Earlier this year, the research group in Peking University developed an electron energy loss spectroscopy technique (termed 4D-EELS) based on a scanning transmission electron microscope. This technique offers high spatial, energy and momentum resolutions with high detection sensitivity, enabling nanoscale phonon dispersion measurements (Nature Communications, 12, 1179 (2021)). It can achieve a good balance between spatial and momentum resolutions, being only ~15% worse than the diffraction limit – the ultimate theoretical bound set by the uncertainty principle.
In their new paper entitled “Measuring phonon dispersion at an interface” on Nature, the authors used a cubic boron nitride / diamond heterointerface as example, first measured the local phonon density of states at the atomic scale, and successfully observed phonon modes localized at the interface and phonon modes isolated from the interface. Then they measured the dispersion relation of interface modes with carefully balanced spatial and momentum resolutions. The experimental observations are in good agreement with first-principle calculations. The observed interface modes are expected to substantially affect thermal conductance across the interface and carrier mobility of the two-dimensional electron gas at the interface. The ability to measure local phonon dispersion should also be useful in studying topological interface phonon modes and even in understanding interface superconductivity.