Scanning tunneling spectroscopy at the single atom scale

This thesis reports measurements at the single atom scale by using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different sample systems where analyzed with normal conducting and superconducting tips. Chapter 2 presents the theoretical aspects which have to be taken into account for a detailed analysis and a consistent interpretation of the STS measurements. In chapter 3 the creation of a hexagonally ordered superlattice of single Ce adatoms on Ag(111) is reported and understood within a scattering model of the surface state electrons with the adatoms. Furthermore, the change in the local density of states of the surface state in ordered and slightly disordered superlattices is measured and theoretically explained within a tight-binding model which allows to understand the creation and stability of the superlattice by an energy gain of the participating surface-state electrons. Because Ce atoms have a non-vanishing magnetic moment which is expected to interact with the continuous states of the supporting surface leading to a Kondo resonance, chapter 4 presents measurements on single Ce adatoms on different Ag surfaces. This chapter shows the difficulties to interpret the obtained data. For instance, bistable Ce adatoms are detected on Ag(100) which show drastical changes in their apparent height and spectral signature depending on the tunneling conditions. The possible physical processes behind these phenomena are discussed. While the results presented in the first chapters were obtained with a normal conducting tip, chapter 5 intensively discusses the opportunities superconducting tips offer in low-temperature STS measurements. Novel insight in and thorough understanding of Andreev reflection processes are obtained by using the unique possibility of having different superconducting gaps in the tip and the sample. Detailed analyses of the supercurrent at low tunneling resistances reveal tunneling currents which are not described within the standard resistivity shunted junction model, and are presumably due to self-induced tunneling or due to an additional quasiparticle tunneling channel which only exist in asymmetric junctions. Furthermore, the influence of single magnetic Co atoms inbetween the superconducting tunnel junction on the obtained spectrum is discussed.

Spectroscopic manifestations of the Kondo effect on single adatoms

Wolf-Dieter Schneider, Markus Ternes

The present topical review focuses on recent advances concerning an intriguing phenomenon in condensed matter physics, the scattering of conduction electrons at the localized spin of a magnetic impurity: the Kondo effect. Spectroscopic signatures of this effect have been observed in the past by high-resolution photoemission which, however, has the drawback of averaging over a typical surface area of 1 mm(2). By combining the atomic-scale spatial resolution of the scanning tunneling microscope (STM) with an energy resolution of a few tens of mu eV achievable nowadays in scanning tunneling spectroscopy (STS), and by exposing the magnetic adatom to external magnetic fields, our understanding of the interaction of a single magnetic impurity with the conduction electrons of the nonmagnetic host has been considerably deepened. New insight has emerged by taking advantage of quantum size effects in the metallic support and by decoupling the magnetic adatom from the supporting host metal, for instance by embedding it inside a molecule or by separating it by an ultrathin insulating film from the metal surface. In this way, Kondo resonances and Kondo temperatures can be tailored and manipulated by changing the local density of states of the environment. In the weak coupling limit between a Kondo impurity and a superconductor only a convolution of tip and sample DOS is observed while for strongly coupled systems midgap states appear, indicating superconducting pair breaking. Magnetic impurities with co-adsorbed hydrogen on metallic surfaces show pseudo-Kondo resonances owing to very low-energy vibrational excitations detected by inelastic tunneling spectroscopy. One of the most recent achievements in the field has been the clarification of the role of magnetic anisotropy in the Kondo effect for localized spin systems with a spin larger than S = 1/2.

2009

Ultra-thin dielectric layers on metal substrates studied by scanning tunneling microscopy and scanning tunneling spectroscopy

In this thesis, ultra-thin dielectric layers on metal substrates are studied mainly by scanning tunneling microscopy and scanning tunneling spectroscopy. In chapter 2, field emission resonances, i.e. bound states in the potential well between sample and tip with an applied voltage, are used to determine the local work function on a sample with patches of different structural composition. To do so, a simple theoretical model is developed and used to simulate the peak positions of the measured dI/dV spectra. In the present case the sample is a Ag(100) surface covered by up to three monolayers of NaCl. The presented method is also applicable to other systems suitable for STM measurements. Additionally, the surface potential is locally probed in the dipole layer region between domains of different work function. In chapter 3, new molecular structures on silver surfaces are presented, that are formed by a mixture of gases. This mixture contains the small molecules H2, H2O, CO, CO2, and N2. No final conclusion can be drawn on the chemical nature of these structures due to the impossibility of the scanning tunneling microscope to determine chemical species. But already the presence of these structures indicates that there is a multitude of unknown interactions, that are possible between a transition metal surface and even smallest molecules. In chapter 4, the moiré pattern is described that a BC3 film creates on a NbB2(0001) surface. As the growth of the BC3 sheet is incommensurate, a moiré pattern is formed that modulates the apparent height of the atoms on the STM images, i.e. the local density of states. The BC3 layer exhibits a completely different behavior than the closely related graphite layer, because it is transparent for the tunnel current. In the present case, the BC3 layer is visible on the STM images only by its moiré effect.

EPFL2007