project title: Multiple Andreev reflections in single-walled carbon nanotubes
project leader: Prof. Juan Carlos Cuevas
access given (in days): 7
access used (in days): 11
local host: Docent Tero Heikkilä
home institution: Departemento de Fisica Teórica de la Materia Condensada C-V, Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049, Madrid, Spain
country of institution: Spain
starting date (yyyy-mm-dd): 2007-02-26
In this project, the aim is to quantitively describe the results of the measurements of current-voltage curves through carbon nanotubes placed between superconducting contacts, obtained in the Nano group of the Low Temperature Laboratory. Such measurements show signatures of multiple Andreev reflections (MAR) through the few energy levels of the nanotube. Moreover, our aim is to find out the modification to the standard MAR theory due to the fact that some of the measured nanotubes were in the long-junction limit, where the Thouless energy of the nanotube becomes relevant. If time allows, we aim also to work to understand the related problem of supercurrent hysteresis observed in many different types of superconductor - normal metal - superconductor junctions.
Juan Carlos Cuevas visited the LTL in February 2007. During his visit, he gave the Nano group his code which has been used to analyze the measurements on the current-voltage curves through superconductor - carbon nanotube - superconductor junctions. His visit also started an ongoing PhD student project of Juha Voutilainen, to analyze the transport through multi-level quantum dots coupled to superconductors. This project is still on-going; we aim to publish the results in the near future. This collaborative project of Cuevas and Heikkilä continued intensively in 2007-08. At present the project on the properties of carbon nanotubes connected to superconductors is at the stage where more input is needed from the experimentalists. Meanwhile, we have worked on a slightly related problem of the properties of superconductor-normal metal-superconductor junctions in microwave fields. This very general topic was first discussed already in the 1970's, but a satisfactory theory has not yet been posed. We have incorporated the microwave field into Usadel equations, the standard way to describe such structures, and explored the weak-coupling limit in detail. Our theory, to be published in the summer 2008, is in good agreement with existing experiments.