Multipole phases in a type of spin/fermion ladders with local conserved quantities and generalizations

We study spin/fermion ladder models with exact multipole phases, which are traditional spin phases formed by multipole moments. These phases feature non-trivial order with zero magnetization. The multipole models have dimer local conserved quantities that are Ising terms of spin. The Hilbert spaces are locally fragmented into independent sectors described effectively by higher-order spin. For dipole models, we consider two ladder geometries with quadratic spin couplings and work out the phase diagrams. Different phases of the model can be distinguished experimentally with the dynamical structure factor. Higher-order multipole models are obtained by introducing more dimer conserved quantities. The phases are characterized by the values of the local conserved quantities and the traditional spin phases of the higher-order spin. Physical properties of traditional spin phases, such as zero modes, are inherited in the multipole phases.