Velocity collapse and non-conformal spiral phase in the sawtooth spin chain

Abstract: Recent matrix-product-state calculations show that the spiral phase in the sawtooth chain has numerical signatures that are difficult to reconcile with an ordinary conformal critical point: a large apparent central charge, slow dynamical scaling, nearly flat excitations, and no detectable dimerization. We develop a bosonization theory for this phenomenology by embedding the sawtooth limit in a zigzag ladder described by two coupled SU(2)$_1$ conformal field theories characterized by an extreme velocity ratio. We show that the sawtooth geometry cancels the leading staggered interaction, leaving a marginal twist interaction that selectively collapses the slow apical spin velocity. Crucially, as this velocity vanishes, the generated apical backscattering interaction diverges only in dimensionless units, causing the energy scale to collapse independently of the spatial correlation length. This mechanism naturally accounts for many of the numerical anomalies and we interpret the perturbative flow as an entrance to local quantum criticality in the strong-coupling regime.