The charge state of individually addressable impurities in semiconductor material was manipulated with a scanning tunneling microscope. The manipulation was fully controlled by the position of the tip and the voltage applied between tip and sample. The experiments were performed at low temperature on the (110) surface of silicon doped GaAs. Silicon donors up to 1 nm below the surface can be reversibly switched between their neutral and ionized state by the local potential induced by the tip. By using ultrasharp tips, the switching process occurs close enough to the impurity to be observed as a sharp circular feature surrounding the donor. By utilizing the controlled manipulation, we were able to map the Coulomb potential of a single donor at the semiconductor-vacuum interface.

The Kondo effect, one of the oldest correlation phenomena known in condensed matter physics [1], has regained attention due to scanning tunneling spectroscopy (STS) experiments performed on single magnetic impurities [2,3]. Despite the sub-nanometer resolution capability of local probe techniques one of the fundamental aspects of Kondo physics, its spatial extension, is still subject to discussion. Up to now all STS studies on single adsorbed atoms have shown that observable Kondo features rapidly vanish with increasing distance from the impurity [4,5,6,7,8,9]. Here we report on a hitherto unobserved long range Kondo signature for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We present a theoretical interpretation of the measured signatures using a combined approach of band structure and many-body numerical renormalization group (NRG) calculations. These are in excellent agreement with the rich spatially and spectroscopically resolved experimental data. The interaction of a single magnetic impurity with the surrounding electron gas of a non-magnetic metal leads to fascinating phenomena in the low temperature limit, which are summarized by the term Kondo effect [1]. Such an impurity has a localized spin moment that interacts with the electrons of the conduction band. If the system is cooled below a characteristic temperature, the Kondo temperature T K , a correlated electronic state develops and the impurity spin is screened. The most prominent fingerprint of this many body singlet state is a narrow resonance at the Fermi energy ε F in the single particle spectrum of the impurity, called Kondo or Abrikosov-Suhl resonance. The existence of this Kondo resonance has been experimentally confirmed for dense systems with high resolution photoemission electron spectroscopy and inverse photoemission [10,11]. Due to their limited spatial resolution these measurements always probe a very large ensemble of magnetic atoms. With its capability to study local electronic properties with high spatial and energetic resolution, Scanning Tunneling Spectroscopy (STS) has paved the way to access individual impurities [2,3].A theoretical prediction for the local density of states (LDOS) -the key quantity measured in STS experiments -was first provided byÚjsághy et al [12]. According to their calculations the Kondo resonance induces strong spectroscopic signatures at the Fermi energy whose line shape is oscillatory with distance to the impurity. Since the first STS studies in 1998 [2, 3] a lot of experiments on magnetic atoms and molecules on metal surfaces were performed, all revealing Kondo fingerprints [5,6,7,8,9]. However, it is worth noting that all previous STS experiments on isolated ad atoms have reported that the Kondo signature rapidly vanishes within a few angstrom and no variation of the line shape occurs (for a review on ad atom Kondo systems see [13]).In the present work we follow a novel route and investigate single isolated magnetic impurities buried below the surface with a low temperature STM ope...

The Fermi surface that characterizes the electronic band structure of crystalline solids can be difficult to image experimentally in a way that reveals local variations. We show that Fermi surfaces can be imaged in real space with a low-temperature scanning tunneling microscope when subsurface point scatterers are present: in this case, cobalt impurities under a copper surface. Even the very simple Fermi surface of copper causes strongly anisotropic propagation characteristics of bulk electrons that are confined in beamlike paths on the nanoscale. The induced charge density oscillations on the nearby surface can be used for mapping buried defects and interfaces and some of their properties.

We investigate the structural, electronic, and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultralow energy ion implantation, the influence of different ion species (boron, nitrogen, and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and can reduce or raise the position of the Fermi level, respectively, (12)C(+) carbon ions are used to study possible defect creation by the bombardment. For low-temperature transport, the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples, we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for (11)B(+)- and (12)C(+)-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, because they result in weak localization effects at low temperatures.

We measured the ionization threshold voltage of individual impurities close to a semiconductor-vacuum interface, where we use the STM tip to ionize individual donors. We observe a reversed order of ionization with depth below the surface, which proves that the binding energy is enhanced towards the surface. This is in contrast to the predicted reduction for a Coulombic impurity in the effective mass approach. We can estimate the binding energy from the ionization threshold and show experimentally that in the case of silicon doped gallium arsenide the binding energy gradually increases over the last 1.2 nm below the (110) surface.