Probing Spin-Dependent Ballistic Charge Transport at Single-Nanometer Length Scales.

The coherent transport of charge and spin is a key requirement of future devices for quantum computing and communication. Scattering at defects or impurities may significantly reduce the coherence of quantum-mechanical states, thereby affecting the device functionality. While numerous methods exist to experimentally assess charge transport, the real-space detection of a material's ballistic spin transport properties with nanometer resolution remains a challenge. Here we report on a novel approach that utilizes a combination of spin-polarized scanning tunneling microscopy (SP-STM) and the recently introduced molecular nanoprobe (MONA) technique. It relies on the local injection of spin-polarized charge carriers from a magnetic STM tip and their detection by a single surface-deposited phthalocyanine molecule via reversible electron-induced tautomerization events. Based on the particular electronic structure of the Rashba alloy BiAg2, which is governed by a spin-momentum-locked surface state, we prove that the current direction inverses upon tip magnetization reversal.

Interaction Effects in a 1D Flat Band at a Topological Crystalline Step Edge.

Step edges of topological crystalline insulators can be viewed as predecessors of higher-order topology, as they embody one-dimensional edge channels embedded in an effective three-dimensional electronic vacuum emanating from the topological crystalline insulator. Using scanning tunneling microscopy and spectroscopy, we investigate the behavior of such edge channels in Pb1-xSnxSe under doping. Once the energy position of the step edge is brought close to the Fermi level, we observe the opening of a correlation gap. The experimental results are rationalized in terms of interaction effects which are enhanced since the electronic density is collapsed to a one-dimensional channel. This constitutes a unique system to study how topology and many-body electronic effects intertwine, which we model theoretically through a Hartree-Fock analysis.

Diboraperylene Diborinic Acid Self-Assembly on Ag(111)-Kagome Flat Band Localized States Imaged by Scanning Tunneling Microscopy and Spectroscopy.

Replacement of sp2-hybridized carbon in polycyclic aromatic hydrocarbons (PAHs) by boron affords electron-deficient π-scaffolds due to the vacant pz-orbital of three-coordinate boron with the potential for pronounced electronic interactions with electron-rich metal surfaces. Using a diboraperylene diborinic acid derivative as precursor and a controlled on-surface non-covalent synthesis approach, we report on a self-assembled chiral supramolecular kagome network on an Ag(111) surface stabilized by intermolecular hydrogen-bonding interactions at low temperature. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal a flat band at ca. 0.33 eV above the Fermi level which is localized at the molecule center, in good agreement with tight-binding model calculations of flat bands characteristic for kagome lattices.

Controlled Formation of Porous 2D Lattices from C3 -symmetric Ph6 -Me-Tribenzotriquinacene-OAc3.

The on-surface self-assembly of molecules to form holey nanographenes is a promising approach to control the properties of the resulting 2D lattice. Usually, planar molecules are utilized to prepare flat, structurally confined molecular layers, with only a few recent examples of warped precursors. However, control of the superstructures is limited thus far. Herein, we report the temperature-controlled self-assembly of a bowl-shaped, acetylated C3 -symmetric hexaphenyltribenzotriquinacene derivative on Cu(111). Combining scanning tunneling microscopy (STM) and density functional theory (DFT) confirms the formation of highly differing arrangements starting with π-stacked bowl-to-bowl dimers at low coverage at room temperature via chiral honeycomb structures, an intermediate trigonal superstructure, followed by a fully carbon-based, flattened hexagonal superstructure formed by on-surface deacetylation, which is proposed as a precursor for holey graphene networks with unique defect structures.

Real-space resolved surface reactions: deprotonation and metalation of phthalocyanine.

Upon deposition on a surface, molecules can undergo a plethora of changes, such as reactions with adsorbates and surface atoms and catalytic decomposition. Since different reaction pathways may coexist, spatially averaging techniques can be insufficient for the characterization and distinction of all on-surface products. Here, we present a study of single phthalocyanine molecules on a Cu(111) surface which was performed using high-resolution low-temperature STM. Upon deposition of metal-free H2Pc, we can identify three distinct molecular species. A thorough investigation reveals that temperature-driven on-surface reactions partially convert H2Pc into H0Pc and CuPc. The individual species are differentiated by their topographic appearance and can unambiguously be identified by their STM-induced rotational behavior. While H2Pc shows a switching between two orientations at low energies, a third orientation can be observed above E > 800 meV, which is induced by tautomerization. Around the Fermi level, the rotational behavior is asymmetric, owing to the excitation of vibrational modes in unoccupied states whereas resonant tunneling occurs in occupied states. A two-step deprotonation of H2Pc confirms that the second species is H0Pc. By comparison with CuPc evaporated on Cu(111), we unambiguously reveal that the third species is indeed CuPc, which exhibits an exceptionally low threshold for rotational switching accompanied by an asymmetric behavior around the Fermi level. Varying the post-annealing temperature, we found a sharp threshold for the H2Pc → CuPc on-surface metalation at around 100 °C. In contrast, the competing process of thermal decomposition from H2Pc to H0Pc only increases weakly.

Anisotropic coupling of individual vibrational modes to a Cu(110) substrate.

We present a study on the excitation of individual vibrational modes with ballistic charge carriers propagating along the Cu(110) surface. By means of the molecular nanoprobe technique, where the reversible switching of a molecule-in this case tautomerization of porphycene-is utilized to detect excitation events, we reveal anisotropic coupling of two distinct vibrational modes to the substrate. The N-H bending mode, excited below |E| ≈ 376 meV, exhibits maxima perpendicular to the rows of the Cu(110) substrate and minima along the rows. In contrast, the N-H stretching mode, excited above |E| ≈ 376 meV, displays maxima along the rows and is constant otherwise. This inversion of the anisotropy reflects the orthogonality between the N-H bending and stretching mode. Additionally, we observe an energy-dependent asymmetry in the propagation direction of charge carriers injected into the Cu(110) surface state. Hereby, the anisotropic band structure results in a correlation between the group velocity and the tunneling probability into electronic states of the substrate.

Brain proteome profiling implicates the complement and coagulation cascade in multiple system atrophy brain pathology.

BACKGROUND: Multiple system atrophy (MSA) is a rare, progressive, neurodegenerative disorder presenting glia pathology. Still, disease etiology and pathophysiology are unknown, but neuro-inflammation and vascular disruption may be contributing factors to the disease progression. Here, we performed an ex vivo deep proteome profiling of the prefrontal cortex of MSA patients to reveal disease-relevant molecular neuropathological processes. Observations were validated in plasma and cerebrospinal fluid (CSF) of novel cross-sectional patient cohorts. METHODS: Brains from 45 MSA patients and 30 normal controls (CTRLs) were included. Brain samples were homogenized and trypsinized for peptide formation and analyzed by high-performance liquid chromatography tandem mass spectrometry (LC-MS/MS). Results were supplemented by western blotting, immuno-capture, tissue clearing and 3D imaging, immunohistochemistry and immunofluorescence. Subsequent measurements of glial fibrillary acid protein (GFAP) and neuro-filament light chain (NFL) levels were performed by immunoblotting in plasma of 20 MSA patients and 20 CTRLs. Finally, we performed a proteome profiling of 144 CSF samples from MSA and CTRLs, as well as other parkinsonian disorders. Data were analyzed using relevant parametric and non-parametric two-sample tests or linear regression tests followed by post hoc tests corrected for multiple testing. Additionally, high-throughput bioinformatic analyses were applied. RESULTS: We quantified more than 4,000 proteins across samples and identified 49 differentially expressed proteins with significantly different abundances in MSA patients compared with CTRLs. Pathway analyses showed enrichment of processes related to fibrinolysis and complement cascade activation. Increased fibrinogen subunit ß (FGB) protein levels were further verified, and we identified an enriched recognition of FGB by IgGs as well as intra-parenchymal accumulation around blood vessels. We corroborated blood-brain barrier leakage by a significant increase in GFAP and NFL plasma levels in MSA patients that correlated to disease severity and/or duration. Proteome profiling of CSF samples acquired during the disease course, confirmed increased total fibrinogen levels and immune-related components in the soluble fraction of MSA patients. This was also true for the other atypical parkinsonian disorders, dementia with Lewy bodies and progressive supra-nuclear palsy, but not for Parkinson's disease patients. CONCLUSION: Our results implicate activation of the fibrinolytic cascade and immune system in the brain as contributing factors in MSA associated with a more severe disease course.

Molecular Chains: Arranging and Programming Logic Gates.

One particularly fascinating vision for charge-operated devices is the controlled assembly of structures from single surface-deposited molecules. Here, we report on the assembly of linear clusters that consist of phthalocyanine (H2Pc) molecules on a Ag(111) surface. The molecules are imaged as well as manipulated with a low-temperature scanning tunneling microscope (STM). Upon deprotonation of every second H2Pc, the resulting HPc molecule exhibits an isomeric bistability which can be used as inputs in logic gates. Combining our STM measurements with density functional theory calculations we show that the HPc isomers exhibit a repulsive electrostatic interaction with adjacent H2Pc molecules which, due to the asymmetric charge distribution on HPc, results in a counterclockwise or clockwise molecule tilt of the latter, thereby defining the logic 0 and 1 of the output. It is shown that information can be relayed along molecule chains over distances equivalent to at least nine molecules.

Anisotropic Ballistic Transport Revealed by Molecular Nanoprobe Experiments.

Atomic-scale charge transport properties are not only of significant fundamental interest but also highly relevant for numerous technical applications. However, experimental methods that are capable of detecting charge transport at the relevant single-digit nanometer length scale are scarce. Here we report on molecular nanoprobe experiments on Pd(110), where we use the charge carrier-driven switching of a single cis-2-butene molecule to detect ballistic transport properties over length scales of a few nanometers. Our data demonstrate a striking angular dependence with a dip in the charge transport along the [11[over ¯]0]-oriented atomic rows and a peak in the transverse [001] direction. The narrow angular width of both features and distance-dependent measurements suggest that the nanometer-scale ballistic transport properties of metallic surfaces are significantly influenced by the atomic structure.

Systematic Investigation of the Coupling between One-Dimensional Edge States of a Topological Crystalline Insulator.

The interaction of spin-polarized one-dimensional (1D) topological edge modes localized along single-atomic steps of the topological crystalline insulator Pb_{0.7}Sn_{0.3}Se(001) has been studied systematically by scanning tunneling spectroscopy. Our results reveal that the coupling of adjacent edge modes sets in at a step-to-step distance d_{ss}≤25 nm, resulting in a characteristic splitting of a single peak at the Dirac point in tunneling spectra. Whereas the energy splitting exponentially increases with decreasing d_{ss} for single-atomic steps running almost parallel, we find no splitting for single-atomic step edges under an angle of 90°. The results are discussed in terms of overlapping wave functions with p_{x}, p_{y} orbital character.

A novel PDGFRB sequence variant in a family with a mild form of primary familial brain calcification: a case report and a review of the literature.

BACKGROUND: Primary familial brain calcification is a rare autosomal dominant or recessive neurodegenerative disease, characterized by bilateral brain calcifications in different areas of the brain. It is a clinically heterogeneous disease and patients are reported to exhibit a wide spectrum of neurological and psychiatric symptoms. Mutations in five genes have been identified so far including SLC20A2, PDGFRB, PDGFB, XPR1, and MYORG. PDGFRB encodes the platelet-derived growth factor receptor-beta, and is expressed in neurons, vascular smooth muscle cells and pericytes. Patients with a PDGFRB mutation seem to exhibit a milder phenotype and milder brain calcification on brain imaging than patients with SLC20A2 and PDGFB mutations. However, this is based on a few observations so far. CASE PRESENTATION: We present a Danish family with bilateral brain calcifications and mild clinical symptoms of primary familial brain calcification, segregating with a novel PDGFRB sequence variant: c.1834G > A; p.(Gly612Arg), detected by whole exome sequencing. The variant results in physiochemical changes at the amino acid level, and affects a highly conserved nucleotide as well as amino acid. It is located in the tyrosine kinase domain of PDGFRß. Segregation analysis and in silico analyses predicted the missense variant to be disease causing. CONCLUSION: Our study confirms that PDGFRB mutation carriers in general have a mild clinical phenotype, and basal ganglia calcifications can be detected by a CT scan, also in asymptomatic mutation carriers.

Analyzing the Wave Nature of Hot Electrons with a Molecular Nanoprobe.

We report on a novel method, the molecular nanoprobe (MONA) technique, which allows us to measure the nanoscale quasiparticle transport between two arbitrary surface points. In these experiments, hot electrons are injected into the sample surface from the probe tip of a scanning tunneling microscope (STM) and detected by tautomerization switching events of a single deprotonated phthalocyanine (H2Pc) molecule. By making use of atom-by-atom-engineered interferometers on a Ag(111) surface, we demonstrate that the quantum-mechanical wave nature of hot electrons leads to characteristic oscillations of the molecule tautomerization probability. Two interferometers can be combined to build an energy-dependent selector, which allows it to selectively switch one out of two molecules without changing the position of the STM tip. The MONA technique is compared with conventional d I/d U measurements, where the injection and detection point of hot electrons is intrinsically tied to the same tip location.

Jahn-Teller Splitting in Single Adsorbed Molecules Revealed by Isospin-Flip Excitations.

Scanning tunneling spectroscopy measurements of Mn phthalocyanine (MnPc) molecules adsorbed on (sqrt[3]×sqrt[3]) surface alloys show single inelastic steps at exclusively positive or negative bias strongly depending on the tip position. This is in contrast to conventional molecular excitation thresholds, which are independent of the current direction and therefore always occur at both positive and negative bias. This polarity selectivity is found to coincide with the spatial distribution of occupied and empty orbitals. Because of the interaction with the substrate, charge transfer into the doubly degenerate d_{π} orbitals of MnPc takes place. The resulting Jahn-Teller effect lifts the degeneracy and leads to an isospin- or pseudospin-flip excitation, the inelastic analogue of an orbital Kondo resonance.

Magnetic Ground State Stabilized by Three-Site Interactions: Fe/Rh(111).

We report the direct observation of a theoretically predicted magnetic ground state in a monolayer Fe on Rh(111), which is referred to as an up-up-down-down (↑↑↓↓) double-row-wise antiferromagnetic spin structure, using spin-polarized scanning tunneling microscopy. This exotic phase, which exists in three orientational domains, is revealed by experiments with magnetic probe tips performed in external magnetic fields. It is shown that a hitherto unconsidered four-spin-three-site beyond-Heisenberg interaction distinctly contributes to the spin coupling of atoms with S≥1 spins. The observation of the ↑↑↓↓ order substantiates the presence of higher-order, in particular, three-site interactions, in thin magnetic films of itinerant magnets.

Remote Single-Molecule Switching: Identification and Nanoengineering of Hot Electron-Induced Tautomerization.

Molecular electronics where single molecules perform basic functionalities of digital circuits is a fascinating concept that one day may augment or even replace nowadays semiconductor technologies. The tautomerization of molecules, that is, the bistable functional position of hydrogen protons within an organic frame, has recently been intensively discussed as a potential avenue toward nanoscale switches. It has been shown that tautomerization can be triggered locally or nonlocally, that is, by a scanning tunneling microscope (STM) tip positioned directly above or in close vicinity to the molecule. Whereas consensus exists that local switching is caused by inelastic electrons that excite vibrational molecular modes, the detailed processes responsible for nonlocal tautomerization switching and, even more important in the context of this work, methods to control, engineer, and potentially utilize this process are largely unknown. Here, we demonstrate for dehydrogenated H2Pc molecules on Ag(111) how to controllably decrease or increase the probability of nonlocal, hot electron-induced tautomerization by atom-by-atom designed Ag nanostructures. We show that Ag atom walls act as potential barriers that exponentially damp the hot electron current between the injection point and the molecule, reducing the switching probability by up to 83% for a four-atom wide wall. By placing the molecule in one and the STM tip in the other focal point of an elliptical nanostructure, we could coherently focus hot electrons onto the molecule that led to an almost tripled switching probability. Furthermore, single and double slit experiment based on silver atom structures were used to characterize the spatial extension of hot electron packets. The absence of any detectable interference pattern suggests that the coherence length of the hot electrons that trigger tautomerization processes is rather short. Our results demonstrate that the tautomerization switching of single molecules can remotely be controlled by utilizing suitable nanostructures and may pave the way for designing new tautomerization-based switches.

Relevance of hybridization and filling of 3d orbitals for the Kondo effect in transition metal phthalocyanines.

Magnetic organic molecules, such as 3d transition metal phthalocyanines (TMPc), exhibit properties which make them promising candidates for future applications in magnetic data storage or spin-based data processing. Due to their small size, however, TMPc molecules are prone to quantum effects. For example, the interaction of uncompensated molecular spins with conduction electrons of the substrate may lead to the formation of a many-body singlet state, which gives rise to the so-called Kondo effect. Although the Kondo effect of TMPc molecules has been the object of several investigations, a consistent picture to describe under which conditions a Kondo state is formed is still missing. Here, we study the Kondo properties of MnPc on Ag(001) by means of the low-temperature scanning tunneling spectroscopy (LT-STS) measurements. Differential conductance dI/dU spectra reveal a zero-bias peak that is localized on the Mn ion site. Ab initio calculations combined with a many-body treatment of the multiorbital interaction show that the local Hund coupling favors the high-spin configuration on the 3d shell of the central TM atom. Therefore, each orbital gets close to its individual half-filling creating the necessary condition for many of the 3d orbitals to contribute to the observed Kondo resonance. This, however, happens only for the 3dz(2) orbital, whose hybridization to the substrate is much stronger than for the other orbitals thanks to its shape and its orientation.

Hysteretic melting transition of a soliton lattice in a commensurate charge modulation.

We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe2 from transport and scanning tunneling microscopy (STM) studies. Below the transition (TC≈275 K on cooling), a q=1/5 charge modulation was observed, which is consistent with previous studies. Additional modulations [qn=(3n+2)(-1)] appear below a second transition at TS≈180 K on cooling. The coexistence of various modulations persists up to TC on warming. The atomic structures of charge modulations and the temperature-dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice that partially melts below TS on cooling. Our results provide compelling evidence that the ground state of IrTe2 is a commensurate 1/6 charge modulation, which originates from the periodic dimerization of Te atoms visualized by atomically resolved STM images.

Spin-polarized scanning tunneling microscopy: breakthroughs and highlights.

The principle of scanning tunneling microscopy, an imaging method with atomic resolution capability invented by Binnig and Rohrer in 1982, can be adapted for surface magnetism studies by using magnetic probe tips. The contrast mechanism of this so-called spin-polarized scanning tunneling microscopy, or SP-STM, relies on the tunneling magneto-resistance effect, i.e. the tip-sample distance as well as the differential conductance depend on the relative magnetic orientation of tip and sample. To illustrate the working principle and the unique capabilities of SP-STM, this compilation presents some key experiments which have been performed on various magnetic surfaces, such as the topological antiferromagnet Cr(001), a double-layer of Fe which exhibits a stripe- domain pattern with about 50 nm periodicity, and the Mn monolayer on W(110), where the combination of experiment and theory reveal an antiferromagnetic spin cycloid. Recent experimental results also demonstrate the suitability of SP-STM for studies of dynamic properties, such as the spin relaxation time of single magnetic nanostructures.

Temperature dependence of metal-organic heteroepitaxy.

The nucleation and growth of 2D layers of tetraphenyl porphyrin molecules on Ag(111) are studied with variable-temperature scanning tunneling microscopy. The organic/metal heteroepitaxy occurs by strict analogy to established principles for metal heteroepitaxy. A hierarchy of energy barriers for diffusion on terraces and along edges and around corners of adislands is established. The temperature is key to activating these barriers selectively, thus determining the shape of the organic aggregates, from a fractal shape at lower temperatures to a compact shape at higher temperatures. The energy barriers for the terrace diffusion of porpyrins and the molecule-molecule binding energy were determined to be 30 meV < E(terrace) < 60 and 130 meV < E(diss) < 160 meV, respectively, from measurements of island sizes as a function of temperature. This study provides an experimental verification of the validity of current models of epitaxy for the heteroepitaxy of organics and is thus expected to help establish design principles for complex metal-organic hybrid structures.

Patterning graphene at the nanometer scale via hydrogen desorption.

We have demonstrated the reversible and local modification of the electronic properties of graphene by hydrogen passivation and subsequent electron-stimulated hydrogen desorption with an scanning tunneling microscope tip. In addition to changing the morphology, we show that the hydrogen passivation is stable at room temperature and modifies the electronic properties of graphene, opening a gap in the local density of states. This insulating state is reversed by local desorption of the hydrogen, and the unaltered electronic properties of graphene are recovered. Using this mechanism, we have "written" graphene patterns on nanometer length scales. For patterned regions that are roughly 20 nm or greater, the inherent electronic properties of graphene are completely recovered. Below 20 nm we observe dramatic variations in the electronic properties of the graphene as a function of pattern size. This reversible and local mechanism for modifying the electronic properties of graphene has far-reaching implications for nanoscale circuitry fabricated from this revolutionary material.