Nonlinear photoelectron emission from metallic nanotips is explored in the strong-field regime. The passage between the multiphoton and the optical field emission regimes is clearly identified. The experimental observations are in agreement with a quantum mechanical strong-field model.

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...

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All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron.

The Kondo resonance of single Co atoms embedded in a Cu matrix has been investigated with tunneling spectroscopy at Tϭ8 K. Dilute magnetic alloys were prepared by homoepitaxial growth of Cu͑111͒ films incorporating approximately 0.1% Co atoms as magnetic scattering centers. The Co impurities in the first layer of the Cu matrix show a characteristic, symmetric dip in the differential conductance around zero bias, indicating the presence of the many-body Abrikosov-Suhl resonance. The corresponding Kondo temperature is found to be T K ϭ405Ϯ35 K, which is much higher than previously reported values for Co adsorbate atoms.Nonmagnetic host metals with a dilute concentration of transition-metal impurities show an unusual asymptotic lowtemperature behavior in many of their physical properties. 1 A prominent anomaly is the increase in resistivity with decreasing temperature below a characteristic value T K . A theoretical explanation given by Kondo in 1964 recognized that resonant spin-flip scattering is the origin of the peculiar behavior of the alloys, 2 and was followed by steady theoretical research. Kondo systems have recently gained new attention, since local probe methods allow us to investigate isolated impurities with high spatial resolution in experiment. 3,4 Magnetic impurity atoms at the surface of noble metals are directly accessible with a scanning tunneling microscopy ͑STM͒ tip, and atomic-scale resolution for single Kondo systems on a surface has been achieved at low temperatures. Until now, scanning tunneling spectroscopy ͑STS͒ work has been focused on isolated 3d-and 4 f -impurity atoms adsorbed on noble-metal surfaces. Several material combinations have been examined, 3-8 but only some of the adsorbed impurities have shown a clear Kondo-resonance feature in the tunneling spectra. The most characteristic fingerprint of the Kondo effect, recorded with the tip on top of the impurity atom, is a small and narrow dip in the differential conductance at zero bias. In the special case of a Co adsorbate atom on the Cu͑111͒ surface, a slightly asymmetric dip has been observed independently in two previous STS studies. 6,7 The width of this feature is directly related to the Kondo temperature T K of the system. In comparison with the Kondo temperature T K bulk ϳ500 K determined with macroscopic methods, 1 the value for Co adsorbates was found to be very small (T K Ͻ100 K). 7 According to the defining relation 9the Kondo temperature of a single impurity is decisively determined by 0 (r ជ )ϭ(E F ,r ជ ), the local density of states ͑LDOS͒ at the Fermi-energy E F and the position r ជ . ͉J͉ denotes the effective exchange interaction between the localized spin and the spins of the conduction electrons. It was proposed by Knorr et al. that adsorbed Kondo impurities, as probed in their STS experiments, should constitute a system with a reduced ͉J͉ 0 (r ជ ). 7 According to Eq. ͑1͒, this leads to a lower T K than that obtained from macroscopic resistivity in bulk samples. The coordination number of impurities embedded in the...