## Wednesday, March 30, 2016

### Energy technology and research opportunities - DOE quadrennial review

If you are interested in the US Department of Energy's take on the current status and trends in energy technology and related research, I strongly encourage you to watch this talk, by Franklin "Lynn" Orr, current US undersecretary of energy for science and energy.  It's a 40 minute talk, full of a lot of information.

If you want to see the actual, detailed, referenced document with graphs and bibliography, see here.

## Wednesday, March 23, 2016

### Colloquium: Pluto and New Horizons

We had an excellent colloquium here today from John Spencer, one of the investigators on the New Horizons mission to Pluto.   Amazing stuff - if you ever get the chance to hear a talk by one of the mission members, don't pass it by.  A few facts that were striking:

• The ambient surface temperature on Pluto is something like 40 K, basically because of the slow release of energy from residual radioactive material in there.  I guess that makes it too warm for the Outsiders, so we'll just have to wait longer to purchase a hyperdrive.
• The surface of much of Pluto is geologically young - there seems to be something like a "nitrogen cycle" analogous to the earth's water cycle, whereby nitrogen ice sublimes, precipitates out on km-tall water ice mountains, and eventually flows in glacial form back down to nitrogen ice seas.
• The New Horizons spacecraft was the fastest thing ever launched directly from the earth's surface, and it passed the moon within 9 hours after launch, having already been boosted to solar escape velocity.  (It's slower in the end than Voyager 1 and 2 because those spacecraft got close gravity assists from both Jupiter and Saturn.)
• Pluto's moons other than Charon are, well, complicated.  Their rotational axes are nearly in the plane of their orbit about the Pluto system barycenter, and they're not all round, so they rotate and interact in complicated ways.
• Space is big.  Really big.  You just won't believe how vastly hugely mindbogglingly bit it is.  I mean, you may think it's a long way down to the road to the chemist's, but that's just peanuts to space.

## Thursday, March 17, 2016

### APS March Meeting, day 4

I spent a big chunk of my last day at the meeting having conversations with a couple of my collaborators, but I did get to see a couple of impressive talks.

Prof. Martin Aeschlimann of Kaiserslautern presented the remarkable work by his group using time-resolved 2-photon photoemission microscopy (PEEM) to drive and monitor plasmons on the nanoscale and femtosecond timescale.  The technique is a mouthful.  It's like electron microscopy, only instead of shooting an electron beam at the sample and looking at the secondary electrons that come out, you illuminate the sample with ultrafast, intense pulses of 800 nm light.  If these excite a plasmon mode, then the very intense local electromagnetic field leads to nonlinear two-photon processes that cause photoemission of electrons from the sample, and those photoelectrons are collected by a high resolution electrostatic column similar to that in an electron microscope.  The result is, you can "see" plasmons with ~ 10 nm or better spatial resolution, and by varying the time delay between pump and probe optical pulses, you can watch plasmons decay, or transport energy coherently, or interfere with each other.  Amazing stuff.

After watching some talks about spin Hall physics (hugely growing activity there, and definitely worth multiple blog posts down the line), I watched a fascinating talk by Scott Kemp of MIT about the Iran nuclear deal - he was one of the US negotiators.  It was great to get a sense of the scientific and political reasoning behind the negotiations and their outcome, and there was information in the talk that I hadn't seen anywhere else.

Final thoughts on the meeting:

• The variety of topics and the level of activity in condensed matter physics these days is great to see.  It's an active, thriving field, with deep ideas, open questions, and some topics that could well have major technological impact.  More than ever, I feel like there is an untapped potential here for informing the public about this stuff.
• The meeting is almost too big at this point.  It's unwieldy, and often there are multiple great talks on similar topics scheduled simultaneously.   I'm curious to learn what the long-term plans are in terms of meeting (re)organization and abstract sorting.  It feels like there has to be a better way to do some of these things, but if there were easy answers they would have been implemented already.
• Finally there was coffee and tea available without making everyone pay through the nose.  Whoo-hoo!

### APS March Meeting, day 3

(Note that I'm leaving out the parts of the meeting where I did things like chat with friends and colleagues, and visit the trade show - I doubt anyone wants to read that stuff.)

I started day 3 with some plasmonics talks.  A particularly remarkable piece of work was presented by Teri Odom, discussing her group's plasmonic lasing efforts.   Metal nanoparticles excited at their local plasmon resonance can support very large local enhancements of the electromagnetic field, effectively confining light to incredibly small, sub-wavelength volumes.  However, usually the plasmon modes are relatively broad, so that a photon doesn't "live" very long in those tiny volumes.  By combining many nanoparticles in a regular array, the interparticle coupling can lead to a collective, coherent narrowing of those resonances.  When combined with a gain medium (in this case IR-140, an infrared dye with emission commensurate with the resonance of the metal nanoparticle array), the result is an optically pumped laser, with emission that can be tuned across the dye's bandwidth by changing the index of refraction of the surrounding medium.

I then tried to learn about Weyl fermions. This is another example of a particle originally proposed in the high energy physics context, with some peculiar relationships between energy, momentum, and angular momentum, and then seen in the emergent properties of a condensed matter system.  Truth be told, the talks I saw focused much more on the photoemission techniques, materials, and the steady stream of high profile publications than on providing a pedagogical approach to these funky (quasi)particles.

Eli Yablonovitch gave a fun, informative Buckley Prize talk, on the history of photonic band gap systems and their use to engineer spontaneous emission, optical antennas, and lastly structural color in nature.  Regarding optical antennas, he argues strongly that it's useful to think of these things in the context of classical antenna theory (basically modeling the antenna as an equivalent circuit made from discrete inductors, capacitors, and resistors) rather than other approaches involving quantum optics concepts.  I'm sure he's right in many cases, but fundamentally it seems to me that lumped element models can't really work well when worrying about a number of problems.

Nadya Mason gave a compelling talk about the nature of superconductivity in islands of granular Nb, as a test case to better understand the low-T metallic state of many thin systems in which superconductivity can be suppressed.  It's elegant work gaining new insights into a classic problem.  Many aspects can be explained with a simple model involving the distribution of grain sizes (and hence local superconducting transition temperatures), though mysteries remain, such as how nearby islands coupled by a normal metal film really talk to each other.

After a fun lunch with blogger extraordinaire Chad Orzel, I heard Yong Chen from Purdue present his group's work on transport in small devices made from 3d topological insulators of sufficiently high quality that the bulk is actually insulating, like it's supposed to be.  The favorite materials are apparently BiSbTeSe2, which can be exfoliated from bulk or grown in film form, and vapor-grown nanowires of Bi2Te3.  That work is here and here, respectively.

After some talks on VO2 (it's still complicated), I rounded out the day by going to the end of the Kavli Frontiers symposium.  My colleague Naomi Halas gave an extremely impressive talk about plasmonic particles for heat transfer and steam generation, and this was followed by an exhuberent lecture from Duncan Brown, who presented the LIGO gravitational wave detection experiment.  His excitement and joy about the result were infectious.

Next:  my last half-day of the meeting, + final thoughts.

## Tuesday, March 15, 2016

### APS March Meeting, day 2

Another eclectic bunch of talks today:

• There was a very interesting session this morning about coupling superconductors to semiconductors - this is a topic that has a long history and has enjoyed a huge resurgence as people have figured out ways to create composite systems with wild properties, like Majorana fermions.  Amir Yacoby gave a talk about what happens when a superconductor (Al) is coupled to a strong spin-orbit semiconductor, a HgCdTe quantum well.  The superconducting order parameter leaks into the semiconductor (the proximity effect), and more interestingly, it oscillates in space between $s$-wave pairing (the electrons in each Cooper pair form an antisymmetric spin configuration,  $(1/\sqrt{2})(| \uparrow \downarrow\rangle - |\downarrow \uparrow \rangle)$, that flips sign if you swap the electrons ) and $p$-wave pairing (the electrons forming a symmetric spin configuration, like $(1/\sqrt{2})(|\uparrow \uparrow\rangle + |\downarrow \downarrow \rangle)$.  From current data as a function of in-plane magnetic field and out-of-plane magnetic field, plus some disorder in the contact region, you can explain almost everything.  The next talk, by Dale van Harlingen, discussed superconductors coupled to the 2d surface of a 3d topological insulator, Bi2Se3, making Josephson junctions.  These things end up playing host to Majorana fermions, and can be used to push them around in interesting ways.
• Later, after seeing some contributed talks, chatting with folks, and visiting the trade show to get literature from a bunch of vendors, I stood through a talk about trying to detect evidence of dark energy with a (comparatively) "tabletop" atomic physics experiment.   A very cool topic, but the room was so claustrophobic I couldn't stay for the talk about the gravity-decoherence paper I'd mentioned here.
• After learning about "Advanced undergraduate labs:  why bother?", I went to the extremely dense session about spintronic devices beyond spin-transfer torque.   The metal spin device toolkit is now very extensive, and it will be interesting to see if the materials issues can be worked out well enough to produce devices that will really revolutionize information storage and processing.  Power dissipation remains a big issue.  Here is a recent review article on this stuff (sorry - I didn't want to direct-link to someone's private copy of the pdf).  I should write a separate post on this stuff.
More tomorrow....

## Monday, March 14, 2016

### APS March Meeting, day 1

First, hat tip to Chad Orzel for this article, and ZapperZ for his.  While condensed matter physics is harder to describe to a general audience, it's shaped your everyday life far more than string theory or neutrino oscillations.  We as a community need to do a better job getting that across, as well as the wonder that some of these topics inspires.  Interesting talks that I saw today (aside from those of my group members, of course):

• There is a lot of interest in trying to capture optical energy (e.g., from the sun) and not waste so much of it.  Plasmons in metals provide one way of converting a photon into electron-hole excitations in a metal - the trick is to then do something useful with those "hot" electrons and holes.  Lisa Krayer spoke about a clever approach of putting a metal film grating on the back of a Si photovoltaic system, to grab photons too low in energy for the Si itself into plasmons, and then kick "hot" electrons back into the Si.  As an added bonus, the optical properties of the Si (high index of refraction) end up implying that the grating can capture light over a much larger range of incident angles than if the grating was on the front side.  Similar in spirit, Prinaha Narang spoke about theoretical modeling of the electrons in these and similar plasmonic structures, with an eye toward manipulating (through geometry) the momentum and energy distributions of the hot electrons and holes.
• Hsin-Zon Tsai gave an interesting talk about using an underlying gate electrode to change not just the charge density in a layer of graphene, but also to manipulate the amount of charge on a molecule (called F4TCNQ) tethered to the graphene.  Measuring by scanning tunneling microscope, Tsai and coworkers showed that the highest occupied molecular level always sat lower in energy than the Dirac point of the graphene, and made a nice argument in support of this involving image charges.
• In his talk in honor of receiving the Adler Prize, Harry Atwater gave a nice overview of his group's plasmonics efforts, including a discussion of their concept of the plasmoelectric effect:  Illuminating a plasmonic object in an environment where it can gain or lose charge can drive charge transfer, as explained here.
• We are used to employing ferromagnets in electronic devices.  Maxim Tsoi gave a very clear talk about some remarkable work using antiferromagnets, both for magnetoresistive devices and for the manipulation of and by spin currents.  The next talk in that session, by Wei Zhang, described recent work where antiferromagnetic alloys were used as sources of spin currents.  Very pretty stuff.
• I also caught part of the session where various historians of science (and a noted blogger) critiqued/commented on Steven Weinberg's latest book, with Weinberg in the room to offer rebuttal.
More tomorrow....

### APS March Meeting 2016

It's that time of year again, when a bit under 10,000 condensed matter/materials/polymer physicists gather in a meeting that is now 1.7 times as large as it was when I first started going to these things.   This year the festivities are in Baltimore, and as I've done in the past I will try to give some snapshot of bits that caught my interest (though my session attendance is of course partly driven by my group's talks).  If there are particular things my readers think I should see, hopefully they will point them out in the comments.  If you are at the meeting, I encourage you to stop by the Cambridge booth at the exhibition and pick up some copies of my book as gifts for your friends.

## Monday, March 07, 2016

### Unidentified Superconducting Objects

The search for new superconductors has been going on for decades, because the potential promise of room temperature superconductors (with useful properties, like high critical fields, high critical currents, chemical stability, the ability to be integrated in some way into wires, ribbons, or tapes) is so enormous.  Littering the metaphorical laboratory floor are various claims over the years of "unidentified superconducting objects" - a term attributed to Paul Chu to describe one-off, irreproducible hints of 200-300 K superconductivity, often features in resistivity or magnetization that look like they could originate in some unknown impurity phase of an already complex material.  I was reminded of this by a paper that showed up on the arxiv last night.  Most likely this will fade away, but these things are always intriguing.  Extraordinary claims require extraordinary evidence, of course.

## Wednesday, March 02, 2016

### Google scholar question

Readers:  I suspect many of you are familiar with Google Scholar, Google's free approximation of what Thomson-Reuters offer for a fee.   Scholar is a nice tool for searching references, though as a Google product it uses something similar to their pagerank algorithm, meaning that it can heavily bias searches in favor of papers that have been highly cited (though this can be tweaked or avoided in many ways).

Google Scholar gives you the opportunity to create a public profile as well, so that people can see at a glance your publications and their citations, keywords that you choose to describe your research area, instantly calculated metrics such as citation counts and the h-index, etc.  (Some people like the Google Scholar h-index because it is systematically higher than the one from Thomson-Reuters, since it does a better job of catching bibliographic references in books and online resources.  That, and our culture of encapsulating complex things in single numbers biases us toward preferring higher numbers.)  I do have a profile, though I have mixed feelings about the score-keeping aspects of these things.

One reason I do have a profile is that Google Scholar has a feature that I've found interesting (if not necessarily useful) in the past:  Based on your papers, where they're being cited, your research interests, etc., every few days Google Scholar comes up with suggested literature that it lists under a "My Updates" tab on your profile.  These are new papers that either cite your work or Google's algorithm computes that you would likely be interested in the subject matter.