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

The ability to probe the full dynamic response of quantum materials on the length- and time-scales (Å to attoseconds on up) fundamental to charge, spin and phonon interactions is leading to a host of new discoveries. Under thermal equilibrium conditions, materials can be tuned by varying the temperature, pressure, chemical doping or dimensionality. Recently, ultrafast light sources have undergone remarkable advances in recent years, achieving what was merely a dream three decades ago, i.e., full coherent control of light fields spanning the THz to the X-ray regions. These new capabilities are providing powerful new tools for coherently manipulating and probing quantum materials using light. We have developed a powerful new method called ultrafast electron calorimetry that can uncover hidden phases in magnetic and charge density wave materials.[1-5] 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. This makes it possible to coherently manipulate the structure, electron-phonon couplings, and expand the phase diagram of 2D charge density wave (CDW) materials.[1,3] We can also use light coherently transfer spin polarization from one element to another in a Heusler alloy, on few-femtosecond timescales.[2]
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