Towards the Optical Control of Resonantly Bonded Materials: An Ultrafast X-Ray Study (Springer Theses)

This thesis describes key contributions to the fundamental understanding of materials structure and dynamics from a microscopic perspective. In particular, the thesis reports several advancements in time-domain methodologies using ultrafast pulses from X-ray free-electron lasers (FEL) to probe the interactions between electrons and phonons in photoexcited materials. Using femtosecond time-resolved X-ray diffraction, the author quantifies the coherent atomic motion trajectory upon sudden excitation of carriers in SnSe. This allows the reconstruction of the nonequilibrium lattice structure and identification of a novel lattice instability towards a higher-symmetry structure not found in equilibrium. This is followed by an investigation of the excited-state phonon dispersion in SnSe using time-resolved X-ray diffuse scattering which enables important insight into how photoexcitation alters the strength of specific bonds leading to the novel lattice instability observed in X-ray diffraction. Finally, by combining ultrafast X-ray diffraction and ARPES, the author performs quantitative measurements of electron-phonon coupling in Bi2Te3 and Bi2Se3. The findings highlight the importance of time-resolved X-ray scattering techniques based on FELs, which reveals the details of interplay between electron orbitals, atomic bonds, and structural instabilities. The microscopic information of electron phonon interaction obtained from these methods can rationalize ways to control materials and to design their functional properties. Supervisor's Foreword Acknowledgments Parts of This Thesis Have Been Published in the Following Journal Articles Contents 1 Ultrafast X-Ray Scattering and Non-equilibrium States of Matter 1.1 Ultrafast Diffraction and Emergent Order 1.2 Uniform Coherent Dynamics 1.3 Going Beyond the Zone Center References 2 Lattice Dynamics: Excitation and Probe 2.1 Structural Instabilities from a Chemical Bonding Perspective 2.2 Lattice Dynamics Dynamical Matrix and Phonons Electron Response Function Electron-Phonon Interaction Raman Scattering 2.3 Pump: Excitations of Phonons 2.4 Probe: X-Ray Scattering A General Formalism X-Ray Scattering in the Equilibrium Time-Resolved Diffraction Time-Resolved Diffuse Scattering References 3 Resonantly Bonded Semiconductors 3.1 Resonant Bonding Orbital Hybridization, Bonding Coordination, and Structural Tunability Long-Range Interatomic Interactions, Soft Phonons, and Low Thermoconductivity Phase Change Materials SnSe as a Representative of IV–VI Bi2Te3 and Bi2Se3 as Representatives of V2VI3 3.2 Topological Phases References 4 Ultrafast Lasers and X-Ray Pump-Probe Experiment 4.1 Ultrafast Near-Infrared Lasers 4.2 Free-Electron Lasers 4.3 The Integrated Setup References 5 Photoinduced Novel Lattice Instability in SnSe 5.1 Structural Phases of SnSe 5.2 Ultrafast X-Ray Diffraction of SnSe 5.3 Quantitative Reconstruction of Photoexcited Atomic Motion 5.4 Identification of a Novel Lattice Instability 5.5 Selectivity of the Photoexcitation Appendix Raman Spectroscopy and Pump-Probe Reflectivity Measurements Density Functional Theory Linear Prediction and Phonon Mode Decomposition References 6 Nonthermal Bonding Origin of the Novel Lattice Instability 6.1 Probing the Excited-State Lattice Dynamics 6.2 Nonequilibrium Interatomic Interactions 6.3 A Nonthermal Driving Force for the Lattice Instability Appendix Oscillator Parameter Extraction for Massive Data Phonon Dispersion and Branch Sorting Coherent Phonons in SnSe Excited Along -X References 7 Quantifying Deformation Potentials of Bi2Te3 and Bi2Se3 7.1 Electron Phonon Lock-in Detection 7.2 Experimental Details 7.3 Calculation of Deformation Potentials Calculations for Bi2Te3: Intrinsic Effects Calculations for Bi2Te3: Extrinsic Effects Calculation for Bi2Se3 7.4 Benchmarking the Experimental Method and Theoretical Calculations Appendix Phonon Eigenvector Reconstruction k-Dependence of Surface State Energy Shifts Simulations of Phonon Fields and Probed Signals Ellipsometry Theory Calculations References 8 Outlook References