Resolving the two-dimensional ANNNI model using transfer matrices

The phase diagram of the two-dimensional ANNNI model has long been theoretically debated. Extremely long structural correlations and relaxation times further result in numerical simulations making contradictory predictions. Here, we introduce a numerical transfer matrix treatment that bypasses these problems, and thus overcome various ambiguities in the phase diagram. In particular, we confirm the transition temperatures and clarify the order of the transition to the floating incommensurate phase. Our approach motivates considering transfer matrices for resolving long-standing problems in related statistical physics models.