## Tuesday, January 23, 2007

### The most powerful idea in condensed matter physics?

To get some science discussion going, I thought I'd throw this out there. There are many candidates, but based purely on citations alone, one could make a credible argument that the most powerful idea in condensed matter physics is the (first) Hohenberg-Kohn theorem: the external potential V(r) of an electronic system can be determined exactly (to within a trivial additive constant) by the ground state electronic density rho(r). This means that, in principle anyway, if you know the ground state rho(r), you know everything - you've exactly specified the Hamiltonian, which means you've specified all the many-body wavefunctions for the ground and excited states of the system, all just by knowing the ground state density. Pretty impressive. It's the basis for all of density functional theory. The original paper's been cited 5059 times (as of this morning), and the followup paper that proposed a practical approximation method to make this useful for calculating electronic structure has been cited 11963 times (as of this morning).

On the other hand, I suspect that if you asked a modern CM theorist, they'd list other choices before getting to that one.

## Thursday, January 18, 2007

### This week in cond-mat

One paper this week, because real life continues to kick my butt.

cond-mat/0701119 - Minot et al., Single quantum dot nanowire LEDs
This is a really nice example of how impressively refined semiconductor nanowire growth has become. By carefully varying the growth conditions and precursors, the composition of growing InP nanowires can be tailored to form a single wire with a p-doped InP lead, an n-doped InP lead, and an interaction region in between formed from InP(1-x)As(x), which has a smaller bandgap than either of the InP segments. The result is a single nanowire LED, potentially well-suited for single-photon emission experiments and the like. Since the whole active region is vastly smaller than the relevant wavelength, the surrounding medium is essentially free space, and screening is comparatively poor in such tiny 1d structures, quantum confinement and charging effects both play a role. Very pretty.

## Monday, January 08, 2007

### Innumeracy or hypocrisy

According to the NY Times, some in the Senate (on both sides of the aisle) are unhappy about attempts by House Democrats to legislate the recommendations of the 9/11 commission. Now, that's not necessarily unreasonable - it's always easier to "recommend" something than actually implement it in real life. However, one of the objections is that inspecting all air freight coming into the US would cost "$3.6B over the next decade, while ship inspections could cost even more." Wait a second here. We're spending$3.6B every two weeks in Iraq. How on earth can a similar expense spread out over 10 years be too much, while the same amount every couple of weeks is an acceptable cost for the War on Terror? I'm not trying to make a value judgment about either one, but I don't see how one can hold both points of view at the same time.

## Friday, January 05, 2007

### The internet "memory hole" and Jan Hendrik Schon

First post of the new year. Whoo-hoo.

While revising the course webpage for the class that I'm teaching this coming semester, I noticed something interesting and disturbing. Anyone out there remember Jan Hendrik Schon? This fellow was the focus of arguably the most serious fraud in the physical sciences in the last 50 years while he was working at Bell Labs. As you can see from the wikipedia entry above, there was a thorough investigation and the fraud was discovered, though only after hundreds of person-years had been wasted by people around the world trying to replicate work that turned out to have been fabricated in the first place. The results of that investigation had been archived on the Bell Labs website. However, now that Alcatel has taken over Lucent, those links are dead, and a seach of the new Alcatel-Lucent site does not find any trace of Herr Dr. Schon. At the moment the links on the Internet Wayback Machine still work, but there is no guarantee that these will last forever. I know that old links to, e.g., previous years' problem sets from my courses go away after some time.

This is bad. It should be the responsibility of Bell Labs to maintain this information in an accessible way for at least ten years, or some other reasonable period. In the meantime, I will host the documents on my own university account. Here is the executive summary, and here is the full report of the investigating committee.