Signatures of an annular Fermi sea

We report Shubnikov-de Haas oscillations measurements revealing experimental signatures of an annular Fermi sea that develops near the energy band edge of the excited subband of two-dimensional holes confined in a wide GaAs quantum well. As we increase the hole density, when the Fermi level reaches the excited subband edge, the low-field magnetoresistance traces show a sudden emergence of new oscillations at an unexpectedly large frequency whose value does $\textit{not}$ correspond to the (negligible) density of holes in the excited subband. There is also a sharp and significant increase in zero-field resistance near this onset of subband occupation. Guided by numerical energy dispersion calculations, we associate these observations with the unusual shape of the excited subband dispersion which results in a "ring of extrema" at finite wavevectors and an annular Fermi sea. Such a dispersion and Fermi sea have long been expected from energy band calculations in systems with strong spin-orbit interaction but their experimental signatures have been elusive.