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Have you ever heard of the phonon? It's a concept that may not be familiar to most people, but it's just as fascinating as the photon. While the photon is the quantum description of the electromagnetic wave, the phonon is a quantum of energy associated with the vibrational motion of atoms, which propagates through the material as a wave responsible for the transmission of heat and sound in solids.

According to its definition, a phonon cannot traverse a vacuum gap between solids due to the lack of material to vibrate. However, my research has led to the development of a groundbreaking theory that utilizes piezoelectricity to create an electric field through the deformation caused by lattice vibration, allowing for acoustic phonons to tunnel across the vacuum. Our formulation can calculate the energy transmission of acoustic phonons for anisotropic materials with all possible crystal orientations. It has allowed us to study the behavior of acoustic phonons in a variety of materials and has provided us with a deeper understanding of the underlying physics.

Through our research, we revealed that the excitation of surface waves tightly bound at the material interface is responsible for such tunneling. This finding led to an intriguing discovery that unity transmission can be achieved through resonant tunneling. This is a significant finding because it suggests the potential for complete tunneling of acoustic phonons and could have practical applications in proximity sensing and nanoscale imaging.

In addition, we have conducted a theoretical analysis of the heat transfer carried by the piezoelectrically mediated acoustic phonon tunneling and found that it can enable heat transfer over significant distances. This effect becomes more pronounced at lower temperatures and can even surpass all other heat transfer mechanisms at temperatures in the tens of Kelvin range with sub-micrometer vacuum distances.

Building upon this theoretical understanding, we have conducted experiments at sub-Kelvin temperatures using nanometer-scale devices equipped with superconducting sensors. Our results demonstrate, for the first time, the transfer of heat through a vacuum gap facilitated by acoustic phonon tunneling. This provides conclusive evidence of the existence of this phenomenon.

The research and discovery presented in my dissertation fills in many gaps in the understanding of acoustic phonon tunneling. It opens up a new field of study that is crucial for further advancements in nanotechnology. As thermal issues have become a critical blockade for scaling transistor density, our findings offer a deeper understanding and potential solution for managing heat at the nanoscale. Furthermore, the results of our study can have practical applications in thermal switches, sensors, and meta-materials, among others. These applications can be utilized in various fields, including nano-imaging, cryogenic detectors, and energy harvesting.

The opponent is Professor Achim Kittel (Carl von Ossietzky University of Oldenburg, Germany) and custos is Professor Ilari Maasilta (Ģ��ֱ��). The language of the dissertation is English.

The dissertation has been published in an online publication series and is available in the JYX publication archive: