Showing papers on "Scanning tunneling spectroscopy published in 2007"
TL;DR: The use of tunneling microscopy and spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors as discussed by the authors.
Abstract: Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.
790 citations
TL;DR: It is shown that, when its source is atomic-scale lattice defects, wave functions of different symmetries can mix and reflect both intravalley and intervalley scattering.
Abstract: A single sheet of carbon, graphene, exhibits unexpected electronic properties that arise from quantum state symmetries, which restrict the scattering of its charge carriers. Understanding the role of defects in the transport properties of graphene is central to realizing future electronics based on carbon. Scanning tunneling spectroscopy was used to measure quasiparticle interference patterns in epitaxial graphene grown on SiC(0001). Energy-resolved maps of the local density of states reveal modulations on two different length scales, reflecting both intravalley and intervalley scattering. Although such scattering in graphene can be suppressed because of the symmetries of the Dirac quasiparticles, we show that, when its source is atomic-scale lattice defects, wave functions of different symmetries can mix.
695 citations
TL;DR: In this article, the authors studied the transport properties of charge carriers through graphene superlattices consisting of monolayer or bilayer graphene on the basis of the transfer-matrix method.
Abstract: This paper studies the transport properties of charge carriers through graphene superlattices consisting of monolayer or bilayer graphene on the basis of the transfer-matrix method. Emphasis is placed on the relationship between the Klein paradox and resonant tunneling in double-barrier junctions. It is shown that normal incidence transmission probabilities for two kinds of graphene structure exhibit different features. Independent of structure parameters, they are always perfectly transmitted in a monolayer graphene structure. In contrast, the transmission resonances occur in a bilayer graphene structure. However, the angularly averaged conductivities for both depend on the thickness and height of the barriers as well as the width and number of the well. That is to say, the angularly averaged conductivities in monolayer and bilayer graphene superlattices can be controlled by changing the structure parameters even if Klein tunneling exists.
266 citations
TL;DR: In this article, a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface is presented.
Abstract: We present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface. Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001). STS spectroscopy shows a ~100meV gap-like feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity. These results are important for potential devices involving electronic transport or tunneling in graphene/SiC.
242 citations
TL;DR: In this article, the authors present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface.
Abstract: The authors present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001) STS spectroscopy shows an ∼100meV gaplike feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity These results are relevant for potential devices involving electronic transport or tunneling in graphene/SiC
221 citations
TL;DR: The present approach provides a technologically feasible way for single spin manipulation by precise thickness control of thin films by placing magnetic molecules on silicon-supported nanostructures.
Abstract: Manipulating the Kondo effect by quantum confinement has been achieved by placing magnetic molecules on silicon-supported nanostructures. The Kondo resonance of individual manganese phthalocyanine (MnPc) molecules adsorbed on the top of Pb islands was studied by scanning tunneling spectroscopy. Oscillating Kondo temperatures as a function of film thickness were observed and attributed to the formation of the thickness-dependent quantum-well states in the host Pb islands. The present approach provides a technologically feasible way for single spin manipulation by precise thickness control of thin films.
202 citations
TL;DR: The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy and adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.
Abstract: The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 \AA{} we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.
157 citations
TL;DR: In this article, the interaction of individual DNA nucleosides with a carbon nanotube (CNT) in vacuum and in the presence of external gate voltage was investigated and a scheme to discriminate between nucleoside on CNTs based on measurement of electronic features through a local probe such as scanning tunneling spectroscopy.
Abstract: We investigate the interaction of individual DNA nucleosides with a carbon nanotube (CNT) in vacuum and in the presence of external gate voltage. We propose a scheme to discriminate between nucleosides on CNTs based on measurement of electronic features through a local probe such as scanning tunneling spectroscopy. We demonstrate through quantum mechanical calculations that these measurements can achieve 100% efficiency in identifying DNA bases. Our results support the practicality of ultrafast DNA sequencing using electrical measurements.
157 citations
TL;DR: The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs, and the local density of states in a half metal has a large peak at the Fermi energy.
Abstract: Motivated by a recent experiment [Keizer et al, Nature (London) 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions using the recursive Green function method The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs As a consequence, the local density of states in a half metal has a large peak at the Fermi energy Therefore the odd-frequency pairs can be detected experimentally by using the scanning tunneling spectroscopy
133 citations
TL;DR: In this article, density-functional calculations of a monolayer of 3,4,9,10-perylene-tetracarboxylic-dianhydride adsorbed on the Ag(111) surface are presented, yielding detailed insight into the structural and electronic properties of this prototypical adsorption system.
Abstract: We present density-functional calculations of a monolayer of 3,4,9,10-perylene-tetracarboxylic-dianhydride adsorbed on the Ag(111) surface, yielding detailed insight into the structural and electronic properties of this prototypical adsorption system. Using the local-density approximation as the best choice for the exchange-correlation functional, we discuss the bond lengths inside the molecules, the distortion of the molecules due to adsorption, and their position and orientation relative to the substrate and to each other. Based on the calculated geometric and electronic structures, we calculate scanning tunneling microscopy and spectroscopy data within the Tersoff-Hamann framework [Phys. Rev. B. 31, 805 (1985)]. To this end, two-dimensional Fourier transform methods and spatial extrapolation techniques are employed to evaluate the sample wave functions at the tip position. We obtain constant-current images and spectral data that reveal detailed information about the electronic structure of the system. In addition, we have measured the same data by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy. Our measured and calculated data are in good agreement with one another.
130 citations
TL;DR: In this paper, the initial stages of oxidation of Cu(1) surfaces exposed to O 2 at low pressure ( −6 ǫmbar) and room temperature were studied using scanning tunneling microscopy (STM).
TL;DR: In this article, the eigenstates in the potential well formed between the tip and the sample are calculated within a one-dimensional model, which permits to locally determine a work function reduction of 1.3 eV in going from the bare substrate to NaCl islands of up to 3 ML.
Abstract: Electrons trapped in field emission resonances (FERs) in front of a Ag(100) surface covered with ultrathin NaCl islands are probed by scanning tunneling spectroscopy. The eigenstates in the potential well formed between the tip and the sample are calculated within a one-dimensional model. This approach permits to locally determine a work function reduction of 1.3 eV in going from the bare substrate to NaCl islands of up to 3 ML. Spatial mapping of the FERs across a NaCl island edge at typical distances of 1 nm from the surface yields a lateral resolution for the surface potential changes of 1 nm.
TL;DR: Intermolecular chain-to-chain tunneling in metal−alkanethiol−metal junctions was investigated by measuring the molecular-tilt dependence of the tunneling through a molecular ensemble of alkanethiols using conducting atomic force microscopy.
Abstract: Intermolecular chain-to-chain tunneling in metal−alkanethiol−metal junctions was investigated by measuring the molecular-tilt dependence of the tunneling through a molecular ensemble of alkanethiols using conducting atomic force microscopy. A variable tip-loading force was applied to tilt the molecular configuration while measuring the current−voltage characteristics of the molecular junctions. The observed transport through the molecules exhibited a tilt angle dependent intermolecular charge transfer in addition to the tunneling along the molecular chains.
TL;DR: In this paper, the authors observed a quantum-confinement effect in individual Ge1−xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature.
Abstract: The authors observed a quantum-confinement effect in individual Ge1−xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ∼1.5eV with a decrease in QD diameter from 35to4nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two dimensionality in density of states of quasi zero-dimensional QDs with the QD flattening.
TL;DR: Both theory and experiment indicate repulsive interactions between vacancies on a given BBO row, and the first measurements and calculations of the intrinsic mobility of bridge-bonded oxygen (BBO) vacancy on a rutile TiO2(110).
Abstract: We report the first measurements and calculations of the intrinsic mobility of bridge-bonded oxygen (BBO) vacancies on a rutile ${\mathrm{TiO}}_{2}(110)$. The sequences of isothermal (340--420 K) scanning tunneling microscope images show that BBO vacancies migrate along BBO rows. The hopping rate increases exponentially with increasing temperature with an experimental activation energy of 1.15 eV. Density functional theory calculations are in very good agreement giving an energy barrier for hopping of 1.03 eV. Both theory and experiment indicate repulsive interactions between vacancies on a given BBO row.
TL;DR: In this paper, density functional theory based electron transport calculations are performed to explain the nature of the break-junction behavior of amine-terminated molecules, and they find that amines can be adsorbed only on the apex Au atom, while the thiolate group can be attached equally well to undercoordinated and clean Au surfaces.
Abstract: Amine-terminated molecules show well-behaved conductance in the scanning tunneling microscope break-junction experimental measurements. We performed density functional theory based electron transport calculations to explain the nature of this phenomenon. We find that amines can be adsorbed only on the apex Au atom, while the thiolate group can be attached equally well to undercoordinated and clean Au surfaces. Our calculations show that only one adsorption geometry is sterically and energetically possible for the amine-anchored junction whereas three different adsorption geometries with very distinct transport properties are almost equally probable for the thiolate-anchored junction. We calculated the conductance as a function of the junction stretching when the molecules are pulled by the scanning tunneling microscope tip from the Au electrode. Our calculations show that the stretching of the thiolate-anchored junction during its formation is accompanied by significant electrode geometry distortion. The amine-anchored junctions exhibit very different behavior—the electrode remains intact when the scanning tunneling microscope tip stretches the junction.
TL;DR: Measurements of the rates for an electron to tunnel on and off a quantum dot are obtained using a quantum point contact charge sensor, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.
Abstract: We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.
TL;DR: First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization over the energy range of the Co surface state.
Abstract: We have used spin-polarized scanning tunneling spectroscopy to observe the spin polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. These magnetic adatoms exhibit stationary out-of-plane spin polarization, but have opposite sign of the exchange coupling between electron states of the adatom and the Co island surface state: Fe adatoms exhibit parallel spin polarization to the Co surface state while Cr adatoms exhibit antiparallel spin polarization. First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization for Fe and Cr adatoms over the energy range of the Co surface state.
IBM1
TL;DR: In this article, the authors investigated the influence of Chemical Bonding on spin-polarized tunneling and the relationship of tunneling magnetoresistance to tunneling spin polarization.
Abstract: The sections in this article are
Introduction
Fundamentals of Magnetic Tunnel Junctions (MTJs)
Influence of Chemical Bonding on Spin-Polarized Tunneling
Influence of Wave-Function Symmetry on Spin-Polarized Tunneling
Relationship of Tunneling Magnetoresistance to Tunneling Spin Polarization
Temperature and Bias Voltage Dependence of TMR
Spin-Dependent Tunneling in Other Systems
Conclusion
TL;DR: Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size, and Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoisland.
Abstract: Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.
TL;DR: The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potentialEnergy surface.
Abstract: Quantum dynamics calculations of the ground state tunneling splitting and of the zero point energy of malonaldehyde on the full dimensional potential energy surface proposed by Yagi et al. [J. Chem. Phys. 1154, 10647 (2001)] are reported. The exact diffusion Monte Carlo and the projection operator imaginary time spectral evolution methods are used to compute accurate benchmark results for this 21-dimensional ab initio potential energy surface. A tunneling splitting of 25.7+/-0.3 cm-1 is obtained, and the vibrational ground state energy is found to be 15 122+/-4 cm-1. Isotopic substitution of the tunneling hydrogen modifies the tunneling splitting down to 3.21+/-0.09 cm-1 and the vibrational ground state energy to 14 385+/-2 cm-1. The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potential energy surface.
TL;DR: In this article, the authors report on the resonant tunneling effect and the increase of tunneling magnetoresistance induced by it in ferromagnetic-semiconductor quantum-well (QW) heterostructures.
Abstract: We report on the resonant tunneling effect and the increase of tunneling magnetoresistance induced by it in ferromagnetic-semiconductor $\mathrm{GaMnAs}$ quantum-well (QW) heterostructures. The resonant tunneling effect was observed when the $\mathrm{GaMnAs}$ QW thickness was from 3.8 to $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which indicates that highly coherent tunneling occurs in these heterostructures. The observed quantum levels of the $\mathrm{GaMnAs}$ QW were successfully explained by the valence-band $k∙p$ model and the $p\text{\ensuremath{-}}d$ exchange interaction. It was also found that the Fermi level of the electrode injecting carriers is important to observe resonant tunneling in this system.
TL;DR: In this paper, an ab initio theory of inelastic electron tunneling spectroscopy along with theoretical analysis of recent experiments on ineelastic transport in molecular tunneling junctions involving a molecule was presented.
Abstract: We present an ab initio theory of inelastic electron tunneling spectroscopy along with the theoretical analysis of recent experiments on inelastic transport in molecular tunneling junctions involving ${\mathrm{C}}_{60}$ molecule. We present a self-consistent procedure for calculating electron charge density and tunneling current in the presence of the electron-phonon interaction. We find that electron tunneling is significantly influenced by several specific vibrational modes. Inelastic scattering suppresses resonance transmission peak and substantially redshifts the peak position. We investigate the microscopic origin of this behavior by calculating the relevant vibrational modes and resonance wave functions under nonequilibrium transport conditions.
TL;DR: In this paper, the authors propose a versatile experimental probe for cold atomic gases analogous to the scanning tunneling microscope (STM) in condensed matter, which uses the coherent coupling of a single particle to the system depending on the measurement sequence.
Abstract: We propose a versatile experimental probe for cold atomic gases analogous to the scanning tunneling microscope (STM) in condensed matter This probe uses the coherent coupling of a single particle to the system Depending on the measurement sequence, our probe allows us to obtain either the local density and spatial density correlations, with a resolution on the nanometer scale, or the single particle correlation function in real time We discuss applications of this scheme to the various possible phases for a two dimensional Hubbard system of fermions in an optical lattice
TL;DR: In this paper, the spin-orbit coupling of two-dimensional surface states can be detected locally by scanning-tunneling spectroscopy (STS) and the spin splitting of the surface state induces a singularity in the local density of states which can then be detected as a distinct peak in the differential conductance spectrum.
Abstract: We demonstrate that the spin-orbit coupling of two-dimensional surface states can be detected locally by scanning-tunneling spectroscopy (STS). The spin splitting of the surface state induces a singularity in the local density of states which can be detected as a distinct peak in the differential conductance spectrum. From the STS spectrum we can determine the Rashba energy as a measure of the strength of the spin splitting. Its detection and imaging are demonstrated for the surface alloys Bi and Pb on Ag(111), which exhibit particularly large spin-split band structures. The influence of the spin splitting on the surface-state STS spectra of close-packed noble metal surfaces is also discussed.
TL;DR: In this article, the influence of oxygen pressure during the cyclic annealing used for the cleaning of W(1/0) surfaces was studied by means of scanning tunneling microscopy (STM) and spectroscopy.
TL;DR: This atomistic study provides an experimental basis for probing the electronic structure of nanometer-sized graphene which can assist the development of graphene-based nanoelectronics.
Abstract: We have developed a method for depositing graphene monolayers and bilayers with minimum lateral dimensions of 2-10 nm by the mechanical exfoliation of graphite onto the Si(100)-2x1:H surface. Room temperature, ultra-high vacuum (UHV) tunnelling spectroscopy measurements of nanometer-sized single-layer graphene reveal a size dependent energy gap ranging from 0.1-1 eV. Furthermore, the number of graphene layers can be directly determined from scanning tunnelling microscopy (STM) topographic contours. This atomistic study provides an experimental basis for probing the electronic structure of nanometer-sized graphene which can assist the development of graphene-based nanoelectronics.
TL;DR: Low-temperature scanning tunneling spectroscopy measurements on Mn acceptors in InAs show a strong (001)-mirror asymmetry of the bound hole wave function close to the (110) surface and signs of anisotropic scattering of the conduction band states by neutral acceptors.
Abstract: We present low-temperature scanning tunneling spectroscopy measurements on Mn acceptors in InAs in comparison with tight-binding calculations. We find a strong (001)-mirror asymmetry of the bound hole wave function close to the (110) surface. In addition, multiple acceptor-related peaks are observed and are attributed to a spin-orbit splitting of the acceptor level. Because of the p-d exchange interaction the local density of states near the acceptors is enhanced in the valence band and suppressed in the conduction band. We also observe signs of anisotropic scattering of the conduction band states by neutral acceptors.
TL;DR: It is proposed that the isomerization is initiated by simultaneous excitation of two vibrational molecular modes via inelastically tunneling electrons, which results in a shift of the distribution probability of chlorine and hydrogen positions, which facilitates their mutual exchange.
Abstract: A scanning tunneling microscope operating at 5 K is used to induce the isomerization of single chloronitrobenzene molecules on Cu(111) and verify the reaction. The threshold voltage of $(227\ifmmode\pm\else\textpm\fi{}7)\text{ }\text{ }\mathrm{mV}$ for this reaction is explained based on electron-induced vibrational heating. We propose that the isomerization is initiated by simultaneous excitation of two vibrational molecular modes via inelastically tunneling electrons. This excitation results in a shift of the distribution probability of chlorine and hydrogen positions, which facilitates their mutual exchange.
TL;DR: It is demonstrated that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits, and higher order Gundlach oscillations can be applied to determine the workfunction of thin metal films precisely.
Abstract: Gundlach oscillation (or the standing-wave state) is a general phenomenon manifesting in the tunneling spectrum acquired from a metal surface using scanning tunneling spectroscopy. Previous studies relate the energy shift between peaks of the lowest-order Gundlach oscillation observed on the thin film and the metal substrate to the difference in their work functions. By observing Gundlach oscillations on Ag/Au(111), Ag/Cu(111), and Co/Cu(111) systems, we demonstrate that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits. Higher order Gundlach oscillations can thus be applied to determine the work function of thin metal films precisely.