Sunday, June 18, 2006

Recently on cond-mat

Here are a couple of recent preprints that caught my eye. I'm going to try to get back to chronicling these weekly, if I can find the self-discipline....

cond-mat/0606430 - Streed et al., Continuous and pulsed Quantum Zeno Effect
This experiment is really an atomic physics experiment, but it is on cond-mat, and the physics is very cool. The Quantum Zeno Effect gets its name from Zeno's Paradox: in order to get from point A to point B, a person would first have to get half-way; however, to get to the midpoint between A & B, a person would first have to get half-way to that spot, and so on. Thus, noone can ever get anywhere. While the solution to this apparent paradox lies in the idea of rates and limits (at a given instant, there is something called the velocity that is the rate of change of distance per unit time), one can set up a quantum case where a system really never does get from state A to state B. This is a result of the basic postulates of quantum mechanics: after a measurement of some observable, the system is left in an eigenstate of that observable. If the same observable is measured again before the system has had a chance to evolve (via the Schroedinger equation and whatever the Hamiltonian is), the system will still be in that same eigenstate that was just found. So, if one keeps measuring the system continuously, the state of the system can't evolve. The act of continuous measurement locks the system in that one eigenstate. Ketterle's group at MIT have managed to implement a version of this using a Bose-Einstein condensate of rubidium atoms. Very neat.

cond-mat/0606375 - Reich et al., Observation of magnetism in thin gold films
This paper is already out as an Applied Physics Letter. The authors report sensitive magnetic susceptibility measurements on thin Au films, and find that, depending greatly on substrate and preparation, it is possible for those films to be significantly paramagnetic. This is a bit weird, since Au in bulk is diamagnetic. Of course, there have been reports of weird magnetism in nanostructured Au before, including ferromagnetism in Au clusters and whopping big magnetic effects in the presence of self-assembled monolayers of molecules. All of these effects have been challenging for folks to reproduce and confirm, in part because it really does seem like every little detail about sample prep and interfaces matters. It's always interesting to see how even things that seem like they should be well understood can be rich and complex. My personal theory on these effects is that they involve orbital moments in the Au caused by interfacial charge transfer and the strong spin-orbit scattering in Au. Some theorists seem to have the same idea.

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