Nonequilibrium Phenomena in Mesoscopic Physics

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****NON-TECHNICAL ABSTRACT****Most traditional scientific measurements are performed on systems that are very close to equilibrium; i.e. at nearly the same temperature and pressure as their surroundings. In the case of electrical measurements, "near equilibrium" means that the applied voltage is small enough so that it does not change the properties of the sample being measured. But sometimes one can learn new information by pushing systems far from equilibrium. This project pursues three types of non-equilibrium measurements in solid-state physics, all involving the study of very small (sub-micrometer) structures. The first topic is a study of the statistics of electron flow, or current, through solids. In very small samples the electron flow is not constant, but rather fluctuates in time. Those fluctuations reveal information about the sample that is normally hidden. The second topic is a study of hybrid systems containing both superconducting and non-superconducting metals. Such systems exhibit many remarkable properties when driven far from equilibrium. The third topic is a study of electron-electron interactions in carbon nanotubes, which are ultra-thin wires made of pure carbon. In this case one uses non-equilibrium measurements as a tool to obtain information about the sample that is difficult to obtain from equilibrium measurements. This research provides the conceptual and technical underpinnings for important areas of nanotechnology. The undergraduate and graduate students in this program are trained in the physical principles and experimental techniques that are essential to future progress in the semiconductor industry and in nanoscience research. They also learn how to think critically, troubleshoot experiments, and present their work in written and oral presentations. ****TECHNICAL ABSTRACT****The vast majority of electrical transport experiments measure just the sample resistance. In some systems, shot-noise measurements reveal further information that is not available from the resistance alone. Full Counting Statistics (FCS) is the study of all the moments of the current distribution, not just the first (conductance) and second (shot noise). Theoretical progress in FCS has been remarkable, but there are only a handful of experiments that go beyond the second moment. This project pursues a suggestion by Nazarov to use Josephson junctions as current threshold detectors to measure FCS. A second research area in this program is the behavior of hybrid superconductor/normal systems under nonequilibrium conditions. Several unusual properties of such systems have already been discovered, and more have been predicted. Experiments are needed to test these predictions, and at the same time elucidate the role of electron-electron and electron-phonon interactions, which are often neglected in theoretical calculations. A third topic is electron-electron interactions in carbon nanotubes. These nearly-ideal one-dimensional conductors exhibit strong interactions, sparking debate about whether they represent true Luttinger liquids. This project exploits a new tunnel-spectroscopy technique to measure the distribution function in a tube subjected to a large voltage bias. These three projects in mesoscopic physics train undergraduate and graduate students in the physical principles and experimental tools that are essential to the semiconductor industry and to future nanoscience research.