Ultrafast Probing and Coherent Vibrational Control of a Surface Structural Phase Transition

by Jan Gerrit Horstmann
Doctoral thesis
Date of Examination:2021-06-02
Date of issue:2021-08-26
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Stefan Mathias
Referee:Prof. Dr. Michael Horn-von Hoegen
Persistent Address: http://dx.doi.org/10.53846/goediss-8795

 

 

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Abstract

English

The present thesis explores the coherent control of surface structural phase transitions by all-optical manipulation of key vibrational modes. To this end, ultrafast low-energy electron diffraction (ULEED) in combination with femtosecond pulse sequences and optical pump-probe spectroscopy (OPP) is harnessed to probe and control the Peierls-like transition between the insulating (8x2) and the metastable, metallic (4x1) phase of atomic indium wires on the (111) surface of silicon. Single-pulse optical excitation is used to drive the (8x2)->(4x1) transition well below the critical temperature of T_c = 125 K via the (de-)population of electronic states coupled to shear and rotational phonon modes connecting both phases. Whereas transient reflectivity measurements point to an acceleration of initial atomic motion at high excitation densities, ULEED underlines the impact of nanoscale heterogeneity on the transition and the subsequent recovery of the ground state for the case of a partially excited surface. In a second set of ULEED experiments, a double-pulse optical excitation scheme is employed to exert coherent control over the transition close to its threshold. Here, pronounced oscillations in the delay-dependent switching efficiency evidence the decisive role of long-lived vibrational coherence in shear and rotation modes for governing the structural transformation. The corresponding lifetimes suggest that these modes act as a phonon bottleneck for energy relaxation between electronic and lattice subsystems. Based on the analysis of mode-specific frequency changes, initial phases and amplitudes, two possible coherent control mechanisms are discussed, involving the ballistic motion of the order parameter across the barrier and absorption modulation by Raman-active phonons, respectively. Multi-pulse experiments demonstrate the selective excitation of shear and rotation phonons and the applicability of 2D spectroscopy schemes for the investigation of possible mode couplings. Furthermore, the joint results of ULEED, OPP and density functional theory (DFT) suggest a description of the transition in terms of a two-dimensional potential energy surface (PES) with an off-diagonal transition state. The outcome of this work shows that coherent atomic motion can be harnessed to affect the efficiencies and thresholds of structural phase transitions. Mode-selective coherent control of surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and nonequilibrium states.
Keywords: Ultrafast dynamics; Coherent control; Phase transition; Surface science; Coherent phonons; Ultrafast spectroscopy; Low-energy electron diffraction; Indium on Silicon
 
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