Local Density Of States Images Research Articles

Direct STM measurements of R-type and H-type twisted MoSe2/WSe2

When semiconducting transition metal dichalcogenide heterostructures are stacked, the twist angle and lattice mismatch lead to a periodic moiré potential. As the angle between the layers changes, so do the electronic properties. As the angle approaches 0° or 60°, interesting characteristics and properties, such as modulations in the band edges, flat bands, and confinement, are predicted to occur. Here, we report scanning tunneling microscopy and spectroscopy measurements on the bandgaps and band modulations in MoSe2/WSe2 heterostructures with near 0° rotation (R-type) and near 60° rotation (H-type). We find a modulation of the bandgap for both stacking configurations with a larger modulation for R-type than for H-type as predicted by theory. Furthermore, local density of states images show that electrons are localized differently at the valence band and conduction band edges.

FRACTAL FEATURES OF THE QUANTIZED SCANNING TUNNELING CURRENT AND THE LOCAL DENSITY OF STATES FOR A QUANTUM CORRAL

We found that quantization of the scanning tunneling microscope (STM) current becomes apparent as the size of a quantum corral (QC) decreases and the corral thickness increases. It is shown that STM current images provide the position dependence of the conduction channels and local density of states (LDOS) images provide the spatial distribution of differential conductance channels. We clarified for the first time that there exist fractal structures in the QC size dependence of the [Formula: see text]–[Formula: see text] characteristic and in that of the LDOS. LDOS images also exhibit a fractal structure as a function of bias voltage. These results demonstrate fractal structures in energy space and real space. For an extremely large QC, we found that the [Formula: see text]–[Formula: see text] curve becomes Ohmic and the LDOS converges to a constant corresponding to the bulk two-dimensional density of states.

Nesting-induced local electronic patterns around a single As (Te, Se) vacancy in iron-based superconductors

The local electronic states around a single As (Te, Se) vacancy are investigated in order to shed light on the role of ligands in a series of iron-based superconductors. Such a vacancy can produce a local hopping correction ranging from −0.22 to 0.12 eV and always induce two in-gap resonance peaks in the local density of states (LDOS) at the fixed symmetrical bias voltages, which are rather robust and irrelevant to the phase of superconducting order parameter. The LDOS images near the defect predominantly possess 0° and 45° stripes. These energy-dependent charge modulations created by quasiparticle interference are originated in the nesting effect between the inner (outer) hole Fermi surface around Γ point and the inner (outer) electron Fermi surface around M point.

Three-dimensional scanning force/tunneling spectroscopy at room temperature

We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si–Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.

Quasiparticle scattering off phase boundaries in epitaxial graphene

We investigate the electronic structure of terraces of single layer graphene (SLG) by scanning tunnelling microscopy (STM) on samples grown by thermal decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the perturbations of the local density of states (LDOS) in the vicinity of edges of SLG terraces. Armchair edges are found to favour intervalley quasiparticle scattering, leading to the LDOS superstructure already reported for graphite edges and more recently for SLG on SiC(0001). Using the Fourier transform of LDOS images, we demonstrate that the intrinsic doping of SLG is responsible for a LDOS pattern at the Fermi energy which is more complex than for neutral graphene or graphite, since it combines local superstructure and long range beating modulation. Although these features have already been reported by Yang et al (2010 Nano Lett. 10 943–7) we propose here an alternative interpretation based on simple arguments classically used to describe standing wave patterns in standard two-dimensional systems. Finally, we discuss the absence of intervalley scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene junctions.

Electron states confined within nano-steps on metal surfaces

To elucidate electron states confined within steps on metal surfaces, we demonstrated anew point of view that a linear step on a noble metal surface can be treated as a dipolepotential composed of delta functions. For an electron confined by two pairs of dipolepotentials, we derived quasi-stationary eigenstates whose eigenenergies are complexnumbers which lead to the lifetime of the electron. To derive the local density of states(LDOS), scanning tunneling microscopy (STM) images and scanning tunneling spectra(STS) for stepped surfaces, we incorporated the lifetime effect on them and clarified therelation between the LDOS and the STM current by applying the expression for the STMcurrent derived by Selloni et al (1985 Phys. Rev. B 31 2602). Although, in previous studies,the Fabry–Pérot interference mechanism has been used to explain electron statesconfined within two steps, it requires four fitting parameters, in contrast our methodrequires one fitting parameter which specifies the height of the delta functions.Our results for LDOS images, topographical images and STS are consistent withexperimental ones for both the cases where electrons stay on a terrace confined by twosteps and on a wide terrace outside the step, which confirms the validity of ourmodel.

Electronic structure ofSi(001)−c(4×2)analyzed by scanning tunneling spectroscopy andab initiosimulations

We used atomic scale scanning tunneling spectroscopy to map the surface local density of states (LDOS) of clean $\mathrm{Si}(001)\text{\ensuremath{-}}c(4\ifmmode\times\else\texttimes\fi{}2)$ at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Energetically and spatially resolved LDOS images show that the intensity of the controversial $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ feature in the empty states is high over the troughs between dimer rows and low over the dimers. In contrast, the intensities are inverted in images constructed at $2.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. To understand those results, we undertook ab initio calculations using a real-space implementation of density functional theory to obtain contour plots of the surface charge density and the distribution of eigenstates. The simulations demonstrate that the features at 1.4 and $2.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ arise from the ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ states, respectively, and that $\ensuremath{\pi}$ bond interaction between dangling bonds persists in the buckled dimer configuration.

Energy-dependent checkerboard patterns in cuprate superconductors

Motivated by the recent scanning tunneling microscopy (STM) experiments [J. E. Hoffman {\it et al.}, Science {\bf 297}, 1148 (2002); K. McElroy {\it et al.}, Nature (to be published)], we investigate the real space local density of states (LDOS) induced by weak disorder in a d-wave superconductor. We first present the energy dependent LDOS images around a single weak defect at several energies, and then point out that the experimentally observed checkerboard pattern in the LDOS could be understood as a result of quasiparticle interferences by randomly distributed defects. It is also shown that the checkerboard pattern oriented along $45^0$ to the Cu-O bonds at low energies would transform to that oriented parallel to the Cu-O bonds at higher energies. This result is consistent with the experiments.