Much as it pains me to admit this, I agree with Lubos Motl about something: Neither of us like the new numerical identifier system launched by Paul Ginsparg and company at the arxiv. Lubos nails both of my complaints. While the old system actually conveyed information (the subject area of the paper and how many papers in that category had been submitted that month), the new system manages to be both cryptic and uninformative. Frankly I don't care how many total papers have been submitted from all categories in the arxiv, and I'm not sure why anyone would. Somehow this reminds me of the apparent desire of the Powers that Be to switch NSF proposal submissions from FastLane, which works extremely well and is easy to use, to grants.gov, which is completely arcane and annoying. Prof. Ginsparg, if you see this, please consider switching back to some incrementally changed form of the old system.
Meanwhile, here's one paper in the old numbering format, for old time's sake, that I thought looked interesting. Perhaps a theorist could take a look at this and tell me if it's as clever and neat as it seems to be.
cond-mat/0703768 - Ostlund, The strong coupling Kondo lattice model as a Fermi gas
The Kondo lattice is a model developed in an attempt to understand the heavy fermion compounds. In this model, there are itinerant conduction electrons, and a lattice of localized unpaired moments (f-shell electrons) representing the ion cores of the rare earth constituents of the heavy fermion material. Under the right conditions, the ground state of these materials is a Fermi liquid, meaning that there are distinct, gapless, electron-like (spin 1/2, charge -e) quasiparticles, but they have an effective mass hundreds of times higher than the free electron mass. The idea is that the conduction electrons have formed fully screened Kondo singlets with the rare earth f-electrons. The true ground state of the Kondo problem is a Fermi liquid, and in this limit the ground state of the Kondo lattice is also a Fermi liquid, though the antiferromagnetic screening of the ion cores leads to the high effective mass. Note that this is rather special - in semiconductors, the effective mass is a single-particle effect that comes from the lattice potential; in these systems, the effective mass is the result of many-body correlations. In this paper, the author explicitly writes down a canonical transformation (read: clever change of variables) that directly maps the Kondo lattice Hamiltonian into that of a weakly interacting Fermi gas. It looks clever to me, but I can't judge it in the context of other theoretical treatments of these strongly correlated systems.