Ultrahigh resolution surface phononics

Abstract

Phonons are one of the most important quasiparticles in condensed matter physics; they influence a wide range of physical properties ranging from thermal and electrical conductivity to the propagation of sound. Phonons are also crucial to emerging research fields such as quantum information and superconductivity. This thesis mainly focuses on the measurement of surface phonons using the helium spin echo (HeSE) technique. In Chapter 1, the concept of phonons is introduced. An overview of phonon measurement techniques is also presented, followed by a more detailed introduction of HeSE. Since the energy resolution of HeSE is in µeV regimes, much higher than other techniques, phonon linewidths can be extracted. This demonstrates HeSE's unique superiority because phonon linewidth is a property which is crucial in many research field related to phononics. Phonon linewidths are typically influenced by three different mechanisms, which are phonon-phonon interaction, defect-phonon interaction, and electron-phonon interaction, which are all discussed in this thesis. Chapter 2 provides a theoretical framework for phonons by using a simplified model of a monatomic chain. The theoretical derivation in this chapter uses time-independent perturbation theory, which aims to simplify existing results from the literature. It is shown that crystal anharmonicity is related to phonon-phonon interaction. It is also proved that phonon-phonon interaction will make phonon energy and phonon linewidth change linearly as a function of temperature. Both trends are demonstrated using HeSE phonon measurement data on a Ni(111) surface. A proof is also given to show that phonon lifetime is inversely proportional to phonon linewidth. Chapter 3 presents a direct experimental measurement of how crystal defects broaden the linewidths of Rayleigh wave (RW) mode phonons on a Ni(111) surface. Defects are found to contribute a temperature-independent component to the linewidths of RW phonons on a Ni(111) surface. Chapter 3 also characterised the increase in phonon scattering with both surface defect density and phonon wave vector. A quantitative estimate of the scattering rate between phonon modes and surface line defects is extracted from the experimental data. In Chapter 4, RW phonon linewidths on a Ru(0001) surface are studied. It is found that, contrary to most phonon measurements, the linewidths of RW phonons decrease with temperature below about 400 K. This is due to the interaction between electrons and phonons. A quantitative model combining phonon-phonon interaction, defect-phonon interaction, and electron-phonon interaction is used to explain the data. Chapter 5 is about the interpretation of the intensity, or height, of peaks in HeSE phonon spectra. The logarithm of the height of diffuse elastic peaks in HeSE spectra is found to decrease linearly as a function of temperature. The phenomenon is attributed to Debye-Waller attenuation. Theoretical analysis has also been performed to analyse the temperature dependence of the phonon peak intensity. The ratio between phonon peak height and elastic peak height versus temperature has been found to give the Bose-Einstein distribution of the phonon. Moreover, electron-phonon coupling constants have been extracted from the data. Chapter 6 presents the newly-designed spin manipulation system of HeSE. Combining two new spin precession solenoids, three RPSu power supplies, and a switch system, the new spin manipulation system improves the performance of the Cambridge HeSE instrument. The new system can be used for beam profile measurements, surface diffusion measurements, and surface phonon measurements. This thesis finishes with Chapter 7 discussing a conclusion of the preceding chapters and an outlook of potential research projects that can emanate from them.