Group theoretical analysis of symmetry breaking in two-dimensional quantum dots

We present a group theoretical study of the symmetry-broken unrestricted Hartree-Fock orbitals and electron densities in the case of a two-dimensional N-electron single quantum dot ~with and without an external magnetic field!. The breaking of rotational symmetry results in canonical orbitals that ~1! are associated with the eigenvectors of a Hückel Hamiltonian having sites at the positions determined by the equilibrium molecular configuration of the classical N-electron problem and ~2! transform according to the irreducible representations of the point group specified by the discrete symmetries of this classical molecular configuration. Through restoration of the total-spin and rotational symmetries via post-Hartree-Fock projection techniques, we show that the point-group discrete symmetry of the unrestricted Hartree-Fock wave function underlies the appearance of magic angular momenta ~familiar from exact-diagonalization studies! in the excitation spectra of the quantum dot. Furthermore, this two-step symmetry-breaking and symmetry-restoration method accurately describes the energy spectra associated with the magic angular momenta.