Polaron Dynamics of The Photo-Induced Hidden Metallic Phase in La_2/3Ca_1/3MnO₃

Creating new “hidden” phases that have no analog in thermodynamic equilibrium is becoming increasingly important in condensed matter physics. The use of ultrafast laser with femtosecond pulse width to trigger such quantum phase transition is appealing as some of the hidden phases can be long-lived, with a lifetime of hours or weeks, and reversible with temperature sweeping or extra laser pulses. However, investigating and understanding the microscopic interactions in those hidden phases are still on the early stage due to the lack of high-end spectroscopic techniques that are sensitive to all degrees of freedom, precluding the development of designing high performance devices. Here, by combining the femtosecond ultrafast laser as a switching knob with the state-of-the-art ultrahigh-resolution resonant inelastic X-Ray Scattering as a probe, we reveal the phonon and polaron dynamics, as well as the evolution of crystal-field excitations across the insulating to photoinduced hidden metallic phase transition in a strained La_2/3Ca_1/3MnO₃ thin film. Upon the ultrafast excitation, the Jahn-Teller distortion is suppressed due to the strain relaxiation induced by the increasing itinerancy of the Mn e_g electrons. This renormalizes the energies of the polaron and Mn <i>dd</i> excitations, ultimately leading to the formation of a metastable long-lived ferromagnetic metallic phase. Our results demonstrate that the electronic properties of the mesoscopic photoinduced hidden phase evolve with laser fluence, as an evidence of the systematic softening of the polaron binding energy and Mn <i>dd</i> excitations, consistent with the trend extracted from the bulk phase comparision. However, the spectral weight of the phonon and polaron is different compared to the metallic as-grown film, proving that the photo-induced phase is unique and distinct from the bulk metallic phase. Our findings indicate the strength of electron-phonon coupling plays the key role to determine the conductivity of manganites.