0239748 Finkelstein

1. Evolution of Electron Transport Regimes in Carbon Nanotube Quantum DotsGleb Finkelstein, Duke University, DMR 0239748 In a series of recent studies, we investigate the Kondo and mixed valence effects in carbon nanotube quantum dots. Both effects result from the many-body interactions of electrons in the nanotube. In our quality nanotubes the quantum-mechanical orbitals are doubly degenerate, which results, together with the spin degeneracy, in four-electron “shells” (groups of 4 peaks in the top figure). Therefore the system obeys the theoretically predicted, albeit elusive, SU(4) symmetry. In our work, the mixed valence regime is identified for the first time. In this regime, the four single-electron conduction peaks in a shell, visible at high temperature, merge at low temperatures into a single broad maximum (right side of the top figure). This temperature evolution is rather counterintuitive: the low temperature conductance does not display single electron features, which are uncovered only by raising the temperature. PRL (2007), PRB Rapid Comm. (2007), PRB(2006). Top: Differential conductance of a nanotube as a function of gate voltage, measured at 1.3 to 15 Kelvin (top to bottom). The single-electron conductance peaks cluster in four four-electron shells. The single-electron conductance peaks within the two shells on the right merge to form smooth oscillations (mixed valence regime). Bottom: Conductance map of same sample (T=1.3 K). The “Kondo ridge” of enhanced conductance close to zero bias becomes the dominant feature of the data in the mixed valence regime.

2. Evolution of Electron Transport Regimes in Carbon Nanotube Quantum DotsGleb Finkelstein, Duke University, DMR 0239748 Three graduate students, four undergraduate students and four high school students were involved in various aspects of this project. Their work is documented in two graduate theses and one senior thesis. One of the high school students was attracted to apply to Duke and now majors in Physics, continuing to work in our laboratory. One of the graduate students now works as a research scientist in industry. Top: this home-made 4-channel 16-bit programmable voltage source features low noise and optical decoupling from the computer. In spite of the simplisticappearance, it makes a unique research instrument, which to our knowledge cannot be purchased from a commercial vendor. We heavily use “The Voltage Box” in the laboratory. Last but not least, building this unit makes a perfect summer project for motivated high school students. Bottom: undergraduate student Scott Miller poses “fixing” our home-built low-temperature scanning probe microscope. The undergraduates get fully involved in the research projects.