Vibrationally driven insulator-to-metal transition in magnetoresistive manganites: phonon-mode specificity, anharmonic energy redistribution, spin dynamics, and lifetime of the metastable metallic phase

Resonant mid-infrared excitation of the Mn-O stretching phonon at 71 meV (17.5 µm) switches the charge-ordered, antiferromagnetic insulator Pr $_{0.7}$ Ca $_{0.3}$ MnO $_3$ into a metastable metallic phase within about a picosecond, with a five-order-of-magnitude drop in resistivity and no electronic carrier injection (Rini et al., 2007). The experiment left four mechanistic issues open, which later literature has illuminated but not closed: the relative efficacy of different infrared-active phonon coordinates (the Mn-O-Mn bending mode versus the Mn-O stretch) in driving the transition; the role of ultrafast anharmonic vibrational energy redistribution (into the bend and into non-infrared-active Jahn-Teller modes) in inducing and stabilizing the metallic phase; whether and on what timescale ferromagnetic double-exchange order accompanies the vibrationally induced metallicity; and the lifetime, relaxation pathways, and spatial homogeneity of the metastable metallic state, which remain bounded only by nanosecond-bandwidth transport electronics. No published experiment has resolved these questions in the source system, so the problem — a quantitative microscopic account of how selective vibrational excitation switches the electronic and magnetic phase of a magnetoresistive manganite — remains open.

Background

Perovskite manganites $R_{1-x}A_x$ MnO $_3$ (rare-earth/alkaline-earth mixed valence Mn $^{3+}$ /Mn $^{4+}$ oxides) exhibit colossal magnetoresistance, a huge change of resistivity in magnetic fields that arises from the competition between a charge-and-orbital-ordered insulating phase — a low-temperature state in which the Mn $^{3+}$ /Mn $^{4+}$ valences and the orientations of the occupied Mn $e_g$ orbitals both form a periodic superstructure — and a ferromagnetic metallic phase. In the metallic phase, conduction proceeds by double exchange: an electron hops between Mn sites through the oxygen $2p$ orbitals only when the Mn core spins are aligned, so metallicity and ferromagnetism are tied together. The insulating phases are stabilized by the Jahn-Teller effect, a spontaneous lattice distortion of the MnO $_6$ octahedra that localizes the Mn $e_g$ electrons (the $e_g$ manifold being the twofold-degenerate set of Mn $3d$ orbitals pointing toward the oxygen ligands). Pr $_{0.7}$ Ca $_{0.3}$ MnO $_3$ (PCMO) is insulating at all temperatures and dopings; its low-temperature charge-ordered, antiferromagnetic state adjoins a 'hidden' metallic state reachable only by strong perturbations such as X-ray, above-bandgap light, or current. The key geometric control parameter is the perovskite tolerance factor, set by the Mn-O bond lengths and the Mn-O-Mn bond angle, which fixes the overlap between Mn $3d$ and O $2p$ orbitals and hence the one-electron bandwidth $W$ : small distortions of the lattice translate into large changes of the electronic ground state. The material has three infrared-active (dipole-active) phonons at 23, 42 and 71 meV; the 71 meV mode is the Mn-O stretching vibration and the 42 meV mode the Mn-O-Mn bending vibration, both of which modulate the tolerance-factor coordinates. In 2007, Rini et al. showed that femtosecond mid-infrared pulses resonant with the 71 meV Mn-O stretch drive PCMO into a metallic state within about 1 ps, with a prompt resistance drop from 2 GΩ to 1.25 MΩ, a response that tracks the phonon absorption line (and thus is mode-selective rather than a broadband heating effect), and negligible multiphoton carrier injection. The magnetic character of this vibrationally induced metallic phase was not established by that measurement: Rini et al. noted that resolving whether the transition is synonymous with an antiferromagnetic-to-ferromagnetic transformation remained open, identifying magneto-optic Kerr measurements — the rotation of the polarization of reflected probe light, which is proportional to the sample magnetization — as the probe for it. This was the founding demonstration of what became 'nonlinear phononics': an infrared-active phonon driven to large amplitude couples through lattice anharmonicity (a mechanism called ionic Raman scattering) to other phonon coordinates — including Raman-active modes, which are symmetry-forbidden for direct one-photon infrared excitation — and to the electronic structure, transiently reshaping the lattice and its electronic phases. Subsequent work established ionic Raman scattering in the ferromagnetic manganite La $_{0.7}$ Sr $_{0.3}$ MnO $_3$ (Först et al., 2011, Nature Physics), quenched magnetic and orbital order by mid-infrared lattice excitation in the layered manganite La $_{0.5}$ Sr $_{1.5}$ MnO $_4$ (a Ruddlesden-Popper structure of single MnO $_2$ sheets separated by rock-salt layers; Först et al., 2011, PRB), developed a first-principles theory in which cubic anharmonic coupling of the driven infrared mode to a Raman-active mode explains the manganite insulator-metal transition (Subedi, Cavalleri and Georges, 2014), and found, by time-resolved resonant X-ray diffraction in Pr $_{0.5}$ Ca $_{0.5}$ MnO $_3$ , prompt and highly nonlinear (threshold-like) charge-order melting after mid-infrared excitation that density-functional calculations attribute to direct nonlinear electron-phonon coupling of the driven mode (Esposito et al., 2017). Mode-selective phonon driving has since induced ferromagnetism in the titanate YTiO $_3$ with mode-dependent efficacy (Disa et al., 2023). What no later experiment has done is return to the original manganite question and measure, in a charge-ordered magnetoresistive manganite, which phonon coordinate actually drives the transition, how the deposited vibrational energy flows, what magnetic order forms, and how the metastable metallic phase relaxes.

Problem Statement

For the insulator-to-metal transition of a charge-ordered, magnetoresistive manganite (prototypically Pr $_{0.7}$ Ca $_{0.3}$ MnO $_3$ ) driven by resonant mid-infrared excitation of the 71 meV Mn-O stretching phonon, establish a quantitative microscopic account of the vibrational control of the electronic phase, by answering four linked questions. (1) Phonon-mode specificity: using the now-available intense, narrowband mid-infrared and terahertz sources that reach the lower-frequency infrared-active phonons, drive the Mn-O-Mn bending mode (42 meV) and the 23 meV external mode under otherwise identical conditions and determine their relative efficacy, fluence thresholds and dynamics compared with the Mn-O stretch in driving the insulator-metal transition - that is, which lattice coordinates of the tolerance factor actually switch the phase. (2) Anharmonic energy redistribution: measure and/or compute how the energy deposited in the driven Mn-O stretch redistributes on ultrafast timescales, in particular the anharmonic coupling into the Mn-O-Mn bend and into non-infrared-active Jahn-Teller modes that influence electron localization, and thereby disentangle the competing mechanisms invoked in the literature (phonon-phonon rectification into Raman-active coordinates, which are symmetry-forbidden for direct one-photon infrared driving, versus direct nonlinear electron-phonon coupling of the driven infrared mode), while quantitatively excluding multiphoton carrier generation and heating. (3) Spin dynamics: determine whether the vibrationally induced metallic phase is synonymous with an antiferromagnetic-to-ferromagnetic transformation, as double exchange predicts for a magnetoresistive manganite, and on what timescale any ferromagnetic order is generated. (4) Metastable phase: quantify the lifetime and relaxation pathways of the vibrationally induced metallic state and the spatial homogeneity (or phase-separated character) of the switched volume, using high-bandwidth transport and local probes, superseding the nanosecond-resolution, ringing-limited bounds of the original experiment. The four questions bear on a single objective - the mechanism and physical character of phonon-driven phase switching in manganites - and the microscopic account sought here is one in which the mode comparison, the energy-flow and coupling determination, the magnetic-order measurement, and the relaxation/homogeneity characterization are mutually consistent.

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