Spin Physics in Carbon Nanotube Double Quantum Dots

In this talk, I will discuss recent measurements in which we investigate spin blockade and Kondo physics in carbon nanotube double quantum dots. Spin blockade is observed in weakly coupled double quantum dots, when electron transitions between the dots are forbidden by spin conservation. As such, this phenomenon is of considerable importance in spin-based quantum information processing schemes as a way to convert the spin degree of freedom to a much easier detectable charge state or current. We have, for the first time, investigated spin blockade in carbon nanotubes and used the developed techniques to investigate mixing between the spin-singlet and triplet states in these devices. Mixing between the various spin states is most likely mediated by spin-orbit and hyperfine interaction, an understanding of which is important as they will ultimately limit the spin-coherence times in carbon nanotubes which are generally expected to be very long.

The ability to control the tunnel couplings in the nanotube devices also allows us to investigate carbon nanotube double quantum dots which are much more strongly coupled to their leads. In this case, we observe pronounced Kondo features when one of the two quantum dots contains an odd number of electrons. In the situation when both quantum dots of the double dot contain an odd number of electrons, we observe a pronounced splitting of the Kondo states in the measured differential conductance. This is direct evidence of the formation of a coherent superposition of the Kondo states of each dot, which form bonding and anti-bonding combinations. The effect has been studied by us as function of tunnel coupling, temperature and magnetic field and will be discussed during the second part of the talk.