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We construct energy-optimized resonating valence bond wavefunctions as ameans to sketch out the zero-temperature phase diagram of the square-latticequantum Heisenberg model with competing nearest- (J1) andnext-nearest-neighbour (J2) interactions. Our emphasis is not on achieving anaccurate representation of the magnetically disordered intermediate phase(centred on a relative coupling g = J2/J1 ~ 1/2 and whose exact nature is stillcontroversial) but on exploring whether and how the Marshall sign structurebreaks down in the vicinity of the phase boundaries. Numerical evaluation oftwo- and four-spin correlation functions is carried out stochastically using aworm algorithm that has been modified to operate in either of two modes: one inwhich the sublattice labelling is fixed beforehand and another in which theworm manipulates the current labelling so as to sample various signconventions. Our results suggest that the disordered phase evolves continuouslyout of the (pi,pi) Neel phase and largely inherits its Marshall sign structure;on the other hand, the transition from the magnetically ordered (pi,0) phase isstrongly first order and involves an abrupt change in the sign structure andspatial symmetry as the results of a level crossing.

Resonating valence bond trial wave functions with both static and dynamically determined Marshall sign structure