Berlin 2008 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 35: Transport: Nanoelectronics I - Quantum Dots, Wires, Point Contacts 3
TT 35.8: Vortrag
Donnerstag, 28. Februar 2008, 16:00–16:15, H 3010
Competition between superconductivity and Kondo effect in a carbon nanotube quantum dot — Alexander Eichler1, •Markus Weiss1, Stefan Oberholzer1, Christian Schönenberger1, Alfredo Levy-Yeyati2, Juan Carlos Cuevas2, and Alvaro Martin-Rodero2 — 1Departement Physik, Universität Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland — 2Departamento de Fisica Teorica de la Materia Condensada, Universidad Autonoma de Madrid, E-28049 Madrid, Spain
We present nonlinear transport measurements on a carbon nanotube contacted to superconducting electrodes. At low temperatures, the nanotube acts as a quantum dot with intermediate contact transparencies. While level spacing and the charging energy dominate, the contact transparencies are large enough for higher order processes like the Kondo effect to occur. With superconducting electrodes, signs of the superconducting gap become visible in nonlinear transport, and additional subgap features due to multiple Andreev refections appear. In addition, we see a striking difference between even and odd charge states, with the first Andreev process at V=Δ/e being strongly enhanced in states with odd electron number. Although direct signs of the Kondo effect, as the Kondo ridge around zero bias, are suppressed, we argue that this even-odd asymmetry is due to a hidden Kondo resonance that, due to contact asymmetries, survives only on one contact, and leads to strongly enhanced transport at V=Δ/e.
We find good agreement of our data with a single impurity Anderson model solved in a Slave Boson mean field approach.