Unconventional bond- and current-density waves on hexagonal lattices

Abstract: Charge-density wave (CDW) orders are conventionally described as modulations of on-site charge at an ordering wave vector $\boldsymbol{Q}$ with symmetry-related wave vectors typically forming a multicomponent order-parameter manifold. Recent developments significantly broadened this phenomenology to bond and loop-current density waves, which possess nontrivial, potentially symmetry-breaking textures within the unit cell in addition to their spatial modulation at $\boldsymbol{Q}$. Such textures arise from particle-hole condensates with nonzero angular momentum, analogous to unconventional superconductivity, and their symmetries are described by the little group $G_{\boldsymbol{Q}}$. In this work, we develop a framework for unconventional CDW phases that simultaneously incorporates the local symmetries described by little group and the presence of multiple symmetry-related ordering wave vectors, also known as the star. The resulting multicomponent order parameter transforms under representations of the full space group induced from irreducible representations of $G_{\boldsymbol{Q}}$. We apply this framework to two-dimensional lattices with sixfold symmetry and ordering wave vectors along high-symmetry lines, and derive the corresponding Landau free energies. As a microscopic example, we demonstrate the emergence of unconventional bond and loop-current orders from electronic interactions on the triangular lattice within the random phase approximation, and determine their ground states by microscopically evaluating the relevant coefficients in the free energy. Our framework provides a systematic route to describing unconventional modulated phases and can be readily extended to more complex lattices.