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Project E7: Time resolved diffraction of photon-induced phase transitions in 1D metal wires on semiconductor surfaces

Michael Horn-von-Hoegen

This project aims at a fundamental understanding of the non-equilibrium structural dynamics of quasi 1D-metal wires on semiconductor surfaces upon optical excitation. With a fs-laser pulse a non-equilibrium situation in the electron system is induced. Subsequently also the lattice system responds to the excitation. The 1D-metal wires at the surface may undergo a non-thermally driven phase transition, characteristic phonon modes may be excited, or the surface may simply become heated. Employing the transient response of the system as an additional dimension for the analysis, the interaction strength of electron-phonon coupling, phonon-phonon coupling or electron-spin coupling may be deduced. Thus hidden parameters become available, which are not accessible from experiments under equilibrium conditions. Here we employ ultra-fast time resolved reflection high energy electron diffraction (tr-RHEED) at surfaces as the method of choice to follow changes of symmetry, size of unit cell, position of atoms, and vibrational excitations on the relevant sub-ps timescale. With our studies we focus on two different prototypical atomic wire systems: the first exhibiting an ordered spin chain and the second a Peierls instability. The Si(553)-Au adsorbate system is known for the formation of an anti-ferromagnetic ordered spin chain of Si step edge atoms along the wires which is apparent in diffraction through additional spots or streaks. The destruction, recovery or reorganization of this spin chain upon fs-laser excitation will be studied through a transient spot profile analysis in tr-RHEED. No mass transport or bond-breaking is associated with the order-disorder phase transition. In contrast to this the Si(111)-In adsorbate system exhibits a Peierls like instability giving rise to a first order phase transition between the (8x2) reconstructed ground state and a (4x1) reconstructed high temperature state. The (8x2) ground state is non-thermally driven by a fs-laser pulse into a super-cooled metastable (4x1)-phase. The recovery dynamics towards the ground state should be studied and manipulated through changes of temperature and the controlled introduction of adsorbates. These studies will be complemented with the time resolved electron spectroscopy (tr-ARPES) employed in E5 for the investigation of the electron dynamics in identical systems.