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Project E4: Transport and collective excitations in metallic wires

Annemarie Pucci, Herbert Pfnür, Christoph Tegenkamp

Exploiting the close relationship of relevant electron densities, transport properties and collective excitations in quasi-1D metallic structures grown by self-assembly on Ge and Si single crystal surfaces will be investigated by means of 4-tip STM-SEM, HREELS, and ELS- LEED. With these experimental methods we will elucidate the interplay between coupling of one-dimensional (1D) electronic systems to a substrate and the role of electronic correlations. We further study the consequences of this coupling on dynamic properties and on potential instabilities of the system. Electron-energy loss spectroscopy (EELS) of plasmons as a coherent excitation that appear over a broad wavelength range will be used to study the interaction of wires with different environments, the changes of the chemical potential induced by gating, and finite-size effects. We will combine the 4-tip STM-SEM with an in-situ lithography on the nano-scale in order to flexibly position defined contacts in a close-by geometry on nano-restricted areas. Thus both 1D properties and transport phenomena in the quantum regime are experimentally accessible for metal chains and silicide nanowires. In addition, the role of defects (atomic steps, dopants) will be systematically studied as a function of temperature and magnetic field (4T, only for transport) in order to determine their relevance for typical instabilities such as metalinsulator (e.g Peierls or Mott) transitions. Dispersion relations of collective quasi-particle (surface phonons) excitations in CDW and non-Fermi systems will be investigated with HREELS. Their behavior with temperature is of special interest to see possible phonon softening related to phase transitions.

(Picture: STM-tips placed on graphene for local 4-point transport measurements)