Scanning tunneling spectroscopy

About: Scanning tunneling spectroscopy is a research topic. Over the lifetime, 7886 publications have been published within this topic receiving 213828 citations.

Contrast Reversal and Shape Changes of Atomic Adsorbates Measured with Scanning Tunneling Microscopy

Abstract: Systematic, quantitative comparisons between scanning tunneling microscopy (STM) experiments and first principles simulations of $\mathrm{O}(2\ifmmode\times\else\texttimes\fi{}2)/\mathrm{Ru}(0001)$ have been performed. The shape of the atomic adsorbates in the images depends strongly on the tunneling resistance and changes reversibly from circular (high resistance) to triangular (low resistance). In addition, after adsorption of oxygen on the STM tip we observe a contrast reversal on the surface, confirmed by extensive numerical simulations.

Sample–tip coupling efficiencies of the photon-scanning tunneling microscope

Abstract: The photon-scanning tunneling microscope is the photon analog to the electron-scanning tunneling microscope. It uses the evanescent field due to the total internal reflection of a light beam in a prism, modulated by a sample attached to the prism. The exponential decay of the evanescent field is characterized by the penetration depth dp and depends on the angle of incidence θ, the wavelength, and the polarization of the incident beam. The 1/e decay lengths range from 150 to 265 nm as deduced from the expression of the electric-field intensity in the rarer medium for θ = π/2. If we place another optically transparent medium near the surface, frustrated total reflection occurs. It is shown theoretically and experimentally that, if we choose an appropriate angle of incidence θ(θ ≠ π/2) and change the index of refraction of one of the media, the decay length of the electric field can be extremely small, so that images with an improved resolution can be produced.

Probing the electronic properties of self-organized poly(3-dodecylthiophene) monolayers by two-dimensional scanning tunneling spectroscopy imaging at the single chain scale.

Abstract: Regioregular poly(3-dodecylthiophene) films self-organized on highly oriented pyrolytic graphite have been investigated by scanning tunneling microscopy and two-dimensional scanning tunneling spectroscopy (STS). Simulated spectra in very good agreement with the experimental data have been obtained by a method combining ab initio and semiempirical approaches, which allows a careful discussion of the polymer electronic states. From the experimental data, with the support of modeling, it is shown that the STS spectra give a direct access to the polymer semiconducting band gap without noticeable charge-transfer effects from the substrate. Spectroscopic images are achieved at the single chain scale, which allows scrutinizing the electronic consequences of chain folds and π-stacking effects through spectroscopic contrasts. While chain folds do not locally increase the polymer band gap more than a few tens of millielectonvolt, a striking widening of the STS conductance gap is observed in the case of electronic t...

Two-sphere model of photon emission from the scanning tunneling microscope

Abstract: Photon emission from the scanning tunneling microscope (STM) has been shown to produce spectra that are dependent on the dielectric function of the surface. This presents the exciting possibility of chemical information from the STM, previously impossible albeit with a few exceptions. However, the spectra are also critically dependent on the geometry of the tunnel junction---changes in either tip or sample curvature can have dramatic effects on the spectra. A model is presented here that allows for curvature in the sample as well as the tip, and the results compare favorably with existing experimental data. It predicts that under certain conditions, spectra can be made almost invariant to changes in the geometry, leaving them dependent solely on the sample material---making the STM a chemical probe. The model also predicts that gold and silver particles may be differentiated by comparing their variations of photon intensity with bias, and this prediction is confirmed experimentally. Finally, a strategy for chemical identification of any metal surface is presented.

Probing the superfluid velocity with a superconducting tip: the Doppler shift effect.

Abstract: We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing scanning tunneling spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle (QP) spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe(2), reveals both the vortex cores, on the scale of the SC coherence length xi, and the supercurrents, on the scale of the London penetration length lambda. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.