Tuesday, September 29, 2009

The return of the embarassing news story.

As mentioned previously, the news story about NSF upper level staff surfing for porn while on the job is back.  This would be funny if it weren't so pathetic and sad.  Obviously this is inappropriate behavior, and NSF clearly needs to get their IT staff up to snuff, since it's certainly possible in a corporate environment to detect and stop this kind of activity.  Still, it seems unfair to single out NSF like this.  I'd be surprised if this didn't go on in all large, computer-heavy organizations at some rate.

First principles vs. toy models

One of the hot topics at the workshop I attended was the proper role of "first principles" calculations in trying to understand electronic conduction at the atomic and molecular scale.  In this business, there tend to be two approaches.  The first, which I call for lack of a better term the "toy model" paradigm, constructs models that are highly idealized and minimalistic, and you hope that they contain the essential physics needed to describe real systems.  An example of such a model would be the single-level Anderson-Holstein model of transport through a molecule.  Instead of worrying about all of the detailed electronic levels of a molecule and the many-electron physics there, you would concentrate on a single electronic level that can either be empty, singly occupied, or doubly occupied.  Instead of worrying about the detailed band structure of the electrodes, you would treat them as ideal electronic reservoirs, and there would be some couplings that allows electrons to hop between the level and the reservoirs.  Instead of considering all of the possible molecular vibrations, you would assume a single characteristic vibrational mode that "lives" on the molecule, and there would be some additional energy cost for having that vibration excited while there is an electron occupying the level.  While this sounds complicated, it is still a comparatively idealized situation that can be described by a handful of characteristic energies, and it contains rich physics.

On the other hand, one can consider trying to model a specific molecule in detail, worrying about the precise electronic and vibrational levels appropriate for exactly that molecule bonded in a particular configuration to a specific kind of metal electrode surface.  While this sounds in some ways like it's what you "really" ought to do, this "first principles" approach is fraught with challenges.  For example, just solving for the electronic levels of the molecule and their relative alignment with the electronic levels in the electrodes is extremely difficult in general.  While there are impressive techniques that can work well in certain situations (e.g., density functional theory), very often the circumstances where those methods work best (quasi-equilibrium, far away from resonances, in situations where electron correlation effects are minimal) are often not too interesting. 

It's interesting to watch the gradual convergence of these approaches.  As computing power grows and increasingly sophisticated treatments are developed, it looks like first-principles calculations are getting better.  One direction that seems popular now, as our condensed matter seminar speaker yesterday pointed out, is using such calculations as guidelines for correctly estimating the parameters that should be fed into the essential physics toy models.  Interesting times are on the horizon.

Friday, September 25, 2009

AAAS and advertising

I've received three pieces of fundraising advertising from AAAS in the last two days via US Mail.  This makes me wonder about a few things.  First, in this day and age, why can't they get a mailing database set up that can tell that Douglas Natelson and Dr. Douglas Natelson at the same address are actually the same person? Second, do they really think that I pay a lot of attention to bulk-mailed fundraising appeals? Third, how much money are they spending, how much energy is consumed, and how much pollution is generated in sending out these tree-killing mailings, when they claim to be environmentally conscious and already have my email address as a subscriber to Science? Fourth, this many appeals in one week smacks of desperation - is there something we should know? 

Tuesday, September 22, 2009

Curve fitting

Very often in experimental physics, we're interested in comparing some data to a physical model that may involve a number of unknown parameters, and we want to find the set of parameters that gives the best fit.  Typically "best fit" means minimizing a "cost" function, often the sum of the squares of the deviations between the model and the data.  The challenge is that many models can be very complicated, with nonlinear dependences on the parameters.  This often means that finding the optimal parameters can be very difficult - the cost function in parameter-space can have lots of shallow, local minima, for example.  The cost function may also be extremely sensitive to some parameters (the "stiff" ones) and comparatively insensitive to others (the "sloppy" ones).  In arxiv:0909.3884, James Sethna and Cornell colleagues take a look at this dilemma using the tools of differential geometry, and they propose an improvement to standard techniques based on geodesics on the relevant hypersurface in parameter space.  This looks really cool (if mathematical!), and I wish they'd included an example of an actual minimization problem that they'd done with this (instead of leaving it for an "in preparation" reference).  Any prospect for real improvements in nonlinear fitting is exciting.

Friday, September 18, 2009

Ahh, KLM.

Stuck in Schipol, forced to fly back to Houston via BRE-AMS-DET-IAH, since mechanical difficulties cancelled my early BRE-AMS flight (thus causing me to miss my AMS-IAH direct flight).  The other AMS-IAH direct flight on their schedule is really just a psychological torture device, since it's really a charter that's 100% business class and un-bookable except as a cash purchase (which would set me back $4K on top of everything I've already paid).

Could be worse.  There was another guy on the original BRE-AMS flight that got involuntarily rebooked through Paris.  After hanging out at the Bremen airport for four hours, he got to have his BRE-Paris flight also cancelled due to mechanical difficulties.
At least the workshop was extremely good.

Monday, September 14, 2009

Draconian ISP.

The ISP (netopsie) for my hotel here in Bremen, Germany has apparently decided to block access to all "blogspot.com" domains.  If I try to view my blog, I get redirected to a page that says "Banned Site.  You are seeing this error because what you attempted to access appears to contain, or is labeled as containing, material that has been deemed inappropriate."  Ironically, I can post new entries since that is done from a blogger.com page.   I can't view the blog, however, or see comments.  Idiots.  Makes me wonder what they find objectionable on blogs in particular, or whether they are complete puritans and block lots of stuff.

Tuesday, September 08, 2009

This week in cond-mat

Three quick blurbs from the arxiv this week.  I'm going to a workshop in Germany next week and have a bunch to do in the meantime, so blogging will likely be light.

arxiv:0909.0628 - Bocquet and Charlaix, Nanofluidics, from bulk to interfaces
This paper is an outstanding overview of fluids confined to the nanoscale.  I will definitely be referring to this the next time I teach my graduate course that touches on this topic.  Two of the central questions that comes up when thinking about fluids at the nanoscale are, when do large-scale assumptions about hydrodynamics (e.g., that fluid right at the walls of a container is at rest relative to the walls, even when the fluid away from the walls is flowing - the so-called "no slip" boundary condition) break down, and when does the continuum picture of the fluid (i.e., that fluid may be modeled as a homogeneous medium with some density, rather than a collection of strongly coupled particles) fall apart?  This article looks at these issues in detail, with many useful references.

arxiv:0909.0951 - Saikin et al., On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters
This one is of interest to me because of its relevance to some of the research done in my group.  Raman scattering is inelastic light scattering, where light can lose (or gain) energy to a molecule by exciting (or de-exciting) molecular vibrations.  It's been known for more than 30 years that the Raman scattering process can be greatly (many orders of magnitude) enhanced on nanostructured metal surfaces.  This happens for two reasons.  First, nanostructured metals support local plasmon modes, so that the metal acts like a little optical antenna, helping the molecule to "receive" (and "transmit") light.  This is called electromagnetic enhancement.  Second, there can be additional enhancing effects due to resonances involving charge transfer between the molecule and the nearby metal.  This latter effect is called chemical enhancement, and this paper takes a detailed look at how this can arise, considering specific configurations of molecules on Ag clusters.  It is very challenging to do calculations like this and get realistic results!

arxiv:0909.1205 - Martineau et al, High crystalline quality single crystal CVD diamond
I picked this one because (a) the fact that it is possible to grow high quality single crystal diamond by chemical vapor deposition is just plain cool, as well as of great technological potential; and (b) the x-ray topographs in this paper showing crystallographic defects in the crystals are very pretty.

Thursday, September 03, 2009

If you're reading this, you're probably pretty net-savvy.

Perhaps this feature has always been available, but I just noticed the other night that Google Analytics can tell me stats about what kind of web browsers people use to access this page.  Far and away the number one browser used was Firefox (57%), followed by Safari (16%), Internet Explorer (15%), and Chrome (8%).  Interestingly, the breakdown for those accessing my group webpage was quite different, with IE having more like 30% of the total.  Very educational.  No one using lynx, though.