Session B12: Quantum Metal Films and Overlayer Structures

Growth of Pb nanowires on the Si(111)-In (4x1) a combined STM and SPALEED study

Due to the combined effect of QSE and the anisotropic strain potential of the substrate, Pb deposited on Si(111)--In (4x1) at 180K grows in nanowires of uniform 4-layer height and controllable uniform width of 5w$_{0}$\textbf{ (}where w$_{0}$=1.33nm is the width of the reconstruction unit cell along [1{\_}1{\_}2] direction). SPA-LEED studies confirm this selected 4-layer height (from Intensity vs K$_{z}$ variation) which is unusually stable because it is unchanged even after annealing to room temperature. The same selected 4-layer height is observed on a different interface Si(111)-In $\surd $31x$\surd $31 which independently confirms the unusual 4-layer stability. Differences in the observed corrugation on the tops of the nanowires due to the Moir\'{e} pattern at the metal/semiconductor interface confirm the strain anisotropy on the reconstructed substrate. Further Pb deposition on top of the nanowires results in the completion of the Pb layer with unusually atomically flat film interfaces over mesoscopic distances.   

Novel Phase Separation for Pb/Ge(111)

Using low energy electron microscopy (LEEM), we have elucidated the phase diagram for the growth of Pb on Ge(111). As Pb is deposited on Ge, the Pb atoms substitute into the top layer, causing released Ge atoms to form into c(2x8) adatom islands, with the size and density of these Ge islands controllable by the substrate temperature. During the reversible $\beta $ (dense ($\surd $3x$\surd $3)R30\r{ }) to (1x1) phase transition, we discovered a novel phase separation mechanism. Above the 1.33ML saturation coverage of the $\beta $ phase, a sharp first order phase transition is observed near 295C. For Pb coverage just $<$1.33ML, the phase transition is no longer sharp, with $\beta $ and (1x1) phases coexisting and the transformation occurring from 232C to 181C. Reducing coverage by $\sim $0.01ML causes a dramatic change, with small domains of the new phase appearing and disappearing, due to fluctuations between the two phases. Additional domains appear and fluctuate until the whole surface is completely transformed. We attribute the fluctuating domains to thermal fluctuations of the density of Pb atoms within a domain. By comparing LEEM images of the $\beta $ and (1x1) phases during the phase transition between [$\alpha $+(1x1)] and ($\alpha +\beta )$, the Pb coverage of the (1x1) phase at the eutectic point was determined to be $\sim $1.29 ML.   

Hard superconductivity of a soft metal in the quantum regime

Superconductivity is a collective quantum phenomenon that is inevitably suppressed in reduced dimensionality. Questions of how thin superconducting wires or films can be before losing their superconducting properties have important technological ramifications and go to the heart of understanding formation, coherence, and robustness of the superconducting state in quantum confined geometries. Here, we exploit quantum confinement of itinerant electrons in a soft metal (Pb), to stabilize atomically-flat superconductors with lateral dimensions of mm and vertical dimensions of only a few atomic layers. They show no indication of defect- or fluctuation- driven suppression of superconductivity and support macroscopic super-currents of up to $\sim $10{\%} of the depairing current density. The hardness of the critical state can be attributed to the presence of intrinsic vortex traps that are stabilized by quantum confinement. The study presents a conceptually appealing picture of a model nano-scale superconductor with calculable critical state properties, suggesting the possibility of achieving and exploiting superconductivity in the ultimate low-dimensional limit.   

Modification of the Quantum Electronic Stability of Thin Films by Interfactants

Electronic states are quantized in thin films, resulting in a modulation of physical properties with film thickness. The thermal stabilities of films differing in thickness by even a single monolayer can be dramatically different due to this quantization. The spectrum of allowed energy states depends on the film thickness, but it is also dependent on the phase shift of the wavefunctions reflected from the film-substrate interface. This phase shift in turn can be adjusted by changing the interface using interfactant atoms. This implies that the physical properties of thin films, including the thermal stability, could be controlled by interfacial engineering. We have grown atomically-uniform thin films of Pb on Si(111). Their thermal stabilities show bilayer oscillations with thickness due to the quantization of electronic states. The stabilities are strongly modified by the introduction of Au, In, or Pb at the film/substrate interface. For example, with In as an interfactant, films an odd number of monolayers thick are more stable than ones with an even number of layers, whereas for the other materials this pattern is reversed.   

Novel Coarsening Behavior of Pb nanocrystals on Si(111)

We show that Pb nanocrystals grown on Si(111)7x7 exhibit novel coarsening behavior that cannot be described by the classical Gibbs-Thomson effect. This system is known for quantum size effects (QSE) that lead to preferred island heights which depend on the coverage and temperature. Using complementary surface x-ray diffuse scattering and STM, we find an unexpected and unusual flux rate dependence, a lack of scaling of the island densities, and island decay times that are orders of magnitude faster than expected from the classical analysis. For example, a highly mono-disperse island height distribution is observed if the islands are grown at high rather than low flux rates. These results have important implications for understanding the controlled growth and self-organization of nanostructures.   

The formation of a sharp metal-semiconductor interface for the growth of Quantum Size Effect islands

In order to form Quantum Size Effect (QSE) metal islands on semiconductors, a smooth island-substrate interface is necessary to set up the electron standing waves that lead to the new quantum confined states. How this occurs for the Pb-Si(111)7x7 system is a mystery because of the large lattice mismatch and the highly corrugated 7x7 reconstruction. To understand how QSE islands develop in this system we have performed structural Surface X-ray scattering measurements on the initial formation of Pb islands grown on Si(111). We show how a smooth Pb-semiconductor interface develops through a series of structural arrangements. Once a vertically disordered Pb monolayer is completed, second layer atoms nucleate fcc clusters. These clusters undergo a displacive transition lifting them above the Si adatoms. This allow the Pb islands to ``float'' above the Si substrate so that the first island layer is smooth, thus setting up the proper boundary condition for QSE.   

The Pb corrugation on Si(111) Pb $\alpha -\surd $3x$\surd $3 as a probe of the island crystallography

Although the corrugation on top of the uniform height Pb islands has been studied by STS, quantitative information about the island morphology is still missing. With this complementary study based on SPA-LEED and STM we use the dependence of the corrugation on coverage to deduce the detailed island crystallographic structure. The two types of bilayer islands which have been identified from their opposing contrasts in STM are analyzed with diffraction and extended STM images to deduce their relative population. Islands with one type of FCC faulted stacking (i.e. ACB) are replaced with coverage by the islands with opposite stacking (i.e. ABC). Since the island shapes are triangular (due to inequivalent type of A- and B- steps) the population reversal is also confirmed from changes in the island orientation with coverage. In addition, a change in the rotation of the Pb overlayer relative to the Si substrate (measured from the location of the Pb(10)) is used to explain the changing intensity ``hexagon-like'' to ``star-like'' distribution of the SPA-LEED pattern near the (00) spot. This rotation explains quantitatively the changing corrugation period. As the coverage increases the preferred Pb orientation changes from 0$^{o}$ to 5.6$^{o}$ rotated with respect to the [110] substrate direction.   

Restructuring Due to Quantum Size Effects During Annealing in Ultrathin Films of Ag/Si(111)

Ultrathin films of silver have been epitaxially grown \textit{in situ }at low temperature and studied with scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED). Restructuring occurs during annealing to 300 K producing flat-topped islands two atomic layers in height in accordance with previous work.\footnote{Gavioli, et. al., \textit{Phys. Rev. Lett., }\textbf{82}(1) 1999, p. 129-132.} Further annealing produces a diverse distribution of heights and sizes of flat topped, vertical-sided islands. At higher coverage, a smooth films anneal to dentritic structures of a single height. A RHEED transmission pattern is formed after annealing the films and crystal orientation is determined. Finally, annealing above 550K produces typical 3D island ``wedding cake'' structures atop the $\sqrt{3} \times \sqrt{3}$ wetting layer on Si(111). These results will be discussed in an ``electronic growth'' model where Quantum Size Effects are thought to stabilize particular island heights, producing this unusual flat island growth.   

Kelvin Force Microscopy of $1 \times 1$ and $\sqrt 3 \times \sqrt 3$ phases of Pb/Si(111)

Using non-contact atomic force microscopy, we have determined the contact potential difference (CPD) of $1 \times 1$ and $\sqrt 3 \times \sqrt 3$ phases in the Pb/Si(111) system. Furthermore, we have tracked the barrier formation “layer-by-layer” by observing multilayer structures. We will discuss the origin of the observed CPD contrast between different phases and thicknesses of the Pb films in light of recent theoretical calculations. In addition, we will present the effect of the reduced dimensionality to the observed electrostatic potential variation at boundaries between different phases.   

Fluctuations of Pb/Si(111) Domain Boundaries

We have used variable-temperature scanning tunneling microscopy to study fluctuations of a 1D interface --- the boundary between two coexisting surface phases. We have prepared Pb/Si(111) surfaces that consist of high-density $(1 \times 1)$-Pb domains coexisting with a lower-density $\sqrt 3 \times \sqrt 3 -R30^{\circ}$-Pb phase. The domain boundaries between these phases fluctuate at moderate temperatures, allowing direct observation with STM. Measurement of the temporal correlation function for the fluctuating boundary between 525 K and 625 K allows determination of the mass transport mechanisms below and above the onset of thermal desorption of the $(1 \times 1)$ phase. In the desorption regime, our measured dynamical exponent of $1/2$ provides microscopic confirmation that fluctuations of the $(1 \times 1)$ boundaries occur via exchange of mass with a 2D adatom gas on the $\sqrt 3 \times \sqrt 3$ phase, consistent with the zeroth-order desorption kinetics inferred from macroscopic measurements.   

Influence of Band Structure and Pb/Si Interfacial Property on Transmission Resonance of Thin Pb Films by Scanning Tunneling Spectroscopy

The transmission spectrum of a metal film for free electrons at low energy may reveal resonance, which is the quantum size effect above the vacuum level. We use scanning tunneling spectroscopy to observe the transmission resonance for Pb films grown on incommensurate Pb/Si(111) and Si(111)7x7 surfaces. Our observations demonstrate that Pb band structure and Pb/Si interfaces significantly affect the signal of the transmission in the tunneling spectra. First, the transmission resonance is not detectable in the range of 5$\sim $6 eV above Fermi level, which can be attributed to that Pb band structure along the (111) direction in this energy range is flat. Secondly, the signal of the transmission resonance acquired on Pb films/incommensurate Pb/Si(111) is more obvious than that acquired on Pb films/Si(111)7x7. This difference can be attributed to that the Pb/Si interface for the former is adiabatic but is non-adiabatic for the latter.   

Thin Film Fractal Morphology and the Enhancement of Superconducting Critical Parameters

In superconducting films, it is known that the use of artificial defects can enhance a films' superconducting critical parameters. In particular, it has recently been reported that regular arrays of sub-micron sized holes produced by means of lithographic techniques can substantially increase the critical temperature for all fields. [1] We report here our observations that careful control of Pb film deposition conditions can result in film texture that has naturally occurring ``holes'' and enhanced critical parameters reminiscent of the artificially structured films. We characterize the texture of these films via their fractal dimension, and find that it is a useful approach for characterizing a films superconducting critical parameters. This work was funded by NSF and AFOSR. \newline \newline [1] A.V. Silhanek et al., PRB \textbf{72}, 014507 (2005)   

Binding site H3 to T4 occupation switching and the Pb/Si(111) ``Devil's Staircase'' phase diagram

With SPA-LEED and STM it has been observed that there is a switching occupation from only H3 sites to H3 and T4 sites within the unit cell of the DS (``Devil's Staircase'') (n,m) linear phases at the (1,1) phase or theta=1.25ML. This is observed from the doubling of the linear phase period and the ``flipping'' of the triangle diffraction pattern. The transition temperature from linear to HIC shows a minimum at $\sim $120K for the (1,1) phase and follows a U-shaped curved in the whole DS range 6/5ML$<$theta$<$4/3. This unusual dependence (instead of the monotonic decrease of the transition temperature expected for repulsive interactions) indicates the presence of other interactions in the system which can originate from the binding site switching. A statistical mechanical model that includes these two interactions is analyzed and accounts semi-quantitatively for the U-shaped curve and the phase diagram topology. However, a complete treatment should also include the comparison with the free energy of the HIC phases since the linear phases transform to these phases at higher temperatures. The binding site H3 to T4 switching is also relevant to theoretical predictions for the ordered phases in Ba(3x2) grown on stepped Si(111) due to the presence of similar long range interactions.   

Quantum Size Effects in $\delta $ -- Pu (111) and (110) Films*

First-principles full-potential linearized-augmented-plane-wave (FP-LAPW) calculations have been carried out for $\delta $-Pu (111) and (110) films up to seven layers. The layers have been studied at the non-spin-polarized-no-spin-orbit coupling (NSP-NSO), non-spin-polarized-spin-orbit coupling (NSP-SO), spin-polarized-no-spin-orbit coupling (SP-NSO), spin-polarized-spin-orbit coupling (SP-SO), anti-ferromagnetic-no-spin-orbit coupling (AFM-NSO), and anti-ferromagnetic-spin-orbit-coupling (AFM-SO) levels of theory. The ground state of both $\delta $-Pu (111) and (110) films is found to be at the AFM-SO level of theory and the surface energy is found to rapidly converge. The semi-infinite surface energy for $\delta $-Pu (111) and (110) films is predicted to be 1.18 and 1.42 J/m$^{2}$, while the magnetic moments show an oscillating behavior, gradually approaching the bulk value of zero with increase in the number of layers. Work functions indicate a strong quantum size effect up to and including five layers for the (111) surface and seven layers for the (110) surface, respectively. The work functions of $\delta $-Pu (111) and (110) films at the ground state are predicted be 3.41 and 2.99eV, respectively. *This work is supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Department of Energy (Grant DE-FG02-03ER15409) and the Welch Foundation, Houston, Texas (Grant Y-1525).   

Novel Atomic Rearrangement in the Pb Monolayer on Si(111) surfaces Induced by Atomic Hydrogen Adsorption.

Using a scanning tunneling microscopy, we have observed interesting hydrogen-adsorption induced atomic rearrangements on Pb/Si(111) system at room temperature. A hexagonal ring-like pattern with decaying intensity is formed around the hydrogen-induced point defect. Moreover, interference-like patterns can be seen in the region among the H-induced point defects. The detailed pattern depends on the relative position of defects. With certain relative positions, a new superstructure of hexagonal cells can be seen. The phase boundaries are found to either enhance or suppress the formation of the hexagonal ring-like pattern. We believe that the intricate interplay between atomic displacement and electronic structure causes the formation of the patterns. [Ref] : I. S. Hwang, S. H. Chang, C. K. Fang, L. J. Chen, and T. T. Tsong, Phys. Rev. Lett. 94, 045505 (2005)