Exchange Effects in an Artificial Atom at High Magnetic Fields

A droplet of $N\sim 30$ electrons is created by lateral confinement of a two-dimensional electron gas. The evolution of its chemical potential ${\mu }_N$ with magnetic field $B$ is inferred from resonant tunneling experiments and is used to study the dependence of its magnetization on $B$.The experimental data, quantitatively described by Hartree-Fock theory, suggest a divergent spin susceptibility at $B_c$ implying a ground state of minimum total spin just below $B_c$. In the spin polarized regime we observe a new phase also predicted by Hartree-Fock theory.