Wednesday, January 8, 2014
PRB accepted: Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot
When you place a carbon nanotube at low temperature between contacts made from a superconducting metal, lots of interesting things happen. Strongly simplifying, currents in a superconductor are carried by Cooper pairs of two electrons each, while the localized electronic system in the carbon nanotube is normal-conducting and carries single electrons. One mechanism at a superconductor - normal conductor interface that mediates between these two types of charge transport is so-called Andreev reflection. An electron from the normal conductor enters the superconductor, at the same time a "missing electron", i.e. a "hole where an electron should be", is sent back into the normal conductor. The total charge passing through the interface is 2e, just right to form a Cooper pair. The superconductor-nanotube-superconductor system consistis of two such interfaces back to back; analogous to box potential, multiple reflections on both sides lead to the formation of bound quantum states within the nanotube, the so-called Andeev bound states (ABS).
So far, all other observations of ABS involved aluminum, which has a fairly low critical temperature and critical field. What is new in our work is that we use niobium as superconducting material, with higher critical temperature and larger energy gap. We can increase the temperature to over 1K and still see the superconductivity plus the ABS in the transport spectrum. This way, we can observe how thermal population of an excited Andreev state takes place. Additionally we observe a second pair of Andreev states in the larger superconducting energy gap, and a surprising multi-loop behaviour. All these effects are successfully modelled by calculations based on the superconducting Anderson model, in a collaboration with Alfredo Levy Yeyati and Alvaro Martin-Rodero from Universidad Autonoma de Madrid.
"Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot"
A. Kumar, M. Gaim, D. Steininger, A. Levy Yeyati, A. Martin-Rodero, A. K. Hüttel, and C. Strunk
Physical Review B 89, 075428 (2014), arXiv:1308.1020 (PDF)