Twist-controlled thermodynamic anomalies in bilayer q-state clock models

Abstract

Twist has emerged as a geometric means of controlling collective behavior across a wide range of physical systems. Whether it can qualitatively reorganize the thermodynamic anomalies of discrete spin models, and how such changes relate to microscopic spin configurations, remains less explored. Here we demonstrate twist-controlled thermodynamic anomalies in ferromagnetic bilayer q-state clock models using Monte Carlo simulations. With sufficiently strong interlayer coupling, twisting changes the number and structure of specific-heat peaks: a single peak can split into two, or two peaks can merge into one, depending on the number of allowed spin orientations. These changes are nonmonotonic in twist angle and occur over narrower parameter ranges, or are absent, for the larger q values examined. The accompanying spin textures and vortex distributions suggest a physical picture in which moire modulation of the interlayer coupling reorganizes local spin alignment and topological defects. In a representative case, peak splitting coincides with nearly uniform spin regions arranged on the moire scale and a reduced vortex population. These findings identify twist as a control parameter for the collective thermal response of discrete spin systems and raise the possibility of modified phase-transition behavior, including the emergence or suppression of an intermediate Berezinskii-Kosterlitz-Thouless phase.