‘Hidden’ polaronic state across the photoinduced transition in a magnetoresistive manganite

Creating new non-ergodic ‘hidden’ phases that have no analog in thermodynamic equilibrium is an active topic in condensed matter physics. The use of femtosecond laser pulses to trigger these ‘hidden’ transitions is appealing as some of these states can be long-lived, with a lifetime of hours or weeks, and reversible with additional external stimuli. However, the investigation of the microscopic interactions and elementary excitations of these phases is often precluded due to the lack of experimental techniques sensitive to the spin, charge, orbital, and lattice degrees of freedom and compatible with atomically thin films. Here, we investigate the evolution of the elementary excitations across the ultrafast ‘hidden’ transition of La_2/3Ca_1/3MnO₃ by synergically combining femtosecond laser stimuli with ultrahigh-resolution Resonant Inelastic X-Ray Scattering (RIXS). Upon photoexcitation at 1.2 eV, the static Jahn-Teller distortion is suppressed due to the renormalization of the occupancy of the Mn³⁺ e_g electrons which consequently softens the polaron energy and leads to a metastable mesoscopic ‘hidden’ phase that is distinct than the pristine ferromagnetic metallic (FMM) phase. From our spectroscopic results, we can explicitly connect electric resistivity and polaron energy with an exponential relation which proves that the polaron in LCMO falls into the ‘strongly-coupled’ regime across the whole phase diagram of manganites along the strain, temperature, and ‘hidden’ photoexcited axes. Our findings provide comprehensive understanding of the excitations permeating the photoinduced transition in a hallmark magnetoresistive manganite. Ultimately, this novel laser-RIXS approach sets the basis for exploring the physics of photoinduced ‘hidden’ phases in quantum materials in static non-stroboscopic and time-resolved mode with sensitivity to quasi-particles of multiple electronic degrees of freedom.