Quantum Dot coupled to a normal and a superconducting lead

We report on electrical transport measurements in a carbon nanotube quantum dot coupled to a normal and a superconducting lead. Depending on the ratio of Kondo temperature T K and superconducting gap ∆ the zero bias conductance resonance either is split into two side-peaks or persists. We also compare our data with a simple model of a resonant level-superconductor interface. At low temperatures carbon nanotubes act as quantum dots. Different transport regimes, depending on the transparency of the contacts, such as Coulomb blockade and Kondo effect can be realized [1, 2]. Recently it has also been possible to couple a carbon nanotube quantum dot in the Kondo regime to superconducting leads, demonstrating a rich interplay of these two many particle phenomena [3]. In this article we consider a slightly different geometry, namely a quantum dot connected to both a normal and a superconducting lead. These hybrid systems are interesting for two reasons. First, the interplay of Kondo effect and superconductivity can be examined on a different basis. Various predictions have been made for this scenario, e.g. suppression or enhancement of the conductance [4], side-peaks at the position of the superconducting gap [5] and excess Kondo resonances [6]. Second, the structure mentioned above is the basic building block of proposed Andreev entanglers making use of either the 0-dimensional quantum dot charging energy U C [7] or the 1-dimensional Luttinger repulsion energy of a nanotube in order to spatially separate pairs of entangled electrons [8]. In the following we will focus on the interplay of Kondo effect and the superconducting lead. Here we report on electrical transport measurements of a Multi Wall Carbon Nanotube (MWNT) quantum dot connected to a normal and a superconducting lead. The sample is prepared as follows. First MWNTs are spread on a degenerately doped silicon substrate, in the experiment serving as a backgate, which is covered by a 400 nm insulating layer of SiO 2. Then single nanotubes are contacted by means of standard electron-beam-lithography and e-gun-evaporation. Similar to reference [3] the superconducting contact is a 45 nm Au/ 160 nm Al proximity bilayer. However, by using tilt-angle-evaporation for the Al layer one obtains a structure such as the one sketched in figure 1(a). Whereas the left-hand side of the MWNT is coupled to the superconducting Au/Al bilayer, the right-hand electrode is formed simply by the 45 nm