Lecture 22: Antiferromagnets

We finish off the low temperature corrections to the magnetization in a ferromagnet due to spin wave excitations, and also calculate the energy and heat capacity of spin waves. Now, on to antiferromagnets, where neighboring spins are antialigned. We derive the susceptibility, and the spin wave dispersion. Due to technical difficulties, I post last year's audio:Lecture Audio

We talk more about holes today. They don't really exist, you know! But when only a few electrons are missing from the valence band, it's so much more convenient to describe only the missing states that the fictional particles we call "holes" are a very useful concept. We talk more about their mass, velocity, momentum, and other properties. Then we discuss the p-n junction, where a semiconductor surface is donor-doped on, say, the right, and acceptor-doped on, say, the left. We calculate the strength of the permanent electric field that happens at the interface. This permanent electric field produces a real live voltage in the material. Can you use it to run a light bulb?Due to technical difficulties this year, I post last year's lecture:Lecture Audio

We define the heat capacity, and calculate the phonon heat capacity in the high and low temperature limits. We also introduce the density of states. Technical difficulties meant that this lecture did not get recorded this year. In its place, I post last year's lecture 5:Lecture Audio