Electron-phonon and phonon-phonon interactions low-dimensional nanostructures

Abstract

The electron-phonon interaction gives rise to a number of physically measurable quantities in solid state physics, perhaps most notably the heat capacity and the finite electrical resistivity in metals. The effect of extreme quantum confinement on the electron-phonon interaction is investigated for single-atom thick, infinitely long chains of metallic Al, Cu, Ag and Au atoms using density functional perturbation theory. In these atomic wires, the minimum energy geometries are distinct from the bulk configurations and the resulting changes in the inter-atomic force constants quantitatively change the phonon spectrum. The character of the electronic states at the Fermi level determines whether electrons will couple to longitudinal or transverse phonons, and in case of Al the overall strength of the interaction is reduced by two orders of magnitude relative to the bulk.

In both cases, experimental evidence is presented to compliment the predictions made by the theoretical calculations. For monolayer MoS2, experimental Raman measurements confirm a non-uniform redshift of the E2g and A1g modes. In order to confirm the predicted reduction in electron-phonon coupling strength in aluminum, we consider a bulk structure subject to pressure, which allows us to mimic the shorter inter-atomic separation observed in the atomic wires. Experiments confirm a reduction in the electrical resistivity as a function of pressure, due to weakened electron-phonon coupling. These studies suggest new ways of manipulating quantum transport in atomic scale materials.

The phonon-phonon interaction gives rise to the observed temperature-dependence of vibrational quantities, including phonon band structure and Raman spectra. Using finite-temperature molecular dynamics simulations based on density functional theory, an accurate description of the phonon-phonon interaction is made possible by considering anharmonic terms in the lattice potential energy. This technique is applied to the strict two-dimensional geometry in monolayer MoS2, where it is found that the Raman active E2g and A1g phonon modes show a non-uniform redshift with increasing temperature due to the distinct symmetries of these vibrational modes.