Significance: Although superconductivity is well-known in intercalated bulk graphite, the ultimate goal of inducing superconductivity in single-layer graphene has not yet been achieved. We have here developed an experiment that combines ultralow-temperature (5 K) and ultrahigh-vacuum (10 ⁻¹¹ torr) sample preparation with high-resolution angle-resolved photoemission spectroscopy (ARPES). We show that decorating monolayer graphene with a layer of lithium atoms enhances the electron–phonon coupling to the point where a superconducting state can be stabilized at low temperature. Measurements of the size of the superconducting gap by ARPES suggest a T c of about 5.9 K. This result constitutes the first observation, to our knowledge, of superconductivity in monolayer graphene. Given the massive scientific and technological interest in graphene, our findings will have significant cross-disciplinary impact.

The electronic structure of decoupled graphene on SiC(0001) can be tailored by introducing atomically thin layers of germanium at the interface. The electronically inactive (63\ifmmode\times\else\texttimes\fi{}63)R30$\circ${} reconstructed buffer layer on SiC(0001) is converted into quasi-free-standing monolayer graphene after Ge intercalation and shows the characteristic graphene $\pi${} bands as displayed by angle-resolved photoelectron spectroscopy. Low-energy electron microscopy (LEEM) studies reveal an unusual mechanism of the intercalation in which the initial buffer layer is first ruptured into nanoscopic domains to allow the local in-diffusion of germanium to the interface. Upon further annealing, a continuous and homogeneous quasifree graphene film develops. Two symmetrically doped (n- and p-type) phases are obtained that are characterized by different Ge coverages. They can be prepared individually by annealing a Ge film at different temperatures. In an intermediate-temperature regime, a coexistence of the two phases can be achieved. In this transition regime, n-doped islands start to grow on a 100-nm scale within p-doped graphene terraces as revealed by LEEM. Subsequently, the n islands coalesce but still adjacent terraces may display different doping. Hence, lateral p-n junctions can be generated on epitaxial graphene with their size tailored on a mesoscopic scale.

We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.

The growth of epitaxial graphene on SiC has been identified as one of the most promising techniques to produce graphene for electronic applications. In this paper, we present a systematic study of the electronic and structural properties of large-area quasifree standing epitaxial monolayer graphene grown on top of the SiC(0001) surface. For this purpose, we combine the thermal treatment of SiC in Ar atmosphere to achieve a homogeneous coverage of the surface with the hydrogen intercalation process, which leads to the removal of the interaction between the substrate and the carbon layer. The band structure in the vicinity of the K\ifmmode\bar\else\textasciimacron\fi{} point is measured using high-resolution angle-resolved photoelectron spectroscopy. A detailed analysis of the quasiparticle dynamics reveals a renormalization of the band velocity estimated to about 3% at energies around 200 meV below the Fermi level, which mainly originates from electron-phonon interaction. Further analysis of the momentum distribution curves leads to the formulation of a model for the doping reduction in such a system in the course of sample annealing above 650 $\circ${}C. The uniformity and homogeneity of the graphene is demonstrated by means of low-energy electron microscopy (LEEM). Microphotoelectron spectroscopy data confirm the high structural quality and homogeneity of the quasifree standing graphene. Using LEEM and scanning tunneling microscopy, we demonstrate that the hydrogen desorption at elevated temperatures of approximately 750 $\circ${}C sets in on the graphene terraces rather than via the step edges.