Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor

An electron is usually considered to have only one type of kinetic energy, but could it have more, for its spin and charge, or by exciting other electrons? In one-dimension (1D), the physics of interacting electrons is captured well at low energies by the linear Tomonaga-Luttinger liquid model. Recent theoretical work has focused on extending the theory to deal with curved dispersions, yet little has been observed experimentally in the nonlinear regime. Here, we report on measurements of many-body modes in gated 1D wires using a momentum-resolved tunnelling spectroscopy technique. We map their dispersion both in and out of equilibrium, observing the formation of two separate Fermi seas at high energies, associated with spin and charge excitations, which cannot be accounted for by the noninteracting model. The effective interaction strength in the wires is varied from the non-interacting limit g=1 all the way down to g=0.5, by changing the amount of 1D inter-subband screening by over 50%. Our spectroscopy technique offers an important tool to explore nonlinear spinful regimes.