Emergent gapless spiral phases and conformal Lifshitz criticality in the cluster Ising model with off-diagonal interactions

Abstract: We perform a comprehensive analytical study of the exotic quantum phases and phase transitions emerging from the cluster-Ising model with off-diagonal Gamma interactions. Specifically, we map out the ground-state phase diagram by analyzing both local and nonlocal order parameters, together with the energy spectra. The results reveal two pairs of gapped phases, namely and antiferromagnetic (AFM) long-range ordered phases, symmetry-protected topological (SPT) phases, as well as two distinct gapless spiral phases induced by the off-diagonal interactions, which are related by a duality transformation and are numerically confirmed through the long-distance behavior of various order parameters. Remarkably, four distinct phase transition lines emerge in the phase diagram. Two of them, which separate the distinct gapped or gapless phases, are described by the Ising and three copy Ising conformal field theories, respectively. In contrast, the remaining two transition lines, between the gapless spiral and gapped phases, belong to a nonconformal Lifshitz criticality with dynamical critical exponent $z = 2$. More importantly, the intersection of these four transition lines gives rise to a new Lifshitz multicritical point exhibiting emergent conformal symmetry, marking a fundamental departure from all previously known nonconformal Lifshitz points. This work provides a valuable reference for future investigations of exotic gapless phases and their transitions in exactly solvable many-body systems.