Shockley Surface State Research Articles

Artificial kagome lattices of Shockley surface states patterned by halogen hydrogen-bonded organic frameworks

Artificial electronic kagome lattices may emerge from electronic potential landscapes using customized structures with exotic supersymmetries, benefiting from the confinement of Shockley surface-state electrons on coinage metals, which offers a flexible approach to realizing intriguing quantum phases of matter that are highly desired but scarce in available kagome materials. Here, we devise a general strategy to construct varieties of electronic kagome lattices by utilizing the on-surface synthesis of halogen hydrogen-bonded organic frameworks (XHOFs). As a proof of concept, we demonstrate three XHOFs on Ag(111) and Au(111) surfaces, which correspondingly deliver regular, breathing, and chiral breathing diatomic-kagome lattices with patterned potential landscapes, showing evident topological edge states at the interfaces. The combination of scanning tunnelling microscopy and noncontact atomic force microscopy, complemented by density functional theory and tight-binding calculations, directly substantiates our method as a reliable and effective way to achieve electronic kagome lattices for engineering quantum states.

Performance of a photoelectron momentum microscope in direct- and momentum-space imaging with ultraviolet photon sources.

The Photoelectron-Related Image and Nano-Spectroscopy (PRINS) endstation located at the Taiwan Photon Source beamline 27A2 houses a photoelectron momentum microscope capable of performing direct-space imaging, momentum-space imaging and photoemission spectroscopy with position sensitivity. Here, the performance of this microscope is demonstrated using two in-house photon sources - an Hg lamp and He(I) radiation - on a standard checkerboard-patterned specimen and an Au(111) single crystal, respectively. By analyzing the intensity profile of the edge of the Au patterns, the Rashba-splitting of the Au(111) Shockley surface state at 300 K, and the photoelectron intensity across the Fermi edge at 80 K, the spatial, momentum and energy resolution were estimated to be 50 nm, 0.0172 Å-1 and 26 meV, respectively. Additionally, it is shown that the band structures acquired in either constant energy contour mode or momentum-resolved photoemission spectroscopy mode were in close agreement.

Dirac nodal line induced anomalous low-energy acoustic plasmons on beryllium (0001) surface

Beryllium is a simple hexagonal metal but exhibits anomalous low-energy acoustic plasmons on its (0001) surface, which are radically distinct from the conventional metal surface plasmons. Although the plasmons on Be(0001) have been extensively studied, the underlying mechanism remains elusive. Here, through first-principles calculations combined with model analysis, we unambiguously demonstrate that it is the topological Dirac nodal line induced surface states that result in the anomalous low-energy acoustic plasmons on Be(0001), rather than the commonly believed Shockley surface states. Our study not only clarifies the origin of the puzzling anomalous plasmons on Be(0001), but also provides a sound theoretical interpretation for the appearance of low-energy acoustic surface plasmons on the noble metal (111) surfaces. This work suggests that the topological nodal line semimetals offer a potential platform for obtaining low-energy acoustic plasmons.

Focused surface plasmon polaritons coherently couple to electronic states in above-threshold electron emission

When an intense light field strongly interacts with the band structure of a solid, the formation of hybrid light-matter quantum states becomes possible. Examples of such Floquet-Bloch states have been reported, but engineering of the band structure using Floquet states can suffer from dissipation and decoherence. Sustaining the necessary quantum coherence of the light-matter interactions requires robust electronic states in combination with strong fields of suitable polarization and frequency. Here, we explore the quantum coherent coupling of nano-focused surface plasmon polaritons (SPP) to distinct electronic states in the band structure of a solid. We observe above-threshold electron emission from the Au(111) Shockley surface state by the absorption of up to seven SPP quanta. Using time-resolved photoelectron spectroscopy the coherence of the interaction of the SPPs with the surface state during electron emission is investigated and the process is shown to be similar to light-driven above threshold electron emission. Ultimately, our work could render SPP-based Floquet engineering in nano-optical systems feasible.

Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle.

A new spin- and angle-resolved inverse photoemission setup with a low-energy electron source is presented. The spin-polarized electron source, with a compact design, can decouple the spin polarization vector from the electron beam propagation vector, allowing one to explore any spin orientation at any wavevector in angle-resolved inverse photoemission. The beam polarization can be tuned to any preferred direction with a shielded electron optical system, preserving the parallel beam condition. We demonstrate the performances of the setup by measurements on Cu(001) and Au(111). We estimate the energy resolution of the overall system at room temperature to be ∼170meV from kBTeff of a Cu(001) Fermi level, allowing a direct comparison to photoemission. The spin-resolved operation of the setup has been demonstrated by measuring the Rashba splitting of the Au(111) Shockley surface state. The effective polarization of the electron beam is P = 30% ± 3%, and the wavevector resolution is ΔkF ≲ 0.06 Å-1. Measurements on the Au(111) surface state demonstrate how the electron beam polarization direction can be tuned in the three spatial dimensions. The maximum of the spin asymmetry is reached when the electron beam polarization is aligned with the in-plane spin polarization of the Au(111) surface state.

Enhanced Rashba Splitting of Au(111) Surface States with Hydrogen‐Bonded Melamine‐Based Organic Framework

AbstractMolecular adsorption on noble‐metal surfaces influences the Rashba effect in the Shockley surface state (SS), but the underlying mechanism remains unclear. Melamine is a simple molecule with a symmetric backbone consisting of a heterocyclic ring. It self‐assembles into a rosette‐like superstructures on Au(111) via double intermolecular hydrogen bonds. In this study, the growth process and structure of this hydrogen‐bonded organic framework (HOF) using low‐energy electron diffraction and X‐ray photoemission spectroscopy is revealed. Above room temperature, it is impossible for melamine to be adsorbed on Au(111) as a monomer, but it is adsorbed collectively as a hydrogen‐bonding network. The melamine HOF has a characteristic hexagonal honeycomb structure (MHC), which significantly affects both the structure and electronic states of Au(111). A theoretical approach reveals that MHC induces a hexagonal periodic deformation in the structure of Au(111) and introduces a new periodic potential in the surface electronic system. Angle‐resolved photoemission spectroscopy measurements of MHC/Au(111) indicate that the bulk sp band is strongly folded back, enhancing the Rashba splitting of the SS of Au(111). Furthermore, spin‐ and angle‐resolved photoemission spectroscopy reveals that the enhanced Rashba splitting of the SS by the MHC is the largest reported to date for the adsorption system on Au(111).

Distinct Tamm and Shockley surface states on Re(0001) mixed by spin-orbit interaction

Tamm and Shockley states, these two paradigmatic concepts are used to describe surface states not only in electronic systems but also in photonic and phononic crystals. The Re(0001) surface hosts both types of electronic surface states in neighboring but qualitatively different energy gaps. Interestingly, spin-orbit interaction generates a double W-shaped energy vs ${\mathbf{k}}_{\ensuremath{\parallel}}$ dispersion by mixing both types of states and lifting their spin degeneracy. By combining spin- and angle-resolved photoemission, tight-binding model calculations, as well as density functional theory including the photoemission process, we develop verifiable criteria to distinguish between the two types of surface states and arrive at a consistent picture of the role of spin-orbit interaction in such a scenario.

Engineering novel surface electronic states via complex supramolecular tessellations

Interfacial complex supramolecular tessellations were employed to derive novel two-dimensional electronic structures from Shockley surface-state, giving rise to Kagome or Dirac bands as well as hybrid band structures through quantum dots coupling.

On the energy resolution of a GaAs-based electron source for spin-resolved inverse photoemission

The spin resolution in inverse photoemission spectroscopy is achieved by injecting spin-polarized electrons, usually produced by GaAs-based cold cathodes that replace hot-filament electron guns of spin-integrated setups. The overall energy resolution of the system can be enhanced by adjusting either the optical bandpass of the photon detector or the energy distribution of the electron beam. Here we discuss the influence of the photocurrent and the photocathode temperature on the energy broadening of the electron beam through the inverse photoemission spectra of the spin-splitted Shockley surface state of Au(111). First, we find that cooling down the GaAs photocathode to 77 K increases the band gap and reduces the number of allowed vertical transitions, monochromatizing the electron beam with an enhancement of about 30 meV for the energy resolution. Second, we observe a correlation between the generated photocurrent at the electron source, and the space-charge effects at the sample as a reduction of lifetime and spin asymmetry of a polarized bulk state. These observations allow defining a threshold of current density for the optimum acquisition in the measurements of spin-resolved inverse photoemission in Au.

Observation of a Shockley Surface State on Gold Nanoparticles with Sizes Down to 5 nm

Observation of a Shockley Surface State on Gold Nanoparticles with Sizes Down to 5 nm

Rashba-type splitting of the Au(110) surface state: A combined inverse and direct photoemission study

The Shockley surface state located at $\overline{\mathrm{Y}}$ on the $(1\ifmmode\times\else\texttimes\fi{}2)$-reconstructed Au(110) surface is predicted to exhibit a Rashba-type spin splitting. Previous photoemission experiments searched for this splitting but it could not be resolved yet. In order to uncover a possible splitting, the unoccupied surface state on Au(110) is examined with spin- and angle-resolved inverse photoemission, whereas Na-covered Au(110) allows for investigation of the now occupied surface state by means of spin- and angle-resolved direct photoemission. Our data show clear spin splittings in the order of $100\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ with a sign reversal at $\overline{\mathrm{Y}}$ in the surface state's in-plane spin components which is characteristic for a Rashba-type behavior. Furthermore, we deduce an effective mass of ${m}^{*}=(0.27\ifmmode\pm\else\textpm\fi{}0.02){m}_{e}$ and a Rashba parameter of ${\ensuremath{\alpha}}_{R}=(0.46\ifmmode\pm\else\textpm\fi{}0.04)\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$ from direct photoemission measurements.

Surface states and related quantum interference in ab initio electron transport

Shockley surface states (SS) have attracted much attention due to their role in various physical phenomena occurring at surfaces. It is also clear from experiments that they can play an important role in electron transport. However, accurate incorporation of surface states in $\textit{ab initio}$ quantum transport simulations remains still an unresolved problem. Here we go beyond the state-of-the-art non-equilibrium Green's function formalism through the evaluation of the self-energy in real-space, enabling electron transport without using artificial periodic in-plane conditions. We demonstrate the method on three representative examples based on Au(111): a clean surface, a metallic nanocontact, and a single-molecule junction. We show that SS can contribute more than 30\% of the electron transport near the Fermi energy. A significant and robust transmission drop is observed at the SS band edge due to quantum interference in both metallic and molecular junctions, in good agreement with experimental measurements. The origin of this interference phenomenon is attributed to the coupling between bulk and SS transport channels and it is reproduced and understood by tight-binding model. Furthermore, our method predicts much better quantized conductance for metallic nanocontacts.

Influence of surface states on the conductance spectra for Co adsorbed on Cu(111)

We calculate the conductance spectra of a Co atom adsorbed on Cu(111), considering the Co $3d$ orbitals within a correlated multiple configurations model interacting through the substrate band with the Co $4s$ orbital, which is treated in a mean-field like approximation. By symmetry, only the $d_{z^2}$ orbital couples with the $s$ orbital through the Cu bands, and the interference between both conduction channels introduces a zero-bias anomaly in the conductance spectra. We find that, while the Kondo resonance is mainly determined by the interaction of the Co $d$ orbitals with the bulk states of the Cu(111) surface, a proper description of the contribution given by the coupling with the localized surface states to the Anderson widths is crucial to describe the interference line shape. We find that the coupling of the Co $4s$ orbital with the Shockley surface states is responsible of two main features observed in the measured conductance spectra, the dip shape around the Fermi energy and the resonance structure at the surface state low band edge.

Supramolecular Architecturing of Quantum Box Arrays from Functionalized Porphyrins and Exploring Their Quantum States

Our research is based on the analysis of structure-function correlations for a broad spectrum of functionalized porphyrins and phthalocyanines in their propensity to create increasingly complex and functional on surface architectures. [1] Thereby we use porphyrin and phthalocyanine functionalization and synthesis, ‘by design’, or ‘on-surface-supramolecular chemistry’ to create structures with specific physical properties towards e.g. future quantum devices.By using a fluoro-bi-phenyl functionalized porphyrin to create a stable, porous organic network, we manufacture arrays of identical quantum boxes. These boxes contain a specific quantum well state arising from the confinement of the electronic (Shockley) surface state within the pores. We show that the electronic quantum box state as well as the interbox coupling can be modified locally by adsorbates, e.g. C60, interacting with the barrier. In view of the wealth of available and differently acting adsorbates, this approach allows for the engineering of quantum states in on-surface network architectures. Thus, by their dance on the nose of the ‘devil’ [3], porphyrins and phthalocyanines justify the emergence of ‘on-surface supramolecular physics’ as reflected by ongoing and future research work.Fig. 1) Adsorbate-induced modification of the confining barriers in a quantum box array formed by a functionalized fluoro-bi-phenyl porphyrin on Ag(111). (Taken from [2])[1] N. Wintjes et al. Two-dimensional phase behavior of a bimolecular porphyrin system at the solid-vacuum interface, J. Am. Chem. Soc. 2010, 132, 21, 7306-7311; N. Wintjes et al., Supramolecular synthons on surfaces: Controlling dimensionality and periodicity of tetraarylporphyrin assemblies by the interplay of cyano and alkoxy substituents Chem. Eur. J. 2008, 14, 5794.[2] S. Nowakowska et al., Adsorbate-induced modification of the confining barriers in a quantum box array, ACS Nano 2018, 12, 768.[3] W. Pauli “God made the bulk; surfaces were invented by the devil.” Quoted, e.g. in the Preface of A. Zangwill, Physics at Surfaces, Cambridge University Press, 1988. ISBN 978-0-521-34752-5 Figure 1

Surface structure of Gd20Co80 alloy

Gd 20 Co 80 alloy was studied applying experimental methods of spectral ellipsometry, atomic force and scanning tunneling microscopy. The experimental results exhibit the eutectic two-phase structure of this alloy. Clusters of a phase with a lower content had a lesser concentration of free carriers, which resulted in smaller tunnel currents during the measurements. To analyze the experimental data, a theoretical approach was developed, which was based both on the quantum-mechanical methods of configurational interaction and on three-diagonal Toeplitz matrices formalism. This approach allowed us to describe in detail the energy bands formation process in solid clusters with a relatively small quantity of atoms, which as a consequence enabled to describe the Shockley surface states as well as the existence of a surface layer with partially formed energy bands. Spectral-ellipsometric measurements of Gd 20 Co 80 alloy thin films confirmed a significant difference between the measured optical constants for 20-nm thick films and larger films. Quantum-mechanical molecular calculus allowed to obtain optical constants for several supercells of Gd-Co chemical compounds and confirmed the acquired experimental and theoretical results.

Electron pair emission from surfaces: Photon versus electron excitation

We have studied the electron pair emission process from a Cu(111) surface excited by a primary electron or photon beam. Although the photon energy was 30 eV the primary electron beam had an energy of 25 eV. We will discuss the energy and momentum distributions. The experiments with the electron beam demonstrate that the contributions from the Shockley surface state can be clearly identified. This is not the case for the work with photons which we ascribe to the low spectral weight of the contribution from two Shockley state electrons compared to two $3d$ electrons. The momentum distributions reveal a close similarity between the two types of excitation despite the fact that the microscopic origin of the pair emission is very different. There is a distinctive energy dependence of these curves which we describe by an empirical three-parameter polynomial. The numerical values of these parameters are close for both pathways. Additional measurements with different primary electron energies reveals the importance of the initial state on the momentum distributions.

Automated Tip Conditioning for Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopy (STS), a technique that records the change in the tunneling current as a function of the bias (dI/dV) across the gap between a tip and the sample, is a powerful tool to characterize the electronic structure of single molecules and nanomaterials. While performing STS, the structure and condition of the scanning probe microscopy (SPM) tips are critical for reliably obtaining high quality point spectra. Here, we present an automated program based on machine learning models that can identify the Au(111) Shockley surface state in dI/dV point spectra and perform tip conditioning on clean or sparsely covered gold surfaces with minimal user intervention. We employed a straightforward height-based segmentation algorithm to analyze STM topographic images to identify tip conditioning positions and used 1789 archived dI/dV spectra to train machine learning models that can ascertain the condition of the tip by evaluating the quality of the spectroscopic data. Decision tree based ensemble and boosting models and deep neural networks (DNNs) have been shown to reliably identify tips in suitable conditions for STS. We expect the automated program to reduce operational costs and time, increase reproducibility in surface science studies, and accelerate the discovery and characterization of novel nanomaterials by STM. The strategies presented in this paper can readily be adapted to STM tip conditioning on a wide variety of other common substrates.

Evidence for a spin acoustic surface plasmon from inelastic atom scattering

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).

Quantum-confinement-induced periodic surface states in two-dimensional metal-organic frameworks

Recently, a series of single-layer metal–organic frameworks (MOFs) was theoretically predicted to be two-dimensional organic topological materials. However, the experimental evidence of their nontrivial topological states has not been found. Here, combining the use of angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we report the electronic structure studies on a single-layer Cu-coordinated 2,4,6-tri(4-pyridyl)-1,3,5-triazine (Cu-T4PT) MOF supported by a Cu(111) substrate and identify periodic surface states with the period of the Cu-T4PT reciprocal lattice. These periodic surface states, which have identical features to the Cu(111) Shockley surface states, can be attributed to the quantum confinement of the surface states of the underlying Cu(111) substrate by the network lattices of the Cu-T4PT MOF. Our work indicates that the surface states of the metal substrate can be tailored in a controlled manner by the network structures of MOFs with different periodic lattices. The lack of intrinsic bands and the possible topological properties of the single-layer Cu-T4PT MOF may be attributed to the strong electronic coupling between the Cu-T4PT MOF and the Cu(111) substrates. In order to exploit organic topological materials predicted in MOFs, it is necessary to grow them on weak van der Waals interaction substrates in the future.

Quantum-well states for uniform Ag layers on the Ga-induced Si(111)–([formula omitted])R30° surface

The atomic and electronic structures are calculated for atomically uniform Ag layers on the Ga–induced Si(111)–(3×3)R30° surface by using the Density Functional Theory. It is found that when the amount of Ag atoms increases on the 1/3 monolayer Ga–induced Si(111)–(3×3)R30° surface, the equilibrium T4 adsorption site of Ga atom changes to the T1 site. We have determined a single covalent bond between Ga and Si atoms but there is some charge accumulation on the Ga–Ag layer, turning the surface to metallic nature. For 10 Ag monolayers, we have determined evolution of several quantum-well states within the energy range of 1 eV below the Fermi level. The energy separation between the quantum well states increases as their numbers develop below the Fermi level. The Ag film quantum-well states show in-plane parabolic dispersion, with splittings arising due to Umklapp features related to the Si(111)–(1×1) surface Brillouin zone. We have also identified Shockley type surface states, which show small Rashba-type spin-orbit splitting.