Pairing of single electron additions at the edge of an ultraclean mini 2DEG

We have created laterally large, quantum well based, quantum dots in a system with no unscreened dopants. Using single-electron capacitance spectroscopy (SECS) in a dilution refrigerator, we can identify the capacitance peak for the addition of the first electron to a dot and record subsequent capacitance peaks in the addition spectrum up to dot occupancies of thousands of electrons. SECS records the electron addition energies and the tunneling density of states within a dot as a function of magnetic field and energy. Previous measurements showed well defined transitions into integer quantum Hall states in these spectra. To investigate fractional states, we produced this new, cleaner, device with a larger physical size so that the dot would behave as a small 2D electron system. Here we report the observation in the addition spectra of individually localized states, extended states, incompressible Landau gaps, and pairing of single electron additions between nu=2 and nu=3. We observe the pairing over similar filling factor ranges where interferometry experiments display pairing effects as in Choi et al. Nat. Comm. 6, 7435 (2015)