Scanning Tunneling Spectroscopy Experiments Research Articles

Energy-Gap Opening in a Bi(110) Nanoribbon Induced by Edge Reconstruction

Scanning tunnelling microscopy and spectroscopy experiments complemented by first-principles calculations have been conducted to study the electronic structure of 4 monolayer Bi(110) nanoribbons on epitaxial graphene on silicon carbide [4H-SiC(0001)]. In contrast with the semimetal property of elemental bismuth, an energy gap of 0.4 eV is measured at the centre of the Bi(110) nanoribbons. Edge reconstructions, which can facilitate the edge strain energy release, are found to be responsible for the band gap opening. The calculated density of states around the Fermi level are decreased quickly to zero from the terrace edge to the middle of a Bi(110) nanoribbon potentially signifying a spatial metal-to-semiconductor transition. This study opens new avenues for room-temperature bismuth nanoribbon-based electronic devices.

Josephson current and density of states in proximity circuits withs+−superconductors

We study the emergent proximity effect in mesoscopic circuits that involve a conventional superconductor and an unconventional pnictide superconductor separated by a diffusive normal or ferromagnetic wire. The focus is placed on revealing signatures of the proposed ${s}_{+\ensuremath{-}}$ state of pnictides from the proximity-induced density of states and Josephson current. We find analytically a universal result for the density of states that exhibits both the Thouless gap at low energies and peculiar features near the superconducting gap edges at higher energies. The latter may be used to discriminate between ${s}_{+\ensuremath{-}}$ and ${s}_{++}$ symmetry scenarios in scanning tunneling spectroscopy experiments. We also calculate Josephson current-phase relationships for different junction configurations, which are found to display robust 0-$\ensuremath{\pi}$ transitions for a wide range of parameters.

Unraveling the Intrinsic and Robust Nature of van Hove Singularities in Twisted Bilayer Graphene by Scanning Tunneling Microscopy and Theoretical Analysis

Extensive scanning tunneling microscopy and spectroscopy experiments complemented by first-principles and parametrized tight binding calculations provide a clear answer to the existence, origin, and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1° to 10°) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and by calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the role of the layer topographic corrugation and of electronic effects in the apparent moiré contrast measured on the STM images.

Boron Nitride on Cu(111): An Electronically Corrugated Monolayer

Ultrathin films of boron nitride (BN) have recently attracted considerable interest given their successful incorporation in graphene nanodevices and their use as spacer layers to electronically decouple and order functional adsorbates. Here, we introduce a BN monolayer grown by chemical vapor deposition of borazine on a single crystal Cu support, representing a model system for an electronically patterned but topographically smooth substrate. Scanning tunneling microscopy and spectroscopy experiments evidence a weak bonding of the single BN sheet to Cu, preserving the insulating character of bulk hexagonal boron nitride, combined with a periodic lateral variation of the local work function and the surface potential. Complementary density functional theory calculations reveal a varying registry of the BN relative to the Cu lattice as origin of this electronic Moiré-like superstructure.

Topologically protected surface Majorana arcs and bulk Weyl fermions in ferromagnetic superconductors

A number of ferromagnetic superconductors have been recently discovered which are believed to be in the so-called ``equal spin pairing'' state. In the equal spin pairing state, the Cooper pairs condense forming order parameters ${\ensuremath{\Delta}}_{\ensuremath{\uparrow}\ensuremath{\uparrow}},\phantom{\rule{4pt}{0ex}}{\ensuremath{\Delta}}_{\ensuremath{\downarrow}\ensuremath{\downarrow}}$ which are decoupled in the spin sector. We show that these three-dimensional systems should generically support topologically protected surface Majorana arcs and bulk Weyl fermions as gapless excitations. Similar protected low-energy exotic quasiparticles should also appear in the recently discovered noncentrosymmetric superconductors in the presence of Zeeman splitting. The protected surface arcs can be probed by angle-resolved photoemission as well as Fourier transform scanning tunneling spectroscopy experiments.

Effect of spin fluctuations on the electronic structure in iron-based superconductors

Magnetic inelastic neutron scattering studies of iron-based superconductors reveal a strongly temperature-dependent spin-fluctuation spectrum in the normal conducting state, which develops a prominent low-energy resonance feature when entering the superconducting state. Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) allow us to study the fingerprints of fluctuation modes via their interactions with electronic quasiparticles. We calculate such fingerprints in 122 iron pnictides using an experimentally motivated spin-fluctuation spectrum and make a number of predictions that can be tested in ARPES and STS experiments. This includes discussions of the quasiparticle scattering rate and the superconducting order parameter. In quantitative agreement with experiment we reproduce the quasiparticle dispersions obtained from momentum distribution curves as well as energy distribution curves. We discuss the relevance of the coupling between spin fluctuations and electronic excitations for the superconducting mechanism.

Hidden order transition in URu2Si2: Evidence for the emergence of a coherent Anderson lattice from scanning tunneling spectroscopy

Using a large-N approach, we demonstrate that the differential conductance and quasi-particle interference pattern measured in recent scanning tunneling spectroscopy experiments (A.R. Schmidt et al. Nature 465, 570 (2010); P. Aynajian et al., PNAS 107, 10383 (2010)) in URu2Si2 are consistent with the emergence of a coherent Kondo lattice below its hidden order transition (HOT). Its formation is driven by a significant increase in the quasi-particle lifetime, which could arise from the emergence of a yet unknown order parameter at the HOT.

Doping domains in graphene on gold substrates: First-principles and scanning tunneling spectroscopy studies

We have studied the graphene/gold interface by means of density functional theory (DFT) and scanning tunneling spectroscopy (STS). Weak interaction between graphene and the underlying gold surface preserves Dirac cones in the band structure, but they can be shifted with respect to the Fermi level of the whole system, which results in effective doping of graphene. DFT calculations revealed that the interface is extremely sensitive to the adsorption distance and to the structure of a metal's surface. On the other hand, STS experiments have shown the presence of energetic heterogeneity in terms of the changes in the local density of states (LDOS) measured at different places on the sample. They have been identified to be domains of doping by comparing measured and simulated LDOS. Domains of zero-doping and $p$-type (0.25--0.55 eV) doping have been then associated with the heterogeneous character of the gold surface. All these results demonstrate the possibility of engineering the electronic properties of graphene, especially tuning the doping across one flake, which can be useful for applications of graphene in electronic devices.

Anisotropic vortex lattice structures in the FeSe superconductor

In the recent work by Song et al. [Science 332, 1410 (2011)], the scanning tunneling spectroscopy experiment in the stoichiometric FeSe reveals evidence for nodal superconductivity and strong anisotropy. The nodal structure can be explained with the extended s-wave pairing structure with the mixture of the $s_{x^2+y^2}$ and $s_{x^2y^2}$ pairing symmetries. We calculate the anisotropic vortex structure by using the self-consistent Bogoliubov-de Gennes mean-field theory. In considering the absence of magnetic ordering in the FeSe at the ambient pressure, orbital ordering is introduced, which breaks the $C_4$ lattice symmetry down to $C_2$, to explain the anisotropy in the vortex tunneling spectra.

Spin-polarized surface states on Fe-deposited Au(111) surface: A theoretical study

We have studied electronic structure of Fe-deposited Au(111) by performing ab initio density functional theory calculations. We find that the magnetic moment on the deposited Fe layer is enhanced as compared to that in bulk iron. We observe a large number of new states on the Fe-deposited surface — one of which is in the majority spin channel having similar dispersion to that on the clean surface, and others in the minority spin channel. The effective mass of electrons in surface states near the Fermi level increases on Fe deposition. The electronic properties are found to be insensitive to the stacking of near-surface layers. We need to use very thick slabs in our calculations to avoid splitting of surface states due to spurious interactions between the two surfaces of the slab. Using the local density of states profiles for different surface states, we conclude that in scanning tunneling microscope experiments one can detect two of the surface states — one in the majority channel below the Fermi level, and another in the minority channel appearing just above the Fermi energy. We compare our results to those from scanning tunneling spectroscopy experiments.

Grand canonical variational approach for thet−Jmodel

Gutzwiller-projected BCS wave function or the resonating-valence-bond (RVB) state in the 2D extended t-J model is investigated by using the variational Monte Carlo technique. We show that the results of ground-state energy and excitation spectra calculated in the grand-canonical scheme allowing particle number to fluctuate are essentially the same as previous results obtained by fixing the number of particle in the canonical scheme if the grand thermodynamic potential is used for minimization. To account for the effect of Gutzwiller projection, a fugacity factor proposed by Laughlin and Anderson few years ago has to be inserted into the coherence factor of the BCS state. Chemical potential, particle number fluctuation, and phase fluctuation of the RVB state, difficult or even impossible to be calculated in the canonical ensemble, have been directly measured in the grand-canonical picture. We find that except for La-214 materials, the doping dependence of chemical potential is consistent with experimental findings on several cuprates. Similar to what has been reported by scanning tunneling spectroscopy experiments, the tunneling asymmetry becomes much stronger as doping decreases. We found a very large enhancement of phase fluctuation in the underdoped regime.

Electronic Fingerprints of DNA Bases on Graphene

We calculate the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T), deposited on graphene. We observe significant base-dependent features in the LDOS in an energy range within a few electronvolts of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases in scanning tunneling spectroscopy (STS) experiments that perform image and site dependent spectroscopy on biomolecules. Thus the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS.

Scanning tunneling microscopy and spectroscopy of graphene on insulating substrates

AbstractGraphene is a truly two‐dimensional (2D) material with exceptional electronic, mechanical, and optical properties. As such, it consists of surface only and can be probed by the well‐developed surface science techniques such as, e.g., scanning tunneling microscopy (STM). This method bridges the gap between the surface science community and the electronic device community and might lead to novel combined approaches. Here, I review some of the STM and scanning tunneling spectroscopy (STS) experiments on monolayer graphene samples. I will concentrate on graphene samples deposited on insulating substrates, since these are related to graphene device concepts. In particular, I will discuss the morphology of graphene on SiO2 and other emerging substrates, some nanomechanical manipulation experiments using STM, and spectroscopic results. The latter can map the disorder potentials as well as the interaction of the electrons with the disorder, which is most pronounced in the quantum Hall regime.magnified image Three‐dimensional representation of a STM image of graphene; the atomic resolution is displayed as color code, while the rippling is shown in three dimensions (courtesy of M. Pratzer, RWTH Aachen).

Vortex Fusion and Giant Vortex States in Confined Superconducting Condensates

In a direct scanning tunneling spectroscopy experiment we address the problem of the quantum vortex phases in strongly confined superconductors. The strong confinement regime is achieved in in situ grown ultrathin single nanocrystals of Pb by tuning their lateral size to a few coherence lengths. Upon an external magnetic field, the scanning tunneling spectroscopy revealed novel ultradense arrangements of single Abrikosov vortices characterized by an intervortex distance up to 3 times shorter than the bulk critical one. At yet stronger confinement we discovered the giant vortex phase; the spatial evolution of the excitation tunneling spectra in the cores of these unusual quantum objects was explored. We anticipate the giant vortex phase to be a common feature of other confined quantum condensates such as superfluids, Bose-Einstein condensates of cold atoms, etc.

Asymmetric saddling of single porphyrin molecules on Au(111)

The interaction of organometallic molecules with metal surfaces affects their conformation and accordingly their functionality. Surprising conformational complexity is revealed in single Au-porphyrin molecules on Au(111) exhibiting a variety of different asymmetric conformers. A combination of low-temperature scanning tunneling microscopy and spectroscopy experiments with density functional theory calculations discloses different registries of the pyrroles with the substrate lattice (determined and quantified with atomic precision) that cause asymmetric sub-\AA{} buckling of the macrocycle and, furthermore, a redshift of the frontier-orbital energy gap.

Charge State Control of Molecules Reveals Modification of the Tunneling Barrier with Intramolecular Contrast

From scanning tunneling microscopy and spectroscopy experiments it is shown that control over the charge-state of individual molecules adsorbed on surfaces can be obtained by choosing a substrate system with an appropriate workfunction. The distribution of the additional charge is studied using difference images. These images show marked intramolecular contrast.

Long-range Kondo signature of a single magnetic impurity

The Kondo effect describes electrons scattering off a magnetic impurity, which affects the resistivity of a metal at low temperatures. In the case of buried iron or cobalt atoms, the correlations are longer ranged than studies of adatoms have shown. The Kondo effect, one of the first recognized correlation phenomena in condensed-matter physics1, has regained attention because of scanning tunnelling spectroscopy (STS) experiments carried out on single magnetic impurities2,3. Despite the subnanometre resolution capability of local probe techniques, one of the fundamental aspects of Kondo physics, its spatial extension, is still subject to discussion. Until now all STS studies on single adsorbed atoms have shown that observable Kondo features vanish rapidly with increasing distance from the impurity4,5,6,7,8,9. Here we report on a hitherto unobserved long-range Kondo signature for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We present a theoretical interpretation of the measured signatures using a combined approach of band-structure and many-body numerical renormalization group calculations. These are in excellent agreement with the rich spatially and spectroscopically resolved experimental data.

Gated adatoms on graphene studied with first-principles calculations

A first-principles pseudopotential density functional method for studying the gating of adatoms on graphene is presented. A variation in gate voltage is assumed to vary the number of electrons in the adatom-graphene system. The method is applied to the cases of Li and Co on graphene. The projected density of states, charge density, and local electrostatic potential are computed as a function of gate voltage. In the case of Li, the calculations show that the Li adatom can be ionized by changing the gate voltage, and that the ionization is accompanied by a sharp change in the electrotstatic potential of the adatom. In the case of Co, correlation in the $3d$ shell is treated using the LDA $+$ $U$ method, with several values of the $U$ parameter considered. For $U=2$ eV or greater, an ionization effect analogous to the case of Li is found for the Co adatom. This result is consistent with recent scanning tunneling spectroscopy experiments for Co on graphene.

Discriminative Response of Surface-Confined Metalloporphyrin Molecules to Carbon and Nitrogen Monoxide

The binding of small gas molecules to metalloporphyrins is of both fundamental scientific and technological interest. It plays a key role in the transport of respiratory gases, catalytic processes in biological systems, and artificial nanostructures for sensing. Here, we present a detailed molecular-level investigation regarding the interaction of nitrogen monoxide (NO) and carbon monoxide (CO) with metallo-tetraphenylporphyrin (M-TPP, M = Co, Fe) arrays, anchored on a noble metal Ag(111) surface, providing M-TPP species with a distinct saddle-shape conformation. Scanning tunneling microscopy and spectroscopy experiments reveal that the impact of CO and NO is strikingly different on both species. In the case of CO, the M-TPP core can be dressed by either one or two carbon monoxide ligands, whereby the porphyrin geometric and electronic structure remains nearly unaffected. In contrast, following NO exposure exclusively a mononitrosyl species evolves. The NO axial ligation induces a relaxation of the adsorption-induced molecular deformation and markedly modifies the electronic structure of the porphyrin.

High magnetic field theory for the local density of states in graphene with smooth arbitrary potential landscapes

We study theoretically the energy and spatially resolved local density of states (LDoS) in graphene at high perpendicular magnetic field. For this purpose, we extend from the Schr\"odinger to the Dirac case a semicoherent-state Green's-function formalism, devised to obtain in a quantitative way the lifting of the Landau-level degeneracy in the presence of smooth confinement and smooth disordered potentials. Our general technique, which rigorously describes quantum-mechanical motion in a magnetic field beyond the semiclassical guiding center picture of vanishing magnetic length (both for the ordinary two-dimensional electron gas and graphene), is connected to the deformation (Weyl) quantization theory in phase space developed in mathematical physics. For generic quadratic potentials of either scalar (i.e., electrostatic) or mass (i.e., associated with coupling to the substrate) types, we exactly solve the regime of large magnetic field (yet at finite magnetic length, formally, this amounts to considering an infinite Fermi velocity) where Landau-level mixing becomes negligible. Hence, we obtain a closed-form expression for the graphene Green's function in this regime, providing analytically the discrete energy spectra for both cases of scalar and mass parabolic confinement. Furthermore, the coherent-state representation is shown to display a hierarchy of local energy scales ordered by powers of the magnetic length and successive spatial derivatives of the local potential, which allows one to devise controlled approximation schemes at finite temperature for arbitrary and possibly disordered potential landscapes. As an application, we derive general analytical nonperturbative expressions for the LDoS, which may serve as a good starting point for interpreting experimental studies. For instance, we are able to account for many puzzling features of the LDoS recently observed by high magnetic field scanning tunneling spectroscopy experiments on graphene, such as a roughly $\sqrt{m}$ increase in the $m\text{th}$ Landau-level linewidth in the LDoS peaks at low temperatures, together with a flattening of the spatial variations in the Landau-level effective energies at increasing $m$.