Saturday, September 22, 2007

Two seminars this past week

I've been remiss by not posting more interesting physics, either arxiv or published. I'll try to be better about that, though usually those aren't the posts that actually seem to generate comments. For starters, I'll write a little about two interesting condensed matter seminars that we had this week. (We actually ended up with three in one week, which is highly unusual, but I was only able to go to two.)

First, my old friend Mike Manfra from Bell Labs came and gave a talk about the interesting things that one sees in two-dimensional hole systems (2dhs) on GaAs (100). Over the last 25 years, practically a whole subdiscipline (including two Nobel prizes) has sprung up out of our ability to make high quality two-dimensional electron systems (2des). If you have a single interface between GaAs below and AlxGa(1-x)As above, and you put silicon dopants in the AlGaAs close to the interface, charge transfer plus band alignment plus band bending combine to give you a layer of mobile electrons confined in a roughly triangular potential well at the interface. Those electrons are free to move within the plane of the interface, but they typically have no ability to move out of the plane. (That is, the energy to excite momentum in the z direction is greater than their Fermi energy.) Now it's become possible to grow extremely high quality 2dhs, using carbon as a dopant rather than silicon. The physics of these systems is more complicated than the electron case, because holes live in the valence band and experience strong spin-orbit effects (in contrast to electrons in the conduction band). In the electron system, it's known that at relatively low densities, low temperatures, and moderate magnetic fields, there is a competition between different possible ground states, including ones where the electron density is spatially complicated ("stripes", "bubbles", "nematics"). Manfra presented some nice work on the analogous case with holes, where the spin-orbit complications make things even more rich.

Then yesterday we had a talk by Satoru Nakatsuji from the ISSP at the University of Tokyo. He was talking about an extremely cool material, Pr2Ir2O7. This material is a metal, but because of its structure it has very complicated low temperature properties. For example, the Pr ions live on a pyrochlore lattice, which consists of corner-sharing tetrahedra. The ions are ferromagnetically coupled (they want to align their spins), but the lattice structure is a problem because it results in geometric frustration - not all the spins can be satisfied. As a result, the spins never order at nonzero temperature (at least, down to the milliKelvin range) despite having relatively strong couplings. This kind of frustration is important in things like water ice, too. In water ice, the hydrogens can be thought of as being at the corners of such tetrahedra, but the O-H bond lengths can't all be the same. For each tetrahedron, two are short (the covalent O-H bonds) and two are long (hydrogen bonds). The result is a ground state for water ice that is highly degenerate, leading to an unusual "extra" residual entropy at T = 0 of R/2 ln 3/2 per mole (in contrast to the classical third law of thermodynamics that says entropy goes to zero at T = 0. The same kind of thing happens in Pr2I2O7 - the spins on the tetrahedron corners have to be "two-in" and "two-out" (see the link above), leading to the same kind of residual entropy as in water ice. This frustration physics is just the tip of the iceberg (sorry.) of what Nakatsuji discussed. Very neat.

2 comments:

Schlupp said...

We might not comment on these entries, but we still like to read them (and occasionally the papers)!

Jackson said...

Thank you..