Stiven Forti

Evidence for superconductivity in Li-decorated monolayer graphene

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 Tc 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. Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron–phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131–134]. Although experiments have shown an adatom-induced enhancement of the electron–phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron–phonon coupling of up to λ≃0.58. On part of the graphene-derived π∗-band Fermi surface, we then observe the opening of a Δ≃0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with Tc≃5.9 K.

Revealing the electronic band structure of trilayer graphene on SiC: An angle-resolved photoemission study

In recent times, trilayer graphene has attracted wide attention owing to its stacking and electric-field-dependent electronic properties. However, a direct and well-resolved experimental visualization of its band structure has not yet been reported. In this paper, we present angle-resolved photoemission spectroscopy data which show with high resolution the electronic band structure of trilayer graphene obtained on alpha-SiC(0001) and beta-SiC(111) via hydrogen intercalation. Electronic bands obtained from tight-binding calculations are fitted to the experimental data to extract the interatomic hopping parameters for Bernal and rhombohedral stacked trilayers. Low-energy electron microscopy measurements demonstrate that the trilayer domains extend over areas of tens of square micrometers, suggesting the feasibility of exploiting this material in electronic and photonic devices. Furthermore, our results suggest that, on SiC substrates, the occurrence of a rhombohedral stacked trilayer is significantly higher than in natural bulk graphite. (Less)

Epitaxial graphene on SiC: from carrier density engineering to quasi-free standing graphene by atomic intercalation

Epitaxial graphene (EG) on SiC has been proven to be an excellent material to investigate the fundamental physical properties of graphene and also to directly implement new findings into devices realized on the versatile platform of SiC. Within this framework, this work aims to review some of the recent major achievements accomplished in the field of EG on SiC, related to the growth of EG on the SiC(0 0 0 1) surface, the control of its doping level, the decoupling of the graphene from the substrate and the intercalation of foreign atomic species at the interface.

Influence of the degree of decoupling of graphene on the properties of transition metal adatoms

We investigate the adsorption sites of

Local transport measurements on epitaxial graphene

3d

Bipolar gating of epitaxial graphene by intercalation of Ge

transition metal (TM) adatoms by means of low-temperature scanning tunneling microscopy and spectroscopy. Co and Ni adatoms were adsorbed on two types of graphene on SiC(0001), i.e., pristine epitaxial monolayer graphene (MLG) and quasi-free-standing monolayer graphene (QFMLG). In the case of QFMLG, two stable adsorption sites are identified, while in the case of MLG, only one adsorption site is observed. Our experimental results reveal the decoupling efficiency as a crucial parameter for determining the adsorption site as well as the electronic properties of

Ballistic bipolar junctions in chemically gated graphene ribbons.

3d

Intercalation of graphene on SiC(0001) via ion implantation

transition metal atoms on graphene. Furthermore, we show that Co atoms adsorbed on QFMLG are strong scattering potentials for Dirac fermions and cause intervalley scattering in their vicinity.

Manipulation of plasmon electron–hole coupling in quasi-free-standing epitaxial graphene layers

Growth of large-scale graphene is still accompanied by imperfections. By means of a four-tip scanning tunneling and electron microscope (4-tip STM/SEM), the local structure of graphene grown on SiC(0001) was correlated with scanning electron microscope images and spatially resolved transport measurements. The systematic variation of probe spacings and substrate temperature has clearly revealed two-dimensional transport regimes of Anderson localization as well as of diffusive transport. The detailed analysis of the temperature dependent data demonstrates that the local on-top nano-sized contacts do not induce significant strain to the epitaxial graphene films.

Alkali doping of graphene: The crucial role of high-temperature annealing

In this study, the ambivalent behavior of Ge intercalation is studied by means of scanning tunneling microscopy and spectroscopy as well as local 4-point probe transport measurements. In quantitative agreement with angle-resolved photoemission experiments, both p- and n-type doped graphene areas and their doping level were identified by local spectroscopy. The p-doped areas appear higher by 2 A with respect to the n-doped areas suggesting incorporation of thicker Ge-layers accompanied by a modified coupling to the initial SiC-surface. Furthermore, the sheet resistance was measured on each of the patches separately. The intrinsic imbalance between the carrier types in the different areas is well reflected by the transport study. The process of intercalation does not affect the transport properties in comparison to pristine graphene pointing to a sufficient homogeneity of the decoupled graphene layer. Transport measurements across chemically gated pn-junctions reveal increased resistances, possibly due to e...