Thermodynamics and elasticity of emergent crystals

Abstract: Periodic field patterns of atoms and their charges/spins/orbits emerge in crystals, forming novel states of matter called emergent crystals (ECs). In recent years, they are observed in diverse systems such as skyrmion crystals in helimagnets, and periodic ripples in 2D materials. ECs essentially changes the properties of material underneath, and are deformable when subject to various effective fields. A major challenge in application is first to predict what kind of EC will appear in the system of interest, and how to quantify its `elasticity' when subject to an effective field. Here we establish the theoretical framework of thermodynamics for deformable ECs, and derive from it the linear constitutive equations when subject to the primary external field. We provide a systematic study on the ECs that may appear in helimagnets induced by the Dzyaloshinskii-Moriya interaction, and analyze their elasticity when subject to bias magnetic fields. We construct in this work the basis of emergent elasticity, a new branch studying deformable emergent crystals under effective fields.