Probing low-lying excitations of the $\nu=2$ incompressible fluid with a quantum antidot

In the integer quantum Hall (IQH) regime, electrons can be manipulated coherently in edge states. We have begun to control spins in such systems, in order to utilise both long spin-coherence timescales and controllable electron transport along edge states. Using quantum point contacts to selectively inject and detect non-equilibrium edge-state populations, we perform spin-resolved spectroscopy of a quantum antidot (AD) in the IQH regime. At filling factor two, we find that AD transmission resonances are not spin-selective, contrary to the prediction of the conventional non-interacting picture of lowest-Landau-level (LLL) quantum states, and implying a small spin-excitation energy relative to the thermal energy. In apparent contradiction, we also observe a much larger orbital excitation energy scale in non-linear transport spectroscopy measurements. By treating the AD as a `dot of holes' in the LLL, we find that our observations are consistent with the predicted spin-charge separation that occurs at the edge of an interacting maximum-density droplet of the IQH incompressible fluid. Thus, we believe these experiments offer a direct probe of the physics of a large IQH droplet ($\sim$150 particles), with an edge which is well-described by a Luttinger-liquid model in this regime, with different scales for spin and charge excitations.