$α$-RuCl$_3$ intercalated into graphite: a new three-dimensional platform for exotic quantum phases

TL;DR

This work reports the first synthesis of graphite intercalated with α-RuCl$_3$, establishing a three-dimensional platform to explore exotic quantum phases that merge flat-band physics, interfacial magnetism, and charge-transfer phenomena. Using two-zone CVT, XRD, SdH transport, and first-principles calculations, the authors identify stage-2 and stage-4 intercalations, reveal significant c-axis expansions, and detect high-frequency quantum oscillations indicative of a rebuilt Fermi surface. Density functional theory shows minimal hybridization between graphene and RuCl$_3$ layers, but substantial charge transfer that depends on stacking, with unfolding-based analysis aligning computed Fermi-surface frequencies to experiment when small $0.1$ eV shifts are considered. These results demonstrate a viable route to engineer bulk materials that integrate rhombohedral graphene–like flat-band phenomena with Kitaev-like magnetism and interfacial effects, potentially enabling novel correlated and topological bulk phases.

Abstract

Multilayer graphene with different stacking sequences has emerged as a powerful setting for correlated and topological phases. In parallel, progress in graphene heterostructures with magnetic or correlated materials-most notably the Kitaev candidate $α$-RuCl$_3$-has demonstrated charge transfer, magnetic proximity effects, and interfacial reconstruction, creating new opportunities for engineered quantum systems. Motivated by these developments, we explore a three-dimensional analogue in which $α$-RuCl$_3$ layers are inserted directly into the van der Waals gaps of graphite, forming an intercalated system. Here, we report the successful synthesis and comprehensive characterization of graphite intercalated with $α$-RuCl$_3$. Using a combination of X-ray diffraction, quantum oscillation measurements, and first-principles electronic structure calculations, we study the structural and electronic properties of these intercalated crystals. Our results demonstrate that graphite intercalated with $α$-RuCl$_3$ offers a robust route to develop three-dimensional materials with access to novel correlated and topological states.

Figures (5)

• Figure 1: Different intercalation stackings: a) A$\mid$A, b) AB$\mid$AB (stage 2: S2), c) ABAB$\mid$ABAB (stage 4: S4), and d) ABCA$\mid$ABCA (stage 4: S4). e) Stacking top view. The $\alpha$-RuCl$_3$ layer is denoted by a blue layer, while the labels A,B, and C denote the different graphene stackings.
• Figure 2: a) Schematic of two-zone vapor transport using AgCl as a chlorine source to grow C-RuCl$_3$ samples. b) Temperature dependence of the longitudinal resistivity of Kish graphite as received and after annealing in Cl$_2$ gas, and C-RuCl$_3$ stage 2 sample. c), d), and e) Magnetic field dependence of the longitudinal resistivity $\rho_{xx}$ of Kish graphite, C-RuCl$_3$ stage 2, and C-RuCl$_3$ stage 4 samples, respectively. f), g), and h) Fast Fourier transform (FFT) of the oscillatory component of resistivity shown in c), d), and e) for ${\mu_0 H}$ applied out-of-plane. Peaks correspond to the extremal cross-sectional areas of the Fermi surface and are labeled as $\nu$ for Kish graphite, $\alpha$ and $\beta$ for the intercalated samples.
• Figure 3: a) Representative ABCA$\mid$ABCA crystal structure of C-RuCl$_3$ used in the DFT calculations. The blue and yellow iso-surfaces show the charge depletion and accumulation, respectively. $d_{\rm Gr-Gr}$ denotes the graphene-graphene layer distance, and $d_{\rm Gr-RuCl_3}$ the distance $\alpha$-RuCl$_3$-graphene layer distance. b) The planar-averaged charge density redistribution along the c-axis for each geometry as a function of the Ru layer distance, $d_{\rm Ru}$. The reference point is the position of the Ru layer indicated by the dashed blue line, the grey dashed lines indicate the position of the graphene layers. The four-layer intercalations in the ABAB$\mid$ABAB and ABCA$\mid$ABCA stacking are represented by one plot. Crystal structure plots are generated with VESTA VESTA.
• Figure 4: Unfolded electronic band structure of the C-RuCl$_3$ graphene layers in red for the four different intercalated geometries obtained via non-spin polarized GGA calculation, a) A$\mid$A, b) AB$\mid$AB, c) ABAB$\mid$ABAB, and d) ABCA$\mid$ABCA stacking. Each inner panel shows a zoomed region around the K point. Light gray solid lines display the bands of pristine (multi-layered) graphene.
• Figure 5: Unfolded Fermi surface of the graphene layers C-RuCl$_3$ structure calculated via a non-spin polarized GGA calculation. a) Full Brillouin zone. The blue region indicates the calculated unfolded region around the K point. b-e) Fermi surfaces for b) A$\mid$A, c) AB$\mid$AB, d) ABAB$\mid$ABAB, and e) ABCA$\mid$ABCA stackings.