Localization by interference: Square billiard with a magnetic flux

Eigenstates and energy levels of a square quantum billiard in a magnetic field, or with an Aharonov-Bohm flux line, are found in quasiclassical approximation, that is, for high enough energy. Explicit formulas for the energy levels and wavefunctions are found. There are localized states, never before noticed in this well studied problem, whose localization is due to phase interference, even though there is no or negligible classical effect of the magnetic field. These and related states account almost entirely for the magnetic response in certain temperature ranges, and thus have a bearing on the experiments of Levy et al.