Moiré in $Γ$-valley square lattice: Copper- and iron-based superconductor simulation in a single device

Abstract

Novel superconducting phases have been found in various moiré heterostructures based on hexagonal lattices. However, the archetypal high-temperature superconductors (cuprates, iron-based and nickelate families) all share a square lattice foundation. These materials host a rich landscape of correlated phenomena, such as charge and spin stripes, pseudogap behavior, and unconventional metallicity, which continue to challenge our fundamental understanding of strongly correlated electrons. In this work, we investigate the possibility of simulating the effective models governing these high-$T_c$ superconductors using twisted homobilayers of $Γ$-valley square-lattice systems. We develop a universal theoretical framework and carry out a detailed analysis of a promising candidate material ZnF$_2$. We find that the first moiré band realizes a single-orbital square-lattice Hubbard model, widely believed to capture cuprate physics, while the second and third moiré bands map to a $p_x,p_y$ two-orbital square-lattice Hubbard model, which shares common physics to the minimal $d_{xz}, d_{yz}$ models proposed for iron pnictides. Our study combines continuum Hamiltonian modeling, first-principle calculations, and Hartree-Fock mean field theory. The latter focuses on the quarter-filling regime of the two-orbital model and in particular leads to, among others, a stable antiferro-orbital, ferromagnetic insulating phase. These results highlight $Γ$-valley square-lattice moiré systems as a new and important generation of van der Waals heterostructures to realize interesting strongly correlated phases of matter.

Figures (17)

• Figure 1: Orbital configurations for (a) ferro-orbital order, (b) stripe-orbital order, and (c) antiferro-orbital order.
• Figure 2: Crystal structure of monolayer ZnF$_2$.
• Figure 3: Real-space profiles for moiré potentials of twisted bilayer ZnF$_2$ in equation \ref{['eq:TB_Hamiltonian']}, plotted as a function of normalized coordinates within the moiré unit cell in eV. Left: corrugated case where the interalayer distance $d_z$ is a function of the relative displacement $\bm{\tau}$ (left, $V-T$). Right: rigid case, where $d_z$ taken to be a constant, the spatial average of the corrugated case (right, $V'+T'$). The corrugation splits the moiré potential minimum from one point at the unit cell center to four points.
• Figure 4: Moiré band structures at twisting angle $\theta=1^{\circ}$ (left) and $\theta=2^{\circ}$ (right).
• Figure 5: Band structures at $\theta=1^{\circ}$. Left: the lowest-lying band corresponding to moiré $s$-orbital. Right: the next two low-lying bands corresponding to the moiré $p_x,$$p_y$ orbitals.