Single-spin flat bands in monolayer graphene

In the rapidly evolving landscape of electronic materials and devices, the integration of two-dimensional materials, graphene in particular, with magnetic elements has emerged as a promising frontier. The ability to engineer spin-dependent electronic properties not only expands the horizons of spintronics but also holds the potential to revolutionize a wide array of electronic applications. From ultra-fast data processing to energy-efficient memory storage and beyond, the synergistic interplay between graphene and magnetic layers opens new routes for device miniaturization, aiming at reduced power consumption and enhanced performance.

Along these lines, the realization of topological electronic flattened bands near or at the Fermi level represents a promising avenue for the emergence of exotic electronic and magnetic states. One of the key attributes of flat bands is their facile electrical tunability, allowing for the exploration of correlated phases such as unconventional superconductivity, quantum states, and insulating topological states, all within two-dimensional platforms and without the need for an applied magnetic field.

An alternative approach to generate flat bands in graphene involves exploiting the Spin-Orbit Coupling (SOC) effect induced by the proximity to magnetic and/or heavy-metal layers. Monolayer graphene (Gr) interfaced with 3d-ferromagnetic and 4f-materials presents compelling technological prospects, bridging the realms of spintronics, ultra-fast graphene-based electronics, and photonics. While Gr/3d-ferromagnetic systems exhibit strong hybridization, impacting the electronic properties of graphene, Gr/4f-ferromagnetic interfaces demonstrate a weak interaction that preserves graphene's electronic structure.

In this work, by means of spin-resolved ARPES experiments combined with DFT calculations, we investigate the role of europium in modifying the spin-dependent electronic properties of monolayer Gr on Co(0001). Spin-integrated and -resolved ARPES measurements were carried out at the NanoESCA beamline of the Elettra synchrotron in Trieste, using a photoemission microscope (PEEM) operating in reciprocal space mode (k-PEEM). As a starting point, we synthetised monolayer graphene on top of a 10-nanometer-thick cobalt film. Figs. 1a-c show the LEED pattern, the 2D momentum map acquired at the Fermi level and the angle-resolved photoemission spectroscopy (ARPES) spectrum acquired at the K point of the pristine Gr/Co interface. The strong interaction between graphene and cobalt leads to orbital hybridization at the interface. In turn, this determines a shift to higher binding energy of the Dirac cone, as compared to freestanding graphene, and the formation of a hybrid state which is characterized by single spin polarization. Subsequently, metallic europium was deposited on top of Gr/Co and intercalated at the interface via thermal annealing. Eu arranges in a √3x√3R30° structure, as confirmed by the LEED pattern in Fig. 1d. The presence of europium leads to the decoupling of the graphene layer from the metallic support and, in parallel, to the doping due to charge transfer from Eu to Gr. The doping level is sufficient to occupy the graphene π* band up to the van Hove singularity, leading to the observation of the flat band at the M point of the surface Brillouin zone (Fig. 1e). Moreover, the SOC at the interface is responsible for the opening of a sizable band gap of 0.36 eV at the Dirac point (Fig. 1f).

Figure 1: Landscape of the structural and electronic properties upon Eu intercalation in Gr/Co(0001). LEED patterns acquired at a kinetic energy of 69 eV (a, d), 2D momentum maps at the Fermi level (b, e) and ARPES energy vs. momentum maps (c, f) acquired at the K point of the first Brillouin zone for Gr/Co and Gr/Eu/Co, respectively.

Following this, the spin character of the electronic bands near the Fermi level was probed using spin-resolved ARPES. In order to better visualize the flat band, the sample was tilted to accept the K, M and K’ points in the 2D momentum map. The 2D spin-resolved momentum map at the Fermi level (Fig. 2a) allows to clearly visualize the shape and polarization of the π* states, which are occupied due to electron transfer from Eu. It can be appreciated that the electron pockets formed by the π* states present strong trigonal warping, as expected for Dirac cones. These states do not display a predominant spin character. In Figs. 2b and 2c, the non-dispersive Eu 4f band is visible around 1.49 eV binding energy and shows a predominant minority spin character. Interestingly, the flat π* band is characterized by a prevalence of the majority spin polarization, as clearly visible in the spin polarization plot on top of Fig. 2d, in line with the dedicated DFT calculations performed for this system.

Figure 2: Spin-resolved electronic structure of the Eu/Gr/Co system. a) Spin-resolved 2D momentum maps at the Fermi level  of the Eu/Gr/Co system. Spin-resolved energy vs. momentum map acquired along the ΓK direction (b) and at the K point (c). d) Spin-resolved momentum distribution curves acquired along the ΓK direction at the Fermi level (between 0.0 and 0.2 eV and marked by a solid black line at the right side of b)). e) Spin polarization curve obtained by a normalized difference of the two curves in d). According to the 2D color code on the right side of panel c), blue and red intensities correspond to majority and minority electronic states.

Our findings demonstrate the relevance of using Eu for efficiently tuning the Gr electronic properties and driving the interlayer magnetic coupling, opening new perspectives for its application in spintronic devices.

This research was conducted by the following research team:

Matteo Jugovac¹, Iulia Cojocariu^1,2,3, Jaime Sanchez-Barriga^4,5, Pierluigi Gargiani⁶, Manuel Valvidares⁶, Vitaliy Feyer², Stefan Blügel⁷, Gustav Bihlmayer⁷ and Paolo Perna^4
1 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
² Peter Grünberg Institute (PGI-6), Forschungszentrum Jülich GmbH, Jülich, Germany
³ Dipartimento di Fisica, Università degli studi di Trieste, Trieste, Italy
⁴ IMDEA Nanociencia, Campus de Cantoblanco, Madrid, Spain
⁵ Helmholtz-Zentrum Berlin für Materialien und Energie, Elektronenspeicherring BESSY II, Berlin, Germany
⁶ ALBA Synchrotron Light Source, Barcelona, Spain.
⁷ Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, Jülich, Germany

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