QDev seminar: Christian Volk
Transport in graphene quantum dots
JARA-FIT and II. Institute of Physics B, RWTH Aachen, 52074 Aachen, Germany
Peter Grünberg Institut (PGI-8/9), Forschungszentrum Jülich, 52425 Jülich, Germany
Graphene quantum dots are attractive candidates for future spin-based quantum information applications. The predicted weak spin-orbit and hyperfine interaction promise long coherence times.
Our devices consist of etched graphene quantum dots (QDs) and double quantum dots (DQDs) surrounded by electrostatic gates and nearby graphene nanoribbon based charge sensors. These allow the detection of individual charging events in the QD even in regimes where the direct current through the QD is below the detection limit. We characterize the detector and investigate the back action on the QD. 
The lateral gates allow us to individually tune the tunnelling-in and -out rates down to the low MHz regime. In this regime we study relaxation processes by applying high frequency pump-and-probe pulse schemes to the plunger gate. Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60-100 ns. 
We perform tunneling spectroscopy experiments on a bilayer graphene DQD. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected value for bilayer graphene QDs. 
 C. Neumann, C. Volk et al. Graphene-based charge sensors. Nanotechnology, in press (arXiv:1304:0039).
 C. Volk, et al. Probing relaxation times in graphene quantum dots. Nat. Commun. 4, 1753 (2013).
 C. Volk et al. Electronic excited states in bilayer graphene double quantum dots. Nano Lett. 11, 3581 (2011).