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CNST Group Seminars - 2005



Electron Physics Group Seminar

SPIN-POLARIZED SCANNING TUNNELING SPECTROSCOPY OF MAGNETIC NANOSTRUCTURES: CO ON CU(111)


Oswald Pietzsch
Institute of Applied Physics, University of Hamburg. Germany

Monday, November 7, 2005, 10:30AM, Rm. H107, AML 217 Bldg.

The material pair Co and Cu is prototypical for a combination of magnetic and non-magnetic metallic substances. Exciting phenomena have been studied in systems consisting of Co-Cu multilayers, like the giant magneto-resistance effect; individual Co adsorbates on a Cu surface give rise to the many-particle Kondo interaction. In my talk, I will present a low-temperature spin-polarized scanning tunneling microscopy/spectroscopy (SP-STM/STS) study of nanometer-sized Co islands on the Cu(111) face. These islands are ferromagnetic at 13 K with a strong perpendicular anisotropy.Tunneling spectra reveal a particularly rich variety of features depending on structural properties like island stacking as well as on the spin state. In particular, the spin polarization strongy depends on energy, with its sign reversed several times in the observed energy interval. Experimental spectra will be interpreted with the aid of ab initio spin density calculations. As is well known, the Cu(111) surface exhibits a standing wave pattern due to surface state electrons scattered at step edges and other defects. A similar pattern can be observed on the Co islands. It will be discussed how the patterns are affected by confinement to the triangular island geometry and by the spin character of the responsible dispersive state. For further information contact Joseph A. Stroscio, 301-975-3716, joseph.stroscio@nist.gov

 
Electron Physics Group Seminar

PLASMONS IN METALLODIELECTRIC NANOSTRUCTURES


Peter Nordlander
Dept. of Physics and Rice Quantum Institute, Rice University.

Friday, April 29, 2005, 2:00PM, Rm. H107, Bldg 217.

The excitation of nanoparticle plasmons can generate very large electromagnetic fields near their surfaces. These electromagnetic fields can dramatically increase the cross section for many surface sensitive spectroscopies such Surface Enhanced Infrared Absorption (SEIRA) and Surface Enhanced Raman Spectroscopy (SERS). In the case of Raman spectroscopy, various groups have reported cross section enhancements of more than 14 orders of magnitude thus enabling the spectroscopic detection of single molecules. In the talk, I will discuss the physical mechanisms of the electromagnetic field enhancements on nanoparticle surfaces and show that they can be particularly strong at the junctions between two nanoparticles. I will discuss a recent application of the plasmon hybridization method to nanoparticle dimer and nanoparticle on a surface configurations. It will be shown that the plasmons in composite metallic nanostructures can be viewed as resulting from hybridization of the elementary plasmon modes of the constituent structures. This interaction leads to the formation of bonding and anti-bonding dimer plasmons in a manner entirely analogous to the interaction of electronic levels in molecular orbital theory. Finally, I will compare the results with a full electrodynamic simulations using the Finite Difference Time Domain (FDTD) method. For further information contact J. William Gadzuk, 301-975-2548, gadzuk@nist.gov


Electron Physics Group Seminar

LOW-TEMPERATURE SCANNING-TUNNELING MICROSCOPY AND ATOMIC MANIPULATION OF ADSORBATES ON ULTRATHIN INSULATING FILMS


Jascha Repp
Physicist, IBM Zurich Research Laboratory.

Monday, January 10, 2005, 2:00PM, Rm. B211, Metrology Bldg.

By means of low-temperature scanning-tunneling microscopy (STM) it is not only possible to image individual adsorbates on metallic surfaces, one can also position them with atomic precision. This opens up the fascinating possibility to build up artificial nanostructures atom-by-atom. Using a simple model system, Cu/Cu(111), it will be demonstrated how atomic/molecular manipulation can be used as a new tool in surface physics to obtain important physical information of the adsorption site and adsorbate-adsorbate interactions. Adsorbates on metal surfaces are strongly disturbed in their intrinsic properties by the presence of the substrate electrons. To understand the electronic properties of an individual molecule in meso-scale devices and for mono-molecular electronics, an electronic decoupling of the molecules from the supporting substrate is therefore desirable, if not mandatory. Ultrathin insulating NaCl films on copper facilitate an electronic decoupling of this kind. STM experiments provide an insight into fundamental issues like the growth, the binding and electronic states of insulator/metal interfaces. Moreover, they open up new possibilities in the so-called “atomic-scale technologies”, because adsorbates on insulating films can now be studied and even manipulated on the atomic length scale for the first time. For further information contact Joseph Stroscio, 301-975-3716, joseph.stroscio@nist.gov
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