Acoustic Phonon Transport at Nanostructured Interfaces.

Transport of acoustic phonons plays an important role in a number of applications, such as thermal management, terahertz phononic filters, and high-frequency mechanical resonators. At the nanoscale, interfaces affect acoustic transport through boundary scattering. The overall aim of this work is to understand how acoustic phonons interact with nanostructured interfaces. We adopt an optical means based on ultrafast lasers to characterize dynamics of phonon (and thermal) transport through interfaces. We examine heat transfer properties of several semiconductor nanostructures using time-domain thermoreflectance and show that forming nanocrystals or nanochannels on the surface reduces the heat conduction rate because of enhanced phonon boundary scattering. Confined acoustic phonon modes may occur in an acoustically isolated medium that is enabled by high acoustic impedance mismatch at the boundaries. We insert a compliant organic (CuPc) film at the Al/Si interface to form a supported membrane resonator. We use femtosecond laser pulses to excite multiple GHz coherent phonon modes in such a cavity. The interfacial CuPc film acts as an acoustic etalon to select certain modes out of the broadband excitation, according to its Fabry-Perot resonances. Our observations have scientific significance in understanding coherent acoustic phonon transport, and support future studies of manipulating high-frequency acoustic energy in nanostructures. Interface irregularities such as roughness affect transport of acoustic phonons due to their short wavelengths. We develop a theoretical model based on perturbation analysis to calculate the specular and quasi-diffuse fields produced by scattering. We evaluate the effect of interface roughness on phonon mode conversion, scattered field distribution, and interface wave generation. We then estimate the performance of a coherent phonon reflector with roughened interfaces. Our findings have novel implications for the design of phononic devices; in addition, our preliminary experimental results confirm the reduction in coherent phonon reflection at roughened interfaces.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99971/1/hrsun_1.pd

Dissimilar thermal transport properties in κ\kappa-Ga2_2O3_3 and β\beta-Ga2_2O3_3 revealed by machine-learning homogeneous nonequilibrium molecular dynamics simulations

The lattice thermal conductivity (LTC) of Ga$_2$O$_3$ is an important property due to the challenge in the thermal management of high-power devices. We develop machine-learned neuroevolution potentials for single-crystalline $\beta$-Ga$_2$O$_3$ and $\kappa$-Ga$_2$O$_3$, and apply them to perform homogeneous nonequilibrium molecular dynamics simulations to predict their LTCs. The LTC of $\beta$-Ga$_2$O$_3$ was determined to be 10.3 $\pm$ 0.2 W/(m K), 19.9 $\pm$ 0.2 W/(m K), and 12.6 $\pm$ 0.2 W/(m K) along [100], [010], and [001], respectively, aligning with previous experimental measurements. For the first time, we predict the LTC of $\kappa$-Ga$_2$O$_3$ along [100], [010], and [001] to be 4.5 $\pm$ 0.0 W/(m K), 3.9 $\pm$ 0.0 W/(m K), and 4.0 $\pm$ 0.1 W/(m K), respectively, showing a nearly isotropic thermal transport property. The reduced LTC of $\kappa$-Ga$_2$O$_3$ versus $\beta$-Ga$_2$O$_3$ stems from its restricted low-frequency phonons up to 5 THz. Furthermore, we find that the $\beta$ phase exhibits a typical temperature dependence slightly stronger than $\sim T^{-1}$, whereas the $\kappa$ phase shows a weaker temperature dependence, ranging from $\sim T^{-0.5}$ to $\sim T^{-0.7}$.Comment: 8 pages, 7 figure