Tunable quantum anomalous Hall effect in fullerene monolayers

Abstract

Nearly four decades after its theoretical prediction, the search for material realizations of quantum anomalous Hall effect (QAHE) remains a highly active field of research. Many materials have been predicted to exhibit quantum anomalous Hall (QAH) physics under feasible conditions but the experimental verification remains widely elusive. In this work, we propose an alternative approach towards QAH materials design by engineering customized molecular building blocks. We demonstrate this ansatz for a two-dimensional (2D) honeycomb lattice of C26 fullerenes, which exhibits a ferromagnetic ground state and thus breaks time-reversal symmetry. The molecular system is found to be highly tunable with respect to its magnetic degrees of freedom and applied strain, giving rise to a rich phase diagram with Chern numbers C= +/-2, +/-1, 0. Our proposal offers a versatile platform to realize tunable QAH physics under accessible conditions and provides an experimentally feasible approach for chemical synthesis of molecular networks with QAHE.

Figures (3)

• Figure 1: (a) Hexagonal lattice of C26 fullerenes. An isolated molecule is shown on the right. Here, gray colored carbon atoms form the inter- molecular bonds of the quasi- structure. The colors of the remaining ions indicate the sets of weakly hybridized $p$- orbitals that are closed under the action of the point group. (b) Low-frequency phonon dispersion. The and high- symmetry lattice momenta are shown in the top right. (c) Low-frequency magnon dispersion.
• Figure 2: (a) Band structure in the absence of . Bright green (dark blue) bands are spin up (down) polarized. The Fermi energy is set to zero and indicated by the dashed line. The band structures along any other path $\overline{\Gamma \mathrm{M}_j \mathrm{K}^{(\prime)} \Gamma}$ with $j \in \{1,2,3\}$ is equivalent by symmetry. Degeneracies of the two bands in proximity of the Fermi level occur along $\overline{\mathrm{M}_j \mathrm{K}^{(\prime)}}$ (at $\mathrm{K}$) and are due to the two bands transforming under distinct (a ). (b) The hexagonal plane corresponds to the and defines $\theta = \pi/2$. Weyl points occur on the edges and are exemplified along a $\overline{\mathrm{KK}^\prime}$ path. Here, colored surfaces illustrate the dispersion of the two bands close to the Fermi energy around the lattice momentum indicated by the turquoise spheres. Upon incorporating , the direct band gap between the two bands in proximity of the Fermi energy depends on the orientation of magnetization $\mathbf{M}$.
• Figure 3: (a) Illustration of the theoretically predicted phase diagram. Depending on the orientation of magnetization $\mathbf{M}$, different Chern number regimes are identified, indicated by different coloring listed in (c). Along black lines, the two bands in proximity of the Fermi energy form massless Weyl points. (b) Numerically determined direct band gaps and Chern number regimes for $\phi ~ \text{mod} ~ (\pi/3) = \pi / 6$. (c) Phase diagram upon introducing in- plane strain with azimuthal angle $\varphi ~ \text{mod} ~ (2\pi/3) = 2\pi/3$. The figure shows the $\phi = \pi / 4$ projection of the parameter space and the details of the interpolation scheme are provided in the Supplemental Material supp_mat.