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Nanographenes (NGs) and graphene nanoribbons (GNRs) are unique connectors between the domains of 1D-conjugated polymers and 2D-graphenes. They can be synthesized with high precision by oxidative flattening processes from dendritic or branched 3D-polyphenylene precursors. Their size, shape and edge type enable not only accurate control of classical (opto)electronic properties, but also access to unprecedented high-spin structures and exotic quantum states. NGs and GNRs serve as active components of devices such as field-effect transistors and as ideal objects for nanoscience. This field of research includes their synthesis after the deposition of suitable monomers on surfaces. An additional advantage of this novel concept is in situ monitoring of the reactions by scanning tunnelling microscopy and electronic characterization of the products by scanning tunnelling spectroscopy.

An electron is usually considered to have only one type of kinetic energy, but could it have more, for both its spin and charge, or by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the linear Tomonaga- Luttinger Liquid (TLL) model, with hallmark predictions, such as spin-charge separation, having already been observed. Recent theoretical work has focused on extending the theory to deal with more realistic curved dispersions. However, experimental realisations have remained elusive until recently. Here, we report on measurements of many-body modes using a momentum-resolved tunnelling spectroscopy technique in gated 1D wires connected via air-bridges. We map the 1D dispersion in a variety of devices, both in and out of equilibrium, and observe the formation of two separate Fermi seas, associated with spin and charge excitations, at energies up to five times that of the Fermi energy, which cannot be accounted for by the noninteracting model. By reducing the length of the system, we also observe the emergence of higher-order ‘replica’ parabolic dispersions with higher momenta or negative effective mass, which is consistent with one of the leading nonlinear Luttinger theories. Determining the bare electron mass m_0 in crystals is also another problem often hindered by many-body effects. Here, Fermi-liquid physics renormalises the band mass, making the observed values density-dependent. In 1D geometries, however, the effect of interactions is strongly amplified, and they naturally decouple from the mass. By changing the level of confinement in the wires, we are able to tune the electron density down to about 18 electrons per micron or, equivalently, an interaction parameter of rs~4. This allows us to extract a constant bare mass value of m_0 = 0.0545m_e, about 20% lighter than observed in GaAs in geometries of higher dimensionality. Finally, by progressively occupying more 1D subbands, our system also allows us to change the amount of inter-subband screening by over 50%, consequently varying the effective interaction strength, in situ, all the way from the weakly to the strongly interacting regime. This ability is the first of its kind for 1D solid-state system. Our spectroscopy technique therefore offers itself as an important and powerful tool for probing strongly correlated systems where new emerging phenomena are occurring. This knowledge is now being used to develop systems for improved energy efficiency and could prove useful in designing the next generation of sustainable materials., Clare-Yale Travel Grant

We report on the fabrication of a clean, stable in room atmosphere and superconducting surface consisting of graphene on tantalum carbide (TaC) obtained by high-temperature annealing of a tantalum thin film on SiC. Low temperature scanning tunnelling spectroscopy reveals that the surface is superconducting with a superconducting order parameter up to 1.6 meV. In addition, atomically-resolved imaging establishes the chemical inertness of the surface. This result is achieved because few graphene layers protect the surface superconductivity of the buried superconducting TaC formed during the high-temperature annealing.

In this thesis, a technique is developed to manipulate individual atoms on an ionic surface, with great precision and at a large scale, to study the quantum mechanical properties of atomic assemblies on the nanoscale.We use the needle of a scanning tunnelling microscope (STM) to approach missing atoms - vacancies - in a chlorine monolayer on a copper crystal, inducing a neighbouring Cl atom to jump to the vacancy position by ramping up the tunnel current. This procedure is automated - with sometimes up to 99% reliability - to construct a 1 kB memory where each bit is represented by an atom-vacancy pair. The data storage is stable at low temperatures and can be rewritten automatically, leading to an information density of 502 terabits per square inch, or 0.778 bits/nm^2.Atom manipulation is then used to build other one- and two-dimensional structures with varying sizes and atom densities. In artificial crystals made of vacancies, standing wave patterns are observed at certain energies, suggesting that it is possible to tune electronic properties of the material, such as the dispersion, by controlling the local geometry with atomic assembly.In the rest of the thesis, more structures were built by atom manipulation in order to investigate the coupling between assemblies of vacancies that form 'artificial molecules'. Resonances in scanning tunnelling spectroscopy measurements indicate the existence of quantum dots on the apex of the STM tip, of which the properties are explored. The chlorine terminated copper surface is also investigated for its use as a decoupling layer suitable for magnetic adatoms.

The spin Hall effect, which arises from the spin–orbit interaction, is expected to be energy dependent, but experiments typically only characterize electrons near the Fermi surface. A tunnelling spectroscopy method has now been developed to probe the energy dependence.

Understanding the origin of superconductivity in strongly correlated electron systems continues to be at the forefront of unsolved problems in all of physics. Among the heavy f-electron systems, CeCoIn5 is one of the most fascinating, as it shares many of the characteristics of correlated d-electron high-Tc cuprate and pnictide superconductors, including the competition between antiferromagnetism and superconductivity. While there has been evidence for unconventional pairing in this compound, high-resolution spectroscopic measurements of the superconducting state have been lacking. Previously, we have used high-resolution scanning tunneling microscopy techniques to visualize the emergence of heavy-fermion excitations in CeCoIn5 and demonstrate the composite nature of these excitations well above Tc. Here we extend these techniques to much lower temperatures to investigate how superconductivity develops within a strongly correlated band of composite excitations. We find the spectrum of heavy excitations to be strongly modified just prior to the onset of superconductivity by a suppression of the spectral weight near the Fermi energy, reminiscent of the pseudogap state in the cuprates. By measuring the response of superconductivity to various perturbations, through both quasiparticle interference and local pair-breaking experiments, we demonstrate the nodal d-wave character of superconducting pairing in CeCoIn5., Advanced online publication in Nature Physics. 15 pages, 5 figures, Supplementary Information

The electronic properties of the granular and intergranular sites of passive films grown on Ni(1 1 1) single-crystal surfaces in alkaline solution have been studied with scanning tunnelling spectroscopy. The results show no effect of the grain boundaries on the intrinsic semiconductive properties of the single layer hydroxide passive film. In contrast the local properties of the duplex oxide/hydroxide passive film are changed from nearly intrinsic on the grains to p -type at the grain boundaries. The implications of these local modifications of the electronic properties on oxidative and reductive electronic transfer and on the corrosion resistance are discussed.

In this chapter I detail the main experimental technique used in this thesis: Spectroscopic-Imaging Scanning Tunnelling Microscopy (SI-STM). I outline the theoretical basis for its use as a tunnelling spectroscopy of superconductors as well as data analysis techniques specific to cuprate superconductors. I also give details of the cuprate samples studied.

We report the local atomic manipulation properties of chemisorbed toluene molecules on the Si(111)-7x7 surface and of the silicon adatoms of the surface. Charge injected directly into the molecule, or into its underlying bonding silicon adatom, can induce the molecule to change bonding site. The voltage dependence of the rates of these processes match closely with scanning tunnelling spectroscopy of the toluene and adatom species. The branching ratio between toluene molecules which are moved to a neighbouring site, or those that travel further is invariant to voltage, suggesting a common final manipulation step for both injection into the molecule and into the bonding adatom site. At low temperatures the rate of silicon adatom manipulation matches that of toluene manipulation, further suggesting that all these manipulation processes are driven by electronic excitation of the underlying silicon surface. Our results therefore suggest that a common non-adiabatic process mediates atomic and molecular manipulation induced by the STM on the Si(111)-7x7 surface and may also mediate similar manipulation induced by the laser irradiation of the Si(111)-7x7 surface.

On-surface Ullmann coupling reactions of dibromo-biphenyl and diiodo-biphenyl molecules on Ag(111), Cu(111), Au(111) and Au(100) and by deposited extrinsic Ag and Ni atoms were systematically explored. Poly-p-phenylene polymer chains turned out to be the final coupling products. Different organometallic polymer chains, [–Ph2Ag–]n, [–Ph2Cu–]n, and [–Ph2Ni–]n, were synthesized as stable intermediates during the coupling reactions. These intermediates were confirmed and verified by X-ray photoelectronic spectroscopy, scanning tunnelling spectroscopy and tip manipulations. On Au surfaces, coupling products were achieved directly from halogenated precursors without formation of the Au-containing organometallic intermediates. Magnetic organometallic polymer chains were prepared by Ullmann coupling of dibromo-biphenyl activated by extrinsic Ni atoms on Au(100) surface and were confirmed by Kondo resonance measurement. Such an on-surface preparation strategy may provide an efficient approach to preparing unentangled organometallic and pure organic polymer chains and subsequently measuring their properties at surfaces.

We have measured the elastic and inelastic tunnelling properties of epitaxial graphene on SiC(0001) using cryogenic scanning tunnelling spectroscopy. We find that the dominant inelastic channel of the out-of-plane acoustic graphene phonon at 70 mV is spatially localized to particular regions of the graphene–SiC system that contain localized states. At these locations the maximum inelastic tunnelling channel reaches up to half of the total tunnelling current. The local enhancement of the inelastic tunnelling is found at the localized electron states of the graphene/SiC interface layer. Nonequilibrium Green's function formalism theory calculations indicate that this intense inelastic channel arises from graphene phonon modes strongly coupled to narrow electron states.

In the version of this Article originally published, an error during typesetting led to the curve in Fig. 2a being shifted to the right, and the curves in the inset of Fig. 2a being displaced. The figure has now been corrected in all versions of the Article; the original and corrected Fig. 2a are shown below.

Single-walled carbon nanotubes provide an ideal system for studying the properties of one-dimensional (1D) materials, where strong electron-electron interactions are expected. Optical measurements have recently reported the existence of excitons in semiconducting nanotubes, revealing the importance of many-body effects. Surprisingly, pioneering electronic structure calculations and scanning tunnelling spectroscopy (STS) experiments report the same gap values as optical experiments. Here, an experimental STS study of the bandgap of single-walled semiconducting nanotubes, demonstrates a continuous transition from the gap reduced by the screening resulting from the metal substrate to the intrinsic gap dominated by many-body interactions. These results provide a deeper knowledge of many-body interactions in these 1D systems and a better understanding of their electronic properties, which is a prerequisite for any application of nanotubes in the ultimate device miniaturization for molecular electronics, or spintronics.

A first-principles computational method is developed to study the inelastic electron tunnelling spectroscopy (IETS) of 4,4'-biphenyldithiol molecular junction with three different contact structures between the molecule and electrodes in the nonresonant regime. The obtained distinct IETS can be used to resolve the geometrical structure of the molecular junction. The computational results demonstrate that the IETS has certain selection rule for vibrational modes, where the longitudinal modes with the same direction as the tunnelling current have greatest contribution to the IETS. The thermal effect on the IETS is also displayed.

The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multi-Q magnetic ground states, hosting unusual states such as chiral density waves and quantum Hall insulators. Distinguishing single-Q and multi-Q magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states (LDOS) near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between single-Q and multi-Q phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors—one single-Q and two double-Q phases. Our results open a promising avenue to investigate the complex magnetic configurations in itinerant systems via standard scanning tunnelling spectroscopy, without requiring spin-resolved capability., It remains difficult to distinguish single-Q and multi-Q magnetic states experimentally. Here, Gastiasoro et al. show that the magnetic configuration of an itinerant system can be mapped out to the local density of states near a magnetic impurity, distinguishing unambiguously between single-Q and multi-Q phases.

The discovery of high-temperature superconductivity in Fe-based compounds triggered numerous investigations on the interplay between superconductivity and magnetism, and on the enhancement of transition temperatures through interface effects. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture remains elusive because of the lack of atomically resolved data. Here we present spin-polarized scanning tunnelling spectroscopy of ultrathin FeTe1−xSex (x=0, 0.5) films on bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level, indicating superconducting correlations up to Tc∼6 K for one unit cell FeTe grown on Bi2Te3, in contrast to the non-superconducting bulk FeTe. The gap spatially coexists with bi-collinear AFM order. This finding opens perspectives for theoretical studies of competing orders in Fe-based superconductors and for experimental investigations of exotic phases in superconducting layers on topological insulators., The microscopic picture of how superconductivity is linked to antiferromagnetic order in Fe-based compounds remains elusive. Here, Manna et al. report superconducting correlations which spatially coexist with bi-collinear antiferromagnetic order in a one unit cell thin layer of FeTe grown on Bi2Te3.

We report the formation and local electronic structure of Ge clusters on the Si(111)- 7 x 7 surface studied by using variable temperature scanning tunnelling microscopy (VT-STM) and low-temperature scanning tunnelling spectroscopy (STS). Atom-resolved STM images reveal that the Ge atoms are prone to forming clusters with 1.0 nm in diameter for coverage up to 0.12 ML. Such Ge clusters preferentially nucleate at the centre of the faulted-half unit cells, leading to the 'dark sites' of Si centre adatoms from the surrounding three unfaulted-half unit cells in filled-state images. Biasdependent STM images show the charge transfer from the neighbouring Si adatoms to Ge clusters. Low-temperature STS of the Ge clusters reveals that there is a band gap on the Ge cluster and the large voltage threshold is about 0.9 V.

This thesis presents an experimental study of magnetic structures, composed of only a few atoms. Those structures are first built atom-by-atom and then locally probed, both with a low-temperature STM. The technique that we use to assemble them is vertical atom manipulation, while to study their physical properties we use inelastic electron tunneling spectroscopy and spin-polarized STM. In this work we focus on structures made of Fe or Co atoms built on a thin insulating layer (Cu2N).

This thesis presents an experimental study of magnetic structures, composed of only a few atoms. Those structures are first built atom-by-atom and then locally probed, both with a low-temperature STM. The technique that we use to assemble them is vertical atom manipulation, while to study their physical properties we use inelastic electron tunneling spectroscopy and spin-polarized STM. In this work we focus on structures made of Fe or Co atoms built on a thin insulating layer (Cu2N).

A brief review of recent results on interlayer tunnelling spectroscopy of layered charge density wave (CDW) materials is presented, demonstrating the high capability of this method for studies of this electron condensed state. We discuss some features observed in comparison with high temperature superconductors (HTS).

Tunnelling spectroscopy with simultaneous transmission electron microscopy (TEM) was applied to an Fe–MgO composite film in which the Fe nano-particles were sandwiched by two thin MgO layers and nanometer-scale double-barrier tunnel junctions were formed. Using a movable needle-shaped electrode, nano-regions were selected and the geometry of the particle system was deformed. The real-time and simultaneous experiments of the deformation, the tunnel measurement and the TEM observation made it possible to confirm the condition under which the Coulomb staircase clearly appears. The measured staircase period corresponded approximately to the value estimated from the geometry observed by TEM. Thus, the feasibility of this method was verified.

In this paper we present a thorough analysis of two systems magnetically inhomogeneous: the manganite La$_{0.7}$Sr$_{0.3}$MnO$_3$/MgO and the amorphous alloy Cu$_{90}$Co$_{10}$. In both cases, the non-extensive statistics yield a faithful description of the magnetic behavior of the systems. In the model proposed here, the inhomogeneous magnetic system is composed by many Maxwell-Boltzmann homogeneous bits and the entropic parameter $q$ is related to the moments of the distribution of the inhomogeneous quantity. From the analysis of Scanning Tunnelling Spectroscopy (STS) images, the $q$ parameter can be directly measured.

The maximum Josephson current (MJC) is measured for small mesas fabricated on cleaved surfaces of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// single crystals. It is found that the MJC increases dramatically with increasing c-axis conductivity. The behavior indicates that the MJC is proportional to the square of the c-axis conductivity. This implies that the superconductivity in high-T/sub c/ superconductors is likely to be inhomogeneous.

This paper adds to the contents of an earlier review entitled ‘On the early history of field emission including attempts of tunnelling spectroscopy’ (Prog. Surf. Sci. 1993; 42: 101) and covers the period up to 1958. In particular, it notes the work on (what one can now recognize as) field-emission-induced discharge phenomena in the period 1878–1905 and the work on the developing applications of field emission microscopy in the period 1952–1958, in what had become Drechsler's group in Berlin. Copyright © 2004 John Wiley & Sons, Ltd.

Four-probe fine electrodes were fabricated on a Si(100)-2 × 1-H surface using conventional lithographic and scanning-probe nano-fabrication techniques followed by standard cleaning and hydrogenation processes of the surface. It was confirmed by scanning tunnelling microscopy that a Si(100)-2 × 1-H surface is formed in the area except for the fine electrodes. Tunnelling spectroscopy of the electrode surface and electrical conduction measurements of nanoscale wires were performed for evaluating the fabricated electrodes. The results showed that the fabricated fine electrodes are capable of measuring the electrical properties of nanoscale surface objects.

A very peculiar feature of the recently discovered superconductor MgB2 is the multigap nature of the superconducting state, which is now commonly accepted in the scientific community and confirmed by a large number of experiments. We report a systematic scanning tunnelling spectroscopy (STS) study performed on high quality thin films of MgB2. Electron microscopy images and STM topography together with the STS investigation allow a direct correlation between tunnelling direction and the observed tunnelling spectra, confirming that the two-gap state is intrinsic to MgB2.

The observations of unusual edge properties in scanning tunnelling spectroscopy and the predicted band structure in photoemission spectra of a monolayer FeSe superconductor reveal its non-trivial topological nature.

The inelastic electron tunnelling spectroscopy (IETS) technique is performed to provide information concerning the vibrational and excitational modes present in MOS tunnelling junctions (molecular species present in silicon dioxide and phonon modes of both silicon substrate and silicon dioxide). The first relevant structures of silicon substrate phonons are investigated in both (1 1 1) and (1 0 0) orientations for the first time. The separated peaks obtained after deconvolution are identified and compared with literature data for both silicon orientations. The IET spectrum of ultra-thin silicon dioxide is also obtained though this spectrum is more difficult to detect. Finally, two different technologies of manufacturing MOS tunnelling junctions are compared.

We report UHV STM/scanning tunnelling spectroscopy (STS) studies on mesoporous TiO 2 films in the presence and absence of adsorbed dye sensitisers (chlorophyll and a Ru bipyridyl dye). Interconnected and aggregated TiO 2 nanocrystallites with mean diameters typically of 30– 80 nm and different pore sizes are observed in STM images. The results show that, under controlled tunnelling conditions, the surface apparent band gap of as-prepared thin films is consistent with the bulk band gap of 3.2 eV , but that a quite different, lower surface apparent band gap is observed when a sensitising dye is adsorbed. Moreover, the characteristic measured in this case is dependent on the choice of dye, showing that the STM I / V characteristic is sensitive to the adsorbed dye, rather than merely to the effect of surface adsorbed water. We discuss possible mechanisms for this effect, and the use of this effect in the optimisation of dye-sensitised photovoltaic cells.

We have studied charge transport in nanopatterned arrays of PbS colloidal quantum dots using conventional two-probe measurements as well as with an integrated chargesensor.

The hallmark of a time-reversal symmetry protected topologically insulating state of matter in two-dimensions (2D) is the existence of chiral edge modes propagating along the perimeter of the system. To date, evidence for such electronic modes has come from experiments on semiconducting heterostructures in the topological phase which showed approximately quantized values of the overall conductance as well as edge-dominated current flow. However, there have not been any spectroscopic measurements to demonstrate the one-dimensional (1D) nature of the edge modes. Among the first systems predicted to be a 2D topological insulator are bilayers of bismuth (Bi) and there have been recent experimental indications of possible topological boundary states at their edges. However, the experiments on such bilayers suffered from irregular structure of their edges or the coupling of the edge states to substrate's bulk states. Here we report scanning tunneling microscopy (STM) experiments which show that a subset of the predicted Bi-bilayers' edge states are decoupled from states of Bi substrate and provide direct spectroscopic evidence of their 1D 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 that are consistent with strong suppression of backscattering as predicted for the propagating topological edge states., 15 pages, 5 figures, and supplementary material

Two methods of detection of a single electron spin, namely, optical detection (1993) and tunnelling spectroscopy (1998) are described. Both methods are based on the spin chemistry laws. Physical principles and prospects of application of these methods are considered. The bibliography includes 34 references.

The model of superconductivity in the intercalated fullerenes based on arising of the valence bond between two neighbouring alkali-metal atoms is proposed. This model explains all the currently available experimental data concerning superconductivity in these compounds. Among them are the absence of superconductivity in A x C60 with even x, the dependence of the critical temperature on the disorder level and the carrier concentration, and the shape of the density of states near the Fermi level as a function of voltage in tunnelling spectroscopy.

MBE grown InAs/AlSb heterostructures with AlAs- and InSb-type interfaces were investigated by tunnelling spectroscopy using a scanning tunnelling microscope (STM). STM surface topography revealed a different electronic surface morphology of the thin InAs/AlSb heterostructure in the case of AlAs and InSb interfaces. Scanning tunnelling spectroscopy (STS) gives an understanding of the origin of local morphology fluctuations. STS suggests the occurrence of classical and quantum effects influencing the energy-band-structure formation in the case of these thin heterostructures.

The results of scanning tunnelling spectroscopy (STS) studies of wet chemical sulphur passivated InP(100) surfaces are presented. The aim of the study was to attempt to establish a more quantitative measure of the surface passivation using the STM. Current voltage ( I-V ) spectroscopic plots for sulphur passivated and native oxide surfaces displayed distinct differences previously reported for GaAs(100) surfaces. Current distance ( I-S ) spectroscopic results for both passivated and unpassivated InP surfaces and the accompanying apparent barrier height (φ A ) measurements are presented along with a justification of the results. For reference purposes, φ A measurements of the Au(111) surface were made to establish the validity of the ( I-S ) spectroscopic technique in air. The chemical composition of the InP(100) surface before and after sulphur passivation is determined by X-ray photoelectron spectroscopy (XPS) measurements.

A simulation methodology to model tunnelling spectroscopy measurements based on the Price-Radcliffe formalism has been developed within a finite element device simulator. The tip-sample system is modelled self-consistently including tip-induced bending and realistic tip shapes. The resulting spectra of III–V semiconductors are compared against experimental results and a model based on the Bardeen tunnelling approach with very good agreement. We have found that the image force induced barrier lowering increases the tunnelling current by three orders of magnitude when tunnelling to the sample valence band, and by six orders of magnitude when tunnelling to the sample conduction band. The work shows that other models which use a single weighting factor to account for image force in the conduction and valence band are likely to be underestimating the valence band current by three orders of magnitude.

The pairing mechanism in the iron pnictides remains unresolved yet. One of the central issues is the structure of the superconducting order parameter which classifies the community into two different and highly disputed camps. On one hand the picture of pairing based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S+- pairing. On the other hand, a more conventional S++ pairing gap was suggested based on the phonon or orbital fluctuation mediated pairing. In the superconducting state, the impurities may generate a unique pattern of local density of states in space and energy, which are regarded as the fingerprints for checking the structure of the pairing gap. In this study, we successfully identified the non-magnetic and magnetic impurities in Na(Fe0.97-xCo0.03Tx)As (T=Cu, Mn) and investigated the spatial resolved scanning tunneling spectroscopy. We present clear evidence of the in-gap quasiparticle states induced by the nonmagnetic Cu impurities, giving decisive evidence of the S+- pairing. This is corroborated by the consistency between the experimental data and the first-principles calculations based on the S+- pairing gap with a scalar scattering potential., Comment: Body text 17 pages with 5 figures; Supplementary Materials 12 pages with 5 figues

Since the discovery of high temperature superconductivity in the iron pnictides and chalcogenides in early 2008, a central issue has been the microscopic origin of the superconducting pairing. Although previous experiments suggest that the pairing may be induced by exchanging the antiferromagnetic spin fluctuations and the superconducting order parameter has opposite signs in the electron and hole pockets as predicted by the S+- pairing model, it remains unclear whether there is a bosonic mode from the tunneling spectrum which has a close and universal relationship with superconductivity as well as the spin excitation. In this paper, based on the measurements of scanning tunneling spectroscopy, we show the clear evidence of a bosonic mode with the energy identical to that of the neutron spin resonance in two completely different systems Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As with different superconducting transition temperatures. In both samples, the superconducting coherence peaks and the mode feature vanish simultaneously inside the vortex core or above Tc, indicating a close relationship between superconductivity and the bosonic mode. Our data also demonstrate a universal ratio between the mode energy and superconducting transition temperature, that is [mode energy]/kBTc ~ 4.3, which underlines the unconventional mechanism of superconductivity in the iron pnictide superconductors., 13 pages, 10 figures

Topological insulators are a class of semiconductor exhibiting charge-gapped insulating behaviour in the bulk, but hosting a spin-polarized massless Dirac electron state at the surface. The presence of a topologically protected helical edge channel has been verified for the vacuum-facing surface of several topological insulators by means of angle-resolved photoemission spectroscopy and scanning tunnelling microscopy. By performing tunnelling spectroscopy on heterojunction devices composed of p-type topological insulator (Bi1−xSbx)2Te3 and n-type conventional semiconductor InP, we report the observation of such states at the solid-state interface. Under an applied magnetic field, we observe a resonance in the tunnelling conductance through the heterojunction due to the formation of Landau levels of two-dimensional Dirac electrons at the interface. Moreover, resonant tunnelling spectroscopy reveals a systematic dependence of the Fermi velocity and Dirac point energy on the composition x. The successful formation of robust non-trivial edge channels at a solid-state interface is an essential step towards functional junctions based on topological insulators.