Paired Electron Additions to Fractional Quantum Hall Edge States in Large GaAs Quantum Dots

Using single electron capacitance spectroscopy, we determine the energies required to add single electrons to a large (0.8 μm) 2D GaAs quantum dot. We observe capacitance peaks from electron additions to edge states over a wide range of Landau level filling factors. As a function of magnetic flux through the dot, between filling factors ν = 1 and ν = 2, these peaks are regularly spaced with periodicity h/e. Surprisingly, over the range ν = 2 to ν = 5, the peaks double in height and form exact pairs. In this regime, the mean periodicity of capacitance peaks is halved to h/2e, indicating the addition of four electrons to the edge for each flux quantum threading the dot. Remarkably, these pairs themselves bunch together at all filling factors except those close to ν = 5/2. The observed pairing violates the Coulomb blockade picture in which a charging energy would separate the two electron peaks that comprise a pair. Instead the sequence of successive paired tunneling events behaves in the same way as tunneling of electrons into superconducting quantum dots.