This week in cond-mat

Two papers that are fun to talk about because of more than just the science:

cond-mat/0608576 - Klimczuk et al., Superconductivity in Mg_10 Ir_19 B_16
This is just a typical example of the kind of neat stuff that can come out of a really outstanding solid-state chemistry group. Bob Cava, formerly of Bell Labs and these days at Princeton, is an impressive materials chemist who has been involved in the discovery and synthesis of an ungodly large number of new materials. The one in the title of this paper is apparently one of a whole new family of superconductors. If someone told me that room temperature superconductivity was discovered, my first guess at the grower would probably be Cava. Just having someone like this on your campus can really make things happen, just like having a fantastic MBE grower. Of course, the total number of people like this who are this successful is very small. You can't just be edisonian - you have to have impressive insight into the chemistry and materials science issues, and you have to have access to the appropriate characterization tools.

cond-mat/0608492 - Hirsch, Do superconductors violate Lenz's Law?
Jorge Hirsch is a very interesting guy. He's very much a political activist, a person interested in developing useful metrics for measuring academic performance, and a condensed matter theorist with his own ideas about superconductivity. When a (type I) superconductor is brought into a region of magnetic field, the superconductor develops screening currents to exclude the magnetic flux. Those currents flow within a penetration depth of the surface of the material, and the result is essentially perfect diamagnetism - this is called the Meissner effect. When those currents get set up, a torque is exerted on the lattice of the superconductor. Basically the paired electrons making up the supercurrent have some orbital angular momentum about the axis of the magnetic field. Since total angular momentum is conserved, the ions of the lattice have to pick up angular momentum going the other way, so that the total remains zero. Hirsch claims (and for fun, is trying to take bets on this to finance an experiment) that there is a big difference between the bring-a-superconductor-into-a-field case, and the cool-through-the-superconducting-transition-in-a-field case. He argues that the torques on the lattice in those two cases should be in opposite directions. I think he's wrong - at the very least, his treatment of this problem is waaaaay to simple. Anyone?