2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/21691A novel nonlinear sigma model method is proposed for the two-dimensional J1-J2 model, which is extended to include plaquette-type distortion. The nonlinear sigma model is properly derived without spoiling the original spin degrees of freedom. The method shows that a single disordered phase continuously extends from a frustrated uniform regime to an unfrustrated distorted regime. By the continuity and Oshikawa's commensurability condition, the disordered ground states for the uniform J1-J2 model are plaquette states with four-fold degeneracy.4 pages (including 2 figures)Strongly Correlated ElectronsStatistical MechanicsHigh Energy Physics - Theorytext