Structure and growth of epitaxial Pb on Si(111)

Tuning of the Rashba effect in Pb quantum well states via a variable Schottky barrier

Spin-orbit interaction (SOI) in low-dimensional systems results in the fascinating property of spin-momentum locking. In a Rashba system the inversion symmetry normal to the plane of a two-dimensional (2D) electron gas is broken, generating a Fermi surface spin texture reminiscent of spin vortices of different radii which can be exploited in spin-based devices. Crucial for any application is the possibility to tune the momentum splitting through an external parameter. Here we show that in Pb quantum well states (QWS) the Rashba splitting depends on the Si substrate doping. Our results imply a doping dependence of the Schottky barrier which shifts the Si valence band relative to the QWS. A similar shift can be achieved by an external gate voltage or ultra-short laser pulses, opening up the possibility of terahertz spintronics.

Imaging the electron-phonon interaction at the atomic scale

Thin Pb films epitaxially grown on 7×7 reconstructed Si(111) represent an ideal model system for studying the electron-phonon interaction at the metal-insulator interface. For this system, using a combination of scanning tunneling microscopy and inelastic electron tunneling spectroscopy, we performed direct real-space imaging of the electron-phonon coupling parameter. We found that λ increases when the electron scattering at the Pb/Si(111) interface is diffuse and decreases when the electron scattering is specular. We show that the effect is driven by transverse redistribution of the electron density inside a quantum well.

Dynamical Coulomb blockade observed in nanosized electrical contacts

Electrical contacts between nanoengineered systems are expected to constitute the basic building blocks of future nanoscale electronics. However, the accurate characterization and understanding of electrical contacts at the nanoscale is an experimentally challenging task. Here, we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nanocontacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island-substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements.

Lead growth on Si(111) surfaces reconstructed by indium

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis-low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3-In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1-In surface results in metallic Pb(111) crystalline islands after the completion of a double incomplete wetting layer. In addition, the bond arrangement of the adatoms is studied, confirming that In adatoms interact more strongly with the silicon substrate than the Pb ones. This promotes a stronger Pb-Pb interaction and enhances metallization. The onset of the metallization is correlated with the amount of pre-deposited In on the Si(111) surface. The decoupling of the Pb film from the 4 × 1-In interface can also explain the unusual thermal stability of the uniform height islands observed on this interface. The formation of these Pb islands is driven by quantum size effects. Finally, the different results of Pb growth on the two reconstructed surfaces confirm the importance of the interface, and also that the growth morphology, as well as the electronic structure of the Pb film can be tuned with the initial substrate reconstruction.

Surface alloy formation of noble adatoms adsorbed on Si(111)-√3 × √3-Pb surface: a first-principles study

The geometric structures, stability and electronic properties of initial stages of surface alloy formation for noble atoms adsorbed on Si(111)-)-√3 × √3-Pb surfaces have been comparatively and extensively studied by using first-principles calculations within density functional theory. Our results revealed that an Au trimer rather than a tetramer adsorption induces a surface alloy by combining with Pb atoms in covalent bonds, exhibiting semiconducting characteristics due to the localization of surface states. The stability of the two-dimensional (2D) surface alloy obeys the Hume-Rothery rule. The electronic structures of the 2D surface alloy are sensitive to the number of Au adatoms and can be modulated by the quantity of Au adatoms. Unlike the Au atoms, our further calculations indicated that adsorption of Ag or Cu atoms on the surface cannot form a surface alloy with Pb atoms in the surface layer due to a weaker interaction or smaller radius.

Reduction of the superconducting gap of ultrathin Pb islands grown on Si(111)

The energy gap Delta of superconducting Pb islands grown on Si(111) was probed in situ between 5 and 60 monolayers by low-temperature scanning tunneling spectroscopy. Delta was found to decrease from its bulk value as a function of inverse island thickness. Corresponding T_{c} values, estimated using bulk gap-to-T_{c} ratio, are in quantitative agreement with ex situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer-dependent ab initio density functional calculations for freestanding Pb films show that the electron-phonon coupling constant, determining T_{c}, decreases with diminishing film thickness.

Characterization of spectroscopic photoemission and low energy electron microscope using multipolarized soft x rays at BL17SU/SPring-8

Spectroscopic photoemission and low energy electron microscope (SPELEEM) improved its performance after installation at BL17SU/SPring-8, where a multipolarization-mode undulator is employed to produce circularly and linearly polarized soft x rays. This undulator enables us to study the domain structures of ferromagnetic and antiferromagnetic materials by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism. SPELEEM is used to study light elements (C, N, and O), 3d transition-metal elements and 4f rare earth elements, utilizing a wide range of photon energies. The two cylindrical mirrors adopted in front of SPELEEM ensure an illumination area of 14 x 14 microm(2) on the samples. The lateral resolution of a secondary electron photoemission electron microscope image is estimated to be better than 85 nm, whereas the energy resolution of the instrument is better than 0.4 eV.

Building Pb nanomesas with atomic-layer precision

We demonstrate a novel scheme for manipulating metallic nanostructures involving a macroscopic number of atoms, yet with precise control in their local structures. The scheme entails a two-step process: (a) a triggering step using a scanning tunneling microscope, followed by (b) self-driven and self-limiting mass-transfer process. By using this scheme, we construct Pb nanomesas on Si(111) substrates whose thickness can be controlled with atomic-layer precision. The kinetic barrier for the mass transfer and the underlying mechanism behind this novel manipulation are determined.

Alternating layer and island growth of Pb on Si by spontaneous quantum phase separation

Real-time in situ x-ray studies of continuous Pb deposition on Si(111)-(7x7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first-principles calculation of the system energy.

Competing classical and quantum effects in shape relaxation of a metallic island

Pb islands grown on a Si substrate transform at room temperature from a flattop facet geometry into an unusual ring shape. The volume-preserving mass transport is catalyzed by the electrical field from the tip of a scanning tunneling microscope. The ring morphology results from the competing classical and quantum effects in the shape relaxation. The latter is enhanced by the large anisotropy of the effective mass, and leads to a sequential strip-flow growth on alternating strips of the same facet defined by substrate steps, showing its dynamical impact on the stability of a nanostructure.

Imaging subsurface reflection phase with quantized electrons

Lead quantum wells (QW) epitaxially grown on annealed Pb/Si(111) interface form a model system for the study of interactions between quantized electrons and adiabatically modulated boundaries. Tunnel spectra of this system reveal a previously unknown adiabatic shift of QW resonances due to lateral variations of the electronic reflection phase at the buried interface. With this effect, lateral distribution of the subsurface reflection phase can be probed, using scanning tunneling microscopy.

Novel interface-mediated metastable oxide phases: vanadium oxides on Pd(111)

In the growth process of ultrathin films of vanadium oxides on Pd(111), a sequence of novel oxide phases with layer-dependent structures and oscillating oxidation states has been detected experimentally and understood theoretically. These phases are interface mediated and metastable with respect to further oxide growth. Transformation into the stable oxide configuration occurs beyond a critical thickness, where energetics combined with kinetic limitations determine the oxide multilayer structure.