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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.

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

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 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.

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.

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.

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 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 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

We have used tunnelling spectroscopy (TS) to probe the density of states (DOS) near the Fermi level of , which shows giant magnetoresistance (GMR). The tunnelling current - voltage (I - V) curves and their derivatives show significant changes for temperatures above and below the ferromagnetic Curie temperature . We have estimated the energy dependence of the DOS near from the tunnelling spectroscopy data, and this shows a strong temperature dependence, for temperatures above and below .

Although the mechanism of superconductivity in the cuprates remains elusive, it is generally agreed that at the heart of the problem is the physics of doped Mott insulators. The cuprate parent compound has one unpaired electron per Cu site, and is predicted by band theory to be a half-filled metal. The strong onsite Coulomb repulsion, however, prohibits electron hopping between neighboring sites and leads to a Mott insulator ground state with antiferromagnetic (AF) ordering. Charge carriers doped into the CuO2 plane destroy the insulating phase and superconductivity emerges as the carrier density is sufficiently high. The natural starting point for tackling high Tc superconductivity is to elucidate the electronic structure of the parent Mott insulator and the behavior of a single doped charge. Here we use a scanning tunneling microscope to investigate the atomic scale electronic structure of the Ca2CuO2Cl2 parent Mott insulator of the cuprates. The full electronic spectrum across the Mott-Hubbard gap is uncovered for the first time, which reveals the particle-hole symmetric and spatially uniform Hubbard bands. A single electron donated by surface defect is found to create a broad in-gap electronic state that is strongly localized in space with spatial characteristics intimately related to the AF spin background. The unprecedented real space electronic structure of the parent cuprate sheds important new light on the origion of high Tc superconductivity from the doped Mott insulator perspective., 26 pages, 4 figures, supplementary information included

The energy gap is the most fundamental property of a superconductor. MgB(2), a superconductor discovered in 2001, exhibits two different superconducting gaps caused by the different electron-phonon interactions in two weakly interacting bands. Theoretical calculations predict that the gap values should also vary across the Fermi surface sheets of MgB(2). However, until now, no such variation has been observed. It has been suggested that two gap values were sufficient to describe real MgB(2) samples. Here we present an electron tunnelling spectroscopy study on MgB(2)/native oxide/Pb tunnel junctions that clearly shows a distribution of gap values, confirming the importance of the anisotropic electron-phonon interaction. The gap values, and their spreads found from the tunnel junction measurements, provide valuable experimental tests for various theoretical approaches to the multi-band superconductivity in MgB(2).

Tunnelling measurements were performed on the new heavy-fermion superconductors UPd2Al3 (single crystal, Tc = 1.8 K) and UNi2Al3 (polycrystal, Tc = 1.2 K) and on the magnetic heavy-fermion superconductor URu2Si2 above and below the antiferromagnetic-ordering temperatures TN for T ≥ Tc. Tunnelling along the (a, b)-planes on UPd2Al3 shows that below TN ≈ 14 K an energy gap of about 13 meV is formed in the density of states. In contrast, no gap is found along the c-direction. For polycrystalline UNi2Al3 a gap of 10 meV is found below TN ≈ 4.8 K. Both the observed anisostropy and the values of the gaps of the new compounds prove surprisingly similar to URu2Si2. The values of the gaps appear to be set by crystal field excitation energies.

We study the homogeneously disordered MoC thin lms with thicknesses of 10 and 5 nm and the superconducting transition temperatures near 6 and 4 K, signi cantly decreased as compared to the bulk Tc = 8.32 K due to a disorder. The scanning tunnelling spectroscopy reveals in the thicker sample a BCS superconducting energy gap ∆ with a broadening parameter Γ equal to about 10 per cent of ∆. Remarkably, Γ increases with temperature. The thinner, more disordered sample shows a gapped superconducting density of states but without any coherence peaks at the gap edge, which could not be approximated by the BCS DOS. Moreover, the reduced DOS around the Fermi level persists above the resistive transition temperature reminding the pseudo-gap known from high-Tc cuprates.

We probe the local quasiparticles density of states in micron-sized ${\mathrm{SmFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ single crystals by means of scanning tunnelling spectroscopy. Spectral features resemble those of cuprates, particularly a dip-hump-like structure developed at energies larger than the gap that can be ascribed to the coupling of quasiparticles to a collective mode, quite likely a resonant spin mode. The energy of the collective mode revealed in our study decreases when the pairing strength increases. Our findings support spin-fluctuation-mediated pairing in pnictides.

How small can a sample of superconducting material be and still display superconductivity? This question is relevant to our fundamental understanding of superconductivity, and also to applications in nanoscale electronics, because Joule heating of interconnecting wires is a major problem in nanoscale devices. It has been shown that ultrathin layers of metal can display superconductivity1,2,3, but any limits on the size of superconducting systems remain a mystery. (BETS)2GaCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene, is an organic superconductor, and in bulk it has a superconducting transition temperature Tc of ∼8 K (ref. 4) and a two-dimensional layered structure5,6,7 that is reminiscent of the high-Tc cuprate superconductors8,9. Here, we use scanning tunnelling spectroscopy to show that a single layer of (BETS)2GaCl4 molecules on an Ag(111) surface displays a superconducting gap that increases exponentially with the length of the molecular chain. Moreover, we show that a superconducting gap can still be detected for just four pairs of (BETS)2GaCl4 molecules. Real-space spectroscopic images directly visualize the chains of BETS molecules as the origin of the superconductivity. Real-space scanning tunnelling spectroscopy has provided new insights into the properties of a well-known two-dimensional organic superconductor, including a superconducting gap that increases exponentially with the length of the molecular chain.

The time dependent dielectric breakdown in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions was investigated by voltage ramp experiments. The measurements were done for two types of junctions: one set of junctions had exchange biased (pinned) bottom electrodes and one set had exchange biased (pinned) top electrodes with an additional artificial ferrimagnet. We found a significant polarity dependence in the dielectric breakdown: top as well as bottom pinned tunnel junctions showed higher breakdown voltage when the top electrode was biased positively compared to negative bias. In contrast to this the differential resistance (dV/dI)-V spectra revealed an asymmetry for the top pinned junctions which was reversed in comparison to the bottom pinned system. This indicates that both asymmetries have different origins. Additionally the bottom pinned junctions showed in general slightly lower breakdown voltages and stronger magnon excitation in the inelastic electron tunneling (d(2)I/dV(2))-V spectra than the top pinned junctions. Possible reasons for these correlations are discussed.

Due to the short space available this article introduces into the actual state of tunnelling spectroscopy mainly by referring to recent literature. The interested reader will find there the full information. Topics discussed in this paper are: separation of intra- and intermolecular contributions to the rotational potential, the calculation of tunnel splittings from transferable pair interaction potentials, the effect of pressure and isotopic substitution, tunnelling molecules as probes in matrices, on surfaces and in complex ions, the coupling of tunnel rotors to phonons, and the corresponding evolution of tunnelling transitions to a quasielastic spectrum with increasing temperature. Finally some recent works on spin conversion are presented.

With UHV‐LT‐STM, we studied the tunneling spectroscopy of single crystals of Bi2Sr2CaCu2O8+δ. We find that the inhomogeneity of electronic structure reported in the literature depends strongly on the thermal loop between different temperatures during the measurement.

The complex behaviour of high-temperature superconductors has inspired some complex models and theories, but a conventional model seems to work just fine for scanning tunnelling spectroscopy.

Using a scanning tunnelling microscope, the tunnelling currentvs. voltage is measured between a gold tip and the chemically etched (001) surface of YBa2Cu3Ox crystals at room temperature. The tunnelling barrier height and thickness are derived by modelling the system in the normal state as a conventional N-I-N junction. TheI(V) andG(V) curves are computed for the same junction in the superconducting state (N-I-S junction). The predicted gap structure for a single BCS-Dynes-like gap is simple and has low zero-bias conductance. The complexG(V) spectra measured belowTcon the etched junctions with ∼ 50% zero-bias conductance suggest a possible multilayer contribution in the unit cell.

Vacuum-tunnelling spectroscopy on superconducting heavy-fermion compounds should be a suitable method to gain information on the superconducting order parameter. We constructed a scanning-tunnelling microscope for the use in a dilution refrigerator. Its miniaturised design provides good thermal coupling of the sample and excellent mechanical stability, yielding atomic resolution. We present topographic and spectroscopic results on graphite (HOPG), as well as on classical and heavy-fermion superconductors.

Experiments have used inelastic electron tunnelling spectroscopy as a method of measuring the phonon density of states of the tunnelling barrier. The magnitude of the measured d2I/dV2 characteristic is proportional to the density of phonon states multiplied by a weighting factor, which depends on the strength of the interaction between the electron and the phonon. Without a knowledge of this weighting factor the experiments can only yield qualitative results. The authors present a study of the tunnelling process and derive a weighting factor which allows the phonon density of states to be deduced from the experimental data.

SUMMARY We have designed and built a scanning tunnelling microscope (STM) in order to work at temperatures ranging from 1·2 to 300 K. Tunnelling spectroscopy has been performed in Au and Pb with this STM. Our results on Au show different conductance-voltage behaviours, their relation with the cleaning state of the tip being discussed. For Pb, the fitting of our I-V characteristics to a BCS density of states gives A = 1·25 meV for the superconducting energy gap at about 5 K.

SUMMARY Investigations of charge-density waves (CDW) in 4Hb-TaS2 and the shifted Fermi level in a CoCl2 graphite intercalation compound (GIC) were performed by scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) A discrimination of the surface electronic structure of the octahedral structure and of the trigonal-prismatic structure in 4Hb-TaS2 was made successfully in terms of the electronic properties. The measured value of the charge-density-wave gap in 4Hb-TaS2 at room temperature was about 85 meV. The shifted Fermi level in CoCl2-GIC was measured to be about 400 meV.

Proximity electron tunnelling spectroscopy (PETS) is applied to Ta, yielding results in agreement with those obtained by conventional tunnelling. The inapplicability of a recent 'empirical model' to such clean NS proximity junctions is commented upon.

Multi-terminal superconducting Josephson junctions based on the proximity effect offer the opportunity to tailor non-trivial quantum states in nanoscale weak links. These structures can realize exotic topologies in several dimensions, for example, artificial topological superconductors that are able to support Majorana bound states, and pave the way to emerging quantum technologies and future quantum information schemes. Here we report the realization of a three-terminal Josephson interferometer based on a proximized nanosized weak link. Our tunnelling spectroscopy measurements reveal transitions between gapped (that is, insulating) and gapless (conducting) states that are controlled by the phase configuration of the three superconducting leads connected to the junction. We demonstrate the topological nature of these transitions: a gapless state necessarily occurs between two gapped states of different topological indices, in much the same way that the interface between two insulators of different topologies is necessarily conducting. The topological numbers that characterize such gapped states are given by superconducting phase windings over the two loops that form the Josephson interferometer. As these gapped states cannot be transformed to one another continuously without passing through a gapless condition, they are topologically protected. The same behaviour is found for all of the points of the weak link, confirming that this topology is a non-local property. Our observation of the gapless state is pivotal for enabling phase engineering of different and more sophisticated artificial topological materials., The European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013)/ERC Grant agreement No. 615187-COMANCHE and MIUR-FIRB2013 – Project Coca (Grant No. RBFR1379UX) are acknowledged for partial financial support. The work of E.S. is funded by a Marie Curie Individual Fellowship (MSCA-IFEF-ST No. 660532-SuperMag). The work of F.S.B was partially supported by the Spanish Ministerio de Economia y Competitividad under Project No. FIS2014-55987-P.

We present phonon spectra of very thin evaporated amorphous films of SiOx, obtained by inelastic electron tunnelling spectroscopy. The broad features of the results are similar to those found in neutron-scattering, Raman and infra-red measurements on bulk vitreous SiO2 but there are differences of detail. The results show inelastic tunnelling spectroscopy to be a powerful technique for the study of amorphous systems.

The ability to gate-induce superconductivity by electrostatic charge accumulation is a recent breakthrough in physics and nanoelectronics. With the exception of LaAlO3/SrTiO3 interfaces, experiments on gate-induced superconductors have been largely confined to resistance measurements, which provide very limited information about the superconducting state. Here, we explore gate-induced superconductivity in MoS2 by performing tunnelling spectroscopy to determine the energy-dependent density of states (DOS) for different levels of electron density n. In the superconducting state, the DOS is strongly suppressed at energy smaller than the gap Δ, which is maximum (Δ ~2 meV) for n of ~1 × 1014 cm−2 and decreases monotonously for larger n. A perpendicular magnetic field B generates states at E

A non-trivial temperature evolution of superconductivity including a temperature-induced phase transition between two superconducting phases or even a time-reversal symmetry breaking order parameter is in principle expected in multiband superconductors such as iron-pnictides. Here we present scanning tunnelling spectroscopy data of LiFeAs which reveal two distinct superconducting phases: at "Equation missing" = 18 K a partial superconducting gap opens, evidenced by subtle, yet clear features in the tunnelling spectra, i.e. particle-hole symmetric coherence peak and dip-hump structures. At Tc = 16 K, these features substantiate dramatically and become characteristic of full superconductivity. Remarkably, the distance between the dip-hump structures and the coherence peaks remains practically constant in the whole temperature regimeT ≤ "Equation missing". This rules out the connection of the dip-hump structures to an antiferromagnetic spin resonance.

Improvements to the stability and automatic balancing of an active bridge circuit for inelastic electron tunnelling spectroscopy are outlined. Provision is made in a new bridge circuit for programmable modulation waveforms and bipolar junction biasing.

The reported results of the superconducting energy gap Δ measurements for high-temperature superconductors vary over a wide range even when the same technique is used e.g. the electron tunnelling spectroscopy. More reasonable results are obtained from IR-spectroscopy. This paper gives the results of the energy gap calculation for high-temperature superconductors M Ba 2 Cu 3 O 7−x , (M = Y, H o , Gd, Sm, Eu) and La 1.85 Sr 0.15 CuO 4−x . It is shown that Δ can be calculated when the experimental data on the superconducting transition temperature T c , the effective coupling parameter λ and the Debye temperature Q D of the HTS are known. The calculated values of the superconducting energy gap Δ are consistent with the experimental results of Δ obtained by the IR-technique.

Thermal smearing in inelastic tunnelling spectroscopy is studied down to 80 mK for phenol-doped AlIPb sandwiches.