Ultrafast Electron Calorimetry: Uncovering New Light-Induced Phases in Magnetic and 2D Materials

We have developed a powerful new method called ultrafast electron calorimetry that can uncover hidden
phases in magnetic and charge density wave materials. By using time and angle-resolved photoemission
spectroscopy to measure the dynamic electron temperature and full band structure as the laser excitation
is varied, one can clearly identify when changes in state or couplings occur in a material. Recently we
used this approach to coherently manipulate the structure, electron-phonon couplings, and state of the 2D
charge density wave (CDW) material 1T-TaSe 2 . Instead of the two material phases normally accessible
using temperature tuning (CDW and normal phases), ultrafast laser excitation expands the phase diagram
to include at least two new phases: a new metastable CDW state, and an inverted CDW state. Moreover,
the electron temperature, bandwidth, band gap and band shift are all modulated by the CDW breathing
mode, and this modulation changes in phase as the material enters a new state. In addition to the ability to
map the state of a material, we can use pulse sequences to coherently guide a material from one phase to
another.
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