Electron wave function at a vicinal surface: switch from terrace to step modulation

Resonant Charge-Transfer in Grazing Collisions of H− with Vicinal Nanosurfaces on Cu(111), Au(100) and Pd(111) Substrates: A Comparative Study

We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated via the resonant charge transfer (RCT) between the surface and a H − ion that flies by at grazing angles. A quantum mechanical wave packet propagation approach is used to simulate the motion of the active electron, and time-evolved wave packet densities are used to visualize the dynamics through the superlattice. The survived ion fraction in the reflected beam generally exhibits modulations as a function of the vicinal terrace size and shows peaks at those energies that access the image state subband dispersions. Differences in magnitudes of the ion-survival as a function of the particular substrate selection and the ion-surface interaction time, based on the choice of two ion-trajectories, are examined. A square well model, producing standing waves between the steps on the surface, explains the energies of the maxima in the ion survival probability for all the metals considered. This indicates that the primary process of confinement induced subband formation is robust. The work may motivate measurements and applications of shallow-angle ion-scattering spectroscopy to access electronic substructures in periodically nanostructured surfaces.

Protected surface state in stepped Fe (0 18 1)

Carbon (C) surface segregation from bulk stabilizes the Fe(0 18 1) vicinal surface by forming a c(3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sqrt{2}$$\end{document}2 × \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sqrt{2}$$\end{document}2 reconstruction with C zig-zag chains oriented at 45° with respect to the iron surface steps. The iron surface electronic states as measured by high resolution ARPES at normal emission with polarized synchrotron radiation split in two peaks that follow distinct energy dispersion curves. One peak follows the dispersion of the carbon superstructure and is photoexcited only when the polarization vector is parallel to the steps, the second peak disperses similarly to the pristine Fe(0 0 1) surface. Such surface electronic structure is robust as it persists even after coating with an Ag overlayer. The robustness of this surface electronic structure and its similarity with that of the clean Fe(0 0 1) surface make this system of interest for magnetic and spintronic properties such as magneto tunnel junctions based on Fe/MgO interface.

Size dependence of the dispersion relation for the interface state between NaCl(100) and Ag(111)

This study investigates the interface state electron dispersion relation between NaCl(100) islands and Ag(111) dependent upon NaCl island size. Both onset energy and effective mass are size dependent. However, these dependencies are relevant at different island sizes. We trace back this effective mass dependency to a misfit-induced strain based on atomically resolved images. Our results open up new avenues for the development of nanodevices by tuning the effective electron mass via strain of the insulating component.

Tuning magnetic anisotropies of Fe films on Si(111) substrate via direction variation of heating current

We adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.

Robust spin polarization and spin textures on stepped Au(111) surfaces

The influence of structural defects, in the form of step lattices, on the spin polarization of the spin-orbit split Shockley surface state of Au(111) has been investigated. Spin- and angle-resolved photoemission data from three vicinal surfaces with different step densities are presented. The spin splitting is preserved in all three cases, and there is no reduction of the spin polarization of individual subbands, including the umklapp bands induced by the step lattice. On the sample with the highest step density studied, where the wave functions are delocalized over several terraces, the spin splitting is enhanced substantially, likely as an effect of the effective surface corrugation as on related surface alloys. The spin texture shows in all cases spin polarization vectors tangential to the Fermi circles, with the same helicities as on Au(111).

Tuning the ferromagnetic coupling of Fe nanodots on Cu(111) via dimensionality variation of the mediating electrons

Using in situ magneto-optical Kerr effect measurements and phenomenological modeling, we study the tunability in both the magnetization anisotropy and magnetic coupling of Fe nanodots on a curved Cu(111) substrate with varying vicinity. We observe that, as the terrace width w decreases, the magnetization anisotropy increases monotonically, faster when w is smaller than the nanodot size d. In contrast, the magnetic coupling strength also increases until w approximately d, after which it decreases steeply. These striking observations can be rationalized by invoking the counterintuitive dimensionality variation of the surface electrons mediating the interdot coupling: the electrons are confined to be one dimensional (1D) when w > or = d, but become quasi-2D when w < d due to enhanced electron spillover across the steps bridged by the nanodots.

Recent ARPES experiments on quasi-1D bulk materials and artificial structures

The spectroscopy of quasi-one-dimensional (1D) systems has been a subject of strong interest since the first experimental observations of unusual line shapes in the early 1990s. Angle-resolved photoemission (ARPES) measurements performed with increasing accuracy have greatly broadened our knowledge of the properties of bulk 1D materials and, more recently, of artificial 1D structures. They have yielded a direct view of 1D bands, of open Fermi surfaces, and of characteristic instabilities. They have also provided unique microscopic evidence for the non-conventional, non-Fermi-liquid, behavior predicted by theory, and for strong and singular interactions. Here we briefly review some of the remarkable experimental results obtained in the last decade.

One-dimensional 3d electronic bands of monatomic Cu chains

The electronic structure of an array of monatomic Cu chains grown on the Pt(997) surface has been examined by angle-resolved photoemission. The monatomic wires exhibit properties associated with 3d electron confinement in one dimension. Along the wire direction, the 3d bands states display a dispersive character, with periodicity in reciprocal space defined by the wire array geometry. These observations are compared and analyzed with ab initio calculations based on the full-potential linearized augmented plane-wave method.

Morphology Selected Molecular Architecture: Acridine Carboxylic Acid Monolayers on Ag (111)

The molecular architecture of acridine-9-carboxylic acid (ACA) grown on Ag (111) by physical vapor deposition was characterized by using UHV-STM and XPS. At lower coverage, ACA molecules exist in a 2-d gas phase on the surface at room temperature. With increased coverage (>0.4 ML), ACA molecules self-organize into distinctive adlayer structures that are correlated with underlying substrate morphology. On step-free Ag (111) regions, ACA molecules form large islands in coexistence with the 2-d ACA gas. These islands are commensurate with the Ag (111) substrate, indexed as (4 0, 2 4) in matrix notation, and can exceed 100 nm in size. There are two nonequivalent ACA molecules in each unit cell. XPS core level measurements reveal a hydrogen-bonding interaction between ACA molecules, with the ring nitrogen acting as the H-bond acceptor and the carboxyl proton acting as the H-bond donor. A structural model for this phase consists of chains of ACA molecules linked by head-to-tail hydrogen bonds along the substrate [10] direction. Alternating ACA tilting angles account for the two nonequivalent ACA molecules and the observed high packing density. Completely different molecular arrangements are observed on Ag (111) surface regions roughened by a higher density of crystallographic steps (terrace widths < or = 6 nm). Pairs of ACA molecules arrange in a zigzag pattern in a (12 2, 6 5) overlayer structure with a diluted packing density. The structural model for this lower density phase consists of carboxyl-carboxyl linked ACA dimers in a flat-lying molecular orientation.

Photoemission from stepped W(110): initial or final state effect?

The electronic structure of the (110)-oriented terraces of stepped W(331) and W(551) is compared to the one of flat W(110) using angle-resolved photoemission. We identify a surface-localized state which develops perpendicular to the steps into a repeated band structure with the periodicity of the step superlattices. It is shown that a final-state diffraction process rather than an initial-state superlattice effect is the origin of the observed behavior and why it does not affect the entire band structure.

Tuning the surface state dimensionality of Cu nanostripes

Stepped Cu nanostripes with varying terrace widths are self-assembled during Ag-induced periodic faceting of vicinal Cu(111). By changing Ag coverage the average terrace size within individual Cu stripes is readily tuned, making it possible to select the one-dimensional or two-dimensional character of surface states. Furthermore, the average terrace size can be smoothly switched from 10 to 30 A, thereby tracking the transition from step-lattice, quasi-two-dimensional umklapp bands to terrace-confined one-dimensional quantum well states.

Localization of surface states in disordered step lattices

The character of the surface state wave function on regularly stepped Cu(111) is reinvestigated. It is shown that the qualitative change at terrace lengths around 17 A observed previously by Ortega et al. [Phys. Rev. Lett. 84, 6110 (2000)]] must necessarily be described as a change from a propagating superlattice state to a terrace-confined quasi-one-dimensional state. This reconciles previous, apparently contradictory experimental results and sheds new light on the behavior of nearly free electrons in nanostructures. Possible mechanisms driving the localization are discussed on the basis of the surface state bulk penetration depth, which has been measured in both regimes.

Step-lattice-induced band-gap opening at the fermi level

The interaction of the Shockley surface state with the step lattice of vicinal Cu(111) leads to the formation of an electronic superlattice state. On Cu(443), where the average terrace length forms a "shape resonance" with the Fermi wavelength, we find a step-lattice-induced band-gap opening at the Fermi level. A gap magnitude >200 meV is inferred from high resolution photoemission experiments and line shape analysis. The corresponding energy gain with respect to a gapless case is approximately 11 meV/unit cell, and is a substantial contribution to the stabilization of the step lattice.

Electronic structure of carbon nanostripes

Carbon nanostripes of graphene structure prepared on the stepped Ni(771) surface have been studied by angle-resolved photoemission. The electronic structure is anisotropic: parallel to the stripe direction, a graphite-type dispersion is measured, whereas the perpendicular direction displays two entangled band structures shifted in energy with respect to each other. These are experimentally identified as the microsurface-centered band structure and its umklapp scattered image caused by the superlattice.

Surface-state depopulation on small Ag(111) terraces

The dependence of the local density of states near the Fermi energy E(F) on the width of terraces T is investigated by tunneling scanning spectroscopy on Ag(111) at 7 K. With decreasing T, the electronic density in the occupied surface state shifts monotonically towards E(F), leading to a depopulation at T=3.2 nm in quantitative agreement with a Fabry-Pérot model. Depopulation coincides with a switch from confinement by terrace modulation to step modulation.

Tailoring confining barriers for surface states by step decoration: CO /vicinal Cu(111)

The influence of CO adsorption on the Shockley type surface state on vicinal Cu(111) surfaces is investigated using angle resolved photoemission. As the steps are decorated with CO the surface state shifts to higher binding energies, which is opposite to the known behavior on flat Cu(111). This is described within a one-dimensional potential model in which clean steps represent repulsive barriers and decorated steps become attractive wells. From the coverage dependence the integrated CO well potential can be quantified. It is U(CO)a = -2.9 eV A on both Cu(332) and Cu(221) surfaces. Density functional calculations reveal that this attractive potential is due to the very local charge transfer from the Cu step atom to the adsorbed molecule.

Electron confinement in surface states on a stepped gold surface revealed by angle-resolved photoemission

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width ( approximately 3.8 nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM.