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The creation of three-dimensional (3D) geometrical shapes with atomically ordered surfaces and the investigation of their physical properties are major steps contributing to the development of a new paradigm in surface science. We produced a 3D-patterned Si sample with atomically flat and reconstructed {111} facet surfaces, and investigated its structural and physical properties. To apply the conventional techniques in surface science to 3D samples with various oriented surfaces, instead of two-dimensional planar samples, an appropriate relationship between the crystallographic surface ordering on the 3D-architected surfaces and the angle-resolved photoelectron spectroscopy (ARPES) setup considering the configuration in 3D space is indispensable. The distinctive and complex low-energy electron diffraction (LEED) patterns reflecting the 3D-arranged facet surfaces showed the realization of atomically reconstructed facet surfaces on 3D-patterned Si. Surface states of the 3D-patterned Si{111} surfaces are mapped by ARPES by considering the 3D geometrical relationship. The selection of the appropriate alignment of the incident electron beam (light) for the target surfaces allows the clear observation of the band dispersion from the produced {111}7×7 facet surfaces in 3D space. Our demonstration of accessibility of ARPES technique could provide useful guidelines for new methodologies, giving a fundamental understanding of 3D-shape-induced novel functionalities.

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 (

We use laser-based spin- and angle-resolved photoemission spectroscopy (laser-SARPES) with high-resolution, and experimentally determine, for the first time, the Rashba-parameters of quantum well states (QWSs) systematically changing with the film thickness and the quantum numbers, through the observation of the Ag films grown on an Au(111) substrate. The data are very well reproduced by the theoretical calculations based on the density functional theory. Most importantly, we find a scaling law for the Rashba parameter ($\alpha_{\rm R}$) that the magnitude of $\alpha_{\rm R}$ is scaled by the charge density at the interface and the spin-orbit coupling ratio between the film and the substrate, and it is expressed by a single straight line regardless of the film thickness and the quantum numbers. The new finding not only is crucial to understand the Rashba effect in QWSs but also gives a foundation of film growth engineering to fine-tune the spin splitting in 2D heterostructure systems., Comment: 6 pages, 4 figures

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.

The symmetry of a surface or interface plays an important role in determining the spin splitting and texture of a two-dimensional band. Spin-polarized bands of a triangular lattice atomic layer (TLAL) consisting of Sn on a SiC(0001) substrate is investigated by spin- and angle-resolved photoelectron spectroscopy. Surprisingly, both Zeeman- and Rashba-type spin-split bands, without and with spin degeneracy, respectively, coexist at a K point of the Sn TLAL. The K point has a threefold symmetry without inversion symmetry according to the crystal structure including the SiC periodicity, meaning that the Zeeman-type is consistent with the symmetry of the lattice while the Rashba-type is inconsistent. Our density functional calculations reveal that the charge density distribution of the Rashba-type (Zeeman-type) band shows (no) inversion symmetry at the K point. Therefore, the symmetry of the charge density distribution agrees with both types of the spin splitting.

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 topology of pure Bi is controversial because of its very small ($\sim$10 meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14$-$202 bilayers Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to $\sim$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., Comment: 6 pages, 5 figures

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 \beta 12 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 \beta 12-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., Comment: accepted for publication in Physical Review Letters

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 \b{eta}12-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boron-derived 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 \b{eta}12 sheet opens the way to novel physics and chemistry in material science., Comment: 5 pages, 4 figures

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.

Surface structures of tin (Sn) adsorbed Cu(0 0 1) have been studied by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Sn 4 d core-level spectroscopy. A structural model for the 5 8 mono-atomic-layer (ML) Sn-adsorbed Cu(0 0 1) surface is proposed on the basis of the observed atomic images and the core-level spectrum. The model is consistent with a height profile of the phase boundary between 1 2 ML Sn on Cu(0 0 1) and 5 8 ML Sn on Cu(0 0 1). After annealing to 410 K, the surface structure of 5 8 ML Sn on Cu(0 0 1) changed to a mixture of 1 2 ML Sn on Cu(0 0 1) areas and a new phase, which has a Sn coverage of about 2 ML. From the STM observation, the surface of the new phase consists of 4 3 - 3 4 domains separated by a disordered area. We propose a structure model consistent with the observed results.

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.

On a Cu (001) surface vicinal to a [100] direction, single rows of square patches c(2×2)N in the direction of [010] can be formed on narrow terraces. We deposited Co on the vicinal Cu(001)–c(2×2)N surface, and investigated the magnetic properties of the Co nano-dot array at a temperature range of between 90 and 300 K, using an in situ surface magneto optical Kerr effect. When an average of 1.4 ML of Co was deposited on the surface, rectangular shape hysteresis loops of in-plane magnetization, parallel to the direction of [010], were observed below 130 K. This indicates that the easy magnetization axis is along the step edge.

Cs adsorption on flat Cu(0 0 1) surfaces with respect to Cs coverage at low temperature was studied by LEED observations. A new structure of Cs on Cu(0 0 1) was observed at the coverage about 60% of the saturated single atomic layer. The atom density at the saturated single layer is higher than that for a vicinal surface. In addition, the absence of rotationally incommensurate phase of Cs overlayer on a flat Cu(0 0 1) surface was confirmed.

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.

Structure and magnetic properties are studied for Cs adsorbed Co(0 0 1) thin films epitaxially grown on clean Cu(0 0 1) surfaces with steps along [1 0 0] direction. A quasi-hexagonal order was found in the structure of single atomic-layer Cs on the surface at 120 and 300 K as observed for Cs on clean Cu(0 0 1) surfaces. The magnetic hysteresis loops were measured at 120 and 300 K using magneto optical Kerr effect. The magnetic anisotropy induced by the steps along [1 0 0] direction changes at 120 K when the density of Cs at the surface exceeds the half of the single atomic-layer. Simultaneously, the coercive field along [1 0 0] direction increases. The charge transfer from Cs to the substrate is discussed as a possible origin of these changes.

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 present the magnetic properties of Co dot arrays grown on the Cu(001)c(2×2)-N substrate prepared by activated nitrogen bombardment. These Co dot arrays show ferromagnetic behavior above average Co thickness of 1.3 monolayer (ML), as evidenced by the existence of remanent magnetization which was measured by magneto-optical Kerr effect. It is also found that at Co coverages below 2 ML the magnitude of the remanent magnetization is always smaller than that of the saturated magnetization between 95 and 400 K. We discuss possible origins of the magnetic behavior, considered to be different from those of ultrathin films grown directly on the clean Cu(001) surface, in connection with the morphology of these dot arrays.

Using scanning tunneling microscopy, we studied Ge(001) surfaces covered with an average of approximately 0.8 monolayers of silver. In the deposition at room temperature, we observed that Ag predominantly grew in a three-dimensional (3D) mode on bare Ge substrates, corresponding to Volmer–Weber growth. At the same time, we found two-dimensional (2D) Ag islands elongated in the dimer-row direction with one monolayer height, though the density of the 2D islands was smaller than one-fifth of that of the 3D islands. Images with atomic resolution showed stripe patterns on the 2D islands and enhancement of asymmetry of the Ge dimers on both sides of the islands at high positive bias voltages.

Germanium(001) surfaces deposited with less than 0.3 nm thick Ag on average were studied by scanning tunneling microscopy. After the small amount of Ag deposition at 100 K, thin two-dimensional (2D) islands with 0.18 nm height are predominantly found to elongate to the dimer-row direction on clean Ge(001)-2×1 surfaces. With increasing temperature during the deposition, the density of three dimensional (3D) islands with more than 0.6 nm height increases, although few 2D islands are still formed at room temperature. With increasing average thickness of deposited Ag, the density of 3D islands increases at any temperature between 100 K and 300 K. These features are explained in terms of the diffusion of Ag atoms on the surface during deposition.

We have studied atomic-hydrogen-induced chlorine extraction on Si(111)-7×7 surfaces using photoemission spectroscopy and scanning tunneling microscopy (STM). We exposed the surface with mono- and polychloride Si to atomic hydrogen at room temperature. Photoemissions from Si 2 p core level and Cl 3 s level were measured before and after the atomic hydrogen dosage on the surfaces. Signals with both silicon-chlorides and Cl atoms decrease with increasing atomic hydrogen dosage. After annealing the hydrogen-exposed surface at 720 K, the STM images are very different from those of chlorine adsorbed Si(111) surface, and similar to those of hydrogen-covered Si(111) surfaces after the annealing. We conclude that chlorine atoms are extracted from the Cl/Si(111) surface by atomic hydrogen, and the surface Si atoms are terminated by hydrogens.

We have studied ultraviolet laser-induced monolayer etching process on Cl-adsorbed Si(111)-7×7 surfaces with X-ray photoemission spectroscopy. We keep the fluence as low as 1 mJ/cm2 to discuss electronic excitation and relaxation at the surface during this process. After irradiation, silicon di- and tri-chloride species on the surface disappear and monochlorides remain at the surfaces. We conclude that surface adatom polychlorides and rest-atom polychlorides are desorbed primarily as SiCl2 species, and that adatom monochlorides and rest-atom monochlorides are stable against irradiation. Photo-excited electrons and/or holes in bulk induces the SiCl2 desorption at the surfaces.

We have studied hydrogen and chlorine exchange reaction on Si(111)-7×7 surfaces with STM. We found that atomic hydrogen extracts chlorine from a monochloride rest-layer surface, while molecular chlorine would not react with a monohydride rest-layer surface. Multi-bilayer pits are found on hydrogen-saturated surfaces followed by Cl dose and annealing.

Surface structure changes of hydrogen- or chlorine-adsorbed Si(111)-(7 × 7) caused by ultra-violet laser light have been studied with a scanning tunneling microscope. Experiments were performed in ultra-high vacuum conditions with a small fluence of laser light in order to avoid gas-phase excitation and strong thermal effects. Polyhydrides are found to decompose slowly to monohydrides, and polychlorides are desorbed by electronic excitation with the light. On the other hand, monohydride and monochloride Si atoms on the surface are stable against irradiation. The results are discussed in terms of hot carriers excited by light in the substrate, and their relaxation to the vibration of the adsorbates at surfaces.

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.

Valence electronic structure of the Ge(111)($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)$R{30}^{\ensuremath{\circ}}$-Au surface was studied by angle-resolved photoemission spectroscopy and density functional calculations. Two metallic surface bands were observed around $\overline{\ensuremath{\Gamma}}$, and parts of them split into spin-polarized bands owing to the spin-orbit interaction. One is a holelike band and originates only from the Ge atoms. The other is an electronlike band and is made of the surface Au and Ge atoms. The polarized spin in these bands has the component normal to the surface as well as that parallel to the surface. These results are attributed to spin-orbit interaction in a large and anisotropic potential gradient at this surface.

The electronic structure of the $\text{Ge}(001)c(8\ifmmode\times\else\texttimes\fi{}2)\text{-Au}$ surface has been investigated by means of angle-resolved photoelectron spectroscopy. The atomic structure of the surface includes nanowires along the $⟨110⟩$ direction separated by deep grooves, which were observed by scanning tunneling microscopy. The dispersion relation along the $⟨110⟩$ direction showed the existence of a metallic surface-state band in accordance with $8\ifmmode\times\else\texttimes\fi{}$ 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. Sch\"afer, C. Blumenstein, S. Meyer, M. Wisniewski, and R. Claessen, Phys. Rev. Lett. 101, 236802 (2008)]. A structural model including periodic arrangement of Au adsorbed (111) nanofacets was examined by comparing the electronic structure with that of the flat $\text{Ge}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-Au}$ surface.

We developed a scanning tunneling microscope (STM) operating below 2 K in a vacuum of less than 10\t-8 Pa. This STM was constructed to study low-temperature properties such as superconductivity and charge density waves in artificial material on surfaces. The sample is prepared in an ultra-high-vacuum preparation chamber separated by a gate valve from the STM chamber, and is transferred to the microscope which is attached to a liquid He pot. By pumping liquid He, the sample is cooled down to 2 K. At low temperature, we observed an atomic image of graphite surface with its constant current mode, and an superconducting gap of Pb film with its tunneling spectroscopy mode.

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.

Submonolayer growth of Co on the reconstructed Cu(001)(2×22)R45°–O surface has been investigated by scanning tunneling microscopy. Cu atoms are displaced from the Cu(001)(2×22)R45°–O structure by incoming Co atoms and subsequently aggregate into elongated islands. The deposited Co atoms are randomly distributed in the oxygen adsorbed surface as individual atoms and clusters at low coverages [⩽0.4 monolayers (ML)]. For larger coverages (⩾0.5 ML), compact fcc Co patches are formed. The adsorbed oxygen acts as a surfactant. Interfacial intermixing is reduced when Co is deposited on the Cu(001)(2×22)R45°–O surface.

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.