The fundamental characteristics of collective interactions in topological band structures can be revealed by the exploration of charge screening in topological materials. In particular, distinct anisotropic screening behaviors are predicted to occur in Dirac nodal line semimetals (DNLSMs) due to their peculiar anisotropic low-energy dispersion. Despite the recent extensive theoretical research, experimental observations of exotic charge screening in DNLSMs remain elusive, which is partly attributed to the coexisting trivial bands near the Fermi energy. This study reports the first direct observation of highly anisotropic charge-screening behavior in the DNLSM SrAs3. Through atomically resolved conductance measurements, a highly anisotropic charge-screening pattern around charged impurities on a surface is demonstrated. Moreover, the combination of model studies and first-principles calculations reveals the unique nature of the screening anisotropy in DNLSMs. The results of this study are expected to pave the way for understanding the profound collective behavior of interacting low-energy fermions in topological materials.

The discovery of topological states localized at the boundaries and dislocations is of paramount importance in the physics of topological phases. Structural imperfections can be a promising testbed to engineer the symmetries and the concomitant topological states, as they genuinely exist in solid-state matters. Here, we present the direct evidence of hierarchical first-order and second-order topological states in symmetry–enforced grain boundaries (GB) of 1T′–MoTe 2 . Guided by exerted scanning tunneling microscope (STM)–tip press and pulse, we construct two distinct types of GBs, which are differentiated by the underlying symmorphic and nonsymmorphic symmetries. The GBs with the nonsymmorphic rotation symmetry harbor the first-order topological edge state protected by nonsymmorphic band degeneracy. On the other hand, the edge state of the symmorphic GBs attains the band gap. More interestingly, the gapped edge state realizes the second-order topological phase, shown by the additional corner states at the GB ends. We anticipate our experiment will pioneer the material platform for the hierarchical realization of the first-order and higher-order topology.

Structural imperfections can be a promising testbed to engineer the symmetries and topological states of solid-state platforms. Here, we present direct evidence of hierarchical transitions of zero- (0D) and one-dimensional (1D) topological states in symmetry-enforced grain boundaries (GB) in 1T '-MoTe2. Using a scanning tunneling microscope tip press-and-pulse procedure, we construct two distinct types of GBs, which are differentiated by the underlying symmorphic and nonsymmorphic symmetries. The GBs with the nonsymmorphic rotation symmetry harbor first-order topological edge states protected by a nonsymmorphic band degeneracy. On the other hand, the edge state of the symmorphic GBs attains a band gap. More interestingly, the gapped edge state realizes a hierarchical topological phase, evidenced by the additional 0D boundary states at the GB ends. We anticipate our experiments will pioneer the material platform for the hierarchical realization of first-order and higher-order topology. Engineering topological states at defect sites in quantum materials is of great interest in condensed matter physics. Here, the authors demonstrate 0D and 1D topological states at symmetry-engineered grain boundaries in a 2D material.

The commercialization of 3D heterogeneous integration through hybrid bonding has accelerated, and accordingly, Cu-polymer bonding has gained significant attention as a means of overcoming the limitations of conventional Cu-SiO2 hybrid bonding, offering high compatibility with other fabrication processes. Polymers offer robust bonding strength and a low dielectric constant, enabling high-speed signal transmission with high reliability, but suffer from low thermomechanical stability. Thermomechanical stability of polymers was not achieved previously because of thermal degradation and unstable anchoring. To overcome these limitations, wafer-scale Cu-polymer bonding via N-heterocyclic carbene (NHC) nanolayers was presented for 3D heterogeneous integration, affording ultrastable packing density, crystallinity, and thermal properties. NHC nanolayers were deposited on copper electrodes via electrochemical deposition, and wafer-scale 3D heterogeneous integration was achieved by adhesive bonding at 170 degrees C for 1 min. Ultrastable conductivity and thermomechanical properties were observed by the spatial mapping of conductivity, work function, and force-distance curves. With regard to the characterization of NHC nanolayers, low-temperature bonding, robust corrosion inhibition, enhanced electrical conductivity, back-end-of-line process compatibility, and fabrication process reduction, NHC Cu/polymer bonding provides versatile advances in 3D heterogeneous integration, indicating that NHC Cu/polymer bonding can be utilized as a platform for future 3D vertical chip architectures.

Standing wave patterns are formed near potential barriers or steps due to the interference of incident and reflected waves. Graphene systems show standing wave patterns near atomic step edges, defects, and impurities in scanning tunneling microscopy; however, there are still unexplored examples. In this study, we present our experimental results for graphene directly grown on hexagonal-BN/Cu by chemical vapor deposition. Standing wave patterns were observed in our scanning tunneling microscopy and spectroscopy, revealing linear dispersion relations with a Fermi velocity of about 10(6) m/s. Our study shows that graphene grown on hexagonal-BN/Cu provides a useful platform to study the electronic characteristics of graphene systems.

Graphene nanoribbon, which is the one-dimensional form of graphene, is an attractive nano structure for next generation electronic devices. Herein, we studied the electronic properties of pseudo-GNRs in large-scale graphene sheets grown on Ge(110) using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Based on our STM results, the pseudo-GNRs are aligned in the <112> direction of the Ge surface; the alignment of the pseudo-GNRs is controlled by the surface reconstruction of Ge(110) substrate, which can be an important merit in terms of the mass fabrication. Bandgap energies ranging from ∼0.12 to ∼0.3 eV were measured via STS on pseudo-GNRs in graphene/Ge(110), while the surrounding graphene area outside the pseudo-GNR growth region shows the typical electronic structure of graphene, verifying the spontaneous formation of metallic-semiconducting-metallic junction nanostructure. This study unveils the geometric and electronic properties of pseudo-GNRs in graphene/Ge(110), providing essential information for the realization of next-generation nanoelectronic devices.

Recent advances in increasing the spectroscopic energy resolution in scanning tunneling microscopy (STM) have been achieved by integrating electron spin resonance (ESR) with STM. Here, we demonstrate the design and performance of a home-built STM capable of ESR at temperatures ranging from 1 K to 10 K. The STM is incorporated with a home-built Joule-Thomson refrigerator and a 2-axis vector magnet. Our STM design allows for the deposition of atoms and molecules directly into the cold STM, eliminating the need to extract the sample for deposition. In addition, we adopt two methods to apply radio-frequency (RF) voltages to the tunnel junction, the early design of wiring to the STM tip directly, and a more recent idea to use an RF antenna. Direct comparisons of ESR results measured using the two methods and simulations of electric field distribution around the tunnel junction show that, despite their different designs and capacitive couplings to the tunnel junction, there is no discernible difference in the driving and detection of ESR. Furthermore, at a magnetic field of 1.6 T, we observe ESR signals (near 40 GHz) sustained up to 10 K, which is the highest temperature for ESR-STM measurement reported to date, to the best of our knowledge. Although the ESR intensity exponentially decreases with increasing temperature, our ESR-STM system with low noise at the tunnel junction allows us to measure weak ESR signals with intensities in the sub-fA range. Our new design of ESR-STM, which is operational in a large frequency and temperature range, can broaden the use of ESR spectroscopy in STM and enable the simple modification of existing STM systems, which will hopefully accelerate a generalized use of ESR-STM.

The electronic structure of periodic lattice distortion (PLD) in NbTe2 was examined using low-temperature scanning tunneling spectroscopy and microscopy. The striped PLDs with 3 x 1 and 1 x 92 superstructures were characterized in real and reciprocal space. The simultaneous formation of momentum-specific suppressions in the spectral weight and phase shifts of the wavefront related to the superstructures were observed at multiple energies. These unusual energy dependencies are well agreed with the charge density waves (CDWs) formed in the multiband electronic structure of NbTe2. Fermi-surface nesting and reconstruction of the reciprocal lattice were suggested in developing the multiband CDWs in NbTe2.

Indenofluorene, which consists of a 6-5-6-5-6 fused-ring structure, is a semiconducting molecule with possible applications in optoelectronic devices. Bulk crystal structures, molecular front orbitals, and on-surface polymerized indenofluorene have recently been studied, but self-assembled structures on the surface have not yet been reported. Here, we report the array structure of 2,8-dibromoindeno[1,2-b]fluorene-6,12-dione on Au(111) studied using scanning tunneling microscopy. We proposed an alternating-tread stair structure as the molecular model for the monolayer molecular islands, which show strong shape anisotropy. The model can be explained by four O⋅⋅⋅H hydrogen bonds and one Br⋅⋅⋅Br halogen bond per molecule, as supported by density functional theory calculations. Although they prefer intermixed heteroprochiral structures in bulk layers, our study shows that these molecules can form phase-separated homoprochiral structures on the surface, leading to potential applications in molecular chiral separation.

Transition metal dichalcogenides (TMDs) have been widely studied as attractive two-dimensional (2D) materials. In particular, specific TMD materials have attracted increasing attention because of their intriguing features as 2D topological insulators (TIs), which have a metallic edge state and bulk band gap. To realize next-generation devices that employ the metallic edge states of 2D TI materials, precise patterning of the edges is essential. In this study, we demonstrate a simple nanopatterning technique for 1 T'-MoTe2, which is known to be a 2D TI material, using atomic force microscopy (AFM)-based scanning probe lithography (SPL). Our AFM-based SPL method entails delicately scratching a few-layer 1 T'-MoTe2 sample while applying an electric field using a conductive AFM tip. The proposed method enables nanoscale lines, holes, and letters to be reliably patterned on the 1 T'-MoTe2 sample. This study results in the development of a clean method that is compatible with existing mass-production facilities to fabricate various TMD materials for realizing next-generation electronic devices and for studying the underlying physics of these materials.

We used electron spin resonance (ESR) combined with scanning tunneling microscopy (STM) to measure hydrogenated Ti (spin-1/2) atoms at low-symmetry binding sites on MgO in vector magnetic fields. We found strongly anisotropic g-values in all three spatial directions. Interestingly, the amplitude and lineshape of the ESR signals are also strongly dependent on the angle of the field. We conclude that the Ti spin is aligned along the magnetic field, while the tip spin follows its strong magnetic anisotropy. Our results show the interplay between the tip and surface spins in determining the ESR signals and highlight the precision of ESR-STM to identify the single atom's spin states.

Quinone molecules form self-assembled structures in biological systems playing a key role in charge transport. Here, we report on the self-assembled chain structures of three quinone molecules, anthraquinone (AQ), naphthacenequinone (NQ), and pentacenequinone (PQ) on Au(111) studied using scanning tunneling microscopy. They all formed one-dimensional chain structures confined to herringbone structures of Au(111) at the low-coverage limit. The observed structures were explained with O center dot center dot center dot H hydrogen bonds, as revealed by our density functional theory calculations.

In the design of supramolecular structures, Y-shaped molecules are useful to expand the structures in three different directions. The supramolecular structures of Y-shaped molecules with three halogen-ligands on surfaces have been extensively studied, but much less are done for those with six halogen-ligands. Here, we report on the intermolecular interactions of a Y-shaped molecule, 1,3,5-Tris(3,5-dibromophenyl) benzene, with six Br-ligands studied using scanning tunneling microscopy (STM). Honeycomb-like structures were observed on Au(111), and could be explained with chiral triple-nodes made of three Br center dot center dot center dot Br halogen bonds. Molecular models were proposed based on STM images and reproduced with density-functional theory calculations. Although the molecule has six Br-ligands, only three of them form Br center dot center dot center dot Br halogen bonds because of geometrical restrictions. Our study shows that halogenated Y-shaped molecules will be useful components for building supramolecular structures. (C) 2017 Elsevier B.V. All rights reserved.

Molecular networks connected by halogen bonds have been actively studied due to their ubiquity in biological systems and complementary role to hydrogen bonds. Although networks of planar molecules with halogen ligands have been studied using scanning tunneling microscopy (STM), those of non-planar molecules have not. Here, we report on the network structures of non-planar molecules containing Brligands, tetrabromo-spirobifluorene on Au (111), studied using STM. One and two-dimensional networks were observed and their intermolecular interactions were investigated. In two-dimensional networks, a molecule forms 3.5 Br center dot center dot center dot Br halogen bonds and 2 Br center dot center dot center dot H hydrogen bonds, as supported by our density functional theory calculation results. Our study demonstrates that intermolecular structures of non-planar molecules with halogen bonds can be probed on surfaces using STM. (C) 2017 Elsevier B.V. All rights reserved.

The surface states protected by time-reversal symmetry in 3-dimensional topological insulators have recently been confirmed by angle-resolved photoemission spectroscopy, scanning tunneling microscopy, quantum transport and so on. However, the electronic properties of ultra-thin topological insulator films have not been extensively studied, especially when the films are grown on metal substrates. In this paper, we have elucidated the local behaviors of the electronic states of ultra-thin topological insulator Bi2Se3 grown with molecular beam epitaxy on Au(111) using scanning tunneling microscopy/spectroscopy. We have observed linear dispersion of electron interference patterns at higher energies than the Fermi energy that were not accessible by conventional angle-resolved photoemission spectroscopy. Moreover, the dispersion of the interference patterns varies with the film thickness, which is explained by band bending near the interface between the topological insulator and the metal substrate. Our experiments demonstrate that interfacial effects in thin topological insulator films on metal substrate can be sensed using scanning tunneling spectroscopy.

The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have been strongly debated issues of iron-based superconductors (FeSCs). Observation of C$_4$-symmetric antiferromagnetism in violation of the inter-band nesting condition of spin-density waves in superconducting ground state will require significant change in our understanding of the mechanism of FeSC. The superconducting material Sr$_2$VO$_3$FeAs, a bulk version of monolayer FeSC in contact with a perovskite layer with its magnetism (T$_N$ ~ 50 K) and superconductivity (T$_c$ ~ 37 K) coexisting at parent state, has no reported structural orthorhombic distortion and thus makes a perfect system to look for theoretically expected C$_4$ magnetisms. Based on variable temperature spin-polarized scanning tunneling microscopy (SPSTM) with newly discovered imaging mechanism that removes the static surface reconstruction (SR) pattern by fluctuating it rapidly with spin-polarized tunneling current, we could visualize underlying C$_4$ symmetric (2$\times$2) magnetic domains and its phase domain walls. We find that this magnetic order is perfectly consistent with the plaquette antiferromagnetic order in tetragonal Fe spin lattice expected from theories based on the Heisenberg exchange interaction of local Fe moments and the quantum order by disorder. The inconsistency of its modulation Q vectors from the nesting condition also implies that the nesting-based C$_2$ symmetric magnetism is not a unique prerequisite of high-T$_c$ FeSC. Furthermore, the plaquette antiferromagnetic domain wall dynamics under the influence of small spin torque effect of spin-polarized tunneling current are shown to be consistent with theoretical simulation based on the extended Landau-Lifshitz-Gilbert equation.