Low-temperature Scanning Tunneling Spectroscopy Research Articles

Scanning tunneling spectroscopy and manipulation of C60 on Cu(111)

The properties of C60 adsorbed on Cu(111) have been studied using low temperature scanning tunnelling microscopy and spectroscopy. In the electronic spectrum of the molecule, we observe features that can be assigned to molecular orbitals. The LUMO level is split into two states, as a consequence of the charge transfer from the substrate to the carbon cage. The data from the inelastic electron tunnelling spectroscopy reveal two peaks that can be assigned to the intramolecular vibrational modes of the C60 cage. We demonstrate also controlled manipulation of single molecules. The plot of the tip height, recorded during the manipulation process, indicates that the C60 is pushed along the surface.

Imaging of Interference between Incident and Reflected Electron Waves at an InAs/GaSb Heterointerface by Low-Temperature Scanning Tunneling Spectroscopy

We measure the spatial variation of the local density of states normal to an InAs/GaSb heterointerface on a cleaved cross-sectional surface by low-temperature scanning tunneling spectroscopy. Clear standing waves originating from the interference between the incident and reflected electron waves at the interface are observed.

Low-temperature scanning tunneling spectroscopy study of two-dimensional electron systems confined in semiconductor heterostructures

Low-temperature scanning tunneling spectroscopy under ultra-high vacuum was employed to investigate the two-dimensional electron system at the epitaxial surface of Sidoped In0.53Ga0.47As/In0.52Al0.48As(111)A quantum-well structures. The electron density in the near-surface region of the quantum well could be controlled through modulation doping. Spectra of the electronic local density of states in the conduction band showed a clear step-like energy dependence that reveals the subband states. In spectra acquired at some areas of nanometer size, peaks were observed near subband minima, indicating the existence of bound states.

Spatial Imaging of Two-Dimensional Electronic States in Semiconductor Quantum Wells

We measure the spatial distribution of the local density of states (LDOS) at cleaved surfaces of InAs/GaSb isolated quantum wells and double quantum wells (DQWs) by low-temperature scanning tunneling spectroscopy. Distinct standing wave patterns of LDOS corresponding to subbands are observed. These LDOS patterns and subband energies agree remarkably well with simple calculations with tip-induced band bending. Furthermore, for the DQWs, coupling of electronic states between the quantum wells is also clearly observed.

Gap distribution in overdoped La 2− xSr xCuO 4 observed by scanning tunneling spectroscopy

Low-temperature scanning tunneling spectroscopy on the highly overdoped La 1.78Sr 0.22CuO 4 ( T c = 27 K) single crystal reveals gap inhomogeneity at 4.2 K. The energy gap Δ observed in the tunneling spectra distributes around 7 meV: it ranges from 4 to 20 meV. The spectra with smaller Δ (4–10 meV) exhibit gap edge peaks and flat background density-of-states, which is general characteristics of the overdoped cuprate high-temperature superconductors. On the other hand, the spectra with Δ exceeding 10 meV shows broad feature without clear gap edge peaks.

Signatures of modulated pair interaction in cuprate superconductors

Recent low-temperature scanning tunnelling spectroscopy experiments on the surface of Bi2Sr2CaCu2O8+x have revealed a strong positive correlation between the position of localized resonances at −960meV identified with interstitial oxygen dopants and the size of the local spectral gap. We review efforts to understand these correlations within a model where the dopants modulate the pair interaction on an atomic scale. We provide further evidence for this model by comparing the correlations between the dopants and the local density of states with experimental results.

Exchange Interaction between Single Magnetic Adatoms

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 A 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.

Imaging of subbands in InAs/GaSb double quantum wells by low-temperature scanning tunneling spectroscopy

The spatial distribution of the electron local density of states (LDOS) in InAs/GaSb double quantum wells (DQWs) was investigated by low-temperature scanning tunneling spectroscopy on cleaved surfaces. For DQW with a thick central barrier, clear standing wave patterns corresponding to subbands confined to each InAs single quantum well appeared in the spatial variation of LDOS spectra. In contrast, for the DQW with a thin central barrier, the standing wave patterns extended over both quantum wells. The deviation of the pattern arising from the asymmetry due to a slight difference of the well thickness appeared clearly. The observed spectra are well explained by the calculated LDOS taken to be the sum of LDOS contributed from all energetically accessible subbands.

Dramatic dependence of the Fermi level pinning strength on crystal orientation at clean surfaces of n-type In 0.53Ga 0.47As grown by molecular beam epitaxy

The electronic density of states at clean surfaces of n-type In 0.53Ga 0.47As, grown by molecular beam epitaxy on lattice-matched (0 0 1)- and (1 1 1)A-oriented InP substrates, was measured by low-temperature scanning tunneling spectroscopy under ultra-high vacuum. It was found that the surface Fermi level (FL) pinning strength dramatically depends on crystal orientation. The FL at the (0 0 1)-(2×4) surface is pinned near midgap, independently of the dopant concentration in the bulk. In contrast, the FL at the (1 1 1)A-(2×2) surface lies in the conduction band, close to the bulk FL, and increases with dopant concentration.

Tip preparation for usage in an ultra-low temperature UHV scanning tunneling microscope

This work deals with the preparation and characterization of tungsten tips for the use in UHV low-temperature scanning tunneling microscopy and spectroscopy (STM and STS, respectively). These specific environments require in situ facilities for tip conditioning, for further sharpening of the tips, as well as for reliable tip characterization. The implemented conditioning methods include direct resistive annealing, annealing by electron bombardment, and self-sputtering with noble gas ions. Moreover, results from in situ tip characterization by field emission and STM experiments were compared to ex situ scanning electron microscopy. Using the so-prepared tips, high resolution STM images and tunneling spectra were obtained in a temperature range from ambient down to 350 mK, partially with applied magnetic field, on a variety of materials.

Electron confinement in hexagonal vacancy islands: Theory and experiment

Electron confinement in hexagonal vacancy islands on Cu(111) is studied by state of the art ab initio calculations and experimentally by low-temperature scanning tunneling spectroscopy measurements (STS). Both theory and experiment reveal standing wave patterns of the local density of states (LDOS), which are ascribed to the scattering of surface-state electrons at the boundary of the vacancy island. The spatial modulation of the LDOS, its size, and energy dependence as measured by STS are well reproduced by our calculations. This agreement between theory and experiment corroborates the application of our theoretical approach to other confined systems, including spin-polarized effects. Our studies predict that quantum states of vacancy islands with magnetic adatoms can be used to tailor the spin-polarization of surface-state electrons on metal surfaces.

Unpinning of the Fermi level at (111)A clean surfaces of epitaxially grown n-type In0.53Ga0.47As

Low-temperature scanning tunneling spectroscopy under ultrahigh vacuum was employed to investigate the (111)A clean surface of n-type In0.53Ga0.47As⁠, grown by molecular beam epitaxy on lattice-matched InP substrates. It was found that the surface Fermi level is located in the conduction band, close to the bulk Fermi level, and can be controlled by varying the dopant concentration in the bulk. This observation was confirmed by determining the dispersion relation of electron standing waves resulting from scattering interferences. Such an unpinning of the surface Fermi level strongly contrasts with the pinning phenomenon observed at the (001) clean surface.

Scanning tunneling spectroscopy of methyl- and ethyl-terminated Si(111) surfaces

Methyl- and ethyl-terminated Si(111) surfaces prepared by a two-step chlorination/alkylation method were characterized by low temperature scanning tunneling spectroscopy (STS). The STS data showed remarkably low levels of midgap states on the CH3- and C2H5-terminated Si surfaces. A large conductance gap relative to the Si band gap was observed for both surfaces as well as for the hydrogen-terminated Si(111) surface. This large gap is ascribed to scanning tunneling microscope tip-induced band bending resulting from a low density of midgap states which avoid pinning of the Fermi levels on these passivated surfaces.

Spatial Mapping of the Electronic States of a One-Dimensional System

Using low-temperature scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of confined electronic states in the troughs between self-organized Pt nanowires on Ge(001) that are spaced 2.4 nm apart. Two sub-bands are resolved, which correspond to the lowest energy levels of a quantum mechanical particle in a box. As expected, the spatial dI/dV maps exhibit a maximum and a minimum in the middle of the troughs for the n = 1 and n = 2 states, respectively.

Influence of the quasiparticle lifetime effect on local tunneling spectra of high-Tc superconductors

The local electronic states of optimally doped Bi2Sr2CaCu2O8−δ and Ln-doped Bi2Sr2CuO6−δ (Ln-Bi2201, Ln=La, Gd) were measured by low-temperature scanning tunneling spectroscopy. The spectra showed enhanced coherence peaks when the gap amplitude was relatively small, while the peaks in the spectra were gradually suppressed as the gap amplitude became larger. In the case of Ln-Bi2201, the spectra showed the enhancements of residual conductance reflecting the impurity scattering due to doped Ln ions. We demonstrate that these trends are systematically reproduced based on the conductance formula that takes account of the energy dependent damping factor corresponding to the quasiparticle lifetime. The consistency suggests that one of the main factors dominating the spectral line shapes is the energy dependence of the quasiparticle lifetime.

Magnon-broadening of exchange-split surface states on lanthanide metals

Low-temperature scanning tunneling spectroscopy is used to study the electronic structure and the dynamics of $d$-like surface states on trivalent lanthanide metals from La to Lu. We find that the dynamics of these states, as revealed by their lifetime widths, is dominated by electron-phonon scattering in the occupied region and by electron-magnon scattering in the unoccupied region. We also show that the magnetic exchange splitting of these states scales with the $4f$ spin multiplied by an effective exchange-coupling constant that increases with $4f$ occupancy in an approximately linear way.

Bimodal electronic structure of isolated Co atoms on Pt(111)

We have used low-temperature scanning tunneling microscopy and spectroscopy to probe the local electronic structure of Co adatoms on the Pt(111) surface. Two varieties of Co adatoms that differ in their $dI∕dV(V)$ spectra are identified. We find that this contrast in spectral density is due to the adatom binding site on the surface. Atoms at different surface lattice sites (i.e., fcc versus hcp sites) exhibit different local densities of states (LDOS). Manipulation of a Co atom from one kind of lattice site to the other results in the expected change in its LDOS. $dI∕dV(V)$ spectra measured in this study were normalized using a method that compensates for differences in tunneling current that occur at different points on the surface. Such normalized spectra are shown to be useful in predicting the spatial contrast of $dI∕dV$ maps.

Molecules on vicinal Au surfaces studied by scanning tunnelling microscopy

Using low-temperature scanning tunnelling microscopy and spectroscopy we investigatedthe adsorption characteristics of 3,4,9,10-perylenetetracarboxylic-dianhydrideand fullerenes on Au(788), Au(433), and Au(778). On Au(788) and Au(778),3,4,9,10-perylenetetracarboxylic-dianhydride exhibits three coexisting superstructures,which do not reflect the periodicity of the hosting substrate. The adsorption on Au(433)leads to the formation of molecule chains along the step edges after annealing the sample.Fullerene molecules on Au(788) arrange in a mesh of islands, which extends over severalhundreds of nanometres with an extraordinarily high periodicity. A combination offullerene adsorption and annealing leads to facetting of Au(433) and the formation ofextraordinarily long fullerene stripes.

Surface electronic structures of La(0001) and Lu(0001)

Most spectroscopic methods for studying the electronic structure of metal surfaces have the disadvantage that either only occupied or only unoccupied states can be probed, and the signal is cut at the Fermi edge. This leads to significant uncertainties, when states are very close to the Fermi level. By performing low-temperature scanning tunneling spectroscopy and ab initio calculations, we study the surface-electronic structure of La(0001) and Lu(0001), and demonstrate that in this way detailed information on the surface-electronic structure very close to the Fermi energy can be derived with high accuracy.

Low Temperature Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Study at the α-Sn/Ge(111) Surface

A scanning tunneling microscopy (STM) study of the α-Sn/Ge(111) at low temperature is presented. The scanning tunneling spectroscopy (STS) measurements of the surface evidenced a metallic character from the room temperature down to the 3×3 transition temperature. The fluctuation model for the √3×√3 reconstruction is confirmed by dynamical measurements of the tunneling current on top the Sn adatoms. The STM tip was used as a probe to verify the presence of oscillating Sn adatoms by studying the tunneling current as a function of time.