Research of non-equilibrium phase transitions of strongly correlated electrons is built around addressing an outstanding challenge: how to achieve ultrafast manipulation of competing magnetic/electronic phases and reveal thermodynamically hidden orders at highly non-thermal, femtosecond timescales? There is growing evidence that femtosecond laser-induced transient polarization can be used to manipulate magnetism during a laser pulse [1, 2]. Recently we reveal a new paradigm called quantum femtosecond magnetism—photoinduced femtosecond magnetic phase transitions driven by quantum spin flip fluctuations correlated with laser-excited inter-atomic bonding coherence . We demonstrate an antiferromagnetic (AFM) to ferromagnetic (FM) switching in step with fs laser pulse in a colossal magneto-resistive (CMR) manganese oxide. Our results show a huge photoinduced femtosecond spin generation, measured by magnetic circular dichroism, with photo-excitation threshold behavior absent in the picosecond dynamics. This reveals an initial quantum coherent regime of magnetism, while the optical polarization/coherence still interacts with the spins to initiate local ferromagnetic correlations. This research thus provides a framework to merge quantum non-equilibrium dynamics and tuning of exotic ground states in strongly correlated oxides, and raises fundamental questions regarding some accepted rules of phase transitions, such as free energy and adiabatic potential surface.
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 T. Li, et al., Nature, 496, 69 (2013)