Wednesday, February 24, 2010

Amazing technology.

It's not nano-related, but I found this video fascinating.  Clearly technology has gotten to the point where Hollywood can fake just about anything, often on a TV budget.

Friday, February 19, 2010

Claims of priority

I was looking at this week's Phys Rev Letters, and I saw this paper being highlighted as an editor's suggestion.  Now, I don't know anything about this work, but I was struck by the title, which says explicitly that this measurement is the first of its kind.  This is repeated a few paragraphs into the paper at the end of their introduction.  I thought that claims of "first"s were very strongly discouraged by the editors, as mentioned here, let alone being included in the title, regardless of how well founded the statement.  Was this an oversight, or am I missing something?

Wednesday, February 17, 2010

High Tc, pseudogaps, broken symmetries

What distinguishes one phase of matter from another?  A physicist would probably say that different phases possess different symmetries.  More specifically, transitions between phases can be described (when going in the right direction) by the breaking of a symmetry.  For example, when water freezes, the continuous rotational and translational symmetry of the liquid (liquid water looks, on average, the same in every direction and at different points within the liquid) are broken, because crystalline ice has a specific lattice (and therefore certain preferred lattice directions, as well as a spatial periodicity).  Solid ice instead has discrete rotational and translational symmetries, rather than continuous ones.

High temperature superconductors have been confounding physicists for 24 years now.  Progress has been made in understanding these complicated materials (typically layered, multicomponent copper oxides with weird oxygen stoichiometries to control the number of mobile charge carriers), but the situation is still a mess.  These compounds have a complicated phase diagram as a function of, e.g., temperature and chemical doping.  The undoped parent compounds are antiferromagnetic insulators.  Over a range of chemical compositions, the ground state is a d-wave superconductor.  Within a good part of that range of composition, at temperatures above the superconducting transition, these materials show a "pseudogap" below some higher temperature, T*.  That is, the number of available electronic states near the Fermi level is depressed compared to what you'd expect for a metal, but not vanishing as you'd expect for a superconductor.  People have been arguing for years about what the pseudogap is - is this a distinct phase?  Is it a precursor to superconductivity (e.g., pairing of electrons w/o long-range coherence), or does it compete with superconductivity?

This recent paper by Louis Taillefer reports the observation of broken rotational symmetry in the pseudogap phase (mainly via the Nernst effect).  The claim is that below T*, the four-fold rotational symmetry (because it's a square lattice) of the electronic properties of the CuO2 planes is broken, and the system becomes electronically anisotropic.  This is important, because it firmly argues that the pseudogap state is a real thermodynamic phase of some kind, and that kind of broken symmetry apparently places strong constraints on possible theories of high Tc.  Not my direct area of expertise, but it looks very interesting.  I'll admit, though, I was surprised by the strong statements made here.  Unless there's way more to this than meets the eye, it's not clear to me why it's justified to claim that we're now much closer to room temperature superconductivity....

Saturday, February 13, 2010


Shocking news out of Alabama yesterday, where three faculty members (incl. the chair) of the biology department at UA Huntsville were shot, allegedly by a faculty member involved in a tenure decision.  I'm assuming that there will be a flood of articles and blog posts about this, and probably quite a bit of hyperventilating about tenure and the tenure process.  The fact is, some (thankfully very) small percentage of the population is unbalanced and responds to personal setbacks (real, perceived, or imagined) with violence.  It's a terrible shame, but this sort of thing happens across all occupations.  My condolences to the UAH community and the family and friends of those involved.

update:  ...and I was right.  *Sigh*, Christian Science Monitor.

Monday, February 08, 2010

This week in cond-mat, SQUID edition

Superconducting quantum interference devices, or SQUIDs, are fascinating gadgets.  Take a superconducting loop with two weak links (e.g., tunnel junctions, or constrictions with a lower critical current).   Now thread magnetic flux through the loop.  The superconducting wavefunction, which includes a phase factor that involves the vector potential, must be single-valued around the loop.  That means that the phase factor must return to itself modulo 2 pi going around the loop. The phase factor is proportional to the line integral of the vector potential, which itself is the magnetic flux through the loop.  Therefore, the total magnetic flux through the loop must be quantized.  If the external magnetic field doesn't give an integer number of flux quanta, then the superconductor must generate screening currents around the loop that produce flux and make up the difference.  If you had connected the loop to an external current source and run that external current (which splits itself around the two branches of the loop) up to the edge of the critical current, you would find that the screening currents would drive the loop normal and lead to a detectable voltage drop that is periodic in magnetic flux through the loop.  This periodicity allows SQUIDs to be phenomenally good magnetic field detectors.  One can integrate a tiny SQUID onto a movable probe, and make a scanning SQUID microscope, and do amazing things like figure out the pairing symmetry of high-Tc superconductors.
This week a paper appeared on the arxiv relevant to scanning SQUID microscopy:

arxiv:1002.1529 - Koshnick et al.,  Design concepts for an improved integrated scanning SQUID
Here, Koshnick, together with scanning SQUID experts Kirtley and Moler, lay out ideas that they have in the works for refining the technology of these gadgets.  Neat stuff.

Almost simultaneously, a new paper appeared in Nano Letters on an implementation of an aluminum scanning SQUID microscope.  The basic concept, involving the use of a drawn optical fiber tip as a template for deposition of an aluminum ring and leads, hearkens back to the scanning single-electron transistor charge detector worked on previously by one of the coauthors.

Friday, February 05, 2010

The arxiv blog: a good idea gone awry?

When it first began, I was impressed with the arxivblog.  The anonymous authors did a good and remarkably prompt job of surfing the preprint archive, and posting interesting tidbits, on essentially a daily basis.  Moving to the Technology Review website seemed like it could only be a good thing.  Larger readership, greater outreach to a scientifically literate audience, etc.  Now I have to wonder.  The arxivblog frequently seems to feature theory preprints that are rather far out (alternative theories of gravity; exotic quantum entanglement interpretational issues), and doesn't always make clear just how speculative some of these are.  Moreover, it seems that many of the comments, particularly on these more speculative topics, are, umm, not informative.  So, is the purpose of the arxivblog to showcase exciting new science (which is what the Technology Review usually does), or is it to be "gee whiz"/quantum sure is weird/nanobots-will-save-us-all entertainment?

Thursday, February 04, 2010

"Not my job!"

US Secretary of Energy Steven Chu is going to be on "Wait, Wait, Don't Tell Me" this coming Saturday, presumably doing their "Not my job!" game.  For those not in the US, WWDTM is a comedic radio quiz program, and "Not my job!" is a game in which the guest must answer three questions about some subject that is very, very far from their area of expertise.  This should be amusing.

Update:  Here is a link to the relevant part of the show.

Monday, February 01, 2010

Lab mysteries and other annoyances.

One aspect of experimental science that never shows up on TV procedurals (NCIS, CSI) is the "lab mystery" - the simple procedure that's supposed to be a piece of cake, but turns out to be unnecessarily and surprisingly complicated.  Here's an example.  There's a material that is supposed to be photopolymerizable; it starts out as a liquid monomer, and under UV exposure it's supposed to polymerize into a gel.  We have some, and we also have a UV lamp.  As a simple test, we exposed the monomer to the UV for tens of minutes - no response.  I'm sure we'll figure this out, but this sort of thing never happens to Abby or the guys from Mythbusters....  Feel free to leave other examples of lab mysteries in the comments.

update:  Mystery resolved.  In this case, the answer seems to be "more power".   A much (~ 20x) brighter UV lamp works quite well.  The paper we're working from didn't really mention intensities, so I think we can be forgiven.