Wednesday, May 29, 2013

What does "heating" mean at the nanoscale?

I've talked before about what physicists mean when they talk about "temperature", and work has me thinking about this a lot these days. Temperature is inherently a statistical concept.  It doesn't really make sense to talk about the temperature of a single electron.  The electron has some momentum (and therefore some kinetic energy), but temperature is not a meaningful concept for a single particle in isolation.  Now, if you have a whole bunch of electrons, you can talk about how many of them have a certain amount of energy.  That distribution of electrons as a function of energy takes on a particular form when the electron system is in thermal equilibrium.  (That is, if the electron system is weakly coupled somehow to an energy reservoir so that energy can be exchanged freely between the reservoir and the electrons.)  When the electron system is in thermal equilibrium with the reservoir, on average no net energy is transferred as a function of time between the electrons and the reservoir; this is what we mean when we say that the electrons and the reservoir have the same temperature.

The situation gets really tricky when a system is driven out of equilibrium.  For example, you can use a battery to drive electrons through some nanoscale system.  When you do that, and you look at different points within the nanoscale system, you will find that, in general, the distribution of the electrons as a function of energy doesn't necessarily look much like the thermal equilibrium case.  So, is there a sensible way to generalize the idea of temperature to quantify how "hot" the electrons are?  The problem is, there are many ways you might want to do this - you are trying to take a potentially very complicated distribution function and essentially summarize it by a single number, some local effective temperature.   A natural direction to go is to consider a thought experiment:  what if you took a reservoir with a well defined equilibrium temperature, and allowed it to exchange energy with the nonequilibrium system at a location of interest.  What reservoir temperature would you have to pick so that there is no net average energy transfer between the system and the reservoir in steady state?   That is one sensible way to go, but in the nano limit the situation can be very tricky, even in the thought experiment.  The details of how the imagined energy exchange takes place can affect the answers you get.  Tough stuff.

Thursday, May 23, 2013

Instructor opening at Rice

Wiess Instructorship in Physics and Astronomy

The Physics and Astronomy Department at Rice University invites applications for a one-year instructorship position teaching introductory physics, commencing July/August 2013.  The teaching load is equivalent to two courses per semester.  There would also be opportunities to develop innovative teaching methods and pursue independent research or collaborations with existing research programs (see web page  Evaluation of applications will begin immediately and continue until the position is filled. Applicants should send a curriculum vitae, a statement of teaching and research interests, and a list of publications as a single PDF file, and should arrange for three letters of reference to be sent to: with subject line "Wiess Instructorship" (pdf format preferred), or by postal mail to Wiess Instructorship Search, c/o Valerie Call, Physics and Astronomy Department-MS61, Rice University, 6100 Main Street, Houston, TX 77005-1892. Applicants must have a PhD and be eligible to work in the U.S. Rice University is an affirmative action/equal opportunity employer.

Saturday, May 18, 2013

Ask me something.

I realized I've never really tried having my readers just post questions for me.   Have at it!

Tuesday, May 14, 2013

A Scientist Laureate position for the US?

This is at least thought-provoking.  Lamar Smith (R-Texas, chair of the US House science committee, famous for things like this) and Zoe Lofgren (D-CA, about as far from Lamar Smith as I can imagine with the possible exception of Nancy Pelosi) are co-sponsoring a bill that would create a position called Scientist Laureate of the United States.  This person would be appointed by the President following nomination by the National Academy of Sciences, and would be meant to act as an inspirational figure, making public appearances and furthering the cause of science.  This could be a good thing, provided (1) an actual accomplished scientist is chosen, not someone who has to satisfy a political agenda; and (2) the person chosen is charismatic and able to use the bully pulpit effectively.  The Science Laureate should do more than show up at middle schools - they should get major exposure (e.g., late night talk shows; hosting a science program on a major network with actual resources to make it good; having the ear of Congress, perhaps even the limited ability to insist on speaking at a hearing of the House or Senate science-related committees).  (Halftime at the Superbowl is probably out of line.)

While I applaud scientists with great public outreach track records (Neil deGrasse Tyson just spoke at our commencement), that should not be the sole criterion.  If this passes, hopefully Congress will keep in the bit about the NAS making the choice.  Suggestions are invited in the comments.

Thursday, May 02, 2013

MOOC followup

Very briefly, here is an open letter by the San Jose State University philosophy department regarding MOOCs (one in particular).   Food for thought.

update:  my colleague Moshe Vardi pointed out his own editorial on this topic.

I don't agree with everything in either of these documents.  I do think it's worth thinking hard about the purpose of MOOCs.  Are they about idealistically providing access to fantastic educational opportunities at very low cost to the student for millions of potential pupils who have an internet connection?  Are they about cynically slashing the operating costs of universities by restructuring the educational experience and potentially eliminating large numbers of faculty jobs?  These are not mutually exclusive.

Wednesday, May 01, 2013

Fun with single atoms

IBM Almaden research has produced "A Boy and His Atom", a stop-motion movie where the frames are scanning tunneling microscope images of carbon monoxide molecules on the Cu(111) surface.  Here is the "Making of..." movie as well.  Fun stuff.  I was particularly amused by Andreas Heinrich's comment that "if I can get a thousand kids to join science rather than go into law school, I'd be super happy."  Amen :-)