Epitaxial growth and band structure of antiferromagnetic Mott insulator CeOI

The van der Waals material CeOI is predicted to be a layered antiferromagnetic Mott insulator by density-functional theory $+U$ calculation. We successfully grow the CeOI films down to monolayer on graphene/6H-SiC(0001) substrate by using molecular beam epitaxy. Films are studied by in situ scanning tunneling microscopy and spectroscopy, which shows a band gap of 4.4 eV. A metallic phase with composition unidentified also exists. This rare earth oxyhalide adds a member to the two-dimensional magnetic materials.

Figure 1

(a) The crystal structure of CeOI. The directions of magnetic moments are illustrated by the blue and red arrows. (b) and (c) The calculated density of states of monolayer CeOI with $U_{\mathrm{eff}}$ = 0 eV and $U_{\mathrm{eff}}$ =5.8 eV, respectively.

Figure 2

(a), (b), and (c) show the lattice parameter $a$, magnetic moment per Ce atom and band gap of CeOI monolayer as functions of $U_{\text{eff}}$, respectively. The black squares with error bars at the right side of (a) and (c) indicate the measured values by STM.

Figure 3

(a) XPS of cerium (black curve) with background (red curve). (b) XPS of iodine (black curve) with background (red curve). (c) STM image of CeOI films on graphene with a coverage lower than one (bias: 4 V, current: 20 pA). The inset shows the lattice orientation of the graphene substrate. (d) STM image of CeOI films on graphene with a coverage between one and two (bias: 4 V, current: 20 pA). (e) Step height along the black line indicated in (d). (f) Atomic resolution of monolayer CeOI on graphene (bias: 0.3 V, current: 100 pA). Lattice constants are measured as $a=b=4.10\pm 0.03\AA$.

Figure 4

(a) STS spectra of CeOI for monolayer and bilayer (set point for both: 2.5 V and 100 pA) together with thick films (set point: 3.2 V and 100 pA). (b) STS spectra on monolayer CeOI and bilayer graphene (set point for both: 0.5 V and 100 pA).

Figure 5

(a) STM image (bias: 3 V, current: 20 pA) of monolayer CeOI near an step edge. (b) STS acquired on five points indicated in (a). Spectra are shifted vertically for clarity. (c) STM image (bias: 4 V, current: 20 pA) of monolayer CeOI near a grain boundary. (d) STS acquired on five points indicated in (c). Spectra are shifted vertically for clarity. (e) STM image (bias: 4 V, current: 20 pA) near a step between bilayer and monolayer CeOI. (f) Band bending near the step edge along the arrow in (e).

Figure 6

(a) STM image of a film with both metallic phase and insulating CeOI (bias: 4 V, current: 20 pA). (b) Image of the metallic phase (bias: 0.5 V, current: 100 pA). Inset shows the FFT of image with white arrows indicating the Bragg spots of the lattice and the ($3\times 1$) superstructure. (c) STS spectra on the metallic phase.