Tuesday, April 27, 2010

The unexpected - the fun part of experimental science

There's an old quote from Isaac Asimov that is very true:  "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny...'."  In my group, we recently had an experience that supports this, and it highlights what I think is some of the most fun you can have as an experimental scientist:  trying to use the tools at your disposal to learn as much as you can about what's behind some unexpected and surprising phenomenon.

The story starts out several years ago.  We'd been having some nice success making single-molecule electronic junctions and using them as model systems to study a particular piece of physics, the Kondo effect.  Our theorist colleagues pointed out that these molecular devices, unlike typical semiconductor quantum dots, might give us an opportunity to study a particularly rich and interesting piece of physics called a quantum phase transition, because molecular devices are easier to attach to ferromagnetic electrodes.  The idea, not directly germane to this story, is that an unpaired electron on the molecule is torn between two competing "baths" of excitations.  On the one hand, the unpaired electron can undergo Kondo processes with the conduction electrons of the electrodes.  On the other hand, spin waves in the ferromagnetic electrodes can also talk to the unpaired electron.  By varying a gate voltage appropriately, the hope was to tune from one limit (the Kondo regime) into the other (expected to be a more exotic "non-Fermi liquid" state).  

Anyway, for various reasons, it became clear that working with palladium electrodes might be a good place to start.  Palladium is almost ferromagnetic.  That means that it has long-lived spinwave-like excitations (paramagnons).  At the same time, it's chemically friendlier (less prone to forming magnetically complicated oxides) than common ferromagnetic metals like iron, nickel, or cobalt.  So, step zero of this project would be to make some bare palladium tunnel junctions (just two pointy palladium electrodes, without any molecule bridging them) and make sure that they're simple and boring, as expected.  After all, we'd looked at literally thousands of gold tunnel junctions like this, and if properly made (so that you don't have extra metal nanoparticles around), they are dull as dirt:  current-voltage (I-V) curves that are nearly linear and essentially temperature-independent.

Surprise!  My postdoc, Gavin Scott, found that Pd tunnel junctions are very much not boring.  While they look dull at, say, 10 K, if they are cooled down to lower temperatures, all kinds of sharp features appear in their differential conductance (dI/dV as a function of V).  The features appear at voltages symmetric around V = 0, and they evolve with temperature in a very interesting way.  In fact, if you look at the temperature dependence of those features, it looks very much like what you see for the temperature dependence of the order parameter in a "mean field" phase transition.  We spent months trying various things, turning all the easily turned "knobs" like temperature, magnetic field, gate voltage, etc.  One striking trend is the observation that, looking at all of our devices on one set of axes, the voltages where the conductance features appear extrapolate to zero when the conductance of the junction approaches e2/h.  In other words, when the metal tips touch, the whole effect goes away.   

It's been science-as-puzzle-solving, trying to figure out what could be going on here.  We came up with many possible explanations, and tried to come up with ways to test the possibilities, eliminating the ones that didn't fit.   For example, the data look (qualitatively) rather like superconductor tunnel junctions, but the quantitative values (specifically, the relationship between the voltage scale and the temperature scale) are far, far away from numbers that would make sense for a superconductor.  In the end, we think that the most likely physics ingredient is the onset of magnetic order at the tips, though that's not a perfect explanation by any means.  It is clear, though, that Pd is special - other metals (Au, Ni, Pt) just don't seem to show this.  The paper is out here (email me if you want a copy).  Hopefully others will get interested.  Suggestions are always appreciated.  In the meantime, it's a good example of how sometimes systems that you think are dull can surprise you, and how science is supposed to work.

Thursday, April 22, 2010

Nanotechnology-enabled sensing

Last May, I participated in a very well run and informative workshop on nanotechnology for sensing purposes.  The workshop was run by the National Nanotechnology Initiative (and associated Federal agencies).  The report that we ended up producing is now available for download.  Here is a direct link to the pdf.  Other NNI reports and information may be found here.

Tuesday, April 20, 2010

A word of caution re: departmental rankings

The US News rankings of US graduate programs are out again, and I've heard a fair bit of discussion about them.  My department, for example, went up slightly in the rankings, while the chemistry department here slipped a few spots.  I want to point out something that US News makes no effort to broadcast:  The US News graduate rankings are a popularity contest.  What I mean is, the US News rankings are the result of an opinion survey taken of department chairs, not the result of actual quantitative metrics like publication rates, citation rates, research funding, major awards, graduation rates, or the like.  Essentially the rankings give you a snapshot of the perception of the community of department chairs in a discipline, not an actual real ranking of some defined quality.  This has some consequences.  For example, perceptions are very hard to change, so it's unlikely that there will be lots of movement on these rankings unless there are exceptional circumstances (e.g., a particular department wins a couple of Nobel prizes out of the blue).  It is distressing to me to see how much importance some people (prospective students on the one hand, administrators on the other) place on rankings that measure reputation rather than something truly quantitative.

The NRC, by contrast, does survey real data like those mentioned above.  Unfortunately, they seem to be in a mode of continuously delaying the release of their "decadal survey".  Anyone have any more insight into why that is taking so long? 

Monday, April 19, 2010

An ethical dilemma

Here is a scientific ethical dilemma that came up in conversation recently.  What do you do if you get a positive referee report, but it's clear from the comments that the referee completely misunderstood your manuscript, or basically had no clue at all?  Do you point this out to the editors?  I'm actually a bit surprised that this never seems to come up, given how often people complain about the complementary case of a clueless referee that bashes a paper because of a lack of understanding....

Saturday, April 10, 2010

This week in cond-mat

Three interesting pieces of physics (among many) from the arxiv this week:

arxiv:1004.0546 - Mak et al., Atomically thin MoS2:  A new direct-gap semiconductor
The electronic structure of ordinary semiconductors is usually presented, in textbooks, in the context of band theory, neglecting electron-electron interactions and assuming infinitely large crystals.  In the case of the layered dichalcogenide, MoS2, the structure of the bulk is that of an indirect gap semiconductor.  The highest filled (single-particle) electronic states (at the top of the valence band) are labeled by wavevectors k that are near zero.  That is, the wavelike electronic states have very long wavelengths.  The (energetically) lowest empty states happen to have k values that are away from zero.  That means, for example, that the energetically cheapest way to kick an electron from the valence into the conduction band requires enough momentum (probably made up via a phonon) to make up the difference in k vectors.  The authors of this paper have observed something interesting:  as this layered material is made thinner and thinner, down toward the atomic limit, the band structure changes.  The finite-k conduction states go up in energy relative to the k = 0 conduction band states, so that the material becomes instead a direct gap system.  Neat stuff.

arxiv:1004.1233 - Cabrera et al., Oblique propagation of electrons in crystals of germanium and silicon at sub-Kelvin temperature in low electric fields
This paper also involves the concept of indirect-gap semiconductors.  Silicon and germanium are both indirect gap systems, so that the lowest energy electronic states in the conduction band live in "valleys" far from k = 0.  That means that if processes that allow intervalley scattering (such as inelastic interactions w/ phonons) are turned off, the motion of conduction electrons in real space has to reflect these valleys.  Of course, the structure of the valence band in k space is quite different than the conduction band.  That means that holes will propagate differently in real space under the same circumstances.  These folks, part of the CDMS collaboration who have been using cryogenic Si and Ge crystals as dark matter detectors, have completed a clean study of this.  Great stuff.

arxiv:1004.1202 - Cheng and Robbins, Defining contact at the atomic scale
As my students and I have encountered, particularly in a paper currently out for review, sometimes it can be very challenging to define what we mean when we say two pieces of material are touching at the one atom or two atom level, or what me mean when we say they're not touching, but instead are a certain distance apart.  These authors take a hard look at this topic in terms of the forces between the two sides.

Monday, April 05, 2010

Medical physics FTW

Blogging will be on hold for a few days, as I'm going to have an appendectomy very soon. Thank you, medical physicists, for developing the wicked cool CT machine (and blech-tasting contrast agents) key to confirming my diagnosis! Thanks, too, to the scientists and engineers that gave us wireless Internet.....

UPDATE! I had the surgery this morning and it seemed to go smoothly. Barring weirdness, I should be back up to some kind of speed in a couple of more days. Thanks for the sentiments!