Enhanced spin–orbit coupling in dilute fluorinated graphene

The preservation andmanipulation of a spin statemainly depends on the strength of the spin–orbit interaction. For pristine graphene, the intrinsic spin–orbit coupling (SOC) is only in the order of few μeV,whichmakes it almost impossible to be used as an active element in future electricfield controlled spintronics devices. This stimulates the development of a systematicmethod for extrinsically enhancing the SOCof graphene. In this letter, we study the strength of SOC inweakly fluorinated graphene devices.We observe high non-local signals evenwithout applying any external magneticfield. Themagnitude of the signal increases with increasing fluorine adatom coverage. From the length dependence of the non-local transportmeasurements, we obtain SOCvalues of∼5.1meV and∼9.1meV for the devices with∼0.005%and∼0.06% fluorination, respectively. Such a large enhancement, together with the high chargemobility offluorinated samples (μ∼4300 cm V s– 2700 cm V s), enables the detection of the spinHall effect even at room temperature. For pristine graphene (PG), the hopping of π electrons between the two next nearest neighbor carbon atoms is the only source of the SOC. This is a second order process and gives rise to SOC in the order of only a few μeV [1–3]. Such a weak coupling limits the prospect of potential graphene-based spin field effect transistors [4]. Different approaches have been suggested to enhance the SOC of graphene. For example, the creation of a curvature in flat graphene is expected to significantly enhance the intrinsic SOC [1, 3, 5] as in the carbon nanotube case [6]. However, there is no well-developed method for the fabrication of such devices. Recently, the proximity effect at the interface between graphene and WS2 layers has been shown to result in a SOC enhancement [7]. Also the hydrogenation of graphene has been shown to significantly enhance the SOC of graphene [8, 9]. Unfortunately, chemisorbed hydrogen atoms can be easily detached at moderate temperatures and this makes devices less stable at ambient conditions [10, 11]. The stability of devices can be improved with choose of other types of adatoms [12]. On this subject, fluorinated graphene is known to be thermodynamically more stable. Here, the carbon forms strong covalent bonds with fluorine, which is known as the most electronegative element [13]. This chemical bonding depends strongly on graphene doping and hence allows a new route to tailor the electronic and spintronics properties with a local gate similar to the one can be achieved with magnetic adatoms [14–16]. Furthermore, and unlike hydrogen, fluorine acts only as a weak resonant scatterer [17]. Therefore, higher charge mobility is expected in such weakly functionalized graphene. And lastly but most importantly, contrary to the hydrogen atoms, fluorine’s own SOC is not negligible and is expected to cause a large SOC in even weakly fluorinated graphene system [2, 17]. Irmer et al has predicted a SOC strength larger than 10 meV inweakly fluorinated graphene which is ten-fold higher what is expected in hydrogenated graphene and comparable to the atomic spin–orbit interaction in carbon itself [17]. In this letter, we study the strength of spin–orbit interaction in dilute fluorinated graphene by measuring the non-local signals of Hall bar devices. Prior to transport measurements, Raman spectroscopy is utilized to determine the fluorine adatom coverage of graphene. We observe a large non-local signals which RECEIVED