Thursday, October 29, 2015

Phase transitions: Hiding in plain sight

Phase transitions are amazing things.  As some knob is turned (temperature, pressure, magnetic field), a physical system undergoes a sudden, seemingly discontinuous change in material properties.  This doesn't happen when looking only at one or two of the constituents of the system - only when we have a big ensemble.  In our daily experience, we are used to the control parameter being temperature, and we take particular notice of phases that have dramatically different, obvious-to-the-naked-eye properties.  Solids and liquids have completely different responses to shear (or rather, liquids lack rigidity).  Liquids and gases have vastly different densities.

It turns out that there are many more phases out there, distinguished in ways that are more subtle and harder to see.  Gadolinium is a magnet below room temperature, and a nonmagnetic metal above room temperature, but to the naked eye looks the same in both phases.  We only know that the transition is there because it has measurable consequences (e.g., you could actually see magnetic forces from a hunk of cold gadolinium).

This week, there was some media attention paid to work from David Hsieh's group at Cal Tech, where they discovered an example of a particularly subtle transition in strontium iridate (Sr2IrO4).  In that stuff, similar in some ways to the copper oxide superconductors (based on CuO4 motifs), there are unpaired electrons (and therefore unpaired spins) on the iridium atoms.  Below a critical temperature (near 200 K, or about -70 Celsius), these spins somehow spontaneously arrange themselves in a subtle way that picks out special directions in the crystal lattice and breaks mirror symmetry, but is not some comparatively well-known kind of magnetic ordering.  They are only able to identify this weird "hidden" ordered phase via a particular optical measurement, since the broken symmetry of the ordered state "turns on" some optical processes that would otherwise be forbidden.  The interest in this system stems from whether similar things may be happening in the cuprate superconductors, and whether the iridates could be tweaked and manipulated like the cuprates.  Neat stuff, and a reminder that sometimes nature can be subtle:  There can be a major change in symmetry properties (i.e., above the transition temperature, the x and y directions in the crystal are equivalent, but below the transition they aren't anymore) that shows up spontaneously, but is still hard to detect.

Tuesday, October 27, 2015

No, dark matter probably did not do in the dinosaurs.

Resurfacing briefly in the midst of proposal writing, I feel compelled to comment on recent media coverage of Lisa Randall's new book.  The science story is an interesting one, as recounted here.   In brief:  There is some evidence of periodicity in mass extinctions due to impacts, though there are at least three hidden assumptions even in that statement.  One possible source of periodicity could be associated with the motion of the solar system (and by extension the earth) in and out of the galactic plane as the sun orbits the center of mass of the Milky Way.  Lisa Randall and Matthew Reece argued that passage through a comparatively thin disk of dark matter on the galactic plane could lead to gravitational perturbations that could lead to Oort Cloud comets getting dinked toward the inner solar system.  A neat idea, though pretty hard to test except through indirect means.  

So, could this have happened?  Sure.  Is it likely?  Well, that's much trickier to evaluate.  There are plenty of other sources of gravitational perturbations (e.g., the passage of nearby stars) that we know for sure take place.  There is no strong observational evidence right now that the would-be disk of dark matter exists, let alone whether it has the properties needed to provide a significant uptick in cometary impacts.  Lisa Randall is undoubtedly a gifted writer, and there is real science here (witness the published paper that discusses ways to test the idea), but the breathless media reaction is somewhat disappointing.

Sunday, October 18, 2015

STEM education and equality of opportunity

Friday evening I went to the Katy Independent School District's Robert R. Shaw Center for STEAM, where they were having a Science Movie Night (pdf).  The science and technology policy part of Rice's Baker Institute for Public Policy had put the organizers in touch with me.  It was a very fun time.   On a night when there were two (!) homecoming high school football games next door, the movie night drew about 80 highly engaged students.  After the film, they stayed and we talked about the science of the film (what it got right and what they fudged) for another half an hour.  It was a great time.

The Shaw Center is amazing - it's a fantastic space, something like 10000 sq ft of reconfigurable maker-space, with a shop, immersive labs, and it provides a home to more than one of Katy ISD's robotics teams.  Frankly, this place rivals or exceeds the available undergrad lab facilities at many universities.  Katy is a reasonably affluent suburb of Houston, and I was floored to learn that this great science/engineering facility was built with district money, not donations or corporate sponsorship/underwriting.  This is a case where public school funding has been deliberately and consciously dedicated to providing a district-wide resource for hands-on science and engineering learning.

In a study in contrasts, my sons and I then volunteered Saturday morning at the Teachers Aid facility run by the Houston Food Bank.  At the Teachers Aid facility, teachers from qualifying schools (where 70+% of the enrollment is sufficiently low-income that they qualify for free lunches) can arrive, by appointment, and pick up basic school supplies (pencils, pens, notebooks) for their students.  In three hours we helped about 70 teachers who serve more than 3000 students.  These are teachers who chose to come in on their own time, to get basic supplies that neither their schools nor the students themselves can afford.  

It's appalling to see the divergence in basic educational opportunities between the more affluent school districts and the economically disadvantaged.  We have to do better.  Making sure that children, regardless of their background, have access to a good education should be a guiding principle of our society, not something viewed as pie-in-the-sky or politically tainted.  It amazes me that some people seem to disagree with this.

Tuesday, October 13, 2015

Several topics: The Nobel, self-organization, and Marcy

I'm heavily into some writing obligations right now, but I wanted to point out a few things:

  • Obviously this year's Nobel in physics was for neutrino oscillations.  ZZ has thoughtfully provided two nice postings (here and here) with relevant links for further reading.  I remember the solar neutrino problem vividly from undergrad days, including one professor semi-jokingly speculating that perhaps the lack of expected electron neutrinos was because the sun had gone out.  The whole particles-oscillating-into-new-types business, famous in physics circles from the K mesons, is tough to explain to a general audience.  You could probably come up with some analog description involving polarized light....
  • Here is a neat article about a record Chinese traffic jam, and it includes links to papers about self-organization.  I'm happy to see this kind of article - I think there is real value in pointing out to people that there can be emergent, organized states that result from the aggregate of simple rules.  That's the heart of condensed matter and statistical physics.
  • This week buzzfeed helped break the story that Geoff Marcy, an astronomer famed for his role in the discovery of extrasolar planets and mentioned as a likely Nobel candidate, was found guilty of multiple violations of Berkeley's sexual misconduct policy.  Note to those who haven't followed this:  This is the result of a long investigation - it's not hearsay, it's essentially a conviction via a long university investigative process.  Marcy's apology letter is here.   Apparently this bad behavior had been tolerated for many years.  Berkeley's response has been, shall we say, tepid.  Despite a clear finding of years of inappropriate behavior involving students 1/3 his age, the response is "do this any more, and you'll be dismissed".  The initial response from the department head had an inexcusably awful last paragraph that implied that this whole process was hardest on Marcy, rather than on the victims.   This is terrible.   People, including departmental colleagues, are calling for Marcy to step down.  Bottom line:  There is simply no excuse for this kind of behavior, and actual sanctions must be applied when people are found through due process to have violated this level of basic social decency.

Tuesday, October 06, 2015

Table-top electron microscopes

A quick question in the hopes that some people in this blog's readership have direct experience:  Anyone work with a table-top scanning electron microscope (SEM) that they really like?  Any rough idea of the cost and operations challenges (e.g., having to replace tungsten filaments all the time)?  I was chatting with someone about educational opportunities that something like these would present, and I was curious about the numbers without wanting to email vendors or anything that formal.  Thanks for any information.

(Note: It would really be fun to try to develop a really low-budget SEM - the electron microscopy version of this.  On the one hand, you could imagine microfabricated field emitters and the amazing cheapness of CCDs could help.  However, the need for good vacuum and some means of beam focusing and steering makes this much more difficult.  Clearly a good undergrad design project....)

Sunday, October 04, 2015

Annual Nobel speculation

It's getting to be that time of year again.  The 2015 Nobel Prize in Physics will be announced this coming Tuesday morning (EU/US time).  Based on past patterns, it looks like it could well be an astro prize.  Dark matter/galaxy rotation curves anyone?  Or extrasolar planets?  (I still like Aharonov + Berry for geometric phases, perhaps with Thouless as well.  However, it's unlikely that condensed matter will come around this year.)

On Wednesday, the chemistry prize will be awarded.  There, I have no clue.  Curious Wavefunction has a great write-up that you should read, though.

Speculate away!