By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18 ML). The layer-dependent energy gap and transition temperature (Tc) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above Tc and the theoretical calculations, we found that the Tc oscillation correlates directly with the density of states oscillation at E(F) . The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.

We have grown well-ordered graphene adlayers on the lattice-matched Co(0001) surface. Low-temperature scanning tunneling microscopy measurements demonstrate an on-top registry of the carbon atoms with respect to the Co(0001) surface. The tunneling conductance spectrum shows that the electronic structure is substantially altered from that of isolated graphene, implying a strong coupling between graphene and cobalt states. Calculations using density functional theory confirm that structures with on-top registry have the lowest energy and provide clear evidence for strong electronic coupling between the graphene pi-states and Co d-states at the interface.

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We present a scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe3O4(111) surface in ultrahigh vacuum. Gold forms two electrically distinct nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe3O4(111) at 500 °C for 15 min. I (V) curves taken via STS measurements show that large gold nanoparticles (∼8 nm) exhibit a metallic electronic structure and, thus, are likely neutral. Single gold adatoms appear to be strongly bonded to the oxygen sites of the Fe3O4(111) surface, and tunneling electrons are observed to flow predominantly from the STM tip to the Au adatoms and into the oxygen sites of the surface. The site-specific adsorption of the gold adatoms on oxygen surface atoms and the size-sensitive nature of the electronic structure suggest that Au adatoms are likely positively charged. When this Au/Fe3O4(111) system is dosed with CO at 260 K, adsorption of CO molecules normal to the surface atop the gold adatom sites takes place. CO adsorption on the large Au nanoparticles (∼8 nm) could not be confirmed by STM. These observations indicate that nonmetallic, positively charged Au species may play a key role in reactions involving CO, such as the CO oxidation and the water−gas-shift reaction on Au/metal oxide surfaces.

The reduced surface of a natural Hematite single crystal α-Fe(2)O(3)(0001) sample has multiple surface domains with different terminations, Fe(2)O(3)(0001), FeO(111), and Fe(3)O(4)(111). The adsorption of water on this surface was investigated via Scanning Tunneling Microscopy (STM) and first-principle theoretical simulations. Water species are observed only on the Fe-terminated Fe(3)O(4)(111) surface at temperatures up to 235 K. Between 235 and 245 K we observed a change in the surface species from intact water molecules and hydroxyl groups bound to the surface to only hydroxyl groups atop the surface terminating Fe(III) cations. This indicates a low energy barrier for water dissociation on the surface of Fe(3)O(4) that is supported by our theoretical computations. Our first principles simulations confirm the identity of the surface species proposed from the STM images, finding that the most stable state of a water molecule is the dissociated one (OH + H), with OH atop surface terminating Fe(III) sites and H atop under-coordinated oxygen sites. Attempts to simulate reaction of the surface OH with coadsorbed CO fail because the only binding sites for CO are the surface Fe(III) atoms, which are blocked by the much more strongly bound OH. In order to promote this reaction we simulated a surface decorated with gold atoms. The Au adatoms are found to cap the under-coordinated oxygen sites and dosed CO is found to bind to the Au adatom. This newly created binding site for CO not only allows for coexistence of CO and OH on the surface of Fe(3)O(4) but also provides colocation between the two species. These two factors are likely promoters of catalytic activity on Au/Fe(3)O(4)(111) surfaces.

The focusing performance of a multilayer Laue lens (MLL) with 43.4 μm aperture, 4 nm finest zone width and 4.2 mm focal length at 12 keV was characterized with X-rays using ptychography method. The reconstructed probe shows a full-width-at-half-maximum (FWHM) peak size of 11.2 nm. The obtained X-ray wavefront shows excellent agreement with the dynamical calculations, exhibiting aberrations less than 0.3 wave period, which ensures the MLL capable of producing a diffraction-limited focus while offering a sufficient working distance. This achievement opens up opportunities of incorporating a variety of in-situ experiments into ultra high-resolution X-ray microscopy studies.

Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.

Optical conductivity spectra of La0.7−y Pry Ca0.3MnO3 were systematically investigated. For metallic samples, the spectral weight below 0.5 eV, whose magnitude can be represented by the effective carrier number N eff (0.5 eV), increases as temperature becomes lower. Regardless of the Pr doping, all the measured values of N eff (0.5 eV)/TC fall into one scaling curve. This scaling behavior could be explained by the theoretical model by Röder et al. (Phys. Rev. Lett. 76, 1356), which includes spin double exchange and Jahn-Teller lattice coupling to holes. With the Pr doping, far-infrared conductivities were found to be suppressed, probably due to the Anderson localization. PACS number; 72.15. Gd, 75.50.Cc, 75.30.Kz, 78.20.Ci Physics of doped manganites, R 1−x A x MnO 3 (R=rare earth and A=alkaline earth ions) with 0.2≤x≤0.5, has been investigated extensively since the recent discovery of colossal magnetoresistance (CMR) phenomena in these compounds. Even if the double exchange (DE) interaction can qualitatively explain most of their physical properties [1], a lot of recent works have revealed that some additional degrees of freedom should be included to describe them more accurately [2][3][4][5][6][7][8][9][10][11]. Optical techniques have been useful to address such additional degrees of freedom [2,3,[8][9][10].However, there still remain some controversies on how to explain temperature(T)-dependent spectral weight (SW) changes in optical spectra of the doped manganites. Especially, there have been numerous interpretations on the mid-infrared (IR) absorption peak below 0.5 eV, which appears below the Curie temperature T C and becomes stronger at lower temperatures. Okimoto et al.claimed that the SW changes should be understood in the spin-split band picture, and they assigned the mid-IR feature to an intraband excitation within an e g band which was merged below T C from two spin-split bands [2,3]. Other workers suggested that an orbital degree of freedom should be included to explain the SW changes [4,5], and de Brito and Shiba attributed the mid-IR absorption to an interband transition between the e g orbital states [5]. Millis et al. showed that the dynamic Jahn-Teller (JT) interaction could play an important role in the SW changes [6,7], and Kaplan et al. assigned the mid-IR feature to a JT type small polaron absorption [8]. Recently, through detailed studies on optical properties of La 0.7 Ca 0.3 MnO 3 (LCMO), we proposed that the evolution of the low frequency feature below 0.5 eV come from a crossover from small to large polaron states [10]. In other words, the mid-IR feature should be attributed to an incoherent absorption of a large polaron state, whose existence was predicted by Röder et al. [11].To get further insights on this interesting issue, we investigated optical properties of La 0.7−y Pr y Ca 0.3 MnO 3 (LPCMO). Hwang et al. showed that dc transport properties of LPCMO were affected by a carrier hopping parameter which could be varied systematically by the Pr doping [12]. In this paper, we rep...