Two cond-mat papers this week, and one other item.
arxiv:0704.1775 - Yu et al., Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions
This paper is by my former student, Lam Yu, now at NIST in Maryland. It is a systematic study of a relatively long-standing problem in the molecular electronics field. Electrons can undergo off-resonant tunneling through molecules; that is, the electrons tunnel from one electrode to the other via the molecule, even though there are no molecular levels aligned with the filled electronic states of the electrodes. You can think of this as a second-order tunneling process: while actually putting an extra electron on the molecule is classically forbidden by conservation of energy, one can consider a second order tunneling process where the electron-on-the-molecule is a virtual intermediate state. If a sufficient bias voltage exists between the source and drain electrodes, and the nuclear wave functions work out right (i.e., Franck-Condon factors), one can have processes where the electron tunnels on to the vibrational ground state of the molecule and tunnels off a vibrationally excited state of the molecule, all in one process. This is the key to inelastic electron tunneling spectroscopy, where such vibrational excitations result in a signature in the IV characteristics of the electrode/molecule/electrode sandwich (nominally a peak in d^2I/dV^2). The problem is, experiments have shown a variety of lineshapes rather than simple peaks. Lam's work shows that the presence of metal ions within the molecular layer can result in complicated lineshapes like those seen in some experiments.
arxiv:0704.0451 - Ward et al., Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy
This paper comes out of my own group. It has been known for some time that metal nanostructures driven at their plasmon resonances can act like little optical antennas, so that the local electromagnetic field near, e.g., metal nanoparticles can be much larger than the incident electromagnetic field. Since Raman scattering (a common vibrational spectroscopy) is a nonlinear optical process that scales like (roughly) the fourth power of the electric field, it is a prime candidate for these enhancement effects. In some geometries, single-molecule Raman sensitivity has been demonstrated. In this paper, we have shown that the electromigrated nanoscale gaps that we use for single-molecule electronic transport experiments are extremely good for surface-enhanced Raman scattering (SERS). We show that one can make these structures in a scalable way, with a high yield, and they show all the hallmarks of few- or single-molecule sensitivity. We're very excited about where we might go with this....
Finally, this past weekend came the first announcement of results from Gravity Probe B, or as my friends and I liked to call it, "The Project that Ate Stanford". GPB is a satellite-based test of general relativity. My thesis advisor remembers Leonard Schiff coming to Cal Tech in 1964 and talking about how GPB was only a couple of years away. What does this have to do with condensed matter? Well, the folks building this thing produced a lot of good science trying to understand thin film niobium superconductors, including a technique using lead balloons (no, really) to produce volumes with magnetic fields smaller than anywhere else in the known universe. Anyway, the finally have announced some results - so far, they've found that GR does a good job (within 1%, anyway) at describing the bending of space near the earth. It would appear that the analysis of the main effect they're trying to see, "frame dragging" of spacetime by the rotation of the earth, is being hampered by annoying systematics in their experiment. Hopefully they'll get it worked out, though they suffer from a sociology of science problem: if they confirm GR, no one will be surprised; if they refute GR, no one may believe the result because the experiment is so complicated. Ahh well.