Elettra | VUV Photoemission, APE-LE Europium’s effect: Unlocking spintronic potential in graphene Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, continues to attract researchers due to the exceptional physical properties stemming from its atomic-scale thickness. The electronic structure of graphene is particularly noteworthy, featuring linearly crossing bands that form a Dirac cone — a band structure that underlies much of its remarkable behavior. One of the major interests in graphene research is to induce magnetism in this intrinsically nonmagnetic material. Since graphene has a large spin coherence length, achieving spin polarization in graphene would open the door to its integration into spintronic technologies, where electron spin — rather than charge — enables information processing. A common strategy to induce magnetism involves coupling graphene to ferromagnetic metals such as iron, cobalt, or nickel. However, the strong interaction between graphene’s π-electrons and the 3d states of these metals severely distorts the Dirac cone, eliminating the unique features that make graphene so valuable for device applications. Thus, a key objective is to find a way to spin-polarize graphene while preserving its characteristic electronic properties. Here, we demonstrated that this goal can be realized by inserting a thin layer of europium (Eu), a magnetic rare-earth element, between graphene and a nickel (Ni) substrate (Fig. 1 a). First-principles calculations reveal that Eu layer weakens the graphene–Ni coupling, keeping the Dirac cone largely intact, and at the same transfers the spin polarization to the graphene layer. The spin-polarized Dirac states in graphene respond differently to the interaction with the spin-polarized states of Eu, leading to two distinct spin-dependent band gaps opening at the Dirac point (Fig. 1 b). We confirmed this theoretical prediction using Angle-Resolved Photoemission Spectroscopy (ARPES), a powerful technique for directly probing the electronic band structure of solids. In combination with spin detector, it can also give access to the information on the spin of the electronic states, giving unparalleled opportunities for the study of two-dimensional materials. ARPES and spin-resolved ARPES were performed at the VUV-Photoemission and APE-LE beamlines at the Elettra synchrotron facility. The ARPES data clearly showed two gapped conical bands in graphene (Fig. 1 c), consistent with our simulations. Moreover, spin-ARPES revealed that these cones possess opposite spin polarizations (Fig. 1 d), with their energy gaps differing by a factor of two to three (Fig. 1 e). This spin-dependent gap in graphene's Dirac states offers exciting prospects for spintronic devices, including spin filters and other applications where selective transport of spin-polarized electrons is essential. We also explored whether this system could enable more exotic quantum phenomena. Theoretical calculations suggest that the spin-polarized Dirac states near the gap exhibit nontrivial topological properties, such as a finite Berry curvature (Fig. 1 f). This gives rise to quantized values in the spin Hall conductivity (Fig. 1 g), that is an indication of the quantum anomalous Hall effect (QAHE), a phenomenon where dissipationless edge currents flow without an external magnetic field. Such behavior would mark a significant step forward for low-power, high-speed electronic applications. In addition, we found that spin polarization in graphene extends beyond the Dirac cone to another critical feature of its electronic structure — the Van Hove singularity (VHs) or flat band. In free-standing graphene, this prominent electronic feature lies in the conduction band and has been relatively underexplored. Eu intercalation introduces strong electronic doping in graphene, shifting the VHs closer to the Fermi level. Simultaneously, the interaction between Eu and the Ni substrate induces a dominant singlespin polarization in this band as results from Fig. 1 (b). This leads to a high density of states with predominantly one spin near the Fermi level, offering new opportunities for spintronic applications. A spin-polarized Van Hove singularity could potentially lead to exotic phenomena such as unconventional superconductivity, quantum phases, or insulating topological states. We confirmed the presence of this state via ARPES, where it appears as a faint signal connected to the apexes of the triangular cross-sections of the Dirac cone (Figs. 1 h,i). Finally, we observed that the proximity of the Van Hove singularity to the Fermi level contributes to the emergence of a polaronic band — a spectral feature visible just below the VHs in the photoemission spectra (Fig. 1 i). According to many-body simulations, this band arises from enhanced electronphonon coupling in graphene, amplified by the VHs’s proximity to the Fermi level (Fig. 1 j). As a result, this band is clearly visible even near the K point of graphene (Fig. 1 c), a phenomenon previously reported in only a handful of systems. In conclusion, our study demonstrates that europium intercalation in graphene/ Ni(111) system enables a powerful combination of preserved Dirac-like dispersion, together with an opening of a spin dependent gap with nontrivial topological properties, and a singlespin polarized Van Hove singularity that enhances a polaronic band due to the electron-phonon coupling. These properties collectively mark the graphene/Eu/Ni(111) system as a highly promising platform for future spintronic devices and quantum electronic applications, as well as for fundamental studies. Elettra Highlights 2024–2025 132
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