Our scanning tunneling microscopy and electron diffraction experiments revealed that a new twodimensional allotrope of Bi forms on the Si111-7 7 surface. This pseudocubic f012g-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) When the size and shape of the materials are downsized to a nanometer scale, they often reveal anomalous atomic structures as well as exotic functional properties that do not exist in bulk. A variety of novel structures discovered in nanoparticles and nanowires have been attracting broad interest [1][2][3][4]. Group V elements are known to show rich allotropic transformation because their semimetallic bonding character can be easily shifted to either the metallic or covalent side, for example, by changing the applied pressure [5][6][7][8]. In this context, a question arises whether a new allotrope can be realized by tuning the dimensionality or size, such as thickness of the film, instead of tuning the pressure or temperature.In this Letter, we report on our finding that ultrathin film of Bi really shows such an allotropic transformation as a function of thickness on the scale of several atomic layers. Our electron diffraction and scanning tunneling microscopy (STM) experiments revealed that, over the wetting layer formed initially on the Si111-7 7 surface, Bi grows with a new f012g-oriented phase whose structure is significantly different from bulk Bi and that it transforms into the bulk-like single-crystal Bi(001) phase above the critical thickness which increases with the substrate temperature. Our ab initio theory revealed that the f012g phase with even-number layers is stabilized by forming a puckered-layer structure. The resulting film is very flat, compared to the growth of any known metal films, reflecting the inherent two-dimensional (2D) structure of this f012g phase. To the best of our knowledge, puckered-layer structure was never observed before except for the case of black phosphorus, which is a famous teratoid phase in group V elements.Bismuth ultrathin films were grown in ultrahigh vacuum (UHV) and characterized in situ using STM and reflection high-energy electron diffraction (RHEED). The spot-profile-analyzing low-energy electron diffraction (SPA-LEED) was performed with momentum reso-

We have measured one-dimensional (1D) plasmons in an atom wire array on the Si(557)-Au surface by inelastic scattering of a highly collimated slow electron beam. The angular dependence of the excitation energy clearly indicates the strong 1D confinement and free propagation of the plasma wave along the wire. The observed plasmon dispersion is explained very well by a quantum-mechanical scheme which takes into account dynamic exchange-correlation effects, interwire interactions, and spin-orbit splitting of the 1D bands. Although the qualitative feature of the plasmon dispersion is reminiscent of that of a highdensity free-electron gas, we detected the substantial influence of electron correlation due to strong 1D confinement. DOI: 10.1103/PhysRevLett.97.116802 PACS numbers: 73.20.Mf, 71.45.Gm, 72.15.Nj, 73.63.Nm One-dimensional electron systems (1DESs) such as metallic atom wires are expected to sustain a unique collective charge-density excitation which can be called onedimensional (1D) plasmon, or wire plasmon. This excitation mode propagates only along the wire and strongly reflects the confined nature of the electron motion. Such a dimensionality effect will show up clearly in the energy dispersion and the linewidth of a plasmon. For example, 3D-type plasmons (bulk and surface plasmons) have finite energies at q 0 (q denotes momentum) [1], but the energies of 1D and 2D plasmons vanish at q 0 [2,3], since the restoring force for charge-density waves in low dimension vanishes in the long-wavelength limit. However, in spite of its broad interest, experimental observation of a plasmon in an atomic-scale metal wire has been lacking so far and its details remain unexplored.Recently, Au-induced 1D chain structures on stepped Si(111) surfaces were found to comprise 1DESs with metallic electron densities [4 -9]. Among them, Au-Si atom wires formed on the Si(557) surface possess two proximal and deep (1 eV) electron pockets [5][6][7][8][9] and can be regarded as an ideal 1D metal. This system has been providing many intriguing topics such as spin-charge separation in a Tomonaga-Luttinger liquid, lattice distortion by Peierls instability, etc., which originate from the restricted 1D motion of electrons [4 -8]. Moreover, a recent theoretical study by ab initio calculation has proposed a large spinorbit (SO) splitting in the 1D electronic band which results from the strong Au 6p character of this 1D band [10].In this Letter, we report the electron-energy-loss spectroscopy (EELS) of the plasmon in such a model 1D system on the Si(557) surface. The measured plasmon clearly shows strong one-dimensional and metallic characteristics which are expected from a 1D electron gas. We analyzed the observed plasmon dispersion by a quantummechanical scheme including the effects of dynamic exchange correlation (XC), interwire interactions, and SO splitting of the 1D bands. These results demonstrate that, although the Fermi velocity of the 1DES is very high, suggesting its free-electron nature, the dynamic electron correlation effect...

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Thin Bi(001) films grown by ultrahigh vacuum deposition on Si(111)-7×7 surfaces at room temperature, were annealed at ∼400K in order to improve their morphology by reducing the step density on the surface. Annealed, well-ordered Bi(001) films have been subsequently used as substrates for growth of pentacene (Pn). It has been determined using low-energy electron microscope that Pn nucleates on Bi(001) into a highly ordered, crystalline layer, with Pn molecules “standing up” on the Bi surface, and the (001) plane on the growth front. Moreover, the Pn layer is aligned with the Bi(001) surface having a “point-on-line” commensurate relationship with the substrate. The Pn∕Bi(001) film crystallizes in a bulk-like structure directly from the first Pn layer. Formation of the thin film phase reported for the Pn growth on SiO2 and other inert substrates was not observed in our experiments.