Three interesting pieces of physics (among many) from the arxiv this week:
arxiv:1004.0546 - Mak et al., Atomically thin MoS2: A new direct-gap semiconductor
The electronic structure of ordinary semiconductors is usually presented, in textbooks, in the context of band theory, neglecting electron-electron interactions and assuming infinitely large crystals. In the case of the layered dichalcogenide, MoS2, the structure of the bulk is that of an indirect gap semiconductor. The highest filled (single-particle) electronic states (at the top of the valence band) are labeled by wavevectors k that are near zero. That is, the wavelike electronic states have very long wavelengths. The (energetically) lowest empty states happen to have k values that are away from zero. That means, for example, that the energetically cheapest way to kick an electron from the valence into the conduction band requires enough momentum (probably made up via a phonon) to make up the difference in k vectors. The authors of this paper have observed something interesting: as this layered material is made thinner and thinner, down toward the atomic limit, the band structure changes. The finite-k conduction states go up in energy relative to the k = 0 conduction band states, so that the material becomes instead a direct gap system. Neat stuff.
arxiv:1004.1233 - Cabrera et al., Oblique propagation of electrons in crystals of germanium and silicon at sub-Kelvin temperature in low electric fields
This paper also involves the concept of indirect-gap semiconductors. Silicon and germanium are both indirect gap systems, so that the lowest energy electronic states in the conduction band live in "valleys" far from k = 0. That means that if processes that allow intervalley scattering (such as inelastic interactions w/ phonons) are turned off, the motion of conduction electrons in real space has to reflect these valleys. Of course, the structure of the valence band in k space is quite different than the conduction band. That means that holes will propagate differently in real space under the same circumstances. These folks, part of the CDMS collaboration who have been using cryogenic Si and Ge crystals as dark matter detectors, have completed a clean study of this. Great stuff.
arxiv:1004.1202 - Cheng and Robbins, Defining contact at the atomic scale
As my students and I have encountered, particularly in a paper currently out for review, sometimes it can be very challenging to define what we mean when we say two pieces of material are touching at the one atom or two atom level, or what me mean when we say they're not touching, but instead are a certain distance apart. These authors take a hard look at this topic in terms of the forces between the two sides.