This week in cond-mat

Three interesting papers from this past week....

arxiv:0810.1308 - Chen et al., Non-equilibrium tunneling spectroscopy in carbon nanotubes
One persistent challenge in condensed matter physics is the fact that we are often interested in physical quantities (e.g., the entropy of some system) that are extremely challenging or impossible to measure directly (e.g., you can't call up Agilent and buy an entropy measuring box). For example, in nanoscale systems, particularly those with strong electron-electron interactions, we would love to be able to study the energy transfer in nonequilibrium situations. As a thought experiment, this might involve injecting particular electrons at known energies and following them through the nanostructure, watching them scatter. Well, we can't do that, but we can do something close. Using a superconducting electrode as a probe (because it has a particularly sharp feature in its density of states near the edge of the superconducting gap), we can perform tunneling spectroscopy on a system of interest. This assumes that the measured tunneling current is proportional to the product of the tunneling probe's density of states and that of the system of interest. It also assumes that the tunneling process is weak enough that it doesn't strongly influence the system. From the resulting data it is possible to back out what the distribution of electrons as a function of energy is in the system. Here a collaboration between UIUC and Michigan State apply this technique to study electrons moving in carbon nanotubes. I need to think a bit about the technical details, but the experimental data look very nice and quite intriguing.

arxiv:0810.1873 - Tal et al., The molecular signature of highly conductive metal-molecule-metal junctions
This is another in a series of extremely clean experiments by the van Ruitenbeek group, looking at conduction through single molecules via the mechanical break junction technique. Using Pt electrodes, they routinely see extremely strong coupling (and consequently high conductance, approaching the conductance quantum) for small molecules bridging the junctions. They further are able to confirm that the molecule of interest is in the junction via inelastic electron tunneling spectroscopy; molecular vibrational modes show up as features in the conductance as a function of bias voltage. They can see isotope effects in those modes (comparing normal vs. deuterated molecules, for example), and they can see nontrivial changes in the vibrational energies as the junctions are stretched. Neat stuff.

arxiv:0810.1890 - Gozar et al., High temperature interface superconductivity between metallic and insulating cuprates
There is no doubt that the ability to grow complex materials one unit cell at a time is a tremendously powerful technique. Here, a collaboration anchored by growth capabilities at Brookhaven have succeeded in creating a novel superconducting state that lives at the interface between two different cuprate oxides, neither of which are superconducting by themselves. Remember, all kinds of wild things can happen at interfaces (e.g., spontaneous charge transfer, band bending) even without the strong electronic correlations present in this class of materials. There is a real possibility here that with appropriate understanding of the physics, it may be possible to engineer superconductivity above previously accessible temperatures. That would be huge.