Condensed matter physics largely exists at the junction between statistical physics and quantum mechanics. Statistical physics tries to understand the emergence of collective phenomena (whether that's crystalline order, magnetic order, the concept of temperature, or the whole idea of phase transitions and broken symmetry) from a large number of particles obeying relatively simple rules. Throw in the fact that the rules of quantum mechanics are rich and can have profound consequences (e.g., the Pauli principle, which says that no two identical fermions can have identical quantum numbers, leads both to the stability of white dwarf stars and the major properties of most metals), and you get condensed matter physics. It's amazing how many complicated phenomena result from just simple quantum mechanics + large numbers of particles, especially when interactions between the particles become important. It's this richness, which we still do not fully understand, that is a big part of the intellectual appeal of the subject, at least for me.
I will also shamelessly crib Chad's list of points that he likes about AMO physics, and point out that CM physics is also well-described by them:
- "AMO physics is cool because it's the best field for exploring quantum effects." Well, while AMO is a nice, clean area for studying quantum effects, CM is just as good for some topics, and better for others. There's probably just as many people studying quantum computation using solid state systems, for example, as AMO systems.
- "AMO physics is cool because it's concrete." Again, it doesn't get much more concrete that CM physics; it's all atoms and electrons. One fascinating area of study is how bulk properties arise from atomic properties - one gold atom is not a metal, but 1000 gold atoms together are distinctly "metallic". One carbon atom is not an insulator, but 1000 of them together can be a nanodiamond on one hand, or a piece of graphene on the other, How does this work? That's part of what CM is about.
- "Experimental AMO physics is cool because it's done on a human scale." Experimental CM physics is the same way. Sure, occasionally people need big user facilities (synchrotrons, e.g.). Still, you can often do experiments in one room with only one or two people. Very different than Big Science.
- "AMO physics has practical applications." So does CM, and personally that's something that I like quite a bit. The computer and monitor that I'm using right now are applied CM physics.
- "AMO physics provides technologies that enable amazing discoveries in lots of other fields." Again, so does CM. Silicon strip detectors for particle physics, anyone? CCD detectors for all the imaging that the AMO folks do? Superconducting magnets for MRI? Solid-state lasers? Photon-counting detectors for astro?