Ultrafast electron calorimetry to uncover new transient states in charge density wave and magnetic materials

Understanding and harnessing phase transitions in a wide range of quantum materials remains a great challenge. The strongly coupled interactions between the charges, spins and lattice often make it challenging to unambiguously uncover the underlying mechanisms. Here we present a new ultrafast electron calorimetry technique to map the laser-driven phase space of materials. It relies on the small mass and heat capacity of the electrons, which means that they can react very quickly to any phase changes in a material. Specifically, we use time- and angle-resolved photoemission spectroscopy to systematically measure the band structure, electron temperature and heat capacity as a function of both time delay and laser fluence. First, we find a new metastable state in the charge density wave material 1T-TaSe₂, which is characterized by a significantly reduced effective heat capacity. Our results also reveal new manifestations of the electron-phonon coupling in the dynamics of both the electron and phonon bathes. Second, we uncover a highly-excited spin state that launches the ultrafast magnetic phase transition in Ni. Finally, we note that our approach is general, and can be used to uncover the presence of hidden phases in other materials.