Jaeyun Moon, Mechanical Engineering, PhD. Thesis Defense

"Thermal conduction in amorphous materials and the role of collective excitations"

ABSTRACT

The atomic vibrations and thermal properties of amorphous dielectric solids are of

fundamental and practical interest. For applications, amorphous solids are widely

used as thermal insulators in thermopile and other detectors where low thermal

conductivity directly sets the sensitivity of the detector. Amorphous solids are of

fundamental interest themselves because the lack of atomic periodicity complicates

theoretical development. As a result, the lower limits of thermal conductivity in

solids as well as the nature of the vibrational excitations that carry heat remain active

topics of research.

In this thesis, we use numerical and experimental methods to investigate the thermal

conduction in amorphous dielectrics. We begin by using molecular dynamics

to investigate the thermal conductivity of amorphous nanocomposites. We find

that mismatching the vibrational density of states of constituent materials in the

composite is an effective route to achieve exceptionally low thermal conductivity in

fully dense solids.

We then transition to examining the properties of the atomic vibrations transporting

heating in amorphous solids. For decades, normal mode methods have been

used extensively to study thermal transport in amorphous solids. These methods

naturally assume that normal modes are the fundamental vibrational excitations

transporting heat. We examine the predictions from normal mode analysis that are

now able to be tested against experiment, and we find that the predictions from these

methods do not agree with experimental observations. For instance, normal mode

methods predict that the normal modes are scattered by anharmonic interactions as

in single crystalline solids. However, temperature dependent thermal conductivity

measurements demonstrate a typical glassy temperature dependence inconsistent

with normal modes scattering through anharmonic interactions. These discrepancies

suggest that normal modes are not the fundamental heat carriers in amorphous

dielectrics.

To identify the actual heat carriers, we draw on fundamental concepts from manybody

physics and inelastic scattering theory that dictate that the excitation energies of

a many-body interacting system are given by the poles of the single-particle Green's

function. The imaginary part of this function is proportional to the dynamic structure

factor that is directly measured in inelastic scattering experiments. Collective

excitations of a given energy and wavevector can thus be identified from peaks in

the dynamic structure factor; their damping is given by the broadening of the peak.

Using these concepts from many-body physics, the physical picture that emerges is

that heat is carried in large part by a gas of weakly interacting collective excitations

with a cutoff frequency that depends on the atomic structure and composition of the

glass.

We test this picture using numerical and experimental inelastic scattering measurements

on amorphous silicon, a commonly studied amorphous solid. We observe

collective excitations up to 10 THz, well into the thermal spectrum, and far higher

than previous inelastic scattering measurements on other glasses. Our numerical

and experimental evidence also confirms that the collective excitations are damped

by structural disorder rather than anharmonic interactions and that they dominate

the thermal conduction in amorphous silicon. Subsequent analysis shows that these

high frequency acoustic excitations are supported in amorphous silicon due to a

large sound velocity and monatomic composition, suggesting that other monatomic

amorphous solids with large sound velocities may also support these thermal excitations.

Overall, our results provide strong evidence that the heat carriers in amorphous dielectrics

are collective excitations rather than normal modes. This change in physical

picture advances our understanding of atomic dynamics in glasses and also provides a foundation for realizing dielectric solids with ultralow thermal conductivity.