Quantum materials represent one of the most promising frontiers in the quest for faster, lightweight, energy-efficient technologies. However, their inherent complexity and rich phase landscape make them challenging to understand or manipulate. Here, we present a new ultrafast electron calorimetry technique that can systematically uncover new phases of quantum matter. Using time- and angle-resolved photoemission spectroscopy, we measure the dynamic electron temperature, band structure, and heat capacity. This approach allows us to uncover a new long-lived metastable state in the charge density wave material 1T-TaSe 2 , which is distinct from all the known equilibrium phases: It is characterized by a substantially reduced effective total heat capacity that is only 30% of the normal value, because of selective electron-phonon coupling to a subset of phonon modes. As a result, less energy is required to melt the charge order and transform the state of the material than under thermal equilibrium conditions.
Shi, X., You, W., Zhang, Y., Tao, Z., Oppeneer, P. M., Wu, X., … Murnane, M. (2019). Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe 2 mediated by mode-selective electron-phonon coupling. Science Advances, 5(3). https://doi.org/10.1126/sciadv.aav4449