Honeycomb structures of group IV elements can host massless Dirac fermions with non-trivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the β12 boron sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the β12-sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

The search for metallic boron allotropes has attracted great attention in the past decades and recent theoretical works predict the existence of metallicity in monolayer boron. Here, we synthesize the β12-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boronderived bands using angle-resolved photoemission spectroscopy. The Fermi surface is composed of one electron pocket at the S point and a pair of hole pockets near the X point, which is supported by the first-principles calculations. The metallic boron allotrope in β12-sheet opens novel physics and chemistry in material science.

The topology of pure Bi is controversial because of its very small (∼10 meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14−202 bilayer Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to ∼10 meV. Detailed analyses on the phase shift of the confined wave functions precisely determine the surface and bulk electronic structures, which unambiguously show nontrivial topology. The present results not only prove the fundamental property of Bi but also introduce a capability of the quantum-confinement approach. [5]. Even now, numbers of novel quantum phenomena have been intensively reported on this system [6][7][8][9][10][11][12][13]. In spite of the enormous amount of research, one fundamental property of Bi has been controversial: its electronic topology. Because of its huge spin-orbit coupling (SOC) [14], Bi has also been a central element in designing topological materials such as Bi 1−x Sb x , Bi 2 Se 3 , Na 3 Bi, and β-Bi 4 I 4 [15][16][17][18][19]. A combination of SOC and several symmetries produces topologically protected electronic states with inherent spin splitting. Despite the essential role in topological studies, a pure Bi crystal itself had long been believed topologically trivial based on several calculations [20][21][22][23][24][25][26], which had been considered to agree with transport [27] and angleresolved photoelectron spectroscopy (ARPES) measurements [22,28,29]. However, a recent high-resolution ARPES result suggests the surface bands are actually different from previously calculated ones and Bi possesses a nontrivial topology [30,31]. New transport measurements also imply the presence of topologically protected surface states [32,33].Nevertheless, the recent ARPES result has not yet been conclusive because it lacks clear peaks of bulk bands [30,31]. In principle, surface-normal bulk dispersions can be measured by changing the incident photon energy, where the momentum resolution is determined from the uncertainty relation ∆z · ∆k z ≥ 1/2 (Ref.[34]). (∆z is an escape depth of photoelectrons.) However, the Dirac dispersion of Bi is so sharp against this resolution that hν-dependent spectra show no clear peak [29][30][31]. This is a serious problem because Bi has a very small (∼10 meV [21,26]) band gap and a slight energy shift in bulk bands can easily transform a nontrivial case [ Fig. 1(d)] into a trivial case [ Fig. 1(e)]. In short, to unambiguously identify the topology of Bi, one must precisely determine both the surface and bulk electronic structures. One promising approach is using a thin film geometry, where quantumwell state (QWS) subbands are formed inside bulk band projections [35,36]. Although QWSs originate from bulk states, they possess a two-dimensional character and can be clearly observed in ARPES measurements.In this Letter, we performed high-resolution ARPES

The electronic structure of the Ge͑001͒c͑8 ϫ 2͒-Au surface has been investigated by means of angle-resolved photoelectron spectroscopy. The atomic structure of the surface includes nanowires along the ͗110͘ direction separated by deep grooves, which were observed by scanning tunneling microscopy. The dispersion relation along the ͗110͘ direction showed the existence of a metallic surface-state band in accordance with 8ϫ periodicity. Moreover, the Fermi-surface measurement revealed that the band has an ellipsoidal shape, indicating an anisotropic two-dimensional metallic state. This is in contrast to the previously reported one-dimensional character of the system ͓J. Schäfer, C. Blumenstein, S. Meyer, M. Wisniewski, and R. Claessen, Phys. Rev. Lett. 101, 236802 ͑2008͔͒. A structural model including periodic arrangement of Au adsorbed ͑111͒ nanofacets was examined by comparing the electronic structure with that of the flat Ge͑111͒ ͱ 3 ϫ ͱ 3-Au surface.

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

No abstract

Ca-intercalation has enabled superconductivity in graphene on SiC. However, the atomic and electronic structures that are critical for superconductivity are still under discussion. We find an essential role of the interface between monolayer graphene and the SiC substrate for superconductivity. In the Ca-intercalation process, at the interface a carbon layer terminating SiC changes to graphene by Ca-termination of SiC (monolayer graphene becomes a bilayer), inducing more electrons than a free-standing model. Then, Ca is intercalated in between the graphene layers, which shows superconductivity with the updated critical temperature (T C ) of up to 5.7 K. In addition, the relation between T C and the normal-state conductivity is unusual, "dome-shaped". These findings are beyond the simple C 6 CaC 6 model in which s-wave BCS superconductivity is theoretically predicted. This work proposes a general picture of the intercalation-induced superconductivity in graphene on SiC and indicates the potential for superconductivity induced by other intercalants.

Layers of twisted bilayer graphene exhibit varieties of exotic quantum phenomena [1][2][3][4][5] . Today, the twist angle Θ has become an important degree of freedom for exploring novel states of matters, i.e. two-dimensional superconductivity ( Θ = 1.1°) 6, 7 and a two-dimensional quasicrystal (Θ = 30°) 8, 9 . We report herein experimental observation on the photo-induced ultrafast dynamics of Dirac fermions in the quasicrystalline 30° twisted bilayer graphene (QCTBG). We discover that hot carriers are asymmetrically distributed between the two graphene layers, followed by the opposing femtosecond relaxations, by using time-and angleresolved photoemission spectroscopy. The key mechanism involves the differing carrier transport between layers and the transient doping from the substrate interface. The ultrafast dynamics scheme continues after the Umklapp scattering, which is induced by the incommensurate interlayer stacking of the quasi-crystallinity. The dynamics in the atomic layer opens the possibility of new applications and creates interdisciplinary links in the optoelectronics of van der Waals crystals.Carrier dynamics in graphene is determined by Fermions in a linearly dispersing band structure, the Dirac cones, which have successfully offered unique electronic properties and achieved many applications 10 . Non-equilibrium electronic states in a matter, generated by optical pumping, are characterized by the transient temperature. In a case of the n-type graphene layer (Fig.1a), typically produced on a SiC substrate in a large area, the temporal chemical potential reduces with the temperature, as shown in Fig.1b,c. This dynamical phenomenon is specific to the massless Dirac I. Umklapp scattering and replica bands Supplementary Figure 1 | Umklapp scattering and replica bands. Red and blue hexagonal lines show the boundaries of the first Brillouin zone for the upper-layer (UL) and lower-layer (LL) graphene, respectively. The large (small) solid red circle represents the original (replica) band for the UL Dirac cone. and are reciprocal-lattice vectors of the crystal for the UL and LL, respectively.