Session G1: 25 Years of Scanning Probe Microscopy

Classical Computation in Quantum Nanostructures: A Long Road to an Uncertain Future

We have extended the spectroscopic abilities of the scanning tunneling microscope to include the measurement of spin-excitation spectra, making it possible to measure the g-value of single atoms. Utilizing spin-excitation spectroscopy as our primary tool, we are now capable of extracting exchange coupling energies, anisotropy energies, and information on the ground and excited state spin configurations of nanometer-scale structures. These experiments are playing an integral role in our efforts to engineer the ``energy landscape'' of a system of spins in order to achieve nanometer-scale binary logic circuits that operate using only the spin degree of freedom.\newline \newline Work done in collaboration with Cyrus Hirjibehedin, Andreas Heinrich, Christopher Lutz, Jay Gupta, and Bruce Melior.   

Scanning Probe Microscopy for Spin Mapping and Spin Manipulation on the Atomic Scale

A fundamental understanding of magnetic and spin-dependent phenomena requires the determination of spin structures and spin excitations down to the atomic scale. The direct visualization of atomic-scale spin structures [1-4] has first been accomplished for magnetic metals by combining the atomic resolution capability of Scanning Tunnelling Microscopy (STM) with spin sensitivity, based on vacuum tunnelling of spin-polarized electrons [5]. The resulting technique, Spin-Polarized Scanning Tunnelling Microscopy (SP-STM), nowadays provides unprecedented insight into collinear and non-collinear spin structures at surfaces of magnetic nanostructures and has already led to the discovery of new types of magnetic order at the nanoscale [6,7]. More recently, the detection of spin-dependent exchange and correlation forces has allowed a first direct real-space observation of spin structures at surfaces of antiferromagnetic insulators [8]. This new type of scanning probe microscopy, called Magnetic Exchange Force Microscopy (MExFM), offers a powerful new tool to investigate different types of spin-spin interactions based on direct-, super-, or RKKY-type exchange down to the atomic level. By combining MExFM with high-precision measurements of damping forces, localized or confined spin excitations in magnetic systems of reduced dimensions now become experimentally accessible. Moreover, the combination of spin state read-out and spin state manipulation, based on spin-current induced switching across a vacuum gap by means of SP-STM [9], provides a fascinating novel type of approach towards ultra-high density magnetic recording without the use of magnetic stray fields. \newline [1] R. Wiesendanger, I. V. Shvets, D. B\"{u}rgler, G. Tarrach, H.-J. G\"{u}ntherodt, J. M. D. Coey, and S. Gr\"{a}ser, Science \textbf{255}, 583 (1992) [2] S. Heinze, M. Bode, O. Pietzsch, A. Kubetzka, X. Nie, S. Bl\"{u}gel, and R.~Wiesendanger, Science \textbf{288}, 1805 (2000) [3] A. Kubetzka, P. Ferriani, M. Bode, S. Heinze, G. Bihlmayer, K. von Bergmann, O. Pietzsch, S. Bl\"{u}gel, and R. Wiesendanger, Phys. Rev. Lett. \textbf{94}, 087204 (2005) [4] M. Bode, E. Y. Vedmedenko, K. von Bergmann, A. Kubetzka, P. Ferriani, S. Heinze, and R. Wiesendanger, Nature Materials \textbf{5}, 477 (2006) [5] R. Wiesendanger, H.-J. G\"{u}ntherodt, G. G\"{u}ntherodt, R. J. Gambino, and R. Ruf, Phys. Rev. Lett. \textbf{65}, 247 (1990) [6] K. von Bergmann, S. Heinze, M. Bode, E. Y. Vedmedenko, G. Bihlmayer, S. Bl\"{u}gel, and R. Wiesendanger, Phys. Rev. Lett. \textbf{96}, 167203 (2006) [7] M. Bode, M. Heide, K. von Bergmann, P. Ferriani, S. Heinze, G. Bihlmayer, A. Kubetzka, O. Pietzsch, S. Bl\"{u}gel, and R. Wiesendanger, Nature \textbf{447}, 190 (2007) [8] U. Kaiser, A. Schwarz, and R. Wiesendanger, Nature \textbf{446}, 522 (2007) [9] S. Krause, L. Berbil-Bautista, G. Herzog, M. Bode, and R. Wiesendanger, Science \textbf{317}, 1537 (2007)   

Understanding Polymer Properties through Imaging of Molecules.

The unique advantage of Scanning Probe Microscopy (SPM) is that it allows imaging of flexible polymer molecules, whose overall size and local curvature are below the optical resolution limit. The role of molecular visualization has grown to be especially profound with the synthesis of complex macromolecules whose structure is difficult to confirm using conventional techniques such as NMR and light scattering. This is especially true for molecules that are branched, heterogeneous, and polydisperse. Here, SPM images provide unambiguous proof of the molecular architecture along with accurate analysis of size, conformation, and ordering of molecules on surfaces. The unique advantage of SPM is that one obtains molecular dimensions in direct space. This offers more opportunities for statistical analysis including fractionation of molecules by size, branching topology, and chemical composition as well as sorting out the irrelevant species. Unlike molecular characterization of static molecules, it remains challenging to study molecules as they move and react on surfaces. We will discuss pioneering AFM studies of flowing monolayers one molecule at a time. Through use of AFM, the flow process was monitored over a broad range of length scales from the millimeter long precursor film all the way down to the movements of individual molecules within the film. Molecular imaging enabled independent measurements both the driving and frictional forces that control spreading rate. In these studies, one also discovered a new type of flow instability in polymer monolayers caused by flow-induced conformational transitions. Recently, molecular imaging has been successfully used to monitor adsorption-induced degradation of branched molecules. These experiments open an entirely new perspective in chemistry wherein the chemical bonds can be mechanically activated upon the physical contact of a macromolecule with a substrate. This research directly impacts coatings, lubrication, heterogeneous catalysis, and biochemical assays, i.e. technologies that are largely controlled by surface-activated changes in the molecular structure and properties.