Phonons at Phase Boundaries in Electronic and Structural Transitions
Restricted (Penn State Only)
- Author:
- Wang, Huaiyu
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 21, 2022
- Committee Members:
- Venkatraman Gopalan, Chair & Dissertation Advisor
Zhiqiang Mao, Outside Unit & Field Member
Ismaila Dabo, Major Field Member
Long-Qing Chen, Major Field Member
John Mauro, Program Head/Chair
Haidan Wen, Special Member - Keywords:
- Structural perturbation
Phonons
Ultrafast
X-ray Diffraction
Raman spectroscopy
Quantum Materials
Ferroelectric Nanotextures
Phase Transition - Abstract:
- Engineering exotic properties in materials is of great importance and interest in materials science. Driving the response of materials with external stimuli has been the approach ever since the first ’material scientist’ made use of stone to start a fire. There is an existing demand for external driving forces with faster speed and better precision. With the invention of Maser and Laser (Microwave/Light amplification by stimulated emission of radiation) in the 1960s, awarded the Nobel Prize in 1964, we now have access to external controls with precision in energy, photon flux and coherence. Access to a really small timescale came with the invention of pulsed lasers. Upon shining light, materials are pushed to an excited state. Depending on the property of the driving laser and the intrinsic response of material systems, either electron, lattice, orbitals, and/or spin subsystems dominate the response. At the same time, a zoo of particle-like entities called quasiparticles appear after the excitation. Low-energy excitations in interacting systems are known to be characterized by quasiparticles. Understanding quasiparticles hold keys to solving many-body problems in complicated crystalline systems, such as density wave states, Mott insulators, and high Tc superconductors. In this dissertation, we focus on one type of quasiparticles, namely phonons, which are quantized particles of lattice vibrations. The energy of phonons is usually in the range of 10s to 100s meV. This energy is historically hard to reach; visible or near-infrared laser energy is too high and microwave energy is too low to directly access phonons. However, phonons can strongly influence properties such as conductivity, magnetism, ferroelectricity, and many more phenomena. With technology emerging in the last few decades, the generation of sub-cycle intense optical pulses in mid-Infrared and THz range has become available. This opens up a new direction called nonlinear phononics, where phonons are resonantly excited to generate nonlinear effects which induce new properties. Understanding coherent atomic motions is becoming increasingly important in manipulating solid-state matter in a short time scale below hundreds of picoseconds. On the other hand, phonons can cross-talk with electrons via electron-phonon coupling, which can in turn modulate the electronic properties of solid-state matter. Specifically, we report how phonons can induce new phenomena when the material system is placed at the edge of a phase transition. In the first case, we study the Ca3Ru2O7, which is on edge of Mott insulating state. We discovered two zone center phonons that exhibit a cross-over change of Raman scattering across the metal-pseudogap transition. Furthermore, one phonon strongly couples to the pseudogap phase and mediates incoherent dynamical charge and spin density wave fluctuations, which are evidenced by a symmetry-dependent change in the background Raman scattering. This example demonstrates how phonons that strongly couple to electronic properties can induce metastable electronic phases transiently. The second study is on a topological ferroelectric superlattice of (PbTiO3)n/(SrTiO3)n. The competition between electrostatic and elastic energy in PbTiO3/(SrTiO3 superlattices leads to the formation of topologically nontrivial nanoscale polar skyrmions. Using THz-pump, femtosecond x-ray diffraction (XRD) probe technique, we discover the collective dynamics of polar skyrmions, featuring a 0.3-THz mode with coherent vibrating domain walls and a stable core. The wide diffuse scattering peaks from polar skyrmions allow us to probe the dispersion of this collective mode. The Fourier spectra of the time domain response reveals distinct dispersion relations at the first-order and second-order diffuse scattering peaks, indicating possible phonon localization. The dynamical response is significantly reduced at the sample temperature of 360 K, corresponding to the loss of skyrmion topological charge because of a phse transition of the skyrmions into a labyrinth-like phase. The dynamical phase-field modeling reproduces the key experimental observations and gives microscopic insight into the polar skyrmion modes. Our work opens opportunities for ultrafast control of strongly correlated systems and topological ferroelectric nanostructures via structural perturbation.