Scanning Tunneling Microscopy Experiments Research Articles

Direct observation of single-electron solitons and Friedel oscillations in a quasi-one dimensional material with incommensurate charge-density waves

We have performed scanning tunneling microscopy experiments in the quasi-one dimensional charge density wave (CDW) system NbSe3, where we could image and study in detail individual solitons corresponding to the self-trapping of a single electron. Our analysis shows that the type of soliton we observed is an “Amplitude Soliton”, characterized by a vanishing CDW amplitude at the soliton center and by a π-shift of its phase along the chain direction. Pairs of solitons or multiple solitons were also observed. Such observations could be made only in the high-temperature CDW phase. Additionally, one-dimensional Friedel oscillations around charged defects could also be imaged and are found to be superimposed to the CDW. The distinction between amplitude solitons and Friedel oscillations can be made without any ambiguity.

Pseudogap in cuprates in the loop-current ordered state

Scanning tunneling microscopy (STM) has revealed that the magnitude of the pseudo-gap in under-doped cuprates varies spatially and is correlated with disorder. The loop-current order, characterized by the anapole vector Ω, discovered in under-doped cuprates occurs in the same region of the temperature and doping as the pseudo gap observed in STM and ARPES experiments. Since translational symmetry remains unchanged in the pure limit, no gap occurs at the chemical potential. On the other hand for disorder coupling linearly to the different possible orientations of Ω, there can only be a finite temperature dependent static correlation length for the loop-current state at any temperature. This leads to formation of domains of the ordered state with different orientation and magnitude of Ω in each. For the characteristic size of the domains much larger than the Fermi-vectors , the boundary of the domains leads to forward scattering of the Fermions. Such forward scattering is shown to push states near the chemical potential to energies both above and below it leading to a pseudo-gap with an angular dependence which is maximum in the directions because the single-particle energies are degenerate in these directions for all domains. The magnitude of the average gap systematically increases with the square of the average loop order parameter measured by polarized neutron scattering. This result is tested. A unique result of the gap due to forward scattering is the lack of a bump in the density of states at the ‘edge’ of the pseudo-gap so that the depletion of states near the chemical potential is recovered only in integration up to the edge of the band. This is also in agreement with a variety of experiments. Some predictions for further experiments are provided. Due to the finite correlation length, low frequency excitations are expected at long wavelength at all temperatures in the ‘ordered’ phase. Such fluctuations motionally average over the shifts in frequencies of local probes such as NMR and muon resonance expected for a truly static order.

Tuning topological superconductivity in helical Shiba chains by supercurrent

Recent experimental investigations of arrays of magnetic atoms deposited on top of a superconductor have opened a new chapter in the search for topological superconductivity. We generalize the microscopic model derived by Pientka et al. [Phys. Rev. B 88, 155420 (2013)] to accommodate the effects of finite supercurrent in the host material. Previously it was discovered that helical chains with nonplanar textures are plagued by a gapless phase. We show that by employing supercurrent it is possible to tune the chain from the gapless phase to the topological gapped phase. It is also possible to tune the chain between the trivial and the topological gapped phase, the size of which may be dramatically increased due to supercurrent. For planar textures supercurrent mainly contributes to proliferation of the gapless phase. Our predictions, which could be probed in scanning tunneling microscope experiments, are encouraging for the observation and manipulation of Majorana states.

Detection of small exchange fields in S/F structures

Ferromagnetic materials with exchange fields Eex smaller or of the order of the superconducting gap Δ are important for applications of corresponding (s-wave) superconductor/ferromagnet/superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small Eex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal–ferromagnet–insulator–superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=Eex. Thus measuring the subgap conductance one can reliably determine small Eex<Δ. In the opposite case Eex>Δ one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=Eex. This peak can be only visible for small enough exchange fields of the order of few Δ.

Exceptionally stiff two-dimensional molecular crystal by substrate-confinement.

We demonstrated an approach to effectively apply in-plane pressures to molecular layers by utilizing the substrate confinement effect. The compressed crystal structure and mechanical behaviors of carbon monoxide (CO) monolayer subjected to the confinement of Cu(100) substrate were jointly investigated by low temperature scanning tunneling microscopy experiments and first-principles density functional theory calculations. By increasing molecular coverage, an exceptionally large Young's modulus of 33 GPa was derived for the constrained CO monolayer film. This extreme in-plane pressure leads to site-specific tilting geometries, polymeric-like electronic states, and vibrational behaviors of CO molecules in the compressed phases. These results provide an extended understanding of the physical and chemical properties of intermolecular interactions in this fundamental system.

Contrast stability and ‘stripe’ formation in scanning tunnelling microscopy imaging of highly oriented pyrolytic graphite: the role of STM-tip orientations

Highly oriented pyrolytic graphite (HOPG) is an important substrate in many technological applications and is routinely used as a standard in Scanning Tunnelling Microscopy (STM) calibration, which makes the accurate interpretation of the HOPG STM contrast of great fundamental and applicative importance. We demonstrate by STM simulations based on electronic structure obtained from first principles that the relative local orientation of the STM-tip apex with respect to the HOPG substrate has a considerable effect on the HOPG STM contrast. Importantly for experimental STM analysis of HOPG, the simulations indicate that local tip-rotations maintaining a major contribution of the tip-apex state to the STM current affect only the secondary features of the HOPG STM contrast resulting in ‘stripe’ formation and leaving the primary contrast unaltered. Conversely, tip-rotations leading to enhanced contributions from m ≠ 0 tip-apex electronic states can cause a triangular-hexagonal change in the primary contrast. We also report a comparison of two STM simulation models with experiments in terms of bias-voltage-dependent STM topography brightness correlations and discuss our findings for the HOPG(0 0 0 1) surface in combination with tungsten tip models of different sharpnesses and terminations.

Static and dynamic buckling of reconstructions at triple steps on Si(111) surfaces

Triple steps on Si(111) surfaces are popular building blocks for bottom-up nanostructure assembly, conferring size uniformity and precise positioning of growing nanostructures. In this work, we employ the Si(7 7 10) regular stepped surface as model system to study the triple steps by scanning tunneling microscopy (STM) and large-scale first-principles calculations. We find a surprising cohabitation of reconstruction elements at the step edge that either buckles statically or dynamically at room temperature. The driving force for the observed sequence of buckling patterns is traced back to Coulomb interactions involving charged adatoms and rest-atoms lying on a mini-terrace. These results reconcile the Si(111) triple step model with the experimental STM data.

Atom-scale surface reactivity mediated by long-ranged equilibrium charge transfer

The observation of reaction outcomes at the bulk scale provides an average view of chemical processes, obscuring potentially significant differences in the behavior of matter at the atom scale. Through a series of atom-resolved scanning tunneling microscopy experiments, the inhomogeneous reactivity of silicon surface dangling bond states is revealed. The differences in reactivity provide evidence for the coexistence of neutral and negative surface states on these surfaces. It is shown that reactivity can be modulated through the density of surface states and by the bulk dopant level. These findings demonstrate that site-specific surface reactivity at the atom scale can be modulated by nonlocal charge allocation and provide opportunities for controlling chemistry at this scale.

RKKY interaction and intervalley processes inp-doped transition-metal dichalcogenides

We study the Ruderman-Kittel-Kasuya-Yosida interaction in $p$-doped transition-metal dichalcogenides such as ${\mathrm{MoS}}_{2}$ and ${\mathrm{WS}}_{2}$. We consider magnetic impurities hybridized to the Mo $d$ orbitals characteristic of the valence bands. Using the Matsubara Green's-function formalism, we obtain the two-impurity interaction vs their separation and chemical potential of the system, accounting for the important angular dependence which reflects the underlying triangular lattice symmetry. The inclusion of the valence-band valley at the $\ensuremath{\Gamma}$ point results in a strong enhancement of the interaction. Electron-scattering processes transferring momentum between valleys at different symmetry points give rise to complex spatial oscillation patterns. Variable doping would allow the exploration of rather interesting behavior in the interaction of magnetic impurities on the surfaces of these materials, including the control of the interaction symmetry, which can be directly probed in scanning tunneling microscopy experiments.

First-principles analysis of a homochiral cycloidal magnetic structure in a monolayer Cr on W(110)

The magnetic structure of a Cr monolayer on a W(110) substrate is investigated by means of first-principles calculations based on noncollinear spin density functional theory (DFT). As magnetic ground state we find a long-period homochiral left-rotating spin spiral on top of an atomic-scale antiferromagnetic order of nearest-neighbor atoms. The rotation angle of the magnetic moment changes inhomogeneously from atom to atom across the spiral. We predict a propagation direction along the crystallographic $[001]$ direction with a period length of $|\ensuremath{\lambda}|=14.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, which is in excellent agreement with a modulation of the local antiferromagnetic contrast observed in spin-polarized scanning tunneling microscope experiments by Santos et al. [New J. Phys. 10, 013005 (2008)]. We identify the Dzyaloshinskii-Moriya interaction as the origin of the homochiral magnetic structure, competing with the Heisenberg-type exchange interaction and magnetocrystalline anisotropy energy. From DFT calculations we extract parameters for a micromagnetic model and thereby determine a considerable inhomogeneity of the spin spiral, increasing the period length by 6% compared to homogeneous spin spirals. The results are compared to the behavior of a Mn and Fe monolayer and Fe double layer on a W(110) substrate.

Impurity Effect of d-wave High-Tc Superconductors in the Presence of the Charge Modulation

Based on a phenomenological model, we study theoretically the impurity effect in d-wave high-Tc superconductors in the presence of the charge-density-wave (CDW) order. Our results verify that the intensity of the resonance state induced by the impurity depends on the CDW scattering strength. It first increases and then decreases as the CDW scattering strength increases. Moreover, the impurity state exist only near the impurity site without the CDW modulation, while it would appear at quite far away from the impurity site in presence of the CDW modulation. The above results may be tested by later scanning tunneling microscopy experiments.

Scanning tunneling microscopy study of Fe, Co and Cr growth on Re(0001)

Atomically flat terraces of the Re(0001) surface with a contaminant density below 0.5% have been obtained by oxygen annealing followed by a flash to higher temperature. This Re(0001) single crystal has been used as a substrate for the deposition of Fe, Co and Cr atoms. Scanning Tunneling Microscopy experiments characterize the growth mode for the submonolayer coverage regime. Co, Cr and Fe atoms self-assemble to form monolayer high islands. Despite a large lattice mismatch between film and substrate, Co and Fe grow pseudomorphically up to half a monolayer. Cr islands are pseudomorphic only for a size below 10nm. Higher coverage leads to reconstructed islands with an element-dependent reconstruction pattern. Scanning Tunneling Spectroscopy measurements at 8K reveal the electronic properties of Fe and Re. Differential conductance measurements on the Re(0001) show the presence of standing waves, possibly due to a rhenium surface state. Atomic resolution images of Fe attached to a Re step edge lead to the conclusion that the Fe atoms occupy hcp hollow sites. A Néel magnetic ordered state of the Fe hcp monolayer is revealed with Spin-Polarized Scanning Tunneling Microscopy and Magnetic Atom Manipulation Imaging.

Understanding the Kondo resonance in the d-CoPc/Au(111) adsorption system.

By combining the density functional theory (DFT) and a hierarchical equations of motion (HEOM) approach, we investigate the Kondo phenomena in a composite system consisting of a dehydrogenated cobalt phthalocyanine molecule (d-CoPc) adsorbed on an Au(111) surface. DFT calculations are performed to determine the ground-state geometric and electronic structures of the adsorption system. It is found that the singly occupied dz(2) orbital of Co forms a localized spin, which could be screened by the substrate conduction electrons. This screening leads to the prominent Kondo features as observed in the scanning tunneling microscopy experiments. We then employ the HEOM approach to characterize the Kondo correlations of the adsorption system. The calculated temperature-dependent differential conductance spectra and the predicted Kondo temperature agree well with the experiments, and the universal Kondo scaling behavior is correctly reproduced. This work thus provides important insights into the relevant experiments, and it also highlights the applicability of the combined DFT+HEOM approach to the studies of strongly correlated condensed matter systems.

Vernier-templated synthesis, crystal structure, and supramolecular chemistry of a 12-porphyrin nanoring.

Vernier templating exploits a mismatch between the number of binding sites in a template and a reactant to direct the formation of a product that is large enough to bind several template units. Here, we present a detailed study of the Vernier-templated synthesis of a 12-porphyrin nanoring. NMR and small-angle X-ray scattering (SAXS) analyses show that Vernier complexes are formed as intermediates in the cyclo-oligomerization reaction. UV/Vis/NIR titrations show that the three-component assembly of the 12-porphyrin nanoring figure-of-eight template complex displays high allosteric cooperativity and chelate cooperativity. This nanoring–template 1:2 complex is among the largest synthetic molecules to have been characterized by single-crystal analysis. It crystallizes as a racemate, with an angle of 27° between the planes of the two template units. The crystal structure reveals many unexpected intramolecular C–H⋅⋅⋅N contacts involving the tert-butyl side chains. Scanning tunneling microscopy (STM) experiments show that molecules of the 12-porphyrin template complex can remain intact on the gold surface, although the majority of the material unfolds into the free nanoring during electrospray deposition.

One-dimensional topological edge states of bismuth bilayers

The hallmark of a topologically insulating state of matter in two dimensions protected by time-reversal symmetry is the existence of chiral edge modes propagating along the perimeter of the sample. Among the first systems predicted to be a two-dimensional topological insulator are bilayers of bismuth. Here we report scanning tunnelling microscopy experiments on bulk Bi crystals that show that a subset of the predicted Bi-bilayers’ edge states are decoupled from the states of the substrate and provide direct spectroscopic evidence of their one-dimensional nature. Moreover, by visualizing the quantum interference of edge-mode quasi-particles in confined geometries, we demonstrate their remarkable coherent propagation along the edge with scattering properties consistent with strong suppression of backscattering as predicted for the propagating topological edge states.

Visualization of electron orbitals in scanning tunneling microscopy

Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated.

The ReactorSTM: atomically resolved scanning tunneling microscopy under high-pressure, high-temperature catalytic reaction conditions.

To enable atomic-scale observations of model catalysts under conditions approaching those used by the chemical industry, we have developed a second generation, high-pressure, high-temperature scanning tunneling microscope (STM): the ReactorSTM. It consists of a compact STM scanner, of which the tip extends into a 0.5 ml reactor flow-cell, that is housed in a ultra-high vacuum (UHV) system. The STM can be operated from UHV to 6 bars and from room temperature up to 600 K. A gas mixing and analysis system optimized for fast response times allows us to directly correlate the surface structure observed by STM with reactivity measurements from a mass spectrometer. The in situ STM experiments can be combined with ex situ UHV sample preparation and analysis techniques, including ion bombardment, thin film deposition, low-energy electron diffraction and x-ray photoelectron spectroscopy. The performance of the instrument is demonstrated by atomically resolved images of Au(111) and atom-row resolution on Pt(110), both under high-pressure and high-temperature conditions.

Topological dynamics in supramolecular rotors.

Artificial molecular switches, rotors, and machines are set to establish design rules and applications beyond their biological counterparts. Herein we exemplify the role of noncovalent interactions and transient rearrangements in the complex behavior of supramolecular rotors caged in a 2D metal-organic coordination network. Combined scanning tunneling microscopy experiments and molecular dynamics modeling of a supramolecular rotor with respective rotation rates matching with 0.2 kcal mol(-1) (9 meV) precision, identify key steps in collective rotation events and reconfigurations. We notably reveal that stereoisomerization of the chiral trimeric units entails topological isomerization whereas rotation occurs in a topology conserving, two-step asynchronous process. In supramolecular constructs, distinct displacements of subunits occur inducing a markedly lower rotation barrier as compared to synchronous mechanisms of rigid rotors. Moreover, the chemical environment can be instructed to control the system dynamics. Our observations allow for a definition of mechanical cooperativity based on a significant reduction of free energy barriers in supramolecules compared to rigid molecules.

Sulfur dimers adsorbed on Au(111) as building blocks for sulfur octomers formation: A density functional study.

Experimental scanning tunneling microscopy (STM) studies have shown for more than two decades rectangular formations when sulfur atoms are deposited on Au(111) surfaces. The precursors have ranged from simple molecules or ions, such as SO2 gas or sulfide anions, to more complex organosulfur compounds. We investigated, within the framework of the Density Functional Theory, the structure of these rectangular patterns assuming them entirely composed of sulfur atoms as the experimental evidence suggests. The sulfur coverage at which the simulations were carried out (0.67 ML or higher) provoked that the sulfur-sulfur association had to be taken into account for achieving a good agreement between the sets of simulated and experimental STM images. A combination of four sulfur dimers per rectangular formation properly explained the trends obtained by the experimental STM analysis which were related with the rectangles' size and shape fluctuations together with sulfur-sulfur distances within these rectangles. Finally, a projected density of states analysis showed that the dimers were capable of altering the Au(5d) electronic states at the same level as atomic sulfur adsorbed at low coverage. Besides, sulfur dimers states were perfectly distinguished, whose presence near and above the Fermi level can explain both: sulfur-sulfur bond elongation and dimers stability when they stayed adsorbed on the surface at high coverage.

Understanding the Growth and Chemical Activity of Co–Pt Bimetallic Clusters on TiO2(110): CO Adsorption and Methanol Reaction

Pure and bimetallic Co–Pt clusters were grown on TiO2(110) and studied by scanning tunneling microscopy (STM). Despite the lower mobility of Co atoms on TiO2 compared to Pt, STM experiments demonstrate that exclusively bimetallic clusters can be deposited through either order of deposition, provided that deposition of the first metal results in a sufficient density of seed clusters to nucleate all of the metal atoms from the second deposition. Bimetallic clusters of varying compositions were prepared by depositing different Co:Pt ratios at room temperature with a fixed total coverage of 0.25 ML. The average cluster heights decrease with increasing Co fraction since the higher Co coverage results in a greater number of nucleation sites. After heating to 800 K, the pure Pt clusters exhibit the greatest increase in average cluster height; however, both Pt and Co clusters become encapsulated with titania upon annealing to 800 K, which is also the onset temperature for Co desorption. Low energy ion scattering ...