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Molecular vibrational spectroscopy with the scanning tunneling microscope is feasible but usually detects few vibrational modes. We harness sharp Yu-Shiba-Rusinov states observed from molecules on a superconductor to significantly enhance the vibrational signal. From a lead phthalocyanine molecule 46 vibrational peaks are resolved enabling a comparison with calculated modes. The energy resolution is improved beyond the thermal broadening limit and shifts induced by neighbor molecules or the position of the microscope tip are determined. Vice versa, spectra of vibrational modes are used to measure the effect of an electrical field on the energy of Yu-Shiba-Rusinov states. The method may help to further probe the interaction of molecules with their environment and to better understand selection rules for vibrational excitations.

Single Fe atoms on Au(111) surfaces were hydrogenated and dehydrogenated with the Au tip of a low-temperature scanning tunneling microscope (STM). Fe and FeH$_2$ were contacted with the tip of the microscope and show distinctly different evolutions of the conductance with the tip-substrate distance. The current shot noise of these contacts has been measured and indicates a single relevant conductance channel with the spin-polarized transmission. For FeH$_2$ the spin polarization reaches values up to 80\% for low conductances and is reduced if the tip-surface distance is decreased. These observations are partially reproduced using density functional theory (DFT) based transport calculations. We suggest that the quantum motion of the hydrogen atoms, which is not taken into account in our DFT modeling, may have a significant effect on the results., 10 pages

The current I through nickelocene molecules and its noise are measured with a low-temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, namely an abrupt jump to contact and a smooth transition. These data along with conductance spectra (dI/dV) recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-12 state that is Kondo-screened. Many-body calculations show that the smooth transition is also consistent with a renormalization of spin excitations of a spin-1 molecule by Kondo exchange coupling. The shot noise is significantly reduced compared to the Schottky value of 2eI. The noise can be described in the Landauer picture in terms of the spin polarization of the transmission of ≈35% through two degenerate dπ-orbitals of the nickelocene molecule., This work has been supported by the Deutsche Forschungsgemeinschaft, project BE 2132/8. N.L. is grateful for the financial support from MICINN (RTI2018- 097895-B-C44). D.J. acknowledges funding by the grant “Grupos Consolidados UPV/EHU del Gobierno Vasco” (IT1249-19).

Spatially resolved measurements of the apparent tunneling barrier height Φapp in scanning tunneling microscopy have been used to estimate variations of the local work function Φ of surface structures. We experimentally show that Φapp can fail as a measure of Φ. The discrepancies are attributed to a kinetic-energy contribution to Φapp. This contribution depends on the lateral extent of the tunneling current filament and, consequently, on the local surface structure.

Molecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces1,2, which is relevant for molecular spintronics devices3–5. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions6–9. Robust metal–organic platforms, on the other hand, rely on external axial ligands to induce spin switching10–14. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces15. Spin-crossover complexes often lose their functionality upon adsorption on metal surfaces. Here, a metal–organic complex adsorbed on a silver surface undergoes reversible interlocked spin and coordination switching, which is enabled by an intramolecular feedback mechanism controlling the position of an axial ligand strapped to the complex.

The shot noise of the current $I$ through junctions to single trioxatriangulenium cations (TOTA$^+$) on Au(111) is measured with a low temperature scanning tunneling microscope using Au tips. The noise is significantly reduced compared to the Poisson noise power of $2eI$ and varies linearly with the junction conductance. The data are consistent with electron transmission through a single spin-degenerate transport channel and show that TOTA$^+$ in a Au contact does not acquire an unpaired electron. Ab initio calculations reproduce the observations and show that the current involves the lowest unoccupied orbital of the molecule and tip states close to the Fermi level., Comment: 5 pages, 5 figures

In the scanning tunnelling microscope, the many-body Kondo effect leads to a zero-bias feature of the differential conductance spectra of magnetic adsorbates on surfaces. The intrinsic line shape of this Kondo resonance and its temperature dependence in principle contain valuable information. We use measurements on a molecular Kondo system, all- trans retinoic acid on Au(1 1 1), and model calculations to discuss the role of instrumental broadening. The modulation voltage used for the lock-in detection, noise on the sample voltage, and the temperature of the microscope tip are considered. These sources of broadening affect the apparent line shapes and render difficult a determination of the intrinsic line width, in particular when variable temperatures are involved.

Nickel islands are grown on W(110) at elevated temperatures. Islands with a thickness of two layers are investigated with scanning tunneling microscopy. Spectroscopic measurements reveal that nanometer sized areas of the islands exhibit distinctly different apparent heights and dI/dV spectra. Spin polarized and paramagnetic band structure calculations indicate that the spectral features are due to fcc(111) and bcc(110) orientations of the Ni film, respectively., The final publication is available at http://www.springerlink.com

The Kondo effect, one of the oldest correlation phenomena known in condensed matter physics, has regained attention due to scanning tunneling spectroscopy (STS) experiments performed on single magnetic impurities. 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. 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., Comment: 7 pages, 3 figures + 8 pages supplementary material; Nature Physics (Jan 2011 - advanced online publication)

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.

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., 5 pages, 3 figures, accepted for publication in Phys. Rev. Lett

A Mn-porphyrin was contacted on Au(111) in a low-temperature scanning tunneling microscope (STM). Differential conductance spectra show a zero-bias resonance that is due to an underscreened Kondo effect according to many-body calculations. When the Mn center is contacted by the STM tip, the spectrum appears to invert along the voltage axis. A drastic change in the electrostatic potential of the molecule involving a small geometric relaxation is found to cause this observation., 8 pages, 8 figures (including supplemental material); accepted in Phys. Rev. Lett

Single atoms on Au(111) surfaces have been contacted with the Au tip of a low temperature scanning tunneling microscope. The shot noise of the current through these contacts has been measured up to frequencies of 120 kHz and Fano factors have been determined to characterize the transport channels. The noise at Fe and Co atoms, the latter displaying a Kondo effect, indicates spin-polarized transport through a single channel. Transport calculations reproduce this observation.

In devices like the single-electron transistor the detailed transport properties of a nanostructure can be measured by tuning its energy levels with a gate voltage. The scanning tunnelling microscope in contrast usually lacks such a gate electrode. Here we demonstrate tuning of the levels of a donor in a scanning tunnelling microscope without a third electrode. The potential and the position of the tip are used to locally control band bending. Conductance maps in this parameter space reveal Coulomb diamonds known from three-terminal data from single-electron transistors and provide information on charging transitions, binding energies and vibrational excitations. The analogy to single-electron transistor data suggests a new way of extracting these key quantities without making any assumptions about the unknown shape of the scanning tunnelling microscope tip.

A scanning tunneling microscope can be used to visualize in real space Fermi surfaces with buried impurities far below substrates acting as local probes. A theory describing this feature is developed based on the stationary phase approximation. It is demonstrated how a Fermi surface of a material acts as a mirror focusing electrons that scatter at hidden impurities., 10 pages, 4 figures

Nature Physics 7, 203–206 (2011); published online 23 January 2011; corrected after print 9 January 2012. In the version of this Letter originally published, in Fig. 3a the y-axis units should have been 'eV'. This error has been corrected in the HTML and PDF versions of the Letter.

in press The magnetic properties of transition-metal ions are generally described by the atomic spins of the ions and their exchange coupling. The orbital moment, usually largely quenched due the ligand field, is then seen as a perturbation. In such a scheme, S = 1/2 ions are predicted to be isotropic. We investigate a Co(II) complex with two antiferromagnetically coupled 1/2 spins on Au(111) using low-temperature scanning tunneling microscopy, X-ray magnetic circular dichroism, and density functional theory. We find that each of the Co ions has an orbital moment comparable to that of the spin, leading to magnetic anisotropy, with the spins preferentially oriented along the Co–Co axis. The orbital moment and the associated magnetic anisotropy is tuned by varying the electronic coupling of the molecule to the substrate and the microscope tip. These findings show the need to consider the orbital moment even in systems with strong ligand fields. As a consequence, the description of S = 1/2 ions becomes strongly modified, which have important consequences for these prototypical systems for quantum operations.