Nanoscale Energy Transport

Figure: Schematic illustrating the difference between diffusive and quasi-ballistic thermal transport across an interface. When the size of the interface is less than the phonon mean path in the substrate, the Fourier picture of diffusive heat flow breaks down, and energy dissipates quasi-ballistically.

Thermal transport in nano structures is significantly different from macro scale transport due to the fundamental scale length of the energy carriers, set by both classical and quantum size effects. Thermal energy is carried by phonons that travel away from a heat source, scattering and diffusing. However, for a heat source of nanometer-scale dimensions, these carriers travel ballistically for a distance comparable to the source size before scattering, leading to a non-local (i.e. non-diffusive) thermal energy distributions. Understanding nanoscale thermal transport is critical for the design of new energy nano systems for better insulation and thermoelectric energy recovery with significantly enhanced efficiency. Moreover, thermal management in nano-electronics and other nano-systems has become a bottleneck for continued Moore’s law scaling in electronics. As the characteristic dimension of devices continues to decrease to << 50nm, nonlocal heat transport create “heat sink problem” bottleneck that can interfere with reliable device operation. Co-engineering of electron and thermal (phonon) transport is required for designing reliable electronics, and this needs a good understanding of energy flow at the nanoscale. Advanced dense magnetic storage devices that use thermal energy to manipulate the magnetic state also require a precise understanding of thermal transport at the nanoscale.

Though significant progress has been made over the past 20 years, many fundamental aspects of energy flow in nano structures are not well understood, due to very limited experimental tools for probing nano systems (< 30 nm) on fast timescales (≈ ps to fs). Moreover, the basic models describing transport are still under development, and more sophisticated simulations remain difficult to verify. Ultrafast coherent high harmonic (HHG) beams can directly measure energy flow at the nanoscale, showing that energy transport can decrease significantly compared with Fourier law predictions.

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