Search

Interesting electronic properties of a-few-degree-twisted bilayer graphene are caused by the formation of a flat band due to the interlayer interaction and electronic correlation. The authors quantitatively investigated the band structure of wide 3${}^{\mathrm{deg}}$-twisted bilayer graphene with clean interface by angle-resolved photoelectron spectroscopy, and compare the results with those of a band calculation using a recently-developed band unfolding method. The observed band structure indicates strong interlayer coupling that renormalizes the band structure including partial flat band features and gap formation. The observed band structure is in good agreement with the calculated results, indicating the importance of the interlayer coupling for the band renormalization.

Mapping of the local lattice distortion is required to understand the details of the chemical and physical properties of thin films. However, the resolution by the direct microscopic methods was insufficient to detect the local distortion. Here, we have demonstrated that the local distortion of a monatomic film on a substrate is estimated with high resolution using the moire pattern of the topographic scanning tunneling microscopy image. The analysis focused on the apparently low centers of the moire pattern of the hexagonal iron nitride monolayer on the Cu(111) substrate. The local distortion was visualized by estimating the displacement of the experimentally detected center position from the ideal position that is extracted from the adjacent center positions. The map of the local distortion revealed that the subsurface impurities are preferentially located on the largely distorted areas. This approach is widely applicable to other thin films on the substrates.

Layers of twisted bilayer graphene exhibit varieties of exotic quantum phenomena1-5. Today, the twist angle {\Theta} has become an important degree of freedom for exploring novel states of matters, i.e. two-dimensional superconductivity ( {\Theta} = 1.1{\deg})6, 7 and a two-dimensional quasicrystal ({\Theta} = 30{\deg})8, 9. We report herein experimental observation on the photo-induced ultrafast dynamics of Dirac fermions in the quasicrystalline 30{\deg} 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 angle-resolved 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.

Twisted bilayer graphene (TBG), in which two monolayer graphene are stacked with an in-plane rotation angle, has recently become a hot topic due to unique electronic structures. TBG is normally produced in air by the tear-and-stack method of mechanical exfoliation and transferring graphene flakes, by which a sizable, millimeter-order area, and importantly clean interface between layers are hard to obtain. In this study, we resolved these problems by directly transferring the easy-to-exfoliate CVD-grown graphene on SiC substrate to graphene in a high vacuum without using any transfer assisting medium and observed electronic band modulations due to the strong interlayer coupling.

Rectangular iron nitride monatomic layers are fabricated on the threefold symmetric Cu(111) substrate by taking advantage of the stability of the square nitride film. Two different ordered structures are observed on the Cu(111) substrate by scanning tunneling microscopy after annealing at 510 K and 580 K. Their chemical composition and lattice symmetry are investigated by x-ray photoelectron spectroscopy and low energy electron diffraction. The monolayer film prepared at 580 K is a distorted ${\mathrm{Fe}}_{2}\mathrm{N}$ monolayer analogous to a ferromagnetic square ${\mathrm{Fe}}_{2}\mathrm{N}$ monolayer with a clock reconstruction on the Cu(001) substrate. The lattice deformation of the square ${\mathrm{Fe}}_{2}\mathrm{N}$ monolayer is induced by using Cu(111) with threefold symmetry.

Femtosecond luminescence of 1-, 2-, and 6–8-layer graphenes as well as graphite ( n = ∞) is studied in the near- to mid-infrared region, using a luminescence up-conversion method. The luminescence decay profiles show non-exponential behaviors within several picoseconds. Compared to graphite, 1- and 2-layer graphenes exhibit a remarkable reduction of lifetime at 0.3 eV. A two-temperature model that considers an effective Fermi energy successfully reproduces the decay profiles. On the basis of the Fermi energy (0.2 eV) of 1-layer graphene obtained from the photoemission experiment, we ascribe the layer number dependence of lifetime to the effective Fermi energy difference.

Graphene, a two-dimensional monatomic layer of crystal carbon, has recently emerged as a potential material for next-generation optoelectronic devices owing to unique properties arising from its massless Dirac fermions. However, the intrinsic carrier dynamics of graphene has remained a mystery even for the simple case of carrier cooling after the photoexcitation. This is because the observed temporal variations of the nonequilibrium carriers in graphene have been thoroughly described in terms of a defect-induced extrinsic effect known as ``supercollision'' (SC). The SC process is based on defect-mediated electron-acoustic phonon scattering and theoretically has been predicted to reduce with the increase of mobility of a material. Here, the authors have prepared extremely high mobility graphene and traced the dynamics of photoexcited carriers in the Dirac bands directly by time- and angle-resolved photoemission spectroscopy. They successfully observed suppression of SC and extracted the intrinsic dynamical properties of graphene, such as anharmonic decay of the optical phonons and the bottleneck relaxation at the Dirac point. Breaking the limit of SC, their research also has technological significance in developing graphene-based optoelectronic devices.

Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced band deformation. We reveal that deformation of bismuth surface bands is directly correlated with vertical charge-density profiles at each electronic state of bismuth. In contrast, a change in the quantized bulk bands is well described by a conventional rigid-band-shift picture. We discuss these two aspects of the band deformation holistically, considering spatial distributions of the electronic states and cesium-bismuth hybridization, and provide a prescription for applying alkali-metal adsorption to a wide range of materials., Comment: 8 pages, 7 figures

Epitaxial graphene growth on SiC surfaces is considered advantageous in terms of device application. However, the first graphitic layer on SiC transforms to a buffer layer because of strong coupling with the substrate. The properties of several subsequent layers are also significantly degraded. One method to decouple graphene from the substrate is Si intercalation. In the present work, we report observation and analysis of interface structures formed by Si intercalation in between the graphene layer and the SiC(0001) surface depending on Si coverage and influence of these interfaces on graphene electronic structure by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and theoretical first-principles calculations. The STM appearance of observed periodic interface structures strongly resembles previously known Si-rich phases on the SiC(0001) surface. Based on the observed range of interface structures we discuss the mechanism of graphene layer decoupling and differences in stability of the Si-rich phases on clean SiC(0001) and in the graphene/SiC(0001) interface region. We also discuss a possibility to tune graphene electronic properties by interface engineering.

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.

Formation process of ferromagnetic nanodot arrays on a periodically nanopatterned Cu(001) surface with nitrogen atoms are overviewed, and their magnetic properties studied by surface magneto-optical Kerr effect (SMOKE) in ultra high vacuum are described. The square lattice nanopattern induced by the strain due to nitrogen adsorption is formed by the strain-relief mechanism of the Cu lattice near the surface. On top of that, the diffusion and the adsorption of the deposited atoms depend on the substrate lattice strain, and the ferromagnetic metal island preferentially grow at the less strained clean Cu surface. As a result, a square nanodot array of 7 nm × 7 nm is formed on the surface. The hysteresis loops of the arrays were in-situ measured by SMOKE between 400 K and 95 K. The results indicate long range ferromagnetic order is established among the superparamagnetic Co dots of two-monoatomic-layers high by the magnetic interaction through the monolayer Co strips.

Femtosecond luminescence of graphene and graphite is studied from near- to mid-infrared regions. Remarkable reduction of lifetime at 0.3 eV is found in mono- and bi-layer graphenes, and interpreted as an effect of non-zero Fermi energy.

Sn atomic layers attract considerable interest owing to their spin-related physical properties caused by their strong spin–orbit interactions. We performed Sn intercalation into the graphene/SiC(0001) interface and found a new type of Sn atomic layer. Sn atoms occupy on-top sites of Si-terminated SiC(0001) with in-plane Sn–Sn bondings, resulting in a triangular lattice. Angle-resolved photoemission spectroscopy revealed characteristic dispersions at and points, which agreed well with density functional theory calculations. The Sn triangular lattice atomic layer at the interface showed no oxidation upon exposure to air, which is useful for characterization and device fabrication ex situ.

Local electron-phonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron-phonon coupling to the out-of-plane phonon at the K̅ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron-phonon coupling in the surface graphene.

We studied the morphological structure and electrical properties, especially superconductivity, of ultra-thin films (5-15 A) of Ag on Ge(100) surfaces at low temperature (LT) below 4.2 K in ultra-high vacuum (UHV). Using a home-made UHV-LT scanning tunneling microscope, we found that 10 A Ag grows three-dimensionally and with elongation on the substrate. Non-elongated islands were observed on the 5 A Ag/Ge surfaces. The relations of tunneling current to sample bias voltage I-V at elongated silver islands were non-ohmic with energy gaps of from a few tens to a few hundred meV. We ascribe these non-ohmic characteristics to the Coulomb blockade. Moreover, we observed slightly non-linear I-V with an energy gap of a few meV at the non-elongated islands on the 5 A Ag/Ge at 2.9 K. The temperature dependence of the non-linearity suggests superconductivity on the surface.

Hot carrier dynamics in the Dirac band of n-type epitaxial graphene on a SiC substrate were traced in real time using femtosecond-time-resolved photoemission spectroscopy. The spectral evolution directly reflects the energetically linear density of states superimposed with a Fermi–Dirac distribution. The relaxation time is governed by the internal energy dissipation of electron–electron scattering, and the observed electronic temperature indicates cascade carrier multiplication.

We have studied the growth and magnetism of two-dimensionally coupled nanometer-scale Co dots which are squarely arranged on a $\mathrm{Cu}(001)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{N}$ surface. The morphology was analyzed by scanning tunneling microscopy, and the magnetization was studied using the magneto-optical Kerr effect. Well-ordered arrays of two monolayer (ML) thick Co dots interconnected by narrow 1 ML thick Co films can be grown by adjusting the deposited amount of Co. Here we used selective growth of Co at the clean Cu area of the surface, which consists of $5\ifmmode\times\else\texttimes\fi{}5 {\mathrm{nm}}^{2}$ N-adsorbed $c(2\ifmmode\times\else\texttimes\fi{}2)$ patches separated by clean Cu lines of a few nm in width. The observed ferromagnetism in these arrays is considered to be mediated by narrow monolayer thick Co films on clean Cu lines. When the average thickness of Co exceeds 1.5 ML, 2 ML thick Co dots are connected with each other, and 1 ML Co films grow on the $c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{N}$ patches. For Co films with an average thickness between 1.8 ML and 2.1 ML, the magnetization is almost constant between 150 K and 370 K, and increases with decreasing temperature from 150 K. This result suggests that polarization is induced in the 1 ML Co film on $c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{N}$ patches below 150 K.

We have been searching novel superconducting materials only at their surface regions using a homemade ultra high vacuum low temperature scanning tunneling microscope. We found that scanning tunneling current-voltage curves on 5 A thick Ag deposited Ge(100) surfaces at 2.9 K are slight non-linear with an energy gap of a few meV. The temperature dependence of the non-linearity suggests supercond activity on the surface alloys.