Thursday, December 20, 2012

Quantum spin liquids - neat stuff!

There is a new result in this week's issue of Nature that is very neat (and my college classmate Young Lee is the PI - small world!).  The experiment is an inelastic neutron scattering measurement that looks at a material with the unusual name herbertsmithite, and reports evidence that this material is a "quantum spin liquid".  I'll try to break down the physics here into reasonably accessible bite-sized chunks.

First, what is a spin liquid?  Imagine having a bunch of localized spins on a lattice.  You can picture these like little bar magnets.  In this case, the spins are the unpaired d electrons of the copper atoms in the herbertsmithite structure.   In general, the spins in a solid (this particular one is an insulator) "talk" to each other via the exchange interaction.  What this really means is that there are interactions between the spins so that the spins prefer a particular relative orientation to each other.  In this case, the interaction between the electron spins is antiferromagnetic, meaning that for spins on two neighboring Cu atoms, having the spins be oppositely directed saves some energy (17 meV) compared to having the spins aligned.  As the temperature is lowered, an ensemble of spins will tend to find whatever configuration minimizes the total energy (the ground state).  In a ferromagnet, that will be a state with the spins all aligned with their neighbors.  In a perfect antiferromagnet, that would be a state where the spins are all antialigned with their neighbors.  Both of these are ordered ground states, in that there is some global arrangement of the spins (with a particular symmetry) that wins at T = 0.   The problem in herbertsmithite is, because of the spatial arrangement of the Cu atoms (in a Kagome lattice), it's impossible to have every spin antialigned with all of its neighbors.  This is an example of geometric frustration.   As a result, even as T gets very low, it would appear that the spins in herbertsmithite never order, even though they interact with their neighbors very strongly.  This is an analog to the liquid state, where the molecules of a liquid clearly interact very strongly with their neighbors (they bump right into each other!), but they do not form a spatially ordered arrangement (that would be a solid).

Why a quantum spin liquid?  Two reasons.  First, I cheated in my description above.  While we can talk classically about antialigned spins, we really should say that pairs of spins want to form singlets, meaning quantum mechanically entangled antialigned states with net spin zero.  So, you can think of this spin liquid state as involving a big entangled mess of spins, where each spin is somehow trying to be entangled in a singlet state with each of its nearest neighbors.  This is very complicated to treat theoretically.  Second, the fluctuations that dominate in this situation are quantum fluctuations, rather than thermally driven fluctuations.  Quantum fluctuations will persist all the way down to T = 0. 

What's special about a quantum spin liquid?  Well, the low energy excitations of a quantum spin liquid can be very weird.  If you imagine reaching into the material and flipping one spin so that it's now energetically "unhappy" in terms of its neighbors, what you find is that you can start flipping spins and end up with "spinon" excitations that travel through the material, having spin-1/2 but no charge, and other exotic properties.  This is described reasonably well here.  Importantly, these excitations have effects that are seen in measurable properties, like heat capacity and how the system can take up and lose energy.

So what did the experimenters do?  They grew large, very pure single crystals of herbertsmithite, and fired neutrons at them.  Knowing the energies and momenta of the incident neutrons, and measuring the energies and momenta of the scattered neutrons, they were able to map out the properties of the excitations, showing that they really do look like what one expects for a quantum spin liquid. 

Why should you care?  This is a great example of seeing exotic properties (like these weird spin excitations) that emerge because of the collective response of a large number of particles.  A single Cu ion or unit cell of the crystal doesn't do this stuff - you need lots of spins.  Moreover, this is now a system where we can study what this weird, highly quantum-entangled does - I think it's very very far from practical applications, but you never know.   Looks like a very nice piece of work.

Tuesday, December 18, 2012

Just how self-correcting is science?

In an extremely timely article in the new issue of American Scientist, Joseph Grcar looks at what fraction of publications in various disciplines are basically corrective (that is, comments, corrigenda,  corrections, retractions, or refutations).  He finds in the sciences in general the correction rate is about 1-1.5% of publications.  This is probably a bit of an underestimate, in my view, since there are new works published that are essentially soft refutations that may not be detected by the methods used here.  Likewise, some fraction of the body of published work (constituting the denominator of that fraction) has no impact (in the sense of never being cited).  Still, that's an interesting number to see. 

Friday, December 14, 2012

A nano controversy

A couple of my colleagues pointed me to this blog, that of  Raphaël Lévy at Liverpool.  Lately it has become a clearing house for information about a particular controversy in nanoscale science, the question of "stripy nanoparticles".  The ultrashort version of the story:  Back in 2004, Francesco Stellacci (then at MIT, now in Switzerland) published a paper in Nature Materials arguing that his group had demonstrated something quite interesting and potentially useful.  Very often when solution-based materials chemistry is used to synthesize nanoparticles, the nanoparticles end up coated in a monolayer of some molecule (a ligand).  These ligands can act as surfactants to alter the kinetics of growth, but their most important function is to help the nanoparticles remain in suspension by preventing their aggregation (or one of my favorite science terms, flocculation).  Anyway, Stellacci and company used two different kinds of ligand molecules, and claimed that they had evidence that the ligands spontaneously phase-segregated on the nanoparticle surface into parallel stripes.  His group has gone on to publish many papers in high impact journals on these "stripy" particles.  

However, it is clear from the many posts on Lévy's blog, to say nothing of the paper published in Small, that this claim is controversial.  Basically those who disagree with Stellacci's interpretation argue that the scanned probe images that apparently show stripes are in fact displaying a particular kind of imaging artifact.  As an AFM or STM tip is scanned over a surface, feedback control is used to maintain some constant conditions (e.g., constant AFM oscillation frequency or amplitude; constant STM tunneling current).  If the feedback isn't tuned properly, there can be "ringing" so that the image shows oscillatory features as a function of time (and therefore tip position).  

I have no stake in this, though I have to say that the arguments and images shown by the skeptics are pretty persuasive.  I'd have to dig through Stellacci's counterarguments and ancillary experiments, but this doesn't look great.

This whole situation does raise some interesting questions, though.  Lévy points out that many articles seem to be published that take the assertion of stripiness practically on faith or on very scant evidence.  Certainly once there is a critical mass of published literature in big journals claiming some effect, it can be hard as a reviewer to argue that that body of work is all wrong.  Still, if you see (a series of) results in the literature that you really think are incorrectly interpreted, what it is the appropriate way to handle something like this?  Write a "comment" in each of these journals?  How should journals respond to concerns like this?  I do know that editors at high profile journals really don't like even reviewing "rebuttal" papers - they'd much rather have a "comment" or to let sleeping dogs lie.  Interesting stuff, nonetheless.

Update: To clarify, I am not taking a side here scientifically - in the long run, the community will settle these questions, particularly those of reproducibility.  Further, one other question raised here is the appropriate role of blogs.  They are an alternative way of airing scientific concerns (compared to the comment/rebuttal format), and that's probably a net good, but I don't think a culture of internet campaigns against research with which we disagree is a healthy limiting case.

Monday, December 03, 2012

The future of Si: Into the fog

Today we had a visit at Rice from Mike Mayberry, the VP for the Technology and Manufacturing group at Intel.  He gave a very good talk about where semiconductor electronics is going (naturally with the appropriate disclaimers that we shouldn't buy or sell stocks based on anything he said).  The general rule is that there is a metaphorical fog out there about ten years off, beyond which it's not clear what the industry will be doing or how it will be doing it.  However, for the comparatively near term, the path is fairly well known.  In short: complementary metal-oxide-semiconductor (CMOS) based on Si is going to continue for quite a ways longer.  Device design will continue toward improved electrostatic control of the channel - we will likely see evolution from tri-gate finFET structures toward full wrap-around gates.  We are firmly in the materials-limited regime of device performance, and Intel has been playing games with strain to improve charge mobility.  They have even experimented with integration of III-V materials via epitaxy onto Si platforms.  It's pretty clear that there is a healthy skepticism about post-CMOS alternative technologies, particularly given the absurdly low cost and high volumes of Si.  Intel ships something like 4 trillion transistors per minute (!).  Other remarkable facts/figures:  Network traffic right now is around 7 exabytes (1018 bytes) per day, or the equivalent of 17000 HD movies every second, including around 204 million emails per minute on gmail, and these numbers are increasing all the time.  Amazing.