Magnetic-field dependence of the level spacing of a small electron droplet

The temperature dependence of conductance resonances is used to measure the evolution with the magnetic field of the average level spacing Δε of a droplet containing {~}30 electrons created by lateral confinement of a two-dimensional electron gas in GaAs. Δε becomes very small (30 {μ}eV) near two critical magnetic fields at which the symmetry of the droplet changes, and these decreases of Δε are predicted by Hartree-Fock (HF) for charge excitations. Between the two critical fields, however, the largest measured Δε=100 {μ}eV is an order of magnitude smaller than predicted by HF but comparable to the Zeeman splitting at this field, which suggests that excitations of the spin degrees of freedom are important.