Sunday, December 31, 2006

This week in cond-mat

Two papers for this end-of-year post.

cond-mat/0612556 - Vartiainen et al., Nanoampere pumping of Cooper pairs
The single-electron transistor was developed almost twenty years ago, based on the observation that one could now fabricate a metal island (weakly coupled to leads via tunnel junctions) so small that it's capacitive charging energy could significantly exceed kT (this gets easier as T is lowered to within a fraction of a Kelvin of absolute zero, which is now readily achievable). In such a device, the charge on the island is generally well-defined and quantized to an integer number of electrons. By cleverly hooking islands together and cycling gate voltages appropriately, it's possible to make an electron "turnstile", such that one electron at a time may be pumped through the circuit. Doing this at high frequencies, f, would enable (ideally) a noiseless current source (with current ef). That's easier said than done, however, because the intrinsic RC charging timescales of such turnstiles tend to limit the frequency of operation. The Finnish group here has implemented an alternative scheme, using superconducting quantum interference devices (SQUIDs) rather than simple tunnel barriers, and can pump individual Cooper pairs of superconducting electrons through their circuit at a high enough rate to generate nanoamperes of current. This is very impressive, and could lead to real advances in metrology.

cond-mat/0612635 - Pereira et al., Kondo screening cloud and charge quantization in mesoscopic devices
In the Kondo effect, a localized spin coupled to mobile electrons undergoes a spin-flip scattering process that leads to spin correlations in the mobile electrons. At temperatures small compared to the characteristic energy of this process, the local spin is "screened" - that is, it is entangled with a cloud of the mobile electrons, forming a singlet state with no net spin. A question that has been around a long time in the solid state community is, how big is that screening cloud? The only successful attempts to measure the size have been in STM measurements of magnetic impurities on surfaces, as far as I know. In this paper, the authors propose a clever scheme to try this in a model system. One can have the local spin be living in a quantum dot, and use a electrons in a large 1d electronic box instead of truly free electrons to form the Kondo state. The idea is that the size of the Kondo cloud will be detected by looking at the single-particle levels of the 1d system (and varying system effective length). Neat, though tough to do!

Friday, December 29, 2006

Great rhetorical device

From an exchange between CNN's Whitehouse correspondent and a Frances Townsend, the Assistant to the President for Homeland Security and Counterterrorism:
HENRY: You know, going back to September 2001, the president said, dead or alive, we're going to get [Osama bin Laden]. Still don't have him. I know you are saying there's successes on the war on terror, and there have been. That's a failure.

TOWNSEND: Well, I'm not sure -- it's a success that hasn't occurred yet. I don't know that I view that as a failure.
Wow. Cool! I need to start using language that way. My Nobel Prize in Physics is a success that hasn't occurred yet.

Friday, December 22, 2006


This is my first introduction to silly blog games, which I suppose shows that I don't blog too much, since it's taken this long. Anyway, I've been tagged. In this game I'm supposed to grab the nearest book, go to page 123, go to the fifth sentence, and write down the next three sentences. Then I'm to tag three more people, presumably ones that I think will play the game. Hmm. Well, on holiday break the nearest book to me right now is The English Assassin by Daniel Silva. Here are the sentences:
He screamed at the room service boys when they didn't bring his coffee quickly enough. Soon the entire staff and most of the guests at the Hotel Laurens knew about the crazy Boche writer in the attic. On the way to Paris, he had stopped at the airport in Nice, dropped off the rented Mercedes, and collected a Renault.
As for tagging, I suppose I'll go with Rob, the Incoherent Ponderer, and the Female Science Professor. (Wolff, Bernie, I'll get you some other time....)

Saturday, December 16, 2006

This week in cond-mat

Just one paper this week. End-of-semester crunch + trying to write up some new stuff in my group is cutting into my blogging....

cond-mat/0612278 - Jeltes et al., Hanbury Brown Twiss effect for bosons versus fermions.
Hanbury Brown and Twiss did a beautiful experiment using light that has since been extended to examine the quantum statistics of other kinds of particles. Consider a source of particles and a couple of detectors. For Bose particles, the symmetry of the wave function under exchange of the particles implies that particles will tend to bunch. In handwavy language, the Bose distribution favors particles to be in the same state rather than different states, all other things being equal. HB and T showed this bunching in space for photons. Conversely, because of Fermi Dirac statistics (the Pauli principle), fermions tend to anti-bunch. All other things being equal, fermions tend to avoid each other. This antibunching has been seen in electrons in solids as well as in free electrons. The authors of this paper have done a beautiful version of this experiment with cold atoms, using the same trapping setup to look at either 3He or 4He, which are chemically identical but possess Fermi and Bose statistics, respectively. They use a multichannel plate detector to look at the positional correlations between pairs of atoms when they hit the detector, and see the expected HB-T correlations. Extremely clean, like all good atomic physics experiments.

Saturday, December 09, 2006

Rumor mills: don't trust 'em.

Since I'm heavily involved in our faculty search, I can't really talk too much about it online. However, I do want to point out something about rumor mill sites. These have existed for about a decade in the high energy and astrophysics communities, and for the last three years or so there has been a condensed matter/AMO rumor mill site as well. The idea is that this is a way for the candidates (primarily) to keep track of who is interviewing for the various jobs, and who is getting offers. The problem is, these sites are only as good as their sources of information. Now that just about every department puts their seminar calendars online, it's not hard to go through the schedules and try to figure out who is giving job talks. Of course, you have to use a bit of common sense, too. As a cautionary example: yesterday the CM/AMO rumor mill listed two candidates allegedly on the short list for our position. However, (a) both are theorists, (b) one is an AMO person, and (c) our search committee hasn't even had its first meeting yet. Doh! The lesson: don't take the rumor mills very seriously. They can be very wrong, have no fact-checking, and in principle can be manipulated via disinformation.

Saturday, December 02, 2006

This week in cond-mat

Two papers on the arxiv this week that give me an excuse to talk about the standard picture of metals and how bits of it can fail.

cond-mat/0611714 - Pisana et al., Born-Oppenheimer breakdown in graphene
The Born-Oppenheimer approximation is one of the most commonly made in quantum mechanical treatments of atoms, molecules, and solids. It's a specific example of the adiabatic approximation: if the potential energy term V(t) of the single-particle Schroedinger equation changes slowly enough (basically compared to \hbar divided by the energy difference between the single-particle energy levels of the system at some instant in time), then it's ok to say that the true single-particle solutions are well approximated at time t by the solutions to the static Schroedinger equation with V = V(t). The Born-Oppenheimer approximation applies this to electrons around atoms. It assumes that the atoms move slowly compared to the electronic energy timescales, so that one can do calculations of molecular (for example) states by assuming that the ions are fixed in space. This paper reports Raman scattering measurements of the vibrational modes of graphene as a function of gate voltage (and hence electronic density). What they find is that the electronic population affects the lattices vibrational modes in a way that violates the Born-Oppenheimer approximation. I haven't read this very carefully, but this is interesting and surprising, at least to me. Given how well the basic graphene electronic structure can be approximated by a simple tight-binding calculation, a big violation here seems weird.

cond-mat/0611724 - Qazilbash et al., Correlated metallic state of vanadium dioxide
The mean free path is a simple concept: it's the average distance a particle travels before scattering off of something. For a classical gas of hard spheres, the mean free path would be the inverse of (number density times cross-section). For quantum mechanical electrons in a metal, the electrons scatter off anything that breaks the periodicity of the crystal lattice - grain boundaries, defects, impurities, distortions of the lattice due to phonons. The mean free path in a metal is typically found from the conductivity, via something called the Einstein relation. Tacit here is the assumption that the electrons behave like well-defined particles that can propagate along for a while between scattering events. Indeed, a general requirement for the validity of this quasiparticle picture for electronic states in a metal is that the ratio of the mean free path to the wavelength of the electron is much greater than one. If the electron scatters many times before even traveling one wavelength, obviously the traveling wave picture of the electron is not valid. The point of this is that there is a physical lower limit to the mean free path: in a "good metal", the mean free path should never be shorter than the lattice spacing between atoms. This is called the Ioffe-Regel-Mott limit.
Now look at vanadium dioxide, which has a transition at 340 K between a high temperature metallic phase and a low temperature insulating phase. The phase transition is complicated, and includes a change in the unit cell shape. The authors of this paper have used optical techniques to infer the frequency-dependent conductivity in both phases. They confirm that the Ioffe-Regel-Mott limit is violated in the metallic phase at high temperatures, and they infer that the dominant scattering mechanism is due to electron-electron interactions. Basically this is one more nice piece of evidence that VO2 is a "bad metal", in which the quasiparticle way of thinking about distinct electrons isn't really valid.