Ultrafast electron dynamics in low-dimensional materials

Abstract

This work investigates the ultrafast electron dynamics in low-dimensional materials using femtosecond time- and angle-resolved photoemission techniques. In low-dimensional materials, the electronic wave functions are confined to one and two dimensions, (1D) and (2D). This electron confinement can lead to (i) quantum size effects, where the physical properties of a solid state material depend decisively on the size of the considered system, and to (ii) many-body phenomena, where electron electron correlation and coupling to other excitations such as phonons can result in the formation of broken-symmetry ground states like charge density wave (CDW) or superconducting phases. Besides the fundamental scientific significance, low-dimensional materials become increasingly important with the advent of nanotechnology. In this work, several well-defined prototypical quasi-2D and quasi-1D model systems have been studied. The quantized band structure and electron dynamics of the occupied and unoccupied quantum well states (QWSs) in the quasi-2D model system Pb/Si(111) have been investigated with two-photon photoemission (2PPE) and photoemission spectroscopy (PES) directly in the time domain. The general trend of the hot electron lifetimes in the unoccupied QWS is confirmed to be governed by Fermi liquid theory. To describe all observed population decays quantitatively, the quantized electronic system has to be taken into account since the simultaneous population decay and build-up in two adjacent QWSs favors inter-subband scattering from the higher lying into the lower lying state. The investigation of the dynamics of the highest occupied QWS in Pb/Si(111) with time-resolved PES revealed an ultrafast electronic stabilization due to the excitation of electron-hole pairs that induce a sudden change of the screening of the ion cores and displacively excite a coherent phonon mode in the film direction. Potential anisotropies of the hot electron lifetimes of self-assembling metallic quasi-1D nanowires in 4x1-In/Si(111) were investigated using time-resolved 2PPE and a novel position-sensitive electron time-of-flight technique, which was developed as part of this work for efficient angle-resolved photoemission studies of low-dimensional materials with anisotropic band structure. Finally, collective excitations of the electron and lattice system in the prototypical quasi-1D CDW compound TbTe3 have been explored with time- and angle-resolved PES. The temperature-, fluence- and k-dependence of the transient band structure in the CDW nesting region at the Fermi level reveals an oscillation of the valence band, which is assigned to the excitation of the amplitude mode of the CDW phase of TbTe3. At the highest fluences investigated, the charge-ordered CDW melts within 100 fs, evidenced by the ultrafast closing of the CDW gap in the band structure at the Fermi level and the transient recurrence of a nearly free electron dispersion