Lattice Dynamics in Temperature-driven and Photo-induced Phase Transitions
Advancements in inelastic X-ray and neutron scattering techniques nowadays allow for the nearly complete measurement of anharmonic phonon behaviors near phase transitions. It is difficult to explain the observed effects of phonon anharmonicity without using first-principles calculations, for instance to obtain renormalized second- order and higher-order force-constants. Moreover, pump-probe experiments can now resolved femtosecond atomic dynamics by using an ultrafast laser pulse to excite materials out of equilibrium and then track the Bragg or diffuse intensities with a time-resolved pulse of X-rays. However, the response of materials to the laser pulse is complex, necessitating ab initio calculations to achieve a physical understanding of the processes underlying pump-probe experiments. Using ab initio calculations, this thesis investigates the significance of phonon anharmonicity in the thermal phase transition, as well as the lattice dynamics of photo-excited crystals, specifically in VO2 and SnS/SnSe single crystals.
The thermal transport behavior and thermodynamics of VO2, which exhibits a well-studied metal-insulator transition (MIT), are both heavily influenced by its anharmonic lattice dynamics. However, the precise evolution of its phonon dispersions over the MIT has remained unknown. Strong phonon softening and damping in rutile VO2 were revealed by our inelastic X-ray scattering (IXS) measurements. We reproduced the phonon softening and phonon damping in rutile VO2, as well as their absence in M1 VO2 and rutile TiO2, using first-principles simulations. Our simulations reveal that the anti-ferroelectric distortions are important to understand the phonon softening in rutile VO2, and the large mean square displacement contributes to the extensive damping of its low-energy phonon branches.
In addition to the thermally-induced transition, photo-excitation can also drive the insulator-to-metal transition (IMT) of VO2 on an ultrafast time scale. Using X-ray pump-probe (XPP) total scattering and Megaelectron-volt ultrafast electron diffraction (MeV-UED), we investigated the lattice dynamics upon photo-excitation. We used MeV-UED to track the Bragg intensity and structural response of M1 VO2 at laser fluences below saturation. Below the saturation laser fluence, our ab initio calculations reproduce the initial structural deformation towards the rutile structure. We also use ab initio calculations to show the dominant factor that causes the structural deformation below the saturation fluence. Moreover, we used XPP total scattering to track both the Bragg and diffuse scattering intensities of VO2 across the IMT at laser fluences above saturation. Above the saturation fluence, our ab initio calculations reproduce the evolvement of Bragg intensity and diffuse intensity. The photo-induced phase transition was found to proceed as an ultrafast disordering phase transition. The disordering IMT is enabled by the flattening of the potential energy surface following photo-excitation, which quickly enlarges the phase space for atomic motions.
Using inelastic neutron scattering (INS) and high-resolution Raman spectroscopy, we investigated the lattice dynamics across the high-temperature Pnma-Cmcm phase transition of SnS (and SnSe). We performed first-principles simulations to understand the strong phonon anharmonicity and its impact on lattice thermal conductivity. INS detected a drastic softening of the TA phonon branch in the Cmcm phase and a broad reconstruction of the low frequency TA and TO phonon modes in the Pnma phase, revealing high directionality of the bonding strength and anharmonicity. Our first-principles simulations use renormalized force-constants to reproduce the phonon reconstruction and explain the phonon anharmonicity, demonstrating that the substantial phonon softening has a direct impact on the lattice thermal conductivity, beyond perturbative scattering, primarily by decreasing phonon lifetime, through the reconstruction of scattering phase space.
Free-electron based laser pump-probe measurements of Pnma SnSe investigated the atomic displacement directions after photo-excitation. This thesis investigates the photo-excitation mechanism of SnSe using different methods and emphasizes the significance of selecting and comparing different simulation methods when interpreting experimental results. The distribution of electrons after photo-excitation inspires different schemes to approximate the complex photo-excitation process. Following photo-excitation, hole doping and constrained-DFT methods predict a structural distortion from Pnma to Immm, whereas electronic heating predicts a structural distortion from Pnma to Cmcm. In addition to the well-known electronic structure instability in the Pnma-Cmcm phase transition, the electronic structure changes from Pnma to Immm are also examined. This study further demonstrates the importance of first-principles modeling to understand the atomic dynamics following photo-excitation.
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