rapid.
This is analogous to the Barkhausen effect, where a pickup coil wrapped around a piece of, e.g., iron and wired into speakers produces pops and crackling sounds as an external magnetic field is swept. In the Barkhausen case, individual magnetic domains reorient or domain walls propagate suddenly, also giving a big \(d\Phi/dt\). In our version, temperature is causing sudden changes in susceptibility, but it's the same basic idea.
This was great fun to figure out, and I really enjoy that it shows how the simple model of the impedance of an inductor can fail dramatically if the material in the coil does interesting things. The paper is available here.
Where does that come from, though? Well, it's really Faraday's law. The magnetic flux through an inductor is given by \(\Phi = LI\). We know that the voltage induced between the ends of such a coil is given by \(-d\Phi/dt = L (dI/dt) + (dL/dt) I\), and in an ordinary inductor, \(dL/dt\) is simply zero. But not always!
Last fall and into the spring, two undergrads in my lab (aided by two grad students) were doing some measurements of inductors filled with vanadium dioxide powder, a material that goes through a sharp first-order phase transition at about 65 \(^{\circ}\)C from a low temperature insulator to a high temperature poor metal. At the transition, there is also a change in the magnetic susceptibility of the material. What I rather expected to see was a step-like change in the inductive response going across the transition, and an accompanying step-like change in the loss (due to resistive heating in the metal). Both of these effects should be small (just at the edge of detection in our scheme). Instead, the students found something very different - a big peak in the lossy response on warming, and an accompanying dip in the lossy response on cooling. We stared at this data for weeks, and I asked them to run a whole variety of checks and control experiments to make sure we didn't have something wrong with the setup. We also found that if we held the temperature fixed in the middle of the peak/dip, the response would drop off to what you'd expect in the absence of any peak/dip. No, this was clearly a real effect, requiring a time-varying temperature to be apparent, and eventually it dawned on me what was going on: we were seeing the other contribution to \(d\Phi/dt\)! As each grain flicks into the new phase, it makes a nearly singular contribution to \(dL/dt\) because the transition for each grain is so rapid.
This is analogous to the Barkhausen effect, where a pickup coil wrapped around a piece of, e.g., iron and wired into speakers produces pops and crackling sounds as an external magnetic field is swept. In the Barkhausen case, individual magnetic domains reorient or domain walls propagate suddenly, also giving a big \(d\Phi/dt\). In our version, temperature is causing sudden changes in susceptibility, but it's the same basic idea.
This was great fun to figure out, and I really enjoy that it shows how the simple model of the impedance of an inductor can fail dramatically if the material in the coil does interesting things. The paper is available here.
Monday, September 15, 2014
What is a "bad metal"? What is a "strange metal"?
Way back in the mists of time, I wrote about what what physicists mean when they say that some material is a metal. In brief, a metal is a material that has an electrical resistivity that decreases with decreasing temperature, and in bulk has low energy excitations of the electron system down to arbitrarily low energies (no energy gap in the spectrum). In a conventional or good metal, it makes sense to think about the electrons in terms of a classical picture often called the Drude model or a semiclassical (more quantum mechanical) picture called the Sommerfeld model. In the former, you can think of the electrons as a gas, with the idea that the electrons travel some typical distance scale, \(\ell\), the mean free path, between scattering events that randomize the direction of the electron motion. In the latter, you can think of a typical electronic state as a plane-wave-like object with some characteristic wavelength (of the highest occupied state) \(\lambda_{\mathrm{F}}\) that propagates effortlessly through the lattice, until it comes to a defect (break in the lattice symmetry) causing it to scatter. In a good metal, \(\ell >> \lambda_{\mathrm{F}}\), or equivalently \( (2\pi/\lambda_{\mathrm{F}})\ell >> 1\). Electrons propagate many wavelengths between scattering events. Moreover, it also follows (given how many valence electrons come from each atom in the lattice) that \(\ell >> a\), where \(a\) is the lattice constant, the atomic-scale distance between adjacent atoms.
Another property of a conventional metal: At low temperatures, the temperature-dependent part of the resistivity is dominated by electron-electron scattering, which in turn is limited by the number of empty electronic states that are accessible (e.g., not already filled and this forbidden as final states due to the Pauli principle). The number of excited electrons (that in a conventional metal called a Fermi liquid act roughly like ordinary electrons, with charge \(-e\) and spin 1/2) is proportional to \(T\), and therefore the number of empty states available at low energies as "targets" for scattering is also proportional to \(T\), leading to a temperature-varying contribution to the resistivity proportional to \(T^{2}\).
A bad metal is one in which some or all of these assumptions fail, empirically. That is, a bad metal has gapless excitations, but if you analyze its electrical properties and tried to model them conventionally, you might find that the \(\ell\) that you infer from the data might be small compared to a lattice spacing. This is called violating the Ioffe-Mott-Regel limit, and can happen in metals like rutile VO2 or LaSrCuO4 at high temperatures.
A strange metal is a more specific term. In a variety of systems, instead of having the resistivity scale like \(T^{2}\) at low temperatures, the resistivity scales like \(T\). This happens in the copper oxide superconductors near optimal doping. This happens in the related ruthenium oxides. This happens in some heavy fermion metals right in the "quantum critical" regime. This happens in some of the iron pnictide superconductors. In some of these materials, when some technique like photoemission is applied, instead of finding ordinary electron-like quasiparticles, a big, smeared out "incoherent" signal is detected. The idea is that in these systems there are not well-defined (in the sense of long-lived) electron-like quasiparticles, and these systems are not Fermi liquids.
There are many open questions remaining - what is the best way to think about such systems? If an electron is injected from a boring metal into one of these, does it "fractionalize", in the sense of producing a huge number of complicated many-body excitations of the strange metal? Are all strange metals the same deep down? Can one really connect these systems with quantum gravity? Fun stuff.
Another property of a conventional metal: At low temperatures, the temperature-dependent part of the resistivity is dominated by electron-electron scattering, which in turn is limited by the number of empty electronic states that are accessible (e.g., not already filled and this forbidden as final states due to the Pauli principle). The number of excited electrons (that in a conventional metal called a Fermi liquid act roughly like ordinary electrons, with charge \(-e\) and spin 1/2) is proportional to \(T\), and therefore the number of empty states available at low energies as "targets" for scattering is also proportional to \(T\), leading to a temperature-varying contribution to the resistivity proportional to \(T^{2}\).
A bad metal is one in which some or all of these assumptions fail, empirically. That is, a bad metal has gapless excitations, but if you analyze its electrical properties and tried to model them conventionally, you might find that the \(\ell\) that you infer from the data might be small compared to a lattice spacing. This is called violating the Ioffe-Mott-Regel limit, and can happen in metals like rutile VO2 or LaSrCuO4 at high temperatures.
A strange metal is a more specific term. In a variety of systems, instead of having the resistivity scale like \(T^{2}\) at low temperatures, the resistivity scales like \(T\). This happens in the copper oxide superconductors near optimal doping. This happens in the related ruthenium oxides. This happens in some heavy fermion metals right in the "quantum critical" regime. This happens in some of the iron pnictide superconductors. In some of these materials, when some technique like photoemission is applied, instead of finding ordinary electron-like quasiparticles, a big, smeared out "incoherent" signal is detected. The idea is that in these systems there are not well-defined (in the sense of long-lived) electron-like quasiparticles, and these systems are not Fermi liquids.
There are many open questions remaining - what is the best way to think about such systems? If an electron is injected from a boring metal into one of these, does it "fractionalize", in the sense of producing a huge number of complicated many-body excitations of the strange metal? Are all strange metals the same deep down? Can one really connect these systems with quantum gravity? Fun stuff.
Saturday, September 06, 2014
What is the Casimir effect?
This is another in an occasional series of posts where I try to explain some physical phenomena and concepts in a comparatively accessible way. I'm going to try hard to lean toward a lay audience here, with the very real possibility that this will fail.
You may have heard of the Casimir effect, or the Casimir force - it's usually presented in language that refers to "quantum fluctuations of the electromagnetic field", and phrases like "zero point energy" waft around. The traditional idea is that two electrically neutral, perfectly conducting plates, parallel to each other, will experience an attractive force per unit area given by \( \hbar c \pi^{2}/(240 a^{4})\), where \(a \) is the distance between the plates. For realistic conductors (and even dielectrics) it is possible to derive analogous expressions. For a recent, serious scientific review, see here (though I think it's behind a paywall).
To get some sense of where these forces come from, we need to think about van der Waals forces. It turns out that there is an attractive force between neutral atoms, say helium atoms for simplicity. We are taught to think about the electrons in helium as "looking" like puffy, spherical clouds - that's one way to visualize the electron's quantum wave function, related to the probability of finding the electron in a given spot if you decided to look through some experimental means. If you imagine using some scattering experiment to "take a snapshot" of the helium atom, you'd find the two electrons located at particular locations, probably away from the nucleus. In that sense, the helium atom would have an "instantaneous electric dipole moment". To use an analogy with magnetic dipoles, imagine that there are little bar magnets pointing from the nucleus to each electron. The influence (electric field in the real atom; magnetic field from the bar magnet analogy) of those dipoles drops off in distance like \(1/r^{3}\). Now, if there was a second nearby atom, its electrons would experience the fields from the first atom. This would tend to influence its own dipole (in the magnet analogy, instead of the bar magnets pointing on average in all directions, they would tend to align with the field from the first atom, rather like how a compass needle is influenced by a nearby bar magnet). The result would be an attractive force, proportional to \(1/r^{6}\).
In this description, we ignored that it takes time for the fields from the first atom to propagate to the second atom. This is called retardation, and it's one key difference between the van der Waals interaction (when retardation is basically assumed to be unimportant) and so-called Casimir-Polder forces.
Now we can ask, what about having more than two atoms? What happens to the forces then? Is it enough just to think of them as a bunch of pairs and add up the contributions? The short answer is, no, you can't just think about pair-wise interactions (interference effects and retardation make it necessary to treat extended objects carefully).
What about exotic quantum vacuum fluctuations, you might ask. Well, in some sense, you can think about those fluctuations and interactions with them as helping to set the randomized flipping dipole orientations in the first place, though that's not necessary. It has been shown that you can do full, relativistic, retarded calculations of these fluctuating dipole effects and you can reproduce the Casimir results (and with greater generality) without saying much of anything about zero point stuff. That is why while it is fun to speculate about zero point energy and so forth (see here for an entertaining and informative article - again, sorry about the paywall), there really doesn't seem to be any way to get net energy "out of the vacuum".
You may have heard of the Casimir effect, or the Casimir force - it's usually presented in language that refers to "quantum fluctuations of the electromagnetic field", and phrases like "zero point energy" waft around. The traditional idea is that two electrically neutral, perfectly conducting plates, parallel to each other, will experience an attractive force per unit area given by \( \hbar c \pi^{2}/(240 a^{4})\), where \(a \) is the distance between the plates. For realistic conductors (and even dielectrics) it is possible to derive analogous expressions. For a recent, serious scientific review, see here (though I think it's behind a paywall).
To get some sense of where these forces come from, we need to think about van der Waals forces. It turns out that there is an attractive force between neutral atoms, say helium atoms for simplicity. We are taught to think about the electrons in helium as "looking" like puffy, spherical clouds - that's one way to visualize the electron's quantum wave function, related to the probability of finding the electron in a given spot if you decided to look through some experimental means. If you imagine using some scattering experiment to "take a snapshot" of the helium atom, you'd find the two electrons located at particular locations, probably away from the nucleus. In that sense, the helium atom would have an "instantaneous electric dipole moment". To use an analogy with magnetic dipoles, imagine that there are little bar magnets pointing from the nucleus to each electron. The influence (electric field in the real atom; magnetic field from the bar magnet analogy) of those dipoles drops off in distance like \(1/r^{3}\). Now, if there was a second nearby atom, its electrons would experience the fields from the first atom. This would tend to influence its own dipole (in the magnet analogy, instead of the bar magnets pointing on average in all directions, they would tend to align with the field from the first atom, rather like how a compass needle is influenced by a nearby bar magnet). The result would be an attractive force, proportional to \(1/r^{6}\).
In this description, we ignored that it takes time for the fields from the first atom to propagate to the second atom. This is called retardation, and it's one key difference between the van der Waals interaction (when retardation is basically assumed to be unimportant) and so-called Casimir-Polder forces.
Now we can ask, what about having more than two atoms? What happens to the forces then? Is it enough just to think of them as a bunch of pairs and add up the contributions? The short answer is, no, you can't just think about pair-wise interactions (interference effects and retardation make it necessary to treat extended objects carefully).
What about exotic quantum vacuum fluctuations, you might ask. Well, in some sense, you can think about those fluctuations and interactions with them as helping to set the randomized flipping dipole orientations in the first place, though that's not necessary. It has been shown that you can do full, relativistic, retarded calculations of these fluctuating dipole effects and you can reproduce the Casimir results (and with greater generality) without saying much of anything about zero point stuff. That is why while it is fun to speculate about zero point energy and so forth (see here for an entertaining and informative article - again, sorry about the paywall), there really doesn't seem to be any way to get net energy "out of the vacuum".
Thursday, August 28, 2014
Two cool papers on the arxiv
The beginning of the semester is a crazy time, so blogging is a little light right now. Still, here are a couple of recent papers from the arxiv that struck my fancy.
arxiv:1408.4831 - "Self-replicating cracks: A collaborative fracture mode in thin films," by Marthelot et al.
This is very cool classical physics. In thin, brittle films moderately adhering to a substrate, there can be a competition between the stresses involved with crack propagation and the stresses involved with delamination of the film. The result can be very pretty pattern formation and impressively rich behavior. A side note: All cracks are really nanoscale phenomena - the actual breaking of bonds at the tip of the propagating crack is firmly in the nano regime.
arxiv:1408.4831 - "Self-replicating cracks: A collaborative fracture mode in thin films," by Marthelot et al.
This is very cool classical physics. In thin, brittle films moderately adhering to a substrate, there can be a competition between the stresses involved with crack propagation and the stresses involved with delamination of the film. The result can be very pretty pattern formation and impressively rich behavior. A side note: All cracks are really nanoscale phenomena - the actual breaking of bonds at the tip of the propagating crack is firmly in the nano regime.
arxiv:1408.6496 - "Non-equilibrium probing of two-level charge fluctuators using the step response of a single electron transistor," by Pourkabirian et al.
I've written previously (wow, I've been blogging a while) about "two-level systems", the local dynamic degrees of freedom that are ubiquitous in disordered materials. These little fluctuators have a statistically broad distribution of level asymmetries and tunneling times. As a result, when perturbed, the ensemble of these TLSs responds not with a simple exponential decay (as would a system with a single characteristic time scale). Instead, the TLS ensemble leads to a decaying response that is logarithmic in time. For my PhD I studied such (agonizingly) slow relaxations in the dielectric response and acoustic response of glasses (like SiO2) at cryogenic temperatures. Here, the authors use the incredible charge sensitivity of a single-electron transistor (SET) to look at the relaxation of the local charge environment near such disordered dielectrics. The TLSs often have electric dipole moments, so their relaxation changes the local electrostatic potential near the SET. Guess what: logarithmic relaxations. Cute, and brings back memories of loooooong experiments from grad school.
Wednesday, August 20, 2014
Science and engineering research infrastructure - quo vadis?
I've returned from the NSF's workshop regarding the successor program to the NNIN. While there, I learned a few interesting things, and I want to point out a serious issue facing science and engineering education and research (at least in the US).
- The NNIN has been (since 2010) essentially level-funded at $16M/yr for the whole program, and there are no indications that this will change in the foreseeable future. (Inflation erodes the value of that sum as well over time.) The NNIN serves approximately 6000 users per year (with turnover of about 2200 users/yr). For perspective, a truly cutting edge transmission electron microscope, one instrument, costs about $8M. The idea that the NNIN program can directly create bleeding edge shared research hardware across the nation is misguided.
- For comparison, the US DOE has five nano centers. The typical budget for each one is about $20M/yr. Each nano center can handle around 450 users/yr. Note that these nano centers are very different things than NNIN sites - they do not charge user fees, and they are co-located with some truly unique characterization facilities (synchrotrons, neutron sources). Still, the DOE is spending seventeen times as much per user per year in their program as the NNIN.
- Even the DOE, with their much larger investment, doesn't really know how to handle "recapitalization". That is, there was money available to buy cutting edge tools to set up their centers initially, but there is no clear, sustainable financial path to be able to replace aging instrumentation. This is exactly the same problem faced by essentially every research university in the US. Welcome to the party.
- Along those lines: As far as I can tell (and please correct me if I'm wrong about this!), every US federal granting program intended to have a component associated with increasing shared research infrastructure at universities (this includes the NSF MRI program, MRSEC, STC, ERC, CCI; DOE instrumentation grants, DOE centers like EFRCs, DOD equipment programs like DURIPs) is either level-funded or facing declining funding levels. Programs like these often favor acquisition of new, unusual tools over standard "bread-and-butter" as well. Universities are going to have to rely increasingly on internal investment to acquire/replace instrumentation. Given that there is already considerable resentment/concern about perceived stratification of research universities into "haves" and "have-nots", it's hard to see how this is going to get much better any time soon.
- To potential donors who are really interested in the problem of graduate (and advanced undergrad) science and engineering hands-on education: PLEASE consider this situation. A consortium of donors who raised, say, $300M in an endowment could support the equivalent of the NNIN on the investment returns for decades to come. This can have an impact on thousands of students/postdocs per year, for years at a time. The idea that this is something of a return to the medieval system of rich patrons supporting the sciences is distressing. However, given the constraints of government finances and the enormous sums of money out there in the hands of some brilliant, tech-savvy people who appreciate the importance of an educated workforce, I hope someone will take this possibility seriously. To put this in further perspective: I heard on the radio yesterday that the college athletics complex being built at Texas A&M University costs $400M. Think about that. A university athletic booster organization was able to raise that kind of money for something as narrowly focused (sorry, Aggies, but you know it's true).
Sunday, August 17, 2014
Distinguishable from magic?
Arthur C. Clarke's most famous epigram is that "Any sufficiently advanced technology is indistinguishable from magic." A question that I've heard debated in recent years is, have we gone far enough down that road that it's adversely affecting the science and engineering education pipeline? There was a time when young people interested in technology could rip things apart and actually get a moderately good sense of how those gadgets worked. This learning-through-disassembly approach is still encouraged, but the scope is much more limited.
For example, when I was a kid (back in the dim mists of time known as the 1970s and early 80s), I ripped apart transistor radios and at least one old, busted TV. Inside the radios, I saw how the AM tuner worked by sliding a metal contact along a wire solenoid - I learned later that this was tuning an inductor-capacitor resonator, and that the then-mysterious diodes in there (the only parts on the circuit board with some kind of polarity stamped on them, aside from the electrolytic capacitors on the power supply side) somehow were important at getting the signal out. Inside the TV, I saw that there was a whopping big transformer, some electromagnets, and that the screen was actually the front face of a big (13 inch diagonal!) vacuum tube. My dad explained to me that the electromagnets helped raster an electron beam back and forth in there, which smacked on phosphors on the inside of the screen. Putting a big permanent magnet up against the front of a screen distorted the picture and warped the colors in a cool way that depended strongly on the distance between the magnet and the screen, and on the magnet's orientation, thanks to the magnet screwing with the electron beam's trajectory.
Now, a kid opening up an ipod or little portable radio will find undocumented integrated circuits that do the digital tuning. Flat screen LCD TVs are also much more black-box-like (though the light source is obvious), again containing lots of integrated circuits. Touch screens, the accelerometers that determine which way to orient the image on a cell phone's screen, the chip that actually takes the pictures in a cell phone camera - all of these seem almost magical, and they are either packaged monolithically (and inscrutably), or all the really cool bits are too small to see without a high-power optical microscope. Even automobiles are harder to figure out, with lots of sensors, solid-state electronics, and an architecture that often actively hampers investigation.
I fully realize that I'm verging on sounding like a grumpy old man with an onion on his belt (non-US readers: see transcript here). Still, the fact that understanding of everyday technology is becoming increasingly inaccessible, disconnected with common sense and daily experience, does seem like a cause for concern. Chemistry sets, electronics sets, arduinos and raspberry pi-s, these are all ways to fight this trend, and their use should be encouraged!
For example, when I was a kid (back in the dim mists of time known as the 1970s and early 80s), I ripped apart transistor radios and at least one old, busted TV. Inside the radios, I saw how the AM tuner worked by sliding a metal contact along a wire solenoid - I learned later that this was tuning an inductor-capacitor resonator, and that the then-mysterious diodes in there (the only parts on the circuit board with some kind of polarity stamped on them, aside from the electrolytic capacitors on the power supply side) somehow were important at getting the signal out. Inside the TV, I saw that there was a whopping big transformer, some electromagnets, and that the screen was actually the front face of a big (13 inch diagonal!) vacuum tube. My dad explained to me that the electromagnets helped raster an electron beam back and forth in there, which smacked on phosphors on the inside of the screen. Putting a big permanent magnet up against the front of a screen distorted the picture and warped the colors in a cool way that depended strongly on the distance between the magnet and the screen, and on the magnet's orientation, thanks to the magnet screwing with the electron beam's trajectory.
Now, a kid opening up an ipod or little portable radio will find undocumented integrated circuits that do the digital tuning. Flat screen LCD TVs are also much more black-box-like (though the light source is obvious), again containing lots of integrated circuits. Touch screens, the accelerometers that determine which way to orient the image on a cell phone's screen, the chip that actually takes the pictures in a cell phone camera - all of these seem almost magical, and they are either packaged monolithically (and inscrutably), or all the really cool bits are too small to see without a high-power optical microscope. Even automobiles are harder to figure out, with lots of sensors, solid-state electronics, and an architecture that often actively hampers investigation.
I fully realize that I'm verging on sounding like a grumpy old man with an onion on his belt (non-US readers: see transcript here). Still, the fact that understanding of everyday technology is becoming increasingly inaccessible, disconnected with common sense and daily experience, does seem like a cause for concern. Chemistry sets, electronics sets, arduinos and raspberry pi-s, these are all ways to fight this trend, and their use should be encouraged!
Tuesday, August 12, 2014
Some quick cool science links
Here are a few neat things that have cropped up recently:
- The New Horizons spacecraft is finally getting close enough to Pluto to be able to image Pluto and Charon orbiting about their common (approximate, b/c of other moons) center of mass.
- The Moore Foundation announced the awardees in the materials synthesis component of their big program titled Emergent Phenomena in Quantum Systems. Congratulations all around.
- Here's a shock: congressmen in the pockets of the United Launch Alliance don't like SpaceX.
- Cute toy.
- The Fields Medal finally goes to a woman, Maryam Mirzakhani. Also getting a share, Manjul Bhargava, who gave the single clearest math talk I've ever seen, using only a blank transparency and a felt-tip pen.
Saturday, August 09, 2014
Nanotubes by design
There is a paper in this week's issue of Nature (with an accompanying news commentary by my colleague Jim Tour) in which the authors appear to have solved a major, two decade+ challenge, growing single-walled carbon nanotubes of a specific type. For a general audience: You can imagine rolling up a single graphene sheet and joining the edges to make a cylinder. There are many different ways to do this. The issue is, different ways of rolling up the sheet lead to different electronic properties, and the energetic differences between these different tube types are very small. When people have tried to grow nanotubes by any number of methods, they tend to end up with a bunch of tube types of similar diameters, rather than just the one they want.
The authors of this new paper have taken an approach that has great visual appeal. They have used synthetic chemistry to make a planar hydrocarbon molecule that looks like they've taken the geodesic hemisphere end-cap of their desired tube and cut it to lay it flat - like making a funky projection to create a flat map of a globe. When placed on a catalytically active Pt surface at elevated temperatures, this molecular seed can fold up into an endcap and start growing as a nanotube. The authors show Raman spectroscopic evidence that they only produce the desired tube type (in this case, a metallic nanotube). The picture is nice, and the authors imply that they could do this for other desired tube types. It's not clear whether this is scalable for large volumes, but it's certainly encouraging.
This is very cute. People in the nanotube game have been trying to do selective synthesis for twenty years. Earlier this summer, a competing group showed progress in this direction using nanoparticle seeds, an approach pursued by many over the years with limited success. It will be fun to see where this goes. This is a good example of how long it can take to solve some materials problems.
The authors of this new paper have taken an approach that has great visual appeal. They have used synthetic chemistry to make a planar hydrocarbon molecule that looks like they've taken the geodesic hemisphere end-cap of their desired tube and cut it to lay it flat - like making a funky projection to create a flat map of a globe. When placed on a catalytically active Pt surface at elevated temperatures, this molecular seed can fold up into an endcap and start growing as a nanotube. The authors show Raman spectroscopic evidence that they only produce the desired tube type (in this case, a metallic nanotube). The picture is nice, and the authors imply that they could do this for other desired tube types. It's not clear whether this is scalable for large volumes, but it's certainly encouraging.
This is very cute. People in the nanotube game have been trying to do selective synthesis for twenty years. Earlier this summer, a competing group showed progress in this direction using nanoparticle seeds, an approach pursued by many over the years with limited success. It will be fun to see where this goes. This is a good example of how long it can take to solve some materials problems.
Monday, August 04, 2014
Does being a physicist ruin science fiction for me? Generally, no.
For the past few years, as I've been teaching honors freshman mechanics, I've tried to work in at least one homework problem based on a popular sci-fi movie. Broadening that definition to include the Marvel Cinematic Universe, I've done Iron Man, Captain America, the Avengers. Yesterday I saw Guardians of the Galaxy, and I've got a problem in mind already.
I've been asked before, does being a physicist just ruin science fiction books and movies for me? Does bad physics in movies or sci-fi books annoy me since I can't not see it? Generally, the answer is "no". I don't expect Star Trek or Star Wars to be a documentary, and I completely understand why bending physics rules can make a story more fun. Iron Man would be a lot less entertaining if Tony Stark couldn't build an arc reactor with enough storage capacity and power density to fly long distances. Trips through outer space that require years of narrative time just to get to Jupiter are less fun than superluminal travel. If anything, I think well-done science fiction can be creatively inspiring.
One thing that does bug me is internally inconsistent bad physics or bad science. For example, in the book Prey by Michael Crichton, it's established early on that any tiny breach in a window, etc. is enough for the malevolent nanocritters to get in, yet at the climax of the book the author miraculously forgets this (because if he'd remembered it the protagonist would've died). Another thing that gets me is trivially avoidable science mistakes. For example, in a Star Trek:TNG episode (I looked it up - it was this one), they quote a surface temperature less than absolute zero. I'm happy to serve as a Hollywood science advisor to avoid these problems :-)
I've been asked before, does being a physicist just ruin science fiction books and movies for me? Does bad physics in movies or sci-fi books annoy me since I can't not see it? Generally, the answer is "no". I don't expect Star Trek or Star Wars to be a documentary, and I completely understand why bending physics rules can make a story more fun. Iron Man would be a lot less entertaining if Tony Stark couldn't build an arc reactor with enough storage capacity and power density to fly long distances. Trips through outer space that require years of narrative time just to get to Jupiter are less fun than superluminal travel. If anything, I think well-done science fiction can be creatively inspiring.
One thing that does bug me is internally inconsistent bad physics or bad science. For example, in the book Prey by Michael Crichton, it's established early on that any tiny breach in a window, etc. is enough for the malevolent nanocritters to get in, yet at the climax of the book the author miraculously forgets this (because if he'd remembered it the protagonist would've died). Another thing that gets me is trivially avoidable science mistakes. For example, in a Star Trek:TNG episode (I looked it up - it was this one), they quote a surface temperature less than absolute zero. I'm happy to serve as a Hollywood science advisor to avoid these problems :-)
Monday, July 28, 2014
A book, + NNIN
Sorry for the posting drought. There is a good reason: I'm in the final stages of a textbook based on courses I developed about nanostructures and nanotechnology. It's been an embarrassingly long time in the making, but I'm finally to the index-plus-final-touches stage. I'll say more when it's in to the publisher.
One other thing: I'm going to a 1.5 day workshop at NSF in three weeks about the next steps regarding the NNIN. I've been given copies of the feedback that NSF received in their request for comment period, but if you have additional opinions or information that you'd like aired there, please let me know, either in the comments or via email.
One other thing: I'm going to a 1.5 day workshop at NSF in three weeks about the next steps regarding the NNIN. I've been given copies of the feedback that NSF received in their request for comment period, but if you have additional opinions or information that you'd like aired there, please let me know, either in the comments or via email.
Monday, July 14, 2014
My Nerd Nite talk - video
I mentioned back in February that I'd had the chance to speak at Nerd Nite Houston (facebook link - it's updated more frequently than the website). It was a blast, and I encourage people in the area to check it out on the last Thursday of each month, location announced on the page, though so far they've all been at Notsuoh.
Thanks to the fantastic videographic efforts of Jon Martensen, the video of my talk is now available on youtube here. The talk is about 20 minutes and the rest is the audience Q&A. All in all, a very fun experience - thanks again to Amado Guloy and the rest of the Nerd Nite folks for giving me the opportunity.
Thanks to the fantastic videographic efforts of Jon Martensen, the video of my talk is now available on youtube here. The talk is about 20 minutes and the rest is the audience Q&A. All in all, a very fun experience - thanks again to Amado Guloy and the rest of the Nerd Nite folks for giving me the opportunity.
Sunday, July 13, 2014
Interesting links: peer review, falsifiability
Slow blogging - I've got the usual papers plus working on finishing a really big writing project (more about that soon), combined w/ summer travel. The posting rate will pick up again in another week and a half. In the meantime, here are a few interesting links from the last couple of weeks.
- A thoroughly dishonest scientist (and I guess a couple of other people) were exposed as running an awful peer review scam. More about this here. The scam involved creating fake email addresses and identities to mask people essentially reviewing their own and friends' papers. The worst thing about this whole mess is that it gives ammunition to the anti-science crowd who are convinced that scientific research is a corrupt enterprise - people like the person I wrote about here.
- Peter Woit has written an interesting review of a book about string theory and whether the scientific method needs to be revised to deal with "post-emprical" theory verification, whatever that means. I haven't read the book, but the idea of post-empiricism is pretty sketchy to me.
- Natalie Wolchover has written an article about some fluid droplet experiments that show quantum-like behavior of droplets (e.g., interference-fringe-like distributions, for example). The physics here is that the droplets are interacting with associated surface waves of an underlying fluid, and the mechanics of those waves self-consistently guides the droplets. This is similar in spirit to Bohm's ideas about pilot waves as a way of thinking about quantum mechanics. The authors of the fluid paper are clearly high on this idea. These are clearly very cool experiments, but it's a huge stretch to say that they should motivate re-thinking our interpretations of quantum mechanics.
Friday, July 04, 2014
An expression of concern about an expression of concern
There has been a big kerfluffle about Facebook conducting a mass social psychology experiment. At heart is the issue of informed consent. By clicking "ok" on a vaguely worded license agreement, did users really give true informed consent to participate in experiments designed to manipulate them? The study was published in the Proceedings of the National Academy of Sciences here. Now, in hindsight, PNAS has published an "Expression of Concern" here about whether the study was in compliance with the Common Rule regarding informed consent by human subjects. The PNAS editors point out that as a privately funded, for-profit corporation not taking federal funding for this work, Facebook isn't technically bound by this constraint.
This is technically correct (the best kind of correct), but doesn't this have frightening implications? Does this mean that private companies are free to perform experiments on human subjects without asking for informed consent, so long as they don't violate obvious laws like killing people? Seems like there must be some statutes out there about human experimentation, right? Perhaps one of my readers knows this issue....
This is technically correct (the best kind of correct), but doesn't this have frightening implications? Does this mean that private companies are free to perform experiments on human subjects without asking for informed consent, so long as they don't violate obvious laws like killing people? Seems like there must be some statutes out there about human experimentation, right? Perhaps one of my readers knows this issue....
Monday, June 30, 2014
What are universal conductance fluctuations?
Another realization I had at the Gordon Conference: there are plenty of younger people in condensed matter physics who have never heard about some mesoscopic physics topics. Presumably those topics are now in that awkward purgatory of being so established that they're "boring" from the research standpoint, but they are beyond what is taught in standard solid state physics classes (i.e., they're not in Ashcroft and Mermin or Kittel). Here is my attempt to talk at a reasonably popular level about one of these, so-called "Universal Conductance Fluctuations" (UCF).
In physics parlance, sometimes it can be very useful to think about electrons in solids as semiclassical, a kind of middle ground between picturing them as little classical specks whizzing around and visualizing them as fuzzy, entirely wavelike quantum states. In the semiclassical picture, you can think of the electrons as following particular trajectories, and still keep in mind their wavelike aspect by saying that the particles rack up phase as they propagate along. In a typical metal like gold or copper, the effective wavelength of the electrons is the Fermi wavelength, \( \lambda_{\mathrm{F}} \sim 0.1~\)nm. That means that an electron propagating 0.1 nm changes its quantum phase by about \(2 \pi\). In a relatively "clean" metal, electrons propagate along over long distances, many Fermi wavelengths, before scattering. At low temperatures, that scattering is mostly from disorder (grain boundaries, vacancies, impurities).
The point of keeping track of the quantum phase \(\phi\) is that this is how we find probabilities for quantum processes. In quantum mechanics, if there are two paths to do something, with (complex) amplitudes \(A_{1}\) and \(A_{2}\), the probability of that something is \(|A_{1} + A_{2}|^{2}\), which is different than just adding the probabilities of each path, \(|A_{1}|^{2}\) and \(|A_{2}|^{2}\). For an electron propagating, for each trajectory we can figure out an amplitude that includes the phase. We add up all the (complex) amplitudes for all the possible trajectories, and then take the (magnitude) square of the sum. The cross terms are what give quantum interference effects, such as the wavy diffraction pattern in the famous two-slit experiment. This is how Feynman describes interference in his great little book, QED.
Electronic conduction in a disordered metal then becomes a quantum interference experiment. An electron can bounce off various impurities or defects in different sequences, with each trajectory having some phase. The exact phases are set by the details of the disorder, so while they differ from sample to sample, they are the same within a given sample as long as the disorder doesn't change. The conduction of the electrons is then something like a speckle pattern. The typical scale of that speckle is a change in the conductance \(G\) of something like \(\delta G \sim e^{2}/h\). Note that inelastic processes can change the electronic wavelength (by altering the electron energy and hence the magnitude of its momentum) and also randomize the phase - these "dephasing" effects mean that on length scales large compared to some coherence length \(L_{\phi}\), it doesn't make sense to worry about quantum interference.
Now, anything that alters the relative phases of the different trajectories will lead to fluctuations in the conductance on that scale (within a coherent region). A magnetic field can do this, because the amount of phase racked up by propagating electrons depends not just on their wavelength (basically their momentum), but also on the vector potential, a funny quantity discussed further here. So, ramping a magnetic field through a (weakly disordered) metal (at low temperatures) can generate sample-specific, random-looking but reproducible, fluctuations in the conductance on the order of \(e^{2}/h\). These are the UCF.
By looking at the UCF (their variation with magnetic field, temperature, gate voltage in a semiconductor, etc.), one can infer \(L_{\phi}\), for example. These kinds of experiments were all the rage in ordinary metals and semiconductors in the late 1980s and early 1990s. They enjoyed a resurgence in the late '90s during a controversy about coherence and the fate of quasiparticles as \(T \rightarrow 0\), and are still used as a tool to examine coherence in new systems as they come along (graphene, atomically thin semiconductors, 2d electron gases in oxide heterostructures, etc.).
In physics parlance, sometimes it can be very useful to think about electrons in solids as semiclassical, a kind of middle ground between picturing them as little classical specks whizzing around and visualizing them as fuzzy, entirely wavelike quantum states. In the semiclassical picture, you can think of the electrons as following particular trajectories, and still keep in mind their wavelike aspect by saying that the particles rack up phase as they propagate along. In a typical metal like gold or copper, the effective wavelength of the electrons is the Fermi wavelength, \( \lambda_{\mathrm{F}} \sim 0.1~\)nm. That means that an electron propagating 0.1 nm changes its quantum phase by about \(2 \pi\). In a relatively "clean" metal, electrons propagate along over long distances, many Fermi wavelengths, before scattering. At low temperatures, that scattering is mostly from disorder (grain boundaries, vacancies, impurities).
The point of keeping track of the quantum phase \(\phi\) is that this is how we find probabilities for quantum processes. In quantum mechanics, if there are two paths to do something, with (complex) amplitudes \(A_{1}\) and \(A_{2}\), the probability of that something is \(|A_{1} + A_{2}|^{2}\), which is different than just adding the probabilities of each path, \(|A_{1}|^{2}\) and \(|A_{2}|^{2}\). For an electron propagating, for each trajectory we can figure out an amplitude that includes the phase. We add up all the (complex) amplitudes for all the possible trajectories, and then take the (magnitude) square of the sum. The cross terms are what give quantum interference effects, such as the wavy diffraction pattern in the famous two-slit experiment. This is how Feynman describes interference in his great little book, QED.
Electronic conduction in a disordered metal then becomes a quantum interference experiment. An electron can bounce off various impurities or defects in different sequences, with each trajectory having some phase. The exact phases are set by the details of the disorder, so while they differ from sample to sample, they are the same within a given sample as long as the disorder doesn't change. The conduction of the electrons is then something like a speckle pattern. The typical scale of that speckle is a change in the conductance \(G\) of something like \(\delta G \sim e^{2}/h\). Note that inelastic processes can change the electronic wavelength (by altering the electron energy and hence the magnitude of its momentum) and also randomize the phase - these "dephasing" effects mean that on length scales large compared to some coherence length \(L_{\phi}\), it doesn't make sense to worry about quantum interference.
Now, anything that alters the relative phases of the different trajectories will lead to fluctuations in the conductance on that scale (within a coherent region). A magnetic field can do this, because the amount of phase racked up by propagating electrons depends not just on their wavelength (basically their momentum), but also on the vector potential, a funny quantity discussed further here. So, ramping a magnetic field through a (weakly disordered) metal (at low temperatures) can generate sample-specific, random-looking but reproducible, fluctuations in the conductance on the order of \(e^{2}/h\). These are the UCF.
By looking at the UCF (their variation with magnetic field, temperature, gate voltage in a semiconductor, etc.), one can infer \(L_{\phi}\), for example. These kinds of experiments were all the rage in ordinary metals and semiconductors in the late 1980s and early 1990s. They enjoyed a resurgence in the late '90s during a controversy about coherence and the fate of quasiparticles as \(T \rightarrow 0\), and are still used as a tool to examine coherence in new systems as they come along (graphene, atomically thin semiconductors, 2d electron gases in oxide heterostructures, etc.).
Thursday, June 26, 2014
Gordon Conference thoughts
Because of travel constraints I'm missing the last day of the meeting, but here are some thoughts, non-science first:
- These meetings remain a great format - not too big, a good mix of topics, real opportunities for students and postdocs to interact w/ lots of people, chances for older researchers to play soccer and ultimate frisbee, etc. As travel costs rise and internet connectivity improves, there are going to be sensible reasons to have fewer in-person meetings of otherwise distant participants, but there remains no substitute for a good conversation face-to-face over a coffee or a beer.
- College dorm rooms, while better than when I was a student, are still not high on ambiance. Generic fitted and top sheets for bedding appear to be made from dryer lint.
- Food options have become progressively healthier and tastier in general.
- Mount Holyoke is a lovely campus, with very loud and happy frogs.
- A session about cuprate superconductors correlated with the literal gathering of storm clouds in an otherwise sunny week.
- About 30% of the audience got the reference (after about a 5 second delay) when, on a slide about magnetic interactions (\(J_{zz} S^{z}_{i} \cdot S^{z}_{j}\)), there was an unlabeled picture of Jay-Z.
- Cuprate superconductors remain amazingly complicated, even after years of improving sample quality and experimental techniques.
- Looking at driven systems is becoming very exciting. Basically under some circumstances you can use light to flip on or off topological changes in band structure, for example.
- It remains very challenging to figure out how to think about systems with low energy excitations that don't look like long-lived quasiparticles.
Sunday, June 22, 2014
Gordon Conference
I am going to be at the Gordon Research Conference on correlated electrons for the next few days. Should be fun, but blogging about such meetings is generally frowned upon (don't want to discourage people from frank discussions and showing brand new, untried stuff). There are rules about confidentiality for these meetings. I'll write more later in the week on other topics.
Sunday, June 15, 2014
FeSe on SrTiO3: report of 100 K superconductivity
I'd heard rumors about this for a while. I presume that the posting of this on the arxiv means that some form of this paper is in submission out there to a suitably glossy, high impact journal that requires reference citations in its abstracts. Background: Bulk FeSe superconducts below around 8 K at ambient pressure (see here). Under pressure, that transition can be squeezed up beyond 35 K (see here). The mechanism for superconductivity in this material is up for debate, as far as I know (please feel free to add a reference or two in the comments).
These investigators have a very fancy ultrahigh vacuum system, in which they are able to grow single layer FeSe on top of SrTiO3 (with the substrate doped with niobium in this case). This material is not stable in air, and apparently doesn't do terribly well even when coated with some protective layer. However, these folks have a multi-probe scanning tunneling microscope system in their chamber, along with a cold stage, so that they can perform electrical measurements in situ without ever exposing their single layer to air. They find that the electrical resistance measured in their four-point-probe configuration drops to zero below around 100 K (as high as 109 K, depending on the sample). One subtle point that clearly worried them: SrTiO3 is know to have a structural phase transition (the onset of ferroelasticity - see here) at around 105 K, so they wanted to be sure that what they saw wasn't somehow an artifact of that substrate effect. (Makes me wonder what happens to superconductivity in the FeSe depending on the ferroelastic domain orientation underneath it.) For the lay audience: liquid nitrogen boils at ambient pressure at 77 K. This would be the first iron-based superconductor to cross that threshold, a domain previously limited to the copper oxides. Remember, if the bulk transition is at 8 K and the single layer case exceeds 100 K, it doesn't seem crazy to hope for some related system with an additional factor of three or four that takes us beyond room temperature.
Important caveats: Right now, they have resistance measurements and tunneling spectroscopy measurements. Because of the need for in situ measurement they don't have Meissner data. It's also important to realize that the restrictions here (not air stable; only happens in single layer material when ultraclean) are not small. At the same time, this is potentially very exciting, and hopefully it holds up well and can be the foundation for more exciting materials.
These investigators have a very fancy ultrahigh vacuum system, in which they are able to grow single layer FeSe on top of SrTiO3 (with the substrate doped with niobium in this case). This material is not stable in air, and apparently doesn't do terribly well even when coated with some protective layer. However, these folks have a multi-probe scanning tunneling microscope system in their chamber, along with a cold stage, so that they can perform electrical measurements in situ without ever exposing their single layer to air. They find that the electrical resistance measured in their four-point-probe configuration drops to zero below around 100 K (as high as 109 K, depending on the sample). One subtle point that clearly worried them: SrTiO3 is know to have a structural phase transition (the onset of ferroelasticity - see here) at around 105 K, so they wanted to be sure that what they saw wasn't somehow an artifact of that substrate effect. (Makes me wonder what happens to superconductivity in the FeSe depending on the ferroelastic domain orientation underneath it.) For the lay audience: liquid nitrogen boils at ambient pressure at 77 K. This would be the first iron-based superconductor to cross that threshold, a domain previously limited to the copper oxides. Remember, if the bulk transition is at 8 K and the single layer case exceeds 100 K, it doesn't seem crazy to hope for some related system with an additional factor of three or four that takes us beyond room temperature.
Important caveats: Right now, they have resistance measurements and tunneling spectroscopy measurements. Because of the need for in situ measurement they don't have Meissner data. It's also important to realize that the restrictions here (not air stable; only happens in single layer material when ultraclean) are not small. At the same time, this is potentially very exciting, and hopefully it holds up well and can be the foundation for more exciting materials.
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