Multiversality and Unnecessary Criticality in One Dimension

We present microscopic models of spin ladders which exhibit continuous critical surfaces whose properties and existence, unusually, cannot be inferred from those of the flanking phases. These models exhibit either "multiversality"-the presence of different universality classes over finite regions of a critical surface separating two distinct phases-or its close cousin, "unnecessary criticality"-the presence of a stable critical surface within a single, possibly trivial, phase. We elucidate these properties using Abelian bosonization and density-matrix renormalization-group simulations, and attempt to distill the key ingredients required to generalize these considerations.