Precompression engineering of metal-insulator transition and magnetism in designed breathing kagome systems

Abstract

Kagome materials featuring dispersive Dirac cones and topological flat bands exhibit unique electronic and magnetic properties. However, kagome compounds with tunable electrical conductivity remain scarce, which severely impedes their device applications. Here, based on density functional theory (DFT) and Boltzmann transport theory, we introduce the breathing effect into kagome materials $\mathrm{Nb_3XCl_7}$ (X = F, Cl, Br, I) via chemical precompression, thereby inducing a metal-insulator transition and magnetic variation. We determine that the band structures, optical absorption spectra and magnetic ground states agree well with experimental results at the effective correlation strength $U_{\text{eff}} = 2$ eV. The calculated conductivity and magnetic properties reveal that the monolayer $\mathrm{Nb_3Cl_8}$ and $\mathrm{Nb_3XCl_7}$ undergoes transitions from paramagnetic metals to Mott insulators at $U_{\text{eff}} = 1$ eV and $t_{\text{out}}/t_{\text{in}} = 0.6674$, respectively. Our detailed analysis establishes that the stronger breathing effect corresponds to enhanced chemical precompression, which reduces the region of free electron gas between intercell Nb atoms and facilitates the metal-insulator transition. Finally, we propose several viable synthesis routes for $\mathrm{Nb_3FCl_7}$, $\mathrm{Nb_3BrCl_7}$, and $\mathrm{Nb_3ICl_7}$, providing predictive guidance for experimental studies. Our study establishes a practical framework for investigating the breathing effect in correlated kagome systems and yields valuable insights into the mechanisms underlying metal-insulator transition and magnetic properties in real breathing kagome materials.

Figures (6)

• Figure 1: (a) The crystal structure of $\rm Nb_3Cl_8$. The light blue polyhedron represents the coordination polyhedron formed by the central Nb atom and its nearest-neighbor atoms. (b) The top view of the monolayer $\rm Nb_3Cl_8$. The transparent blue region represents a unit cell. The $l_1$ marks the distance between a and b cites in the unit cell. The $l_2$ marks the distance between b cite and the a cite of the next unit cell. (c) The calculated band structures of the monolayer $\rm Nb_3Cl_8$ with the effective correlation strength $U_{\text{eff}}=2$ eV. The light gray regions in the density of states were inferred from the optical absorption spectra acs.nanolett.2c00778. (d) The calculated optical absorption and joint density of states of 20 nm $\rm Nb_3Cl_8$ thin film.
• Figure 2: (a) The calculated electronic structures of monolayer $\rm Nb_3Cl_8$ with the Heyd-Scuseria-Ernzerhof hybrid functional. The light gray regions in the density of states were inferred from the optical absorption spectra acs.nanolett.2c00778. (b)-(f) The calculated electronic structures of monolayer $\rm Nb_3Cl_8$ with the effective correlation strength $U_{\text{eff}}=0,1,3,4,5$ eV. The light gray regions in density of states were inferred from the optical absorption spectra acs.nanolett.2c00778. (g)-(h) The calculated band gaps ($\rm \alpha-\beta$) and band gaps ($\rm \beta-\gamma$). The orange dashed lines represent the experimentally measured band positions acs.nanolett.2c00778. (i) The calculated density of states at the Fermi level of monolayer $\rm Nb_3Cl_8$. (j)-(m) The Fermi lines of monolayer $\rm Nb_3Cl_8$ with $U_{\text{eff}}=0,1,2,3$ eV.
• Figure 3: (a)-(c) The density of states, conductivity, and magnetic susceptibility of monolayer $\rm Nb_3Cl_8$ with the effective correlation strength $U_{\text{eff}}=0,1,3,4$ eV and the temperature range from 20 K to 100 K. (d)-(f) The density of states, conductivity, and magnetic susceptibility of the monolayer $\rm Nb_3XCl_7$ (X = F, Cl, Br, and I) with the temperature range from 20 K to 100 K.
• Figure 4: The phonon dispersion curves for the monolayer $\rm Nb_3XCl_7$ (X = F, Cl, Br, and I).
• Figure 5: (a) The schematic representation of the crystal structure transition from $\rm Nb_3Cl_8$ to $\rm Nb_3FCl_7$. (b)-(d) The contour maps of the electron localization functions on the Nb trimer plane for $\rm Nb_3XCl_7$ (X = F, Cl, Br, and I).