The topological soliton in Peierls semimetal Sb.

Sb is a three-dimensional Peierls insulator. The Peierls instability gives rise to doubling of the translational period along the [111] direction and alternating van der Waals and covalent bonding between (111) atomic planes. At the (111) surface of Sb, the Peierls condition is violated, which in theory can give rise to properties differing from the bulk. The atomic and electronic structure of the (111) surface of Sb have been simulated by density functional theory calculations. We have considered the two possible (111) surfaces, containing van der Waals dangling bonds or containing covalent dangling bonds. In the models, the surfaces are infinite and the structure is defect free. Structural optimization of the model containing covalent dangling bonds results in strong deformation, which is well described by a topological soliton within the Su-Schrieffer-Heeger model centered about 25 Å below the surface. The electronic states associated with the soliton see an increase in the density of states (DOS) at the Fermi level by around an order of magnitude at the soliton center. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements reveal two distinct surface regions, indicating that there are different surface regions cleaving van der Waals and covalent bonds. The DFT is in good agreement with the STM/STS experiments.

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)2Te3 Crystal Doped with Mn.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

Estimating the Orientation of 4f Magnetic Moments by Classical Photoemission.

To use efficiently the magnetic functionalities emerging at the surfaces or interfaces of novel lanthanides-based materials, there is a need for complementary methods to probe the atomic-layer resolved magnetic properties. Here, we show that 4f photoelectron spectroscopy is highly sensitive to the collective orientation of 4f magnetic moments and, thus, a powerful tool for characterizing the related properties. To demonstrate this, we present the results of systematic study of a family of layered crystalline 4f-materials, which are crystallized in the body-centered tetragonal ThCr2Si2 structure. Analysis of 4f spectra indicates that the 4f moments at the surface experience a strong reorientation with respect to the bulk, caused by changes of the crystal-electric field. The presented database of the computed 4f spectra for all trivalent rare-earth ions in their different MJ states will facilitate the estimation of the orientation of the 4f magnetic moments in the layered 4f-systems for efficient control of their magnetic properties.

Organismic Memristive Structures With Variable Functionality for Neuroelectronics.

In this paper, we report an approach to design nanolayered memristive compositions based on TiO2/Al2O3 bilayer structures with analog non-volatile and volatile tuning of the resistance. The structure of the TiO2 layer drives the physical mechanism underlying the non-volatile resistance switching, which can be changed from electronic to ionic, enabling the synaptic behavior emulation. The presence of the anatase phase in the amorphous TiO2 layer induces the resistive switching mechanism due to electronic processes. In this case, the switching of the resistance within the range of seven orders of magnitude is experimentally observed. In the bilayer with amorphous titanium dioxide, the participation of ionic processes in the switching mechanism results in narrowing the tuning range down to 2-3 orders of magnitude and increasing the operating voltages. In this way, a combination of TiO2/Al2O3 bilayers with inert electrodes enables synaptic behavior emulation, while active electrodes induce the neuronal behavior caused by cation density variation in the active Al2O3 layer of the structure. We consider that the proposed approach could help to explore the memristive capabilities of nanolayered compositions in a more functional way, enabling implementation of artificial neural network algorithms at the material level and simplifying neuromorphic layouts, while maintaining all benefits of neuromorphic architectures.

Local Electric Property Modification of Ferroelectric Tunnel Junctions Induced by Variation of Polarization Charge Screening Conditions under Measurement with Scanning Probe Techniques.

Scanning tunneling spectroscopy in ultrahigh vacuum conditions and conductive atomic-force microscopy in ambient conditions were used to study local electroresistive properties of ferroelectric tunnel junctions SrTiO3/La0.7Sr0.3MnO3/BaTiO3. Interestingly, experimental current-voltage characteristics appear to strongly depend on the measurement technique applied. It was found that screening conditions of the polarization charges at the interface with a top electrode differ for two scanning probe techniques. As a result, asymmetry of the tunnel barrier height for the opposite ferroelectric polarization orientations may be influenced by the method applied to study the local tunnel electroresistance. Our observations are well described by the theory of electroresistance in ferroelectric tunnel junctions. Based on this, we reveal the main factors that influence the polarization-driven local resistive properties of the device under study. Additionally, we propose an approach to enhance asymmetry of ferroelectric tunnel junctions during measurement. While keeping the high locality of scanning probe techniques, it helps to increase the difference in the value of tunnel electroresistance for the opposite polarization orientations.

Oxidation of h-BN on strongly and weakly interacting metal surfaces.

We used x-ray photoemission and absorption spectroscopies to study the influence of thermal molecular oxygen exposure on the h-BN/Co(0001) and h-BN/Au/Co(0001) systems. The spectral analysis was supported by density functional theory calculations. It is shown that oxygen can intercalate h-BN on Co(0001) and also be embedded into its lattice, replacing the nitrogen atoms. Upon substitution, the structures containing one (BN2O) and three (BO3) oxygen atoms in the boron atom environment are formed predominantly. In the case of gold-intercalated h-BN, only the (BN2O) structures are formed; the long-lasting oxygen exposures lead to etching of the h-BN layer.

Spin-polarized Fermi surface, hole-doping and band gap in graphene with boron impurities.

Embedding foreign atoms in graphene and interchanging the underlying substrate are proved to be efficient methods for manipulating the properties of graphene. Combining ARPES experiments with DFT calculations we show that boron-doped graphene (B-graphene) grown on a Co(0001) substrate by chemical vapor deposition (CVD) becomes hole doped and its Fermi surface near the K-point reveals strongly spin-polarized states. The latter stems from the spin-polarized mini Dirac cone that is an intrinsic two-dimensional feature of the graphene/Co(0001) interface and is formed by a mixture of C 2pz and Co 3d states. Since the CVD method allows the achievement of up to 20 at% of incorporated B atoms, this provides a certain flexibility for handling the spin-polarized properties of the system. We also show that the bonding of the B-graphene layer to the Co(0001) substrate can be released by intercalation of Li into the interface. This allows the exploration of the doping effect in detail. Finally, our ARPES data indicate a gap opening in the Dirac cone as a result of the highly unbalanced boron concentrations in the two graphene sublattices.

Cobalt-assisted recrystallization and alignment of pure and doped graphene.

Recrystallization of bulk materials is a well-known phenomenon, which is widely used in commercial manufacturing. However, for low-dimensional materials like graphene, this process still remains an unresolved puzzle. Thus, the understanding of the underlying mechanisms and the required conditions for recrystallization in low dimensions is essential for the elaboration of routes towards the inexpensive and reliable production of high-quality nanomaterials. Here, we unveil the details of the efficient recrystallization of one-atom-thick pure and boron-doped polycrystalline graphene layers on a Co(0001) surface. By applying photoemission and electron diffraction, we show how more than 90% of the initially misoriented graphene grains can be reconstructed into a well-oriented and single-crystalline layer. The obtained recrystallized graphene/Co interface exhibits high structural quality with a pronounced sublattice asymmetry, which is important for achieving an unbalanced sublattice doping of graphene. By exploring the kinetics of recrystallization for native and B-doped graphene on Co, we were able to estimate the activation energy and propose a mechanism of this process.

Magneto-Spin-Orbit Graphene: Interplay between Exchange and Spin-Orbit Couplings.

A rich class of spintronics-relevant phenomena require implementation of robust magnetism and/or strong spin-orbit coupling (SOC) to graphene, but both properties are completely alien to it. Here, we for the first time experimentally demonstrate that a quasi-freestanding character, strong exchange splitting and giant SOC are perfectly achievable in graphene at once. Using angle- and spin-resolved photoemission spectroscopy, we show that the Dirac state in the Au-intercalated graphene on Co(0001) experiences giant splitting (up to 0.2 eV) while being by no means distorted due to interaction with the substrate. Our calculations, based on the density functional theory, reveal the splitting to stem from the combined action of the Co thin film in-plane exchange field and Au-induced Rashba SOC. Scanning tunneling microscopy data suggest that the peculiar reconstruction of the Au/Co(0001) interface is responsible for the exchange field transfer to graphene. The realization of this "magneto-spin-orbit" version of graphene opens new frontiers for both applied and fundamental studies using its unusual electronic bandstructure.

Raman Spectroscopy of Lattice-Matched Graphene on Strongly Interacting Metal Surfaces.

Regardless of the widely accepted opinion that there is no Raman signal from single-layer graphene when it is strongly bonded to a metal surface, we present Raman spectra of a graphene monolayer on Ni(111) and Co(0001) substrates. The high binding energy of carbon to these surfaces allows formation of lattice-matched (1 × 1) structures where graphene is significantly stretched. This is reflected in a record-breaking shift of the Raman G band by more than 100 cm-1 relative to the case of freestanding graphene. Using electron diffraction and photoemission spectroscopy, we explore the aforementioned systems together with polycrystalline graphene on Co and analyze possible intercalation of oxygen at ambient conditions. The results obtained are fully supported by Raman spectroscopy. Performing a theoretical investigation of the phonon dispersions of freestanding graphene and stretched graphene on the strongly interacting Co surface, we explain the main features of the Raman spectra. Our results create a reliable platform for application of Raman spectroscopy in diagnostics of chemisorbed graphene and related materials.

Large-Scale Sublattice Asymmetry in Pure and Boron-Doped Graphene.

The implementation of future graphene-based electronics is essentially restricted by the absence of a band gap in the electronic structure of graphene. Options of how to create a band gap in a reproducible and processing compatible manner are very limited at the moment. A promising approach for the graphene band gap engineering is to introduce a large-scale sublattice asymmetry. Using photoelectron diffraction and spectroscopy we have demonstrated a selective incorporation of boron impurities into only one of the two graphene sublattices. We have shown that in the well-oriented graphene on the Co(0001) surface the carbon atoms occupy two nonequivalent positions with respect to the Co lattice, namely top and hollow sites. Boron impurities embedded into the graphene lattice preferably occupy the hollow sites due to a site-specific interaction with the Co pattern. Our theoretical calculations predict that such boron-doped graphene possesses a band gap that can be precisely controlled by the dopant concentration. B-graphene with doping asymmetry is, thus, a novel material, which is worth considering as a good candidate for electronic applications.

A curious interplay in the films of N-heterocyclic carbene Pt(II) complexes upon deposition of alkali metals.

The recently synthesized series of Pt(II) complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned Pt(II) complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination Pt(II) centre to Pt(0) and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system "Pt complex-alkali metal ion"; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the Pt(II) complexes for a variety of biomedical, sensing, chemical, and electronic applications.

Epitaxial B-Graphene: Large-Scale Growth and Atomic Structure.

Embedding foreign atoms or molecules in graphene has become the key approach in its functionalization and is intensively used for tuning its structural and electronic properties. Here, we present an efficient method based on chemical vapor deposition for large scale growth of boron-doped graphene (B-graphene) on Ni(111) and Co(0001) substrates using carborane molecules as the precursor. It is shown that up to 19 at. % of boron can be embedded in the graphene matrix and that a planar C-B sp(2) network is formed. It is resistant to air exposure and widely retains the electronic structure of graphene on metals. The large-scale and local structure of this material has been explored depending on boron content and substrate. By resolving individual impurities with scanning tunneling microscopy we have demonstrated the possibility for preferential substitution of carbon with boron in one of the graphene sublattices (unbalanced sublattice doping) at low doping level on the Ni(111) substrate. At high boron content the honeycomb lattice of B-graphene is strongly distorted, and therefore, it demonstrates no unballanced sublattice doping.

Observation of single-spin Dirac fermions at the graphene/ferromagnet interface.

With the discovery and first characterization of graphene, its potential for spintronic applications was recognized immediately. Since then, an active field of research has developed trying to overcome the practical hurdles. One of the most severe challenges is to find appropriate interfaces between graphene and ferromagnetic layers, which are granting efficient injection of spin-polarized electrons. Here, we show that graphene grown under appropriate conditions on Co(0001) demonstrates perfect structural properties and simultaneously exhibits highly spin-polarized charge carriers. The latter was conclusively proven by observation of a single-spin Dirac cone near the Fermi level. This was accomplished experimentally using spin- and angle-resolved photoelectron spectroscopy, and theoretically with density functional calculations. Our results demonstrate that the graphene/Co(0001) system represents an interesting candidate for applications in devices using the spin degree of freedom.

Epitaxial growth and electronic properties of well ordered phthalocyanine heterojunctions MnPc/F16CoPc.

We have prepared phthalocyanine heterojunctions out of MnPc and F16CoPc, which were studied by means of X-ray absorption spectroscopy. This heterojunction is characterized by a charge transfer at the interface, resulting in charged MnPc(δ +) and F16CoPc(δ -) species. Our data reveal that the molecules are well ordered and oriented parallel to the substrate surface. Furthermore, we demonstrate the filling of the Co 3d(z(2)) orbital due to the charge transfer, which supports the explanation of the density functional theory, that the charge transfer is local and affects the metal centers only.

The chemistry of imperfections in N-graphene.

Many propositions have been already put forth for the practical use of N-graphene in various devices, such as batteries, sensors, ultracapacitors, and next generation electronics. However, the chemistry of nitrogen imperfections in this material still remains an enigma. Here we demonstrate a method to handle N-impurities in graphene, which allows efficient conversion of pyridinic N to graphitic N and therefore precise tuning of the charge carrier concentration. By applying photoemission spectroscopy and density functional calculations, we show that the electron doping effect of graphitic N is strongly suppressed by pyridinic N. As the latter is converted into the graphitic configuration, the efficiency of doping rises up to half of electron charge per N atom.