Ultrafast electron diffraction and time resolved lattice dynamics
- The functional properties of materials often depend on the detailed interactions of the electronic, spin, and lattice degrees of freedom. These interactions can lead to technologically useful behaviors such as optical switching of magnetization, superconductivity, or ultrafast demagnetization. In this dissertation I present both ultrafast x-ray scattering results and ultrafast electron diffraction results that help to demystify the interaction between the electronic, spin, and lattice degrees of freedom. Following laser excitation of the sample, x-ray scattering at the Linac Coherent Lightsource (LCLS) is used to measure demagnetization and electron diffraction at the new MeV Ultrafast Electron Diffraction (UED) beamline is used to measure lattice dynamics. For Granular Iron Platinum (FePt) samples stress contributions from different degrees of freedom can be separated in the time domain by looking at strains within the grains. A combination of LCLS and UED measurements show the existence of a magnetostrictive driving force for the anisotropic lattice motion of the FePt nanoparticles. The magnetoelastic stress builds up on sub-ps timescales, and then on ps timescales the stress from the transiently populated phonons take over. Diffuse scattering measurements on Nickel (Ni) and Gold (Au) show that the increase in phonon occupation varies as a function of momentum exchange. These occupation increases range in speed from much faster than the timescale derived from the average lattice disordering rate (measured by the Debye-Waller effect) to around the same values. This range of values suggests the phonons are not equilibrated at these timescales, bringing into question the validity of the three temperature model. The details of these dynamics agree with the notion that faster energy transfer occurs from the laser-heated electrons to higher energy phonons. And for Ni a non-equilibrium model is proposed that qualitatively matches the experimental results.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Chase, Tyler Foster
|Stanford University, Department of Applied Physics.
|Dürr, Hermann, 1961-
|Dürr, Hermann, 1961-
|Statement of responsibility
|Tyler Foster Chase.
|Submitted to the Department of Applied Physics.
|Thesis (Ph.D.)--Stanford University, 2017.
- © 2017 by Tyler Foster Chase
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