Search

Protected and spin-polarized transport channels are the hallmark of topological insulators, coming along with an intrinsic strong spin-orbit coupling. Here we identified such corresponding chiral states in epitaxially grown zigzag graphene nanoribbons (zz-GNRs), albeit with an extremely weak spin-orbit interaction. While the bulk of the monolayer zz-GNR is fully suspended across a SiC facet, the lower edge merges into the SiC(0001) substrate and reveals a surface state at the Fermi energy, which is extended along the edge and splits in energy toward the bulk. All of the spectroscopic details are precisely described within a tight binding model incorporating a Haldane term and strain effects. The concomitant breaking of time-reversal symmetry without the application of external magnetic fields is supported by ballistic transport revealing a conduction of G = e2/h.

Even though there have been many experimental attempts and theoretical approaches to understand the process of electromigration (EM), it has not been quantitatively understood for ultrathin structures and at grain boundaries. Nevertheless, we showed recently that it can be used reliably for the formation of single atomic point contacts after careful pre-structuring of the initial Ag nanostructures. The process of formation of nanocontacts by EM down to a single-atom point contact was investigated for ultrathin (5 nm) Ag structures at 100 K by measuring the conductance as a function of the time during EM. In this paper, we compare the process of thinning by EM of structures with constrictions below the average grain size of Ag layers (15 nm) with that of structures with much larger initial constrictions of around 150 nm having multiple grains at the centre constriction prior to the formation of a point contact. Even though clear morphological differences exist between both types of structures, quantized conductance plateaus showing the formation of single point contacts have been observed for both. Here we put emphasis on the thinning process by EM, just before a point contact is formed. To understand this thinning process, the semi-classical regime before the contact reaches the quantum regime was analyzed in detail. For this purpose, we used experimental conductance histograms in the range between 2G0 and 15G0 and their corresponding Fourier transforms (FTs). The FT analysis of the conductance histograms exhibits a clear preference for thinning along the [100] direction. Using well-established models, both atom-by-atom steps and ranges of stability, presumably caused by electronic shell effects, can be discriminated. Although the directional motion of atoms during EM leads to specific properties such as the instabilities mentioned, similarities to mechanically opened contacts with respect to cross-sectional stability were found.

Proximity coupling is discussed as a promising approach to tune further graphene’s properties. Organic π-conjugated molecules couple per se weakly to surfaces and the class of phthalocyanines provides manifold possibilities for molecular functionalization. We have investigated the adsorption of lead phthalocyanine (PbPc) on epitaxial graphene by means of scanning tunneling microscopy and in-situ electronic transport measurements in order to quantify both a molecular charge transfer and modifications of the carrier mobility in graphene. Compared to other metallic surfaces, monolayer structures of PbPc adsorbed on graphene solely pointing upwards, apparently, due to the antibonding character of the Pb-states with the pz-orbitals of graphene. The squared lattice of the molecular layer is not commensurate with the honeycomb lattice of graphene and the PbPc molecules are slightly tilted, thus forming a chiral monolayer structure. Despite this interaction with graphene, the charge state of the molecule as well as the mobility of the charge carriers in graphene are not altered. Therefore, monolayer structures of Pb molecules are promising candidates for proximity studies in graphene.

Graphene nanoribbons (GNRs) are considered as major building blocks in future carbon-based electronics. The electronic performance of graphene nanostructures is essentially influenced and determined by their edge termination and their supporting substrate. In particular, semi-conducting, as well as metallic GNRs, can be fabricated by choosing the proper template which is favorable for device architecture designs. This study highlights the impact of microscopic details of the environment of the GNRs on the charge transport in GNRs. By means of lateral force, conductive atomic force and nanoprobe measurements, we explore the charge propagation in both zig-zag and armchair GNRs epitaxially grown on SiC templates. We directly image transport channels on the nanoscale and identify SiC substrate steps and nano-instabilities of SiC facets as dominant charge scattering centers. [Figure not available: see fulltext.].

The complex behavior of magnetoconductance of Bi films grown epitaxially on Si(111) with a thickness of 20--100 bilayers (BL) was measured at $T$= 9 K in magnetic fields up to $B=4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, oriented in-plane parallel and perpendicular to the electric dc current $I$. Contributions to magnetoconductance (MC) by diffuse scattering, by weak localization (WL) as well as by weak antilocalization (WAL) were identified. All these components to MC turned out to be isotropic in two dimensions, i.e., no dependence on angle between $B$ and $I$ within the surface plane was found. Only for $B\phantom{\rule{0.16em}{0ex}}\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}I$ an increase of MC was detected that is, to first approximation, $\ensuremath{\propto}{B}^{2}$. It is ascribed to ballistic scattering between the Rashba-split interfaces that allow Umklapp scattering without spin flip. While MC within the surface states, dominant at small thicknesses, $d$, shows negligible diffuse scattering under the chosen geometry, their quantum corrections are characterized by WAL with $\ensuremath{\alpha}=\ensuremath{-}0.3$ and a coupling strength that decays $\ensuremath{\propto}1/d$ with layer thickness. The admixing of quantized bulk states, which dominates MC above 50 BL, not only increases diffuse scattering, it introduces WL in combination with WAL. Presumably due to hybridization with the surface states, it also modifies strongly the WAL component for $dg60$ BL. Thus our findings suggest an intriguing interplay in magnetotransport between 2D and quantized 3D states at the Fermi surface of ultrathin bismuth quantum films and provide further deep insight into the electronic transport in quantized and partly spin split bands.

This article shows a new way to produce hierarchical microsieves by layering three types of float-cast microsieves, differing from each other in their pore diameters (approximately 68 μm, 7 μm, and 0.24 μm) on top of each other. The unsupported microsieves with 7 and 0.24 μm pore sizes are mechanically fragile. The complete hierarchical sieve composed of all three layers, however, can be handled manually without special precaution. This article further investigates the flow through the hierarchical sieve and filtration via experiment, theory (Hagen-Poiseuille's and Sampson-Roscoe's law), and simulation (numerically solving the Navier-Stokes equations for a predefined set of discrete volumetric elements). The experimental, theoretical, and simulated permeances of the microsieves and the hierarchical sieves are in reasonable agreement with each other and are significantly higher than the permeances of conventional filtration media. In filtration experiments, the hierarchical sieves show the expected sharp size cut-off, retaining particles of diameters exceeding the pore diameter.

Self-organized gold chains on vicinal Si(111) surfaces represent prototype examples of quasi-one-dimensional objects that are stabilized by hybridization with Si surface states. Their plasmons contain important information about the unoccupied bandstructure close to the Fermi level. Using Si(775)–Au as an example, we report here the modifications of the plasmon dispersion by the simple atomic adatom species H and O. Using a combination of low energy electron diffraction and high-resolution electron energy loss spectroscopy, we study the interconnection between plasmonic excitation and the corresponding local surface structure. Both adsorbates do not destroy metallicity, but, similar to Si(553)–Au, atomic hydrogen enhances dimerization of the Au chains, which at small concentrations counteracts the disorder introduced by random adsorption. This effect, most likely caused by electron donation of H to the surface states, is missing in case of adsorbed oxygen, so that only the effect of disorder is observed. For both adsorbates increasing disorder as a function of adsorbate concentration finally results in plasmon localization and opening of a band gap.

In bismuth quantum films, a fascinating interplay between (bulk) quantum well and interface (edge) states is observed in this comprehensive study of DC magnetoconductance and Hall resistivity as a function of film thickness. While for thin layers (up to 40 bilayers, BL) conductance is governed by the surface states, strong contributions of a quantum well state dominate above 70 BL. Quantum corrections in a magnetic field normal to the surface, however, are dominated by the interface states for all thicknesses investigated for reasons discussed in the paper.

The low-temperature spin-orbit density phase of Pb quantum wires on Si(557) was investigated by scanning tunneling microscopy. High resolution images of this phase allow the proposal of an atomistic model for the densely-packed Pb coverages on the (223) facets. A metal-insulator phase transition was confirmed by local spectroscopy and the BCS-like gap feature of $2\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}30\phantom{\rule{0.28em}{0ex}}\text{meV}$ at 10 K correlates with the transition temperature of ${T}_{c}=78\phantom{\rule{0.28em}{0ex}}\text{K}$ seen in surface transport experiments. Moreover, Pb excess coverage adds an extra row of Pb atoms on the mini-(111) terraces. These atoms induce a metallic state located at the step edge at low temperature and quench the charge and spin correlations of the spin-orbit density phase.

A thermal atomic layer deposition process with precursors tris(trimethylsilyl) phosphate (TTMSP), titanium tetrachloride (TiCl4), and water was used with various pulse sequences in order to deposit titanium phosphate onto bundles of carbon fibers (diameter of one filament = 7 μm, 6000 filaments per bundle) and flat silicon substrates. Pulse sequence 1, TTMSP/N2/TiCl4/N2, which comprises no water, yields no significant deposition. Pulse sequence 2, TTMSP/N2/H2O/N2/TiCl4/N2, which comprises a water pulse, yields a mixed phosphate/oxide coating and shows a self-limiting character at 200 °C with a growth per cycle of 0.22 nm cycle-1. Wet chemical analysis of the coating revealed a ratio of Ti:P between 3:1 and 2:1 in reasonable agreement with the composition Ti2.4P1O7 obtained from X-ray photoelectron spectroscopy. Thus, the deposited material can approximately be described as a mixture of Ti¾PO4 and TiO2 in a molar ratio of 1:1.5. The coating shifts the temperature of the onset of oxidation - 3% weight loss in thermogravimetry - of the carbon fibers from 630 °C (uncoated C-fiber) to 750 °C (with the titanium phosphate coating).

Graphene pn-junctions offer a rich portfolio of intriguing physical phenomena. They stand as the potential building blocks for a broad spectrum of future technologies, ranging from electronic lenses analogous to metamaterials in optics, to high-performance photodetectors important for a variety of optoelectronic applications. The production of graphene pn-junctions and their precise structuring at the nanoscale remains to be a challenge. In this work, a scalable method for fabricating periodic nanoarrays of graphene pn-junctions on a technologically viable semiconducting SiC substrate is introduced. Via H-intercalation, 1D confined armchair graphene nanoribbons are transformed into a single 2D graphene sheet rolling over 6H-SiC mesa structures. Due to the different surface terminations of the basal and vicinal SiC planes constituting the mesa structures, different types of charge carriers are locally induced into the graphene layer. Using angle-resolved photoelectron spectroscopy, the electronic band structure of the two graphene regions are selectively measured, finding two symmetrically doped phases with p-type being located on the basal planes and n-type on the facets. The results demonstrate that through a careful structuring of the substrate, combined with H-intercalation, integrated networks of graphene pn-junctions could be engineered at the nanoscale, paving the way for the realization of novel optoelectronic device concepts. Funding Agencies|Deutsche Forschungsgemeinschaft (DFG)German Research Foundation (DFG) [Sta315/9-1, Te386/13-1, Te386/12-1]; Projekt DEAL

The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics., Here, the authors investigate armchair graphene nanoribbons by angle-resolved photoelectron spectroscopy, and show the development of a width-dependent semiconducting gap driven by quantum confinement effects, and an ideal one-dimensional electronic behaviour.

The Si(553)-Au system resembles a heteroatomic chain ensemble with a delicate spin-charge interplay. The ordering of the $\ifmmode\times\else\texttimes\fi{}3$ reconstruction vanishes via a phase transition taking place at ${T}_{c}=100$ K. Our directional-dependent surface transport measurements showed that this order-disorder phase transition is not driven by the formation of a charge-density wave, as previously suggested. Instead, at 65 K there is a pronounced increase of the surface-state conductivity along the wires. We attribute this to activated charge transfer between the localized Si dangling bond states and the proximate Au bands revealing a $\ifmmode\times\else\texttimes\fi{}2$ periodicity. Apparently, a quasiorthogonality between the wave functions of the two proximal reconstructions is also responsible for a missing $\ifmmode\times\else\texttimes\fi{}6$ periodicity along the wires. The electronic charge transfer is in agreement with recent band-structure calculations.

High temperature-resistant fabrics can be used as a reinforcement structure in ceramic matrix composites. They often need a coating for oxidation protection and mechanical decoupling from the matrix. Atomic layer deposition (ALD) provides very thin conformal coatings even deep down into complex or porous structures and thus might be a suitable technique for this purpose. Carbon fiber fabrics (size 300 mm × 80 mm) and SiC fiber fabrics (size 400 mm × 80 mm) were coated using ALD with a multilayer system: a first layer made of 320 cycles of alumina (Al2O3) deposition, a second layer made of 142 cycles of titania-furfuryl alcohol hybrid (TiO2-FFA), and a third layer made of 360 cycles of titanium phosphate (TixPOy). Scanning electron microscopy reveals that the coatings are uniform and that the thickness of each layer is almost independent of the place in the reactor while coating. Appearance and thickness do not show any dependence on the type of fiber used as a substrate. Energy dispersive x-ray spectroscopy confirmed the expected elemental composition of each layer. Thermogravimetric analysis under oxidizing environment revealed that the first layer increases the onset temperature of fiber oxidation significantly, while the following two layers improve the oxidative protection only to a much smaller degree. Varying the geometry and size of the sample holder and especially the stacking of several fabric specimens on top of each other allowed increasing the total area of coated fabric up to 560 cm2 per batch. It was demonstrated that four-layered fiber coatings could be obtained with high uniformity even on these much more complicated geometries.

We studied the adsorption of atomic hydrogen on monolayer graphene and quasi free monolayer graphene, epitaxially grown on SiC(0001). By means of in-situ surface transport measurements, a metal-insulator transition was found on both n- and p-type doped two dimensional electron systems. The detailed analysis of the temperature dependent resistivity revealed that even ultra-low concentrations ( n H ≈ 10 12 cm − 2 ) of locally chemisorbed H-clusters act as effective scattering centers for the propagating electrons and limit the mean-free path L 0 ∝ 1 / n H . Despite the weak disorder due to adsorption, strong localization was found. The activation energy for destroying the phase coherence within the system is around 30 meV. Our analysis rules out the formation of a band insulator or even a”bad metal” due to adsorption of hydrogen.

Generally, it is supposed that the Fermi level in epitaxial graphene is controlled by two effects: p-type polarization doping induced by the bulk of the hexagonal silicon carbide (SiC)(0001) substrate and overcompensation by donor-like states related to the buffer layer. The presented work is evidence that this effect is also related to the specific underlying SiC terrace. Here a periodic sequence of non-identical SiC terraces is fabricated, which are unambiguously attributed to specific SiC surface terminations. A clear correlation between the SiC termination and the electronic graphene properties is experimentally observed and confirmed by various complementary surface-sensitive methods. This correlation is attributed to a proximity effect of the SiC termination-dependent polarization doping on the overlying graphene layer. These findings open a new approach for a nano-scale doping-engineering by the self-patterning of epitaxial graphene and other 2D layers on dielectric polar substrates. (Less)

Electronic properties of low dimensional structures on surfaces can be comprehensively explored by surface transport experiments. However, the surface sensitivity of this technique to atomic structures comes along with the control of bulk related electron paths and internal interfaces. Here we analyzed the role of Schottky-barriers and space charge layers for Si-surfaces. By means of a metal submonolayer coverage deposited on vicinal Si(1 1 1), we reliably accessed subsurface transport channels via angle- and temperature-dependent in situ transport measurements. In particular, high temperature treatments performed under ultra high vacuum conditions led to the formation of surface-near bulk defects, e.g. SiC-interstitials. Obviously, these defects act as p-type dopants and easily overcompensate lightly n-doped Si substrates.

One of the ways to use graphene in field effect transistors is to introduce a band gap by quantum confinement effect. That is why narrow graphene nanoribbons (GNRs) with width less than 50 nm are considered to be essential components in future graphene electronics. The growth of graphene on sidewalls of SiC(0001) mesa structures using scalable photolithography was shown to produce high quality GNRs with excellent transport properties. Such epitaxial graphene nanoribbons are very important in fundamental science but if GNRs are supposed to be used in advanced nanoelectronics, high quality thin (

The multiprobe technique based on scanning tunneling microcopy (STM) has become very popular over the past few years and has demonstrated its capability to reveal fundamental nanoscale charge transport properties . Compared to conventional, i.e. lithographically-driven concepts, the four-tip STM technique can directly contact delicate nanostructures with upmost precision.

Surfaces of insulating single crystals (like Si at and below room temperature) provide perfect templates for ordered arrays of atomic wires, which in many cases exhibit one-dimensional (1D) properties. The physics in these quasi-one-dimensional systems is quite intriguing and gives rise to the observation of new phenomena such as spin and charge order. Here we present a short overview, concentrating on Pb on Si(557) close to monolayer coverage, which a priori is an anisotropic 2D system, as shown, e.g., by magnetoconductance or by plasmonic excitations. At certain critical conditions, however, it turns into a 1D system, as most obvious from dc conductance and angular resolved photoemission. Conditions and stability of its occurrence will be discussed.

Epitaxial Bi(111) films were subject to many and partly even controversial studies on the semimetal-semiconductor transition triggered by a robust quantum confinement. The residual conductance was ascribed to conducting surface channels. We investigated ultrathin crystalline Bi films on Si(111) as a function of film thickness $d$ between 20 and 100 bilayers by means of electric transport measurements. Varying temperature and magnetic field, we disentangled two transport channels. One remains indeed metallic at all thicknesses investigated and exhibits a slightly increasing conductance as a function of $d$, whereas the second is activated with a ${d}^{\ensuremath{-}1}$ dependence of the activation energy, indicating a quasiharmonic confining potential. Both channels reflect the electronic properties of the entire film and do not allow us to strictly separate surface and bulk states. While there is clearly no bulk conductivity, the activated channel is consistently described as electronic excitation into the partly occupied quantum well states, which are also responsible for the metallic conductance and preferentially located close to both interfaces of the film.

Au-induced wire structures grown on Ge(100) were shown to exhibit spectroscopic signatures of strong electronic correlation. However, the atomistic structure of these wires is still controversially discussed. In this study the morphology has been investigated by means of low energy electron diffraction (SPA-LEED). The detailed analysis of diffraction profiles favors strongly to the model of an Au-induced giant missing row (GMR) structure rather than the heterodimer model. Systematic variation of the initial Au-coverage has revealed further that approximately 0.25 ML of the deposited Au coverage penetrates into subsurface positions stabilizing the adsorbate-induced surface structure.

Monolayer structures of Pb on Si(111) attracted recently considerable interest as superconductivity was found in these truly two-dimensional (2D) structures. In this study, we analyzed the electronic surface band structure of the so-called striped incommensurate Pb phase with $\frac{4}{3}$ ML coverage by means of spin-resolved photoemission spectroscopy. Our results fully agree with density functional theory calculations done by Ren et al. [Phys. Rev. B 94, 075436 (2016)]. We observe a local Zeeman-type splitting of a fully occupied and spin-polarized surface band at the ${\overline{\text{K}}}_{\sqrt{3}}$ points. The growth of this densely packed Pb structure results in the formation of imbalanced rotational domains, which triggered the detection of ${C}_{3v}$ symmetry forbidden spin components for surface states around the Fermi energy. Moreover, the Fermi surface of the metallic surface state of this phase is Rashba spin split and revealed a pronounced warping. However, the 2D nesting vectors are incommensurate with the atomic structure, thus keeping this system rather immune against charge density wave formation and possibly enabling a superconducting behavior.

Vicinal semi-metallic Bi-films are expected to reveal topologically protected edge states. In this study the growth of Bi-multilayer structures on Si(557) substrates has been investigated by low energy electron diffraction. Thereby, wetting layer structures formed prior to the film deposition on Si(557) surfaces turned out to be crucial for epitaxial growth. Only in the presence of Bi-wetting layers can well-ordered films be grown. In contrast to growth on Si(111), the pseudo-cubic surface of Bi(110) dominates. In addition, Bi(221) surfaces have been obtained only on wetting layers formed by less than a monolayer. The formation of Si(335)-facets during formation of the wetting layers turns out to be essential for the growth of the these structures.

This chapter will provide an overview of the properties of low-dimensional plasmons, discussing particularly characteristic examples. We will start with two-dimensional sheet plasmons (Sect. 19.1), concentrating on the plasmonic properties of the system most widely investigated in the recent past, graphene. Further emphasis will be given to low-dimensional plasmons coupled to other electron gases, which leads to linearization in the form of acoustic surface plasmons, but also to crossover of dimensionality, depending on plasmonic wavelengths. Finally we turn to quasi-one-dimensional systems and their corresponding plasmons, and try at the end to solve the puzzle of broad loss peaks but still fairly large plasmonic lifetimes.

We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultra-smooth defect- and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of polymer adsorbates which act as a carbon source. With most of the steps well below 0.75 nm pure monolayer graphene without bilayer inclusions is formed with lateral dimensions only limited by the size of the substrate. This makes the polymer assisted sublimation growth technique the most promising method for commercial wafer scale epitaxial graphene fabrication. The extraordinary electronic quality is evidenced by quantum resistance metrology at 4.2 K with until now unreached precision and high electron mobilities on mm scale devices., 20 pages, 6 Figures

In this paper, we give an example how domain walls (DW) within an adsorbed monolayer form one-dimensional localized electronic states. We investigated in detail the Ag 3 × 3 phase on Si (111), a prototype system for a low dimensional electron gas, by means of scanning tunneling microscopy. The doubling of the periodicity along the DW-direction at low temperatures (80 K) suggests a metal–insulator transition of Peierls type. The superimposed intensity modulation in the direction across the DW can be interpreted in terms of lateral quantum well states. However, the simultaneous vanishing of both features at room temperature indicates more complicated changes in band structure and/or wave functions during the transition than described by the simple Peierls model.

For ultra-thin films, electronic transport properties at surfaces and interfaces are dominated by electronic scattering at the interfaces. Therefore, as our example with Pb on Si(557) shows, d.c. transport is sensitive to confinement, to growth modes, and to (electronic) surface roughness. During the growth of Pb on Si(557) up to more than 10 monolayers, we found classical and quantum size effects, a pronounced conductance anisotropy and conductance oscillations directly related to the anisotropic layer-by-layer growth mode of the Pb film on this regularly stepped surface. In fact, it turned out that the interface anisotropy, which extends up to the seventh layer in conductance, is kept as a memory effect even for thick (isotropic) layers. By adding a magnetic field, details of the scattering mechanism at impurities and defects are revealed, since elastic, inelastic, spin–orbit scattering contributions can be separated. Strong changes of scattering properties are evident when going from the multilayers to the monolayer, as demonstrated again for the system Pb on Si(557). Contrary to multilayers, spin–orbit scattering dominates for the monolayer, presumably due to the symmetry reduction at the surface and the appearance of spin-split bands split by the Rashba effect. Also the anomaly in spin–orbit scattering close to monolayer completion, coupled with quasi one-dimensional d.c. conductance, can be ascribed to the Rashba effect. The intriguing interplay between bulk and surface conductance with large Rashba split states at the surface is exemplified by a study of multilayer growth of Bi on Si(111). By magnetoconductance we were able to separate bulk from surface contributions. Scattering between strongly spin-polarized Rashba-split states spin can be effectively suppressed, so that only the “classical” magnetoconductance effect remains, as observed in this system.

The low-energy plasmonic excitations of the Ge(0 0 1)-Au close to one monolayer coverage of Au were investigated by momentum-resolved high resolution electron energy loss spectroscopy. A very weak plasmonic loss was identified dispersing along the chain direction of the [Formula: see text] formed at these Au coverages. The measured dispersion was compared with the Tomonaga-Luttinger-liquid (TLL) model and with a model for an anisotropic Fermi liquid. Using the TLL model both for single and arrays of wires, no consistent picture turned up that could describe all available data. On the contrary, a quasi-one-dimensional model of a confined 2D electron gas gave a satisfactorily consistent description of the data. From these results for the collective low-energy excitations we conclude that the Ge(0 0 1)-Au system is reasonably well described by a strongly anisotropic 2D Fermi liquid, but is incompatible with a TLL.

The adsorption of ferrocene-dithiol on Si(111)– Ag 3 × 3 reconstruction has been studied by scanning tunneling microscopy. Preferred chemisorption was found at defect sites, e.g. grain boundaries, with characteristic distortions of the two dimensional electron gas. Occasionally, adsorption also occurs at perfect terrace sites, where submolecular resolution was achieved and the adsorption geometry was deduced. Due to the formation of Ag–S bonds, the conformational flexibility of the free ferrocene is restricted. Thus, the frozen rotation leaves only two different rotational configurations of ferrocene.

The structure on the atomic scale was found to be directly correlated with the electronic and transport properties of ultra-thin metallic wires. We present examples of both self-organized and artificially structured nanowires in the system Pb/Si(557). We demonstrate that artificial structuring on the nanoscale allows identification and determination of the electrical transport properties of atomic size defects, using Si(557) as a quasi-insulating substrate and a Si double step in a monolayer high Pb nanowire as an example. One-dimensional (1D) properties with strong electron correlation and various instabilities in metallic chains or ribbons generated by metal adsorption of sub-monolayers on insulating substrates like Si(111) or Si(557) turn out to always be modified by the unavoidable coupling to 2D and 3D. The wealth of new and partly unexpected phenomena is exemplified here again in monolayers of Pb on Si(557), which forms wire-like arrays. We identify structural self-stabilization as the origin of 1D electronic transport and show that a new 1D state is generated by step decoration. Magnetoconductance measurements yield even deeper insight into mechanisms of electronic transport, as also exemplified in this specific system.

As shown previously, Pb on vicinal Si(5 5 7) refacets the surface into a (2 2 3) facet orientation at a Pb coverage of 1.31 ML. This facet formation is electronically stabilized by Fermi nesting and leads to one-dimensional conductance. Electronic correlation seems to be responsible also for the periodic arrangement of atomic Pb chains which decorate the step edges at concentrations exceeding 1.31 ML, up to a concentration of 1.5 ML. Instead of random step decoration, periodicities up to six (2 2 3)-terrace widths (28 lattice constants, 93 A) have been found. These depend inversely on excess Pb concentration and end at a concentration of 1.52 ML when all steps are decorated with a line density equal to the Si density at steps. These one-dimensional periodicities can be explained assuming that split-off states from surface bands are completely filled by two electrons per Pb atom with corresponding gap opening. This behavior is reminiscent of the formation of charge density waves with tunable wavelengths as a function of excess Pb concentration, and indicates strong electron correlation in this strongly anisotropic 2d system. The alternative, simple band filling within a rigid band model is expected to destabilize the (2 2 3) facet structure upon further adsorption of Pb, which has not been observed.

Ultrathin metallic silicide nanowires with extremely high aspect ratios can be easily grown, e.g., by deposition of rare earth elements on semiconducting surfaces. These wires play a pivotal role in fundamental research and open intriguing perspectives for CMOS applications. However, the electronic properties of these one-dimensional systems are extremely sensitive to atomic-sized defects, which easily alter the transport characteristics. In this study, we characterized comprehensively TbSi2 wires grown on Si(100) and correlated details of the atomic structure with their electrical resistivities. Scanning tunneling microscopy (STM) as well as all transport experiments were performed in situ using a four-tip STM system. The measurements are complemented by local spectroscopy and density functional theory revealing that the silicide wires are electronically decoupled from the Si template. On the basis of a quasiclassical transport model, the size effect found for the resistivity is quantitatively explained in terms of bulk and surface transport channels considering details of atomic-scale roughness. Regarding future applications the full wealth of these robust nanostructures will emerge only if wires with truly atomically sharp interfaces can be reliably grown. © 2016 American Physical Society. DFG/FOR/1700

We have realized nanometer size constrictions in ballistic graphene nanoribbons grown on sidewalls of SiC mesa structures. The high quality of our devices allows the observation of a number of electronic quantum interference phenomena. The transmissions of Fabry-Perot like resonances were probed by in-situ transport measurements at various temperatures. The energies of the resonances are determined by the size of the constrictions which can be controlled precisely using STM lithography. The temperature and size dependence of the measured conductances are in quantitative agreement with tight-binding calculations. The fact that these interference effects are visible even at room temperature makes the reported devices attractive as building blocks for future carbon based electronics., Comment: to appear in Phys. Rev. Lett

The collective electronic excitations of arrays of Au chains on regularly stepped Si(553) and Si(775) surfaces were studied using electron loss spectroscopy with simultaneous high energy and momentum resolution (ELS-LEED) in combination with low energy electron diffraction (SPA-LEED) and tunneling microscopy. Both surfaces contain a double chain of gold atoms per terrace. Although one-dimensional metallicity and plasmon dispersion is observed only along the wires, two-dimensional effects are important, since plasmon dispersion explicitly depends both on the structural motif of the wires and the terrace width. The electron density on each terrace turns out to be modulated, as seen by tunneling spectroscopy (STS). The effective wire width of 7.5\,\AA\ for Si(553)-Au -- 10.2\,\AA\ for Si(775)-Au -- , determined from plasmon dispersion is in good agreement with STS data. Clear evidence for coupling between wires is seen beyond nearest neighbor coupling., Comment: 5 pages, 4 figures

Four-point conductance measurements of a high temperature annealed monolayer of Pb on Si(557) are combined with tunneling microscopy (STM) and LEED for structural investigations. We found extremely high surface state conductance which becomes quasi-one-dimensional below a critical temperature of T C = 78 K. The change from low to extremely high conductance anisotropy is associated with a reversible order-disorder phase transition with a temperature dependence α1/T+ const. along the chains below the phase transition. Below the phase transition order is found simultaneously in the lateral separation between Pb chains and along the chains. There a 10-fold superperiodicity appears. We suggest that strong two-dimensional coupling, leading to electronic stabilization of terraces with a width of 42 3 lattice constants, results in perfect nesting normal to the chains at E F (below T c ), which is the origin of this switching to one-dimensional behavior. Therefore, no metal-insulator transition is expected at low temperature.

We performed scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) experiments for Dy adsorbed on Mo(1 1 2) in the monolayer regime in order to clarify the concentration dependent reordering of the surface glass that exists for coverages above 0.58 of a monolayer (ML) after annealing to temperatures higher than 400 K. The partial reaction model developed earlier is corroborated. The Dy defect structure formed initially in Dy–Mo surface alloy acts as nucleation sites for Dy so that clusters with a wide distribution of lateral distances are formed, as found in particular at a coverage of 0.28 ML. The change in bonding character at coverages above 0.58 ML leads to reordering of the defects and the concentration dependent modulation of the adsorbed Dy layers. Examples at coverages of 0.7, 0.9 and 1.15 ML are shown and compared.

The unoccupied electronic structure of quasi-one-dimensional reconstructions of Pb atoms on a Si(557) surface is investigated by means of femtosecond time- and angle-resolved two-photon photoemission. Two distinct unoccupied electronic states are observed at E-EF=3.55 and 3.30 eV, respectively. Density functional theory calculations reveal that these states are spatially located predominantly on the lead wires and that they are energetically degenerated with an energy window of reduced electronic density of states in Si. We further find momentum-averaged lifetimes of 24 and 35 fs of these two states, respectively. The photoemission yield and the population dynamics depend on the electron momentum component perpendicular to the steps of the Si substrate, and the momentum-dependent dynamics cannot be described by means of rate equations. We conclude that momentum- and direction-dependent dephasing of the electronic excitations, likely caused by elastic scattering at the step edges on the vicinal surface, modifies the excited-state population dynamics in this system. © 2015 American Physical Society. DFG/FOR/1700

The In/Si(111) system reveals an anisotropy in the electrical conductivity and is a prototype system for atomic wires on surfaces. We use this system to study and tune the interwire interaction by adsorption of oxygen. Through rotational square four-tip transport measurements, both the parallel $({\ensuremath{\sigma}}_{||})$ and perpendicular $({\ensuremath{\sigma}}_{\ensuremath{\perp}})$ components are measured separately. The analysis of the I(V) curves reveals that ${\ensuremath{\sigma}}_{\ensuremath{\perp}}$ is also affected by adsorption of oxygen, showing clearly an effective interwire coupling, in agreement with density-functional-theory-based calculations of the transmittance. In addition to these surface-state mediated transport channels, we confirm the existence of conducting parasitic space-charge layer channels and address the importance of substrate steps by performing the transport measurements of In phases grown on Si(111) mesa structures.

The electrical conductivity of solid-state matter is a fundamental physical property and can be precisely derived from the resistance measured via the four-point probe technique excluding contributions from parasitic contact resistances. Over time, this method has become an interdisciplinary characterization tool in materials science, semiconductor industries, geology, physics, etc, and is employed for both fundamental and application-driven research. However, the correct derivation of the conductivity is a demanding task which faces several difficulties, e.g. the homogeneity of the sample or the isotropy of the phases. In addition, these sample-specific characteristics are intimately related to technical constraints such as the probe geometry and size of the sample. In particular, the latter is of importance for nanostructures which can now be probed technically on very small length scales. On the occasion of the 100th anniversary of the four-point probe technique, introduced by Frank Wenner, in this review we revisit and discuss various correction factors which are mandatory for an accurate derivation of the resistivity from the measured resistance. Among others, sample thickness, dimensionality, anisotropy, and the relative size and geometry of the sample with respect to the contact assembly are considered. We are also able to derive the correction factors for 2D anisotropic systems on circular finite areas with variable probe spacings. All these aspects are illustrated by state-of-the-art experiments carried out using a four-tip STM/SEM system. We are aware that this review article can only cover some of the most important topics. Regarding further aspects, e.g. technical realizations, the influence of inhomogeneities or different transport regimes, etc, we refer to other review articles in this field. DFG/FOR1700 DFG/Te 386/9-1

The electrical transport properties of epitaxial graphene layers are correlated with the SiC surface morphology. In this study we show by atomic force microscopy and Raman measurements that the surface morphology and the structure of the epitaxial graphene layers change significantly when different pretreatment procedures are applied to nearly on-axis 6H-SiC(0001) substrates. It turns out that the often used hydrogen etching of the substrate is responsible for undesirable high macro steps evolving during graphene growth. A more advantageous type of sub-nanometer stepped graphene layers is obtained with a new method: a high-temperature conditioning of the SiC surface in argon atmosphere. The results can be explained by the observed graphene buffer layer domains after the conditioning process which suppress giant step bunching and graphene step flow growth. The superior electronic quality is demonstrated by a less extrinsic resistance anisotropy obtained in nano-probe transport experiments and by the excellent quantization of the Hall resistance in low-temperature magneto-transport measurements. The quantum Hall resistance agrees with the nominal value (half of the von Klitzing constant) within a standard deviation of 4.5*10(-9) which qualifies this method for the fabrication of electrical quantum standards., Comment: This is an author-created, un-copyedited version of an article accepted for publication in J. Phys.: Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it

Adsorption of simple organic molecules on wide band gap insulators is a versatile tool to introduce unoccupied electronic states in the band gap at the surface. Relative energetic positions, varied also by intra-molecular interactions of functional side groups, determine the effective band gap. Specific examples used here are adsorbed molecules of benzoic acid and of its OH-substituted derivates (salicylic acid, etc.) on NaCl, KCl and on MgSO4 · H2O. These acids are mainly physisorbed on NaCl and KCl, whereas on MgSO4 · H2O chemical bond formation is found. Ab initio and semi-empirical calculations are used in order to identify mechanisms of surface-molecule as well as intra-molecular interactions.

We present data of a study of four-point conductance of adsorbed Pb films on Si(557) in the thickness range between 0.6 up to several monolayers (ML) at various annealing stages. These measurements are combined with tunneling microscopy (STM). Onset of conductance is found close to the percolation limit. Pb layers annealed to room temperature are characterized by activated contributions to conductance up to 3 ML, a purely metallic temperature dependence at thicker layers, and an anisotropy of at most a factor of 2. On the contrary, annealing to 640 K, leaving only the first monolayer on the Si(557) surface results in extremely high surface state conductance which is quasi one–dimensional below a critical temperature of Tc=78 K, associated with an order–disorder phase transition. Induced by a 10-fold superperiodicity along the Pb chains and their lateral ordering, the system switches from low to high conductance anisotropy, with a metal–insulator transition in the direction perpendicular to the chain structure, while in the direction along the chains conductance with a 1/T + const. temperature dependence was found.