Lessons from $α$-RuCl3 for pursuing quantum spin liquid physics in atomically thin materials

TL;DR

The paper surveys how atomically thin and van der Waals-engineered RuCl$_3$-based systems are advancing the pursuit of Kitaev quantum spin liquids, highlighting work-function-driven charge transfer, strain, and proximity effects as levers to tune $K$, $J$, and $\Gamma$ toward Kitaev-dominated regimes. It synthesizes experimental progress across electronic transport, tunneling spectroscopy, light–matter interactions, and neutron scattering, together with theoretical models extending Kitaev physics to 2D and moiré lattices, rare-earth and 3$d$ candidates, and Floquet/cavity approaches. A key insight is that heterostructures can enhance Kitaev couplings by up to ~50% and can destabilize competing orders, offering a practical route to approach or realize quantum spin liquid behavior in 2D. The review outlines a coherent framework for discovering superior 2D Kitaev materials, leveraging strain, doping, interlayer coupling, and advanced spectroscopies to push toward fault-tolerant topological phases and quantum technologies.

Abstract

Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from the field of atomically thin materials. We primarily focus on the antiferromagnetic Mott insulator $α$-RuCl3, which exhibits Kitaev couplings and is readily exfoliated to single- or few-layer sheets, and thus serves as a test bed for developing probes of Kitaev phenomena in atomically thin materials and devices. We introduce the Kitaev model and how it is realized in $α$-RuCl3 and other material candidates; and cover $α$-RuCl3 synthesis and fabrication into van der Waals heterostructure devices. A key discovery is a work-function-mediated charge transfer that heavily dopes both the $α$-RuCl3 and proximate materials, and can enhance Kitaev interactions by up to 50%. We further discuss a wide range of recent results in electronic transport and optical and tunneling spectroscopies of $α$-RuCl3 devices. The experimental techniques and theoretical insights developed for $α$-RuCl3 establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases.

Figures (27)

• Figure 1: (color online) Honeycomb lattice in Kitaev's model with bond-direction-dependent couplings $K_i$, $i=x,y,z$. Open and closed circles identify the two triangular sublattices. The dashed outline marks red line the unit cell of the lattice.
• Figure 2: (color online) Crystal field effects, spin-orbit coupling, and electronic correlations lead to the spin-orbit entangled moments $j=1/2$.
• Figure 3: (color online) Top: Illustration of (a) typical vapor transport growth with a large horizontal temperature gradient, and (b) self-selecting vapor transport growth performed with a small temperature gradient around the starting powder. Weak temperature gradients applied in the horizontal and vertical directions help the grain selection. Over time, part of the starting powder is converted to small crystals growing on top of the powder; at the end of an ideal growth, one large single crystal forms consuming all starting powder as well as the intermediate smaller crystals yan2023self. American Physical Society (APS).
• Figure 4: $\alpha$-RuCl$_3$ crystals grown at different stages by J-Q Yan, et al.yan2023self. (a) A conventional vapor transport growth performed in a horizontal tube furnace with a temperature gradient of about 45 $^{\circ}$C between the starting powder at the hot end and crystals at the cold end. (b)-(e) Self-selecting vapor transport growths performed in a box furnace. The increasing crystal dimension results from a better control of the temperature gradient around the starting powder. The number beneath each panel shows in what year the growth was performed. Reproduced with permission from yan2023self, APS.
• Figure 5: (color online) Magnetic order and structure transition in two typical types of $\alpha$-RuCl$_3$ single crystals. (a), (b) Temperature dependence of magnetization and specific heat (C$_p$) below 20 K. The magnetization was measured in a magnetic field of 1 kOe applied along the zigzag direction (perpendicular to the Ru-Ru bond). The vertical dashed lines highlight the Néel temperature, T$_N$, defined as the temperature where C$_p$ peaks. T$_N$ = 7.6 K in (a) is 1.1 K higher than that in (b). (c)-(e) Temperature dependence of magnetization and 005 reflection in the temperature range 20-200 K for the crystal with T$_N$ = 7.6 K. (f)-(h) Temperature dependence of magnetization and 005 reflection in the temperature range 20-200 K for the crystal with T$_N$ = 6.5 K. The temperature dependence of magnetization in (d),(g) was measured in a magnetic field of 10 kOe applied perpendicular to the honeycomb plane. All data in [(c)-(h)] were collected in a temperature sweep at a rate of 2 K/min. Reproduced with permission from PhysRevMaterials.8.014402, APS.